31. pyroxene geothermometry of basalts and an … · group 6, which will be called two-pyroxene...

31. PYROXENE GEOTHERMOMETRY OF BASALTS AND AN ANDESITE FROM THEPALAU-KYUSHU AND WEST MARIANA RIDGES, DEEP SEA DRILLING PROJECT LEG 59

Teruaki Ishii, Ocean Research Institute, University of Tokyo, Minamidai, Nakano-ku, Tokyo 164, Japan

Appendix: "Data on Major-Element Compositions of DSP Leg 59 Tholeiitic Basalt, Calc-Alkalic Andesite,and Glass Obtained by Wet Chemical Analysis, XRF, and EPMA Methods," by Teruaki Ishii and Hiroshi Haramura

ABSTRACT

Pyroxenes contained in island-arc type basalts and a calc-alkalic andesite from DSDP Leg 59, underwent micro-probe analyses; from these data pyroxene geothermometry was calculated. The samples were plagioclase-augite-bronzite-( ± )pigeonite phyric tholeiitic basalt lavas and intrusive units from Holes 448 and 448A on the Palau-Kyushuremnant arc and plagioclase-augite-orthopyroxene-titanomagnetite phyric basalt clasts and a calc-alkalic andesite clastfrom Hole 451 on the West Mariana remnant arc.

The crystallization trend of pyroxene in magma chambers (delineated by tracing the cores of phenocrysts in thelavas) differs from that in the groundmass of lavas.

Magma temperatures, which can be represented by the crystallization temperatures of phenocryst pyroxenes, areestimated to be between 1130° and 1075°C for the Site 448 basalts and about 1O55°C for the Site 451 basalts. Thesebasalts have solidification indexes between 34 and 19. In contrast, a 970°C temperature is estimated for a Site 451calc-alkalic andesite with a solidification index of 20.

Differences in volatile content, particularly water, appear to be the controlling factors in determining thesetemperatures. The water content of tholeiitic magmas is presumably lower than that of calc-alkalic magmas.

INTRODUCTION

Various hypotheses have been proposed regardingthe origin of the ridge-basin system in the PhilippineSea (Karig, 1975; Uyeda and Ben-Avraham, 1972; Hildeet al., 1977). Karig (1975) suggests that a remnantarc-arc system of Palau-Kyushu-West Mariana-Mari-ana ridges was formed by opening of the Parece VelaBasin and Mariana Trough. However, very littlepetrological data have been available on these inactiveremnant arcs.

During the DSDP Leg 59 cruise, orthopyroxene-bearing rocks (lavas, intrusive rocks, and volcaniclasticbreccias and tuffs) were obtained from Site 448 on thePalau-Kyushu Ridge and Site 451 on the West MarianaRidge.

Pyroxenes of six representative two-pyroxene rocksfrom these ridges were analyzed by electron probemicroanalyzer. On the basis of the resulting data,crystallization trends of groundmass pyroxenes in thelavas and phenocryst pyroxenes from the magma cham-ber were determined, and the temperatures of thesemagmas were estimated by pyroxene geothermometry(Ishii et al., 1976, 1979). The relations betweenestimated lava temperatures and the bulk compositionof the calc-alkalic andesite will be reported. The originof the magma will be discussed in comparison with pre-vious results (Ishii, 1974; 1976; 1978). Data on major-element compositions of other rocks analyzed fromcores collected on DSDP Leg 59 are included in theAppendix.

ANALYTIC METHODChemical compositions of rock-forming minerals were determined

with a Japan Electron Optics Laboratory electron probe micro-

analyzer (EPMA) Model JXA-5, which has a 40° take-off angle. Theanalytic method is the same as that described by Nakamura andKushiro (1970a).

Because most of the rock-forming minerals in volcanic rocks are inmany cases zoned, their compositional range was determined bycareful analysis. Initially, their compositional zoning was observed byscanning and partial analyses of three elements; then major elementswere analyzed at several points. Glass was analyzed with a specimencurrent of 0.02 µamps (A) and an electron beam size of 2 to 3 µm. Toavoid light-element volatilization, the glass sample was moved rapidlyover a wide area during analysis. Ten or more counts were averagedfor each glass.

X-ray fluorescence (XRF) analyses of the major elements of rockswere carried out with a Rigaku Memory Controlled Free AutomaticSequential Vacuum X-ray Spectrometer Geigerflex System No. 3064,with a current of 45 Kv and 30 mA. The analytic method is the same asthat described by Matsumoto and Urabe (personal communication).

DESCRIPTION OF ROCKS ANDMICROPROBE ANALYSES

DSDP Site 448 is located on the western edge of thePalau-Kyushu Ridge at a water depth of 3500 meters.Penetration was successful to a sub-bottom depth of584.5 meters for Hole 448 and 914.0 meters for Hole448A. Holes 448 and 448A are treated as one sequence.The first igneous rock was cored at 337.5 meters sub-bottom. The underlying igneous complex consists of analternation of basaltic lavas, intrusive rocks, and pyro-clastic rocks. The 27 igneous rock units are classified into6 petrographic groups, according to the assemblage ofphenocryst minerals : (1) aphyric basalts (6 units), (2)plagioclase-phyric basalt (2 units), (3) plagioclase-oli-vine-phyric basalt (2 units), (4) plagioclase-augite-phyricbasalt (10 units), (5) plagioclase-olivine-augite-phyricbasalt (3 units), and (6) plagioclase-augite-orthopyrox-ene phyric basalt (4 units). Group 6, which will be calledtwo-pyroxene rocks, occurs as pillow lavas or flows andas a dike at sub-bottom depths of 521 to 575 meters and

693

T. ISHII

887 to 897 meters, respectively. Three rocks selectedfrom Group 6 were used for detailed petrologic studies.

Site 451 is located on the eastern side of the WestMariana Ridge at a water depth of 2060 meters. The sitewas cored continuously to a sub-bottom depth of 930.5meters. A thick sequence of massive volcaniclastic brec-cias and tuffs were recovered between 65.5 and 930.5meters. Within the volcaniclastic breccias, four petro-graphic groups of volcanic clasts are recognized: (1)aphyric or sparsely phyric basalt, (2) plagioclase-augite-phyric basalt, (3) plagioclase-augite-titanomagnetite-phyric basalt and (4) plagioclase-augite-orthopyroxene-titanomagnetite-phyric basalt and andesite. Groups 3and 4 contain the most abundant clasts. Three rocksamples from Group 4 were selected for detailed petro-logic studies.

The petrology of the six samples used for thesestudies is described in the following report. Microprobestudies of these rocks were carried out on plagioclase,titanomagnetite, orthopyroxene, and clinopyroxene; se-lected results are presented in Tables 1 through 6 andplotted in Figure 1.

Plagioclase-Augite-Bronzite-Pigeonite-Phyric BasalticGlass (Rock A814) and Bytownite-Augite-Bronzite-Phyric Basalt (Rock B815)

The specimens are from the upper chilled glassymarginal zone (Rock A814) (Table 1) and interior zone(Rock B815) (Table 2) of a massive lava flow more than3 meters thick from Hole 448—Samples 448-59-1, 61-65cm (Rock A814) and 117-120 cm (Rock B815). Mega-scopically, Rock B815 is vesicular and pale brown, with

CaMg CaFe CaMg

Mg

Sample 448-59-1, 61-65 (A814)PI-Aug-Opx•Pig-phyric basalt

O = phenocryst coreo = phenocryst rimc = microphenocryst• = ground mass

50Atomic (

Fe

Sample 448-59-1, 117-120 (B815PPI-Aug-Opx-phyric basalt lava

O= phenocrysto = phenocryst rim• = ground mass

Mg 50Atomic %

CaMg CaFe

* Sample 448A-62-1, 70-83 (C860P

PI•Aug•Opx-phyric basalt intrusive rock

= phenocryst core

= groundmass

Mg 50Atomic'

CaMg CaFe

Sample 451-38-1, 137-140 (D 1050)PI-Aug-Opx-Mt-(CI)-phyric basalt

O= phenocrysto r phenocryst rim• = groundmass

Mg 50Atomic %

CaMg CaFe CaMg

Sample 451-46-1, 24-29 (E 1130)PI-Aug-Mt-Cpx-(OI)-phyric basalt

O= phenocrysto = phenocryst rim• = groundmass

50Atomic %

Fe

CaFe

Sample 451-59-1, 55-57 (F 1200)PI•Opx-Aug-Mt-phyric calc-alkalic

andesite

O = phenocrysto = phenocryst rim• groundmass

Mg 50Atomic '

Fe

Figure 1. A. Ca-Mg-Fe plot of analyzed pyroxenes in Rock A814. B. Ca-Mg-Fe plot of analyzed pyroxenes in Rock B815. C. Ca-Mg-Fe plot ofanalyzed pyroxenes in Rock C860. D. Ca-Mg-Fe plot of analyzed pyroxenes in Rock D1050. E. Ca-Mg-Fe plot of analyzed pyroxenes in RockEl 130. F. Ca-Mg-Fe plot of analyzed pyroxenes in Rock F1200.

694

PYROXENE GEOTHERMOMETRY

Table 1. The pyroxene analyses of augite-bronzite-pigeonite-phyric basaltic glass (Rock A814)from Leg 59, Sample 448-59-1, 61-65 cm.a

Anal. No.b

SiO2

A12O3TiO2Cr2O3FeO*MnOMgOCaONa2OTotal

O = 6.000SiAlAlTiCrFeMnMgCaNaZWXY

Atomic °7oCaMgFe

Contig.Anal. No.c

Orthopyroxene

Phenocryst core

A15

52.811.150.160.04

15.880.37

26.142.010.04

98.60

1.9480.0500.0000.0040.0010.4900.0121.4370.0790.0031.9982.026

4.071.624.4

A16

52.630.800.180.00

16.270.34

25.352.110.02

97.70

1.9630.0350.0000.0050.0000.5070.0111.4090.0840.0011.9982.018

4.270.425.4A17

Phc.

A l l

53.060.910.160.04

16.000.39

25.532.220.06

98.37

1.9630.0370.0020.0040.0010.4950.0121.4070.0880.0042.0002.015

4.470.724.9

A12

m. and microphc.

A18

53.191.200.230.00

18.460.36

23.852.400.11

99.80

1.9600.0400.0120.0060.0000.5690.0111.3100.0950.0082.0002.011

4.866.428.8

A13

51.841.270.240.01

18.170.44

24.042.360.04

98.41

1.9410.0560.0000.0070.0000.5690.0141.3410.0950.0031.9972.029

4.766.928.4

Phc.c.

A17

51.431.800.310.059.300.25

16.0818.830.19

98.24

1.9420.0580.0220.0090.0010.2940.0080.9050.7620.0142.0002.015

38.946.215.0

A16

Phc.

A12

50.971.940.360.03

10.220.33

16.1818.540.21

98.78

1.9240.0760.0100.0100.0010.3230.0110.9100.7500.0152.0002.030

37.845.916.3

A l l

Augite

m. and microphc.

A19

51.512.010.270.04

13.090.22

16.3914.850.22

98.60

1.9470.0530.0370.0080.0010.4140.0070.9240.6010.0162.0002.008

31.047.621.3

A14

50.852.030.340.05

10.120.26

15.6419.060.25

98.60

1.9250.0750.0150.0100.0010.3200.0080.8820.7730.0182.0002.029

39.144.716.2

Gm.

A21

50.353.190.550.03

12.030.28

15.8416.620.20

99.09

1.8990.1010.0400.0160.0010.3790.0090.8900.6720.0152.0002.022

34.645.919.5

Pigeonite

Gm.

A22

52.380.980.270.00

18.550.33

22.433.990.13

99.06

1.9580.0420.0010.0080.0000.5800.0101.2500.1600.0092.0002.018

8.062.829.1

Note: FeO* indicates total Fe expressed as FeO.a Abbreviations often used in Tables 1 through 6 are: Phc. = phenocryst. m. = margin, microphc. = microphenocryst,

Gm. = groundmass, Pig. = pigeonite, c. = core.° Anal. no. in Tables 1 through 7 and 9 = the analysis number author uses for his data.c Contiguous-phase analyzed number. Each pair was used for temperature estimation.

Table 2. The pyroxene analyses of augite-bronzite-phyric tholeiitic basalt (Rock B815) from Leg 59, 448-59-1, 117-120 cm.

Anal. No.

SiO2

A12O3

TiO2Cr2O3FeO*MnOMgOCaONa2OTotal


Atomic %CaMgFe

Contig.Anal. No.

Phenocryst core

B08

54.001.100.150.04

15.120.30

26.432.040.02

99.20

1.9670.0330.0140.0040.0010.4610.0091.4350.0800.0012.0002.005

4.072.723.3

B10

53.201.190.090.04

16.620.36

25.582.080.01

99.17

1.9550.0450.0070.0020.0010.5110.0111.4010.0820.0012.0002.016

4.170.325.6

B09

B12

52.681.170.130.05

17.940.36

24.502.060.02

98.91

1.9540.0460.0050.0040.0010.5570.0111.3550.0820.0012.0002.016

4.168.027.9

Bll

Orthopyroxene

Phenocryst margin and microphenocryst

B13

52.131.730.240.03

18.230.38

23.232.600.03

98.60

1.9460.0540.0220.0070.0010.5690.0121.2930.1040.0022.0002.010

5.365.829.0

B16

52.791.650.160.04

16.380.35

25.241.960.04

98.61

1.9490.0510.0200.0040.0010.5060.0111.3890.0780.0032.0002.012

3.970.425.6

B15

B17

52.551.650.210.03

16.610.35

24.892.150.03

98.47

1.9460.0540.0180.0060.0010.5150.0111.3740.0850.0022.0002.012

4.369.626.1

B23

52.371.620.220.00

18.270.41

24.282.040.04

99.25

1.9400.0600.0110.0060.0000.5660.0131.3410.0810.0032.0002.020

4.167.428.5

B22

B24

52.351.400.240.02

16.300.38

25.162.360.05

98.26

1.9440.0560.0050.0070.0010.5060.0121.3920.0940.0042.0002.020

4.769.925.4

Phc.

B09

51.302.000.330.08

10.060.23

15.5719.200.20

98.97

1.9320.0680.0210.0090.0020.3170.0070.8740.7750.0152.0002.020

39.444.516.1

B10

c.

B l l

50.092.530.400.05

12.400.27

14.3818.000.22

98.34

1.9160.0840.0300.0120.0020.3970.0090.8200.7380.0162.0002.023

37.742.020.3

B12

Augite

Phc. m. and microphc.

B15

51.591.940.310.089.430.23

16.2119.190.20

99.18

1.9330.0670.0180.0090.0020.2950.0070.9050.7700.0152.0002.022

39.145.915.0

B16

B18

51.152.600.290.118.030.19

16.3019.900.17

98.74

1.9170.0830.0310.0080.0030.2520.0060.9100.7990.0122.0002.022

40.746.412.8

B22

50.752.380.380.00

11.250.28

15.0518.760.21

99.06

1.9190.0810.0250.0110.0000.3560.0090.8480.7600.0152.0002.025

38.743.218.1

B23

Gm.

B21

51.521.930.490.04

11.490.28

15.1417.590.23

98.71

1.9480.0520.0340.0140.0010.3630.0090.8530.7120.0172.0002.003

36.944.218.8

Note: See Table 1 for meaning of abbreviations.

695

T. ISHII

rare phenocrysts of plagioclase (up to 3 mm in diameter)and indistinct black pyroxene. Scattered vesicles (up to 2mm) are partly filled with calcite and zeolites. RockA814 has a compact, fresh, black glass margin up to 1.5cm thick containing sparse plagioclase phenocrysts.

Thin-section observations of Rock B815 show thatthe rock consists of phenocrysts and microphenocrystsof plagioclase (15 volume %, exclusive of vesicles,0.4-3.0 mm long), augite (2%, 0.4-1.5 mm), and ortho-pyroxene (1%, 0.5-2.6 mm) with a hyalophitic ground-mass of plagioclase (25%, 0.5-0.1 mm), augite (20%,0.05-0.1 mm), titanomagnetite (7%, 0.01-0.05 mm),and devitrified dark brown glass (30%). Scattered ir-regular vesicles (0.2-3 mm, one third in volume) areusually not filled with secondary minerals.

Plagioclase phenocrysts are euhedral to subhedraland commonly contain rounded brown to black alteredglass. Their composition ranges from An88Ab12 to An80Ab2o (molar ratios) and from An67Ab33 to An62Ab38 inthe cores of phenocrysts and groundmass plagioclase,respectively; both zone toward An57Ab43 in their thinrims. On the other hand, rarely observed anhedral,rounded plagioclase is more calcic (An92Ab8-An9oAblo)and contains abundant glass inclusions. These charac-teristics indicate that the calcic plagioclases are xeno-crysts.

Augite phenocrysts are euhedral to subhedral andcommonly glomerophyric with plagioclase and augiteitself. As shown in Figure IB, the reverse zoning(increasing Mg/Fe ratio) is observed in the augitecrystallization sequence, that is, chemical compositionof augite varies from large phenocryst cores (Ca39Mg45Fe16 in atomic ratio) through small phenocrystcores (Ca38Mg42Fe20) with relatively low Mg/Fe ratios,and through microphenocrysts (Ca41Mg46Fei3) withrelatively high Mg/Fe ratios to groundmass augitecrystals (Ca^Mg^Fe^) with relatively low Mg/Fe andCa/(Mg + Fe + Ca) ratios.

Bronzite phenocrysts are euhedral to subhedral,sometimes glomerophyric with augite, and alwaysrimmed by zoned augite. The chemical composition ofbronzite varies from large phenocryst cores (Ca4Mg73Fe23) through small phenocryst cores (Ca4Mg70Fe26)with relatively low Mg/Fe ratios, and through micro-phenocrysts (Ca4 5Mg7OFe25.5) with reversed trends toMg/Fe ratios, to phenocryst margins (CasMg^Fe^)with relatively low Mg/Fe ratios. Orthopyroxene is notobserved in the groundmass.

The chemical composition of groundmass titanomag-netite varies from Usp33Mt67 (ulvospinel and magnetitemolar ratio) with relatively large grains about 0.02 mmlong, to Usp55Mt45 with grains smaller than about 0.01mm.

Under the microscope, the glassy rock (A814) con-sists of phenocrysts and microphenocrysts of plagio-clase (10%, 0.2-3 mm), augite (3%, 0.1-0.5 mm), andbronzite (1%, 0.1-0.7 mm), and microphenocrysts ofpigeonite (1%, 0.1-0.3 mm) with a groundmass ofplagioclase (3%, 0.01-0.1 mm), augite (1%, 0.01-0.03mm), pigeonite (trace, 0.01-0.03 mm), and scarcely de-vitrified pale brown clear glass (80%).

Plagioclase phenocrysts are euhedral to subhedraland rarely contain brown to black relatively freshglasses. Groundmass plagioclase crystals are lath-shaped in most cases, but have stout prismatic forms ina few large crystals.

Augite phenocrysts are euhedral to subhedral. Thechemical composition of augite varies from phenocrystcores (Ca43Mg48Fe9) with relatively high Mg/Fe ratios,through phenocryst margins or microphenocrysts (Ca36Mg47Fe17-Ca29Mg49Fe22) with relatively low Mg/Fe andCa/(Ca + Mg + Fe) ratios, to groundmass crystals(Ca33Mg46Fe21). This represents an intensive quenchedcrystallization trend.

Orthopyroxene phenocrysts are euhedral to sub-hedral, and usually have no clinopyroxene rim. Al-though some orthopyroxene phenocrysts have a rela-tively thick augite rim, pigeonite rims are very rare. Thechemical composition of Ca-poor pyroxene varies frombronzite phenocryst cores (Ca4Mg73Fe23) with relativelyhigh Mg/Fe ratios, through phenocryst margins ormicrophenocrysts (Ca4Mg66Fe30) with normal trends toMg/Fe ratios, to pigeonite microphenocrysts (Ca8Mg62Fe30). These pyroxene crystallization sequences indicatethat three pyroxenes (augite + orthopyroxene + pigeon-ite) coprecipitated in equilibrium in the micropheno-cryst stage. Opaque minerals are not observed.

Anorthite-Bronzite-Augite Phyric Basalt (Rock C860)

This specimen, from the interior zone of a massivedike with a thickness of more than 2 meters, was col-lected from Hole 448A (Sample 448A-62-1, 70-83 cm)(Table 3). Megascopically, the rock is compact and palegreenish gray, with phenocrysts of abundant plagioclase(up to 4 mm) and distinct black pyroxene (up to 3 mm).

Thin-section observations show that the rock consistsof phenocrysts and microphenocrysts of plagioclase(40%, 0.5-3 mm), orthopyroxene (3%, 0.5-2 mm), andaugite (2%, 0.5-1.5 mm), with an intergranular ground-mass of plagioclase (15%, 0.1-0.3 mm), augite (15%,0.05-0.2 mm), magnetite (5%, 0.02-0.1 mm), and al-tered pale brown glass (20%).

Plagioclase phenocrysts are euhedral and rarelyglomerophyric. The composition ranges from An93Ab7to An89Abn and An70Ab30 to An67Ab33 in the core ofphenocrysts and groundmass plagioclase, respectively;both are zoned toward An53Ab47 in their thin rims. Verysodic rims (An33Ab63Or4) are rarely observed.

Bronzite phenocrysts are subhedral and sometimespartially or wholly replaced by clay minerals, especiallyin the margins. Orthopyroxene and pigeonite are notobserved in the groundmass. Phenocrysts of bronziteare very homogeneous (Ca4 5Mg70 5Fe25) and free fromaugite or pigeonite rims.

Augite phenocrysts are euhedral to subhedral andsometimes glomerophyric with orthopyroxene and pla-gioclase. The chemical composition of augite variesfrom phenocryst cores (Ca38Mg49Fe13) with relativelyhigh Mg/Fe ratios, through phenocryst margins (Ca38Mg^Fe^) with varying Mg/Fe ratios, to groundmassaugite crystals (Ca37Mg38Fe25) with relatively lowMg/Fe ratios.

696


Table 3. The pyroxene analyses of bronzite-augite-phyrictholeiitic basalt (Rock C860) from Leg 59, Sample448A-62-1, 70-83 cm.

Anal. No.

SiO2

A12O3TiO2Cr2O3FeO*MnOMgOCaONa2OTotal

0 = 6.000SiAlAlTiCrFeMnMgCaNaZWXY

Atomic %CaMgFe

Contig.Anal. No.

Orthopyroxene

Phenocryst core

C01

53.270.940.170.01

16.240.38

25.392.160.02

98.58

1.9960.0340.0070.0050.0000.5010.0121.3970.0850.0012.0002.009

4.370.425.3

C03

C02

53.291.600.190.08

15.900.35

25.112.220.03

98.77

1.9590.0410.0280.0050.0020.4890.0111.3760.0870.0022.0002.001

4.570.525.0

C03C04

Augite

Phenocryst core

C03

51.452.370.380.04

10.180.24

16.1717.490.20

98.52

1.9360.0640.0410.0110.0010.3200.0080.9070.7050.0152.0002.008

36.546.916.6

C01C02

C04

51.592.490.350.247.900.21

17.0418.670.18

98.67

1.9260.0740.0350.0100.0070.2470.0070.9480.7470.0132.0002.013

38.548.812.7

C02

Phc. m.

C05

51.542.010.400.04

10.210.27

15.8818.420.21

98.98

1.9370.0630.0260.0110.0010.3210.0090.8900.7420.0152.0002.014

38.045.616.4

Gm.

C07

49.013.000.780.00

15.160.39

12.7617.250.23

98.58

1.8940.1060.0310.0230.0000.4900.0130.7350.7140.0172.0002.023

36.837.925.3


The chemical composition of groundmass titanomag-netite crystals varies from Usp5oMt5o in relatively largegrains about 0.05 mm long to Usp58Mt42 in small grainsabout 0.02 mm long.

Anorthite-Augite-Bronzite-Titanomagnetite-(Olivine)-Phyric Basalt (Rock D1050)

The specimen is a clast in the volcaniclastic brecciafrom Hole 451 (Sample 451-38-1, 137-140 cm). Mega-scopically, the rock is compact and slightly greenishdark gray, with phenocrysts of abundant plagioclases(up to 5 mm) and a few pyroxenes (Table 4).

Microscopic examination of thin sections shows thatthe rock consists of phenocrysts and microphenocrystsof plagioclase (35%, 0.4-7 mm), augite (3%, 0.4-3.0mm), orthopyroxene (2%, 0.4-2.0 mm), titanomag-netite (1%, 0.05-0.1 mm), and pseudomorphed olivine(1%, 0.4-0.8 mm), with a hyalophitic groundmass ofplagioclase (10%, 0.01-0.1 mm), augite (10%, 0.01-0.1mm), titanomagnetite (5%, 0.01-0.05 mm), pigeonite(trace, <O.l mm), and devitrified yellow glass (40%).

Two types of plagioclase phenocrysts can be rec-ognized by the resorption structure in their cores:plagioclase phenocrysts without resorption are euhedraland have very large sizes (up to 7 mm); those with re-sorption are subhedral to euhedral, small in size (up to 3mm), and show a honeycombed structure in the core,followed by overgrowths of more sodic plagioclase. Inthe plagioclase-pnenocryst cores with and without re-

Table 4. The pyroxene analyses of augite-bronzite-phyric tholeiitic basalt (Rock D1050)from Leg 59, Sample 451-38-1, 137-140 cm.

Anal. No.

SiO2

A12O3TiO2Cr2O3

FeOMnOMgOCaONa2O

Total


Atomic "?oCaMgFe

Contig.Anal. No.

Orthopyroxene

Phc. c.

D13

52.281.490.110.00

17.040.46

24.861.770.04

98.05

1.9480.0520.0140.0030.0000.5310.0151.3810.0710.0032.0002.017

3.669.626.8

D12

Phc. m. andmicrophc.

D19

52.611.400.130.00

17.110.44

25.341.990.05

99.07

1.9420.0580.0030.0040.0000.5280.0141.3940.0790.0042.0002.025

3.969.726.4

D18

D14

52.041.050.300.00

18.340.51

23.832.340.04

98.45

1.9490.0460.0000.0080.0000.5740.0161.3300.0940.0031.9952.026

4.766.628.7

D15

Phenocryst core

D12

49.723.150.360.00

10.330.28

14.8119.350.29

98.29

1.8940.1060.0350.0100.0000.3290.0090.8410.7900.0212.0002.036

40.342.916.8

D13

D23

49.703.370.440.009.870.29

14.6719.650.31

98.30

1.8900.1100.0420.0130.0000.3140.0090.8320.8010.0232.0002.033

41.142.716.1

Augite


D18

50.831.980.280.01

10.600.31

16.2918.030.21

98.54

1.9240.0760.0120.0080.0000.3360.0100.9190.7310.0152.0002.032

36.846.316.9

D19

D15

50.222.600.360.01

10.940.34

15.9318.050.22

98.67

1.9030.0970.0190.0100.0000.3470.0110.9000.7330.0162.0002.036

37.045.517.5

D14

Gm.

D16

49.593.090.630.00

13.390.45

14.1217.620.29

99.18

1.8910.1090.0290.0180.0000.4270.0150.8020.7200.0212.0002.033

36.941.221.9

Pigeonite

GroundmassD17

52.380.770.180.00

19.910.60

20.354.120.09

98.40

1.9840.0160.0190.0050.0000.6310.0191.1490.1670.0072.0001.997

8.659.032.4

D21

51.231.200.200.00

19.020.67

22.253.850.06

98.48

1.9370.0530.0000.0060.0000.6010.0211.2540.1560.0041.9902.043

7.862.329.9


697

T. ISHII

sorption, the compositions are near An85Ab15 andAn95Ab5 to An92Ab8 (sodic anorthite), respectively.Both phenocrysts strongly zone toward An67Ab33 neartheir margins. Groundmass plagiocase has a composi-tion range of An66Ab34 to AnóoAb^.

Augite phenocrysts are anhedral and contain titano-magnetite and altered, brown, spherical glass inclu-sions. Augite crystals have chemical compositions ofCa4oMg43Fei7 and Ca37Mg41Fe22 for phenocrysts andgroundmass, respectively.

Bronzite phenocrysts are subhedral to anhedral, haveonly a few clinopyroxene rims, and commonly containrounded titanomagnetite inclusions. Bronzite pheno-crysts are homogeneous (Ca3 5Mg69 5Fe27) in the coreand zone slightly toward the margin (Ca4-5Mg7oFe26.5).

Titanomagnetite phenocrysts are subhedral to an-hedral and free of exsolution. The composition oftitanomagnetite ranges from Usp18Mt82 to Usp22Mt78

for phenocrysts and is about Usp25Mt75 for the ground-mass.

Groundmass pigeonites are subhedral and show smallvariations in composition between grains, as shown inFigure ID. The most magnesian pigeonite in the thinsection has a composition of Ca8Mg62Fe30.

Bytownite-Augite-Titanomagnetite-Hypersthene-(Olivine)-Phyric Basalt (Rock E1130)

The specimen is a clast in the very coarse volcani-clastic conglomerate from Hole 451 (Sample 451-46-1,24-29 cm). Megascopically, the rock is vesicular andreddish brown, with phenocrysts of abundant plagio-

clase (up to 3 mm) and distinct black pyroxene (up to 1mm) (Table 5). It shows some resemblance to the red-dish brown surface of the plagioclase-two-pyroxene-phyric andesite lava, which is very common in calc-alkalic volcanoes of island arcs.

Microscopic examination of thin section shows thatthe rock consists of phenocrysts and microphenocrystsof plagioclase (30%, 0.5-6 mm), augite (3%, 0.5-2.0mm), titanomagnetite (2%, 0.2-0.7 mm), orthopy-roxene (1%, 0.2-1.5 mm), and pseudomorphous olivine(trace, 0.5-2.5 mm), with a hyalophitic groundmass ofplagioclase (15%, 0.02-0.4 mm), augite (5%, 0.02-0.3mm), magnetite (5%, 0.01-0.05 mm), pigeonite (trace,<0.05 mm), and devitrified altered brown glass (40%).

Two types of plagioclase phenocrysts are recognized;one is clear, and the other contains dusty inclusions inthe core (followed by a very thin overgrowth of moresodic plagioclase). Both plagioclases are euhedral tosubhedral. Their composition ranges from An93Ab7 toAn90Abio a n d from An86Abi4 to An8OAb2o in cores withand without dusty inclusions, respectively. Both pheno-crysts strongly zone toward An62Ab38 in the margins.Groundmass plagioclase has a composition range fromAn67Ab33 to An55Ab45.

Augite phenocrysts are subhedral to anhedral andcontain subhedral titanomagnetite inclusions. As shownin Figure IE, the chemical composition of augite variesfrom phenocrysts (Ca39Mg42Fe19) with relatively lowMg/Fe ratios, through microphenocrysts (Ca40Mg44

Fej6) with relatively high Mg/Fe ratios, to groundmassaugite crystals (Ca35Mg43Fe22) with relatively low

Table 5. The pyroxene analyses of augite-hypersthene-phyric tholeiitic basalt (RockE1130) from Leg 59, Sample 451-46-1, 24-29 cm.

Anal. No.

Siθ2A12O3

TiO 2

C r 2 O 3

FeO*MnOMgOCaO

Na2θ

Total


Atomic °7oCaMgFe

Contig.Anal. No.

Phc. c.

E24

51.411.610.250.00

20.620.70

22.071.710.08

98.45

1.9440.0560.0160.0070.0000.6520.0221.2440.0690.0062.0002.016

3.5

63.333.2

E25

Orthopyroxene

E29

52.090.870.190.00

19.220.52

22.962.320.08

98.25

1.9620.0380.0010.0050.0000.6050.0171.2890.0940.0062.0002.016

4.7

64.830.5


E33

51.871.200.210.00

21.210.69

21.362.120.05

98.71

1.9620.0380.0150.0060.0000.6710.0221.2040.0860.0042.0002.008

4.4

61.434.2

E32

E35

52.241.790.150.00

15.940.45

25.662.030.06

98.32

1.9340.0660.0120.0040.0000.4940.0141.4160.0810.0042.0002.025

4.071.224.8

E36

Phenocryst core

E25

50.782.140.420.00

11.710.46

14.7218.900.29

99.42

1.9210.0790.0160.0120.0000.3700.0150.8300.7660.0212.0002.030

39.042.218.8

E24

E34

50.122.490.430.00

12.140.41

13.9519.100.34

98.98

1.9110.0890.0230.0120.0000.3870.0130.7930.7800.0252.0002.033

39.840.419.7

Augite


E32

49.972.120.450.00

12.670.44

14.2218.800.34

99.01

1.9100.0900.0050.0130.0000.4050.0140.8100.7700.0252.0002.042

38.840.820.4

E33

E36

50.252.970.450.039.930.31

15.4319.300.31

98.98

1.8960.1040.0280.0130.0010.3130.0100.8680.7800.0232.0002.036

39.844.216.0

E35

Gm.

E28

50.441.900.430.00

13.680.46

14.8916.520.22

98.54

1.9300.0700.0150.0120.0000.4380.0150.8490.6770.0162.0002.023

34.543.222.3

Pig-

Gm.

E37

51.580.960.230.00

20.690.63

18.944.730.12

97.88

1.9780.0220.0210.0070.0000.6640.0201.0830.1940.0092.0001.998

10.055.834.2


698

Mg/Fe and Ca/(Ca + Mg + Fe) ratios; that is, reversezoning is observed in the augite crystallization sequence.

Titanomagnetite phenocrysts are subhedral and con-tain exsolution ilmenite lamellae. The compositions areUsp26Mt74 to Usp38Mt62 for the phenocrysts, and Il77

Ht23 (ilmenite-hematite molar ratio) and Usp14Mt86 forthe lamellae and host, respectively.

Orthopyroxene phenocrysts are subhedral to an-hedral, relatively fresh, and always surrounded by oneof three types of rims: (1) a dark brown alteration prod-uct after orthopyroxene, followed in order of abun-dance by (2) a relatively thick augite rim and (3) a thinpigeonite rim. Reverse zoning is also observed in the Ca-poor pyroxene crystallization sequence. Chemical com-positions of Ca-poor pyroxene vary from hypersthenephenocrysts (Ca3 5Mg63 5Fe33) with relatively low Mg/Feratios, through bronzite microphenocrysts (Ca4Mg71

Fe25) with high Mg/Fe ratios, and through hypersthenemicrophenocrysts (Ca4 5Mg615Fe34) with relatively lowMg/Fe ratios, to groundmass pigeonite crystals (Ca10

Mg56Fe34) with low Mg/Fe ratios.Pigeonite is observed in the groundmass and in the

rims of the microphenocryst orthopyroxene. Ground-mass pigeonite shows small variation in compositionbetween grains.

Bytownite-Hypersthene-Augite-TitanomagnetitePhyric Calc-Alkalic Andesite (Rock F1200)

The specimen is a clast in the tuff breccia from Hole451 (Sample 451-59-1, 55-57 cm) (Table 6). Mega-scopically, the rock is slightly vesicular and gray, with


phenocrysts of abundant plagioclase (up to 4 mm) anddistinct black pyroxene.

Thin-section observations show that the rock consistsof phenocrysts and microphenocrysts of plagioclase(35%, 0.5-3 mm), hypersthene (7%, 0.2-1 mm), augite(5%, 0.2-0.8 mm), and titanomagnetite (3%, 0.1-0.3mm), with a typical hyalopilitic groundmass of plagio-clase (10%, 0.005-0.1 mm), augite (< 1%, -0.02 mm),titanomagnetite ( < 1 % , -0.01 mm), ilmenite (trace,-0.01 mm), and devitrified glass (40%). Olivinepseudomorphs are not observed.

Plagioclase phenocrysts are subhedral to euhedral,rarely glomerophyric, and show intensive resorption(i.e., honeycombed structure in the core) followed byovergrowth of more sodic plagioclase. Their composi-tion ranges from An78Ab22 to An63Ab37 (mainlybytownite) and near An63Ab37 in the cores of pheno-crysts and groundmass plagioclase crystals, respec-tively. Both are zoned toward An43Ab37 in their thinrims.

Titanomagnetite phenocrysts are subhedral to eu-hedral and are free of exsolution. Their compositionsare near Usp25Mt75 and between Usp31Mt69 and Usp45

Mt55 for phenocrysts and the groundmass titanomag-netite crystals, respectively. Groundmass ilmenite(Il53Mt47) is rarely detected by microprobe.

Hypersthene phenocrysts are euhedral and com-monly contain titanomagnetite inclusions. They are par-tially or wholly replaced by clay minerals in some cases.As shown in Table 6 and Figure IF, hypersthene pheno-crysts are homogeneous (Ca2 5Mg66 5Fe31) and are rarely

Table 6. The pyroxene analyses of hypersthene-augite phyric calc-alkalic andesite (RockF1200) from Leg 59, Sample 451-59-1, 55-57 cm.

Anal. No.

Siθ2A12O3

TiO2

Cr2O3

FeO*MnOMgOCaONa2O

Total

O = 6.000SiAlAlTiCrFeMπ

MgCaNaZWXY

Atomic %CaMgFe

Contig.Anal. No.

F02

51.191.790.210.00

19.730.76

23.161.400.06

98.30

1.9310.0690.0100.0060.0000.6220.0241.0320.0570.0042.0002.026

2.965.731.4

Orthopyroxene

Phenocryst core

F04

52.860.690.110.00

19.770.80

23.681.280.05

99.24

1.9700.0300.0000.0030.0000.6160.0251.3150.0510.0042.0002.014

2.666.331.1

F03

F08

52.810.820.120.00

19.160.77

24.131.320.05

99.18

1.9640.0360.0000.0030.0000.5960.0241.3370.0530.0042.0002.017

2.667.330.0

F10

51.960.840.130.00

19.690.97

23.291.520.07

98.47

1.9570.0370.0000.0040.0000.6200.0311.3080.0610.0051.9942.029

3.165.731.2

F l l

Phc. m.

F05

52.290.800.140.00

20.781.08

22.181.170.05

98.49

1.9570.0250.0110.0040.0000.6560.0351.2490.0470.0042.0002.005

2.464.033.6

Phenocryst core

F03

51.271.970.400.009.830.40

14.2521.160.31

99.59

1.9300.0700.0170.0110.0000.3090.0130.8000.8530.0232.0002.026

43.540.715.8

F04

Fl l

50.791.680.310.009.300.46

14.0521.16

0.30

98.05

1.9400.0600.0160.0090.0000.2970.0150.8000.8660.0222.0002.024

44.140.815.1

F10

Augite

Microphenocryst

F06

51.651.330.230.009.12

0.5914.6821.37

0.30

99.27

1.9470.0530.0060.0070.0000.2870.0190.8250.8630.0222.0002.028

43.741.814.6

F07

51.551.060.170.00

10.590.68

13.5421.140.34

99.07

1.9600.0400.0070.0050.0000.3370.0220.7670.8610.0252.0002.024

43.839.017.1

Gm.

F09

48.244.920.770.00

10.170.24

13.6719.850.27

98.13

1.8440.1560.0660.0220.0000.3250.0080.7790.8130.0202.0002.033

42.440.617.0


699

T. ISHII

rimmed by augite (Ca34Mg44Fe22) Groundmass hyper-sthene is not observed.

Augite phenocrysts, commonly containing titano-magnetite inclusions, are euhedral and sometimesresorbed to a round shape. Augite phenocrysts arehomogeneous (Ca^Mg^Fe^) in the core and arerimmed by strongly zoned augite toward Ca-poor augite(Ca32Mg48Fe2o) in some cases. Groundmass augitecrystals show zoning from cores (Ca42Mg41Fei7) to Ca-poor augite rims (about Ca33Mg49Fei8). Discrete Ca-poor augite is not observed.

PYROXENE GEOTHERMOMETERS

Detailed data and the method used for these pyrox-ene geothermometers have been partly reported (Ishii,1974, 1975; Ishii et al., 1976). These pyroxene geother-mometers are briefly summarized in the followingmaterial.

Three-Pyroxene Geothermometer (POA-GT)

Pyroxenes in common mafic magmas are approx-imated in the three-component system: CaSiO3-MgSiO3-FeSiO3. In this system, the lower stability limitof pigeonite, or the pigeonite eutectoid reaction (PER)line (Ishii and Takeda, 1974), is the isobaric univariantline on which three phases (pigeonite + orthopyroxene+ augite) coexist. A practical application of the PERline to geothermometry was initiated by Ishii (1974).Based on the compositions of such pyroxenes in twolavas in Japan of known eruption temperature as well ason Brown's experimental data (Brown, 1968), the PERline was approximated by

CaMg

T = 1270 - 480XFe (1)

where T is temperature in °C and XFe is atomic ratioFe/(Mg + Fe) (Ishii, 1975).

The PER line can be used as a three-pyroxene geo-thermometer (i.e., if the composition of the pigeonitethat crystallized on or close to the PER line is known, itscrystallization temperature can be determined by usingthis equation). In some basic lavas and intrusives, pyrox-ene assemblages change during crystallization from or-thopyroxene + augite to pigeonite + augite. In thispyroxene crystallization sequence, the most magnesianpigeonite crystallizes close to the PER line. For exam-ple, the crystallization temperature 1118°C was deducedfor the most magnesian pigeonite with XFt = 0.317 inRock A814.

Orthopyroxene-Augite Geothermometer (OA-GT)

Not many experimental low-pressure data on theenstatite-diopside two-phase region are available atpresent. We calibrated Wood and Banno's formula(1973), estimated from high-pressure data, by employ-ing compositions of four orthopyroxene-augite pairs inthree-pyroxene rocks collected from Funagata (Aoki,1960; Ishii, 1974), Hakone (Kuno, 1936; Ishii, 1974),Ashio (Kuno, 1969; Ishii, 1974), and Weiselberg (Naka-mura and Kushiro, 1970b). These data are shown inFigure 2 and Table 7. Crystallization temperatures

Mg 50Atomic'

Figure 2. Ca-Mg-Fe plot of coexisting three-pyroxene phenocrysts.

estimated from the compositions of the coexistingpigeonites (by the PER line just cited) are also shown inTable 7. According to Wood and Banno, the crystalli-zation temperature (in terms of XFe of orthopyroxene) isgiven as:

T = A/ m V M g 2 S i 2 O 6

/ α M g 2 S i 2 O 6 > /

- B - CXFe - DX\t I - 273 (2)

where a is component activity. Accepting the A (- 10202)and B(4.6) values of Wood and Banno, we obtainC(7.10) and £>(-6.02) by employing the least-squaresmethod on the four-pyroxene pairs data (Table 7).

Pigeonite-Augite Geothermometer (PA-GT)

Assuming that equation (2) will still be valid even iforthopyroxene is replaced by pigeonite, the >l(-6232)and 5(2.96) terms at XFe = 0 were estimated from thepigeonite-augite solvus at low pressure (Boyd andSchairer, 1964; Kushiro, 1972; Yang, 1973). TheC(1.141) and Z>(0.68) values were obtained by employ-ing the compositions of pigeonite and augite (Table 7)from the four sets of the three-pyroxene assemblagesand by employing their estimated temperatures.

These three kinds of pyroxene geothermometers canbe used to estimate the crystallization temperature ofpyroxenes in the lavas using a Fortran computer pro-gram coded by Miyamoto (Ishii et al., 1976).

ESTIMATION OF CRYSTALLIZATIONTEMPERATURES OF PYROXENES

The compositions of Ca-poor and Ca-rich pyroxenepairs crystallized at equilibrium in each rock have beenobtained by detailed microprobe analyses. Using thesedata we estimated the equilibrium crystallization tem-peratures of pyroxenes with the previously mentionedpyroxene geothermometers.

In the anorthite-bronzite-augite-phyric basalt (C860),bronzite phenocrysts are partially replaced by clayminerals along the margins. Augite and bronzite pheno-crysts are in contact with each other, as if augite crystalswere molded into a single large bronzite crystal, asshown in Figure 3. Many microprobe-scan traversesacross these crystals for Ca, Mg, and Fe have revealedthat each crystal is very homogeneous and that minorchemical zoning is observed only in augite in the narrow

700


Table 7. The pyroxene analyses of three pyroxene rocks from Funagata,a Hakone° Weiselberg,c andAshio.d

Samplesand Temp.

Anal. No.

SiO2

A12O3TiO2FeO*MnOMgOCaONa2OTotal

O = 6.000SiA!AlTiFeMnMgCaNaZWXY

Atomic %CaMgFe

Funagata (1115

Opx.f

1

53.81.560.17

14.50.34

27.22.540.03

100.1

1.9410.0590.0070.0050.4370.0101.4630.0980.0022.0002.022

4.973.221.9

Pig.2

52.91.530.13

13.90.34

24.85.720.08

99.4

1.9380.0620.0040.0040.4260.0111.3540.2240.0062.0002.028

11.267.621.2

°C)e

Aug.3

52.12.330.288.970.28

17.718.20.15

100.0

1.9240.0760.0250.0080.2770.0090.9740.7200.0112.0002.023

36.549.414.1

Hakone (1073

Opx.4

51.11.700.27

22.40.48

21.42.230.04

99.6

1.9270.0730.0030.0080.7060.0151.2030.0900.0032.0002.029

4.560.235.3

Pig.5

51.01.540.41

22.40.60

18.05.290.11

99.3

1.9470.0530.0160.0120.7150.0191.0240.2160.0082.0002.011

11.152.436.6

°C)

Aug.6

49.82.780.61

14.90.37

14.116.50.35

99.4

1.9000.1000.0250.0180.4750.0120.8020.6750.0262.0002.033

34.641.124.4

Weiselberg (1039°C)

Opx.7

51.60.670.33

27.00.5618.31.86

n.d.

100.3

1.9690.0300.0000.0090.8620.0181.0410.076_2.0002.006

3.852.643.5

Pig.8

51.10.620.29

26.90.66

16.24.07n.d.

99.8

1.9740.0260.0030.0080.8690.0220.9330.168—

2.0002.003

8.547.344.1

Aug.9

50.31.430.68

15.10.38

12.418.1n.d.

98.4

1.9470.0530.0130.0200.4890.0120.7160.751

—2.0002.000

38.436.625.0

Ashio (995°

Opx.10

48.81.580.32

32.20.67

13.62.050.00

99.2

1.9410.0590.0150.0101.0710.0230.8060.0870.0002.0002.012

4.441.054.5

Pig.11

49.21.000.29

30.10.71

12.64.590.15

98.6

1.9650.0350.0120.0091.0050.0240.7500.1960.0122.0002.008

10.138.451.5

C)

Aug.12

48.92.340.48

19.60.449.93

17.00.24

98.9

1.9220.0780.0300.0140.6440.0150.5820.7160.0182.0002.019

36.930.033.2

Note: FeO* = total Fe as FeO.a From Aoki, 1960; Ishii, 1974.b From Kuno, 1936; Ishii, 1974.c From Nakamura and Kushiro, 1970b.d From Kuno, 1969; Ishii, 1974.e Crystallization temperatures estimated by the three-pyroxene geothermometer (see text).

Abbreviations: Opx. = orthopyroxene; Pig. = pigeonite; Aug. = augite.

0.5mm

Figure 3. Photomicrograph of analyzed orthopyroxenes and augites(represented by the black circles).

margin in contact with the groundmass. The chemicalcompositions of the uniform portions of each pyroxeneare given in Table 3 and Figure 1C, and the regionsanalyzed are marked by the black circles in Figure 3.This textural and chemical evidence suggests that thetwo pyroxenes were crystallized from magma in equi-librium.

Compositions determined with the contiguous crys-tals of bronzite and augite are connected by straightlines (Fig. 1C). The crystallization temperatures werededuced with the orthopyroxene-augite geother-mometer using each pyroxene pair according to equa-

tion (2). The estimated temperatures from three pyrox-ene pairs—Opx COl-Aug C03, Opx C02-Aug C03, andOpx C02-Aug C04 in Table 3—are 1129°C, 1134°C,and 1131°C, respectively, and average 1131 °C. Thecrystallization temperature 1131 °C was obtained for thecore of the most magnesian orthopyroxene phenocryst(with X¥e = 0.262). The temperatures deduced by adetailed investigation of pyroxene pairs contained ineach rock are shown in Table 8, and plotted in Figure 4,(where the XFe value is of the most magnesian ortho-pyroxene-phenocryst core).

Hakone Volcano in Central Japan is a typical island-arc type stratovolcano, and its geology and petrologyhave been investigated in detail by Kuno (1950). De-tailed microprobe data on pyroxenes in the tholeiitic

Table 8. Estimated temperature of magma by pyroxene geo-thermometers.

RockSiO2

a

(wt. %)SIa

valueA-pe0 of

opx. phc.c

Phc.stage**

Gm.stagee

Differ-ence

Site 448 A814 53.63 18.97 0.254B815 48.27 23.78 0.243C860 49.20 24.78 0.262

Site 451 D1050 49.99 24.72 0.278El 130 50.6 33.74 0.344F1200 57.2 20.35 0.319

1100 1120 +201075 1095 +201130 — —1055 1115 +601055 1090 +35970 — —

a See Table 9 for values.b A"pe is mole fraction of Fe in orthopyroxene (see text).c Abbreviations: Opx. = orthopyroxene; Phc. = phenocryst; Gm. = ground-

mass.Temperature of magma in the magma chamber.

e Temperature of magma in the effusive stage.

701

T. ISHII

1 ^UU

1100

1000

^ ^ r Pigeonite Eutectoid Reaction (PER) Line

^ A ^ Legend

" \ \ ov CY \ ^ \ •X-TO O N ^ ^ "D X) X ^ \ ^

. . I 1 1

A814B815C860D1050E1130F1200Hakone, tholeiitic rockHakone, calc—alkalic andesite

i

0.2 0.3 0.4 0.5 0.6

X F e = FeO/(MgO + FeO)

Figure 4. Crystallization temperatures of orthopyroxene phenocrystsand XFe values.

and calc-alkalic rocks from Hakone Volcano have beenreported in part (Ishii, 1974, 1976, 1978). These data arealso shown in Figure 4.

The following features are remarkable: (1) the crys-tallization temperature of orthopyroxene phenocrystcores of Rock F1200 is at least 80 °C lower than that ofthe other samples from the Philippine Sea; (2) thetholeiitic rocks from the Hakone Volcano have certainranges of crystallization temperatures, and the lowest ofthe Hakone tholeiites is still higher than those withsimilar XFe values in the calc-alkalic rocks from theHakone Volcano; and (3) Rock F1200 is plotted near the

positions of calc-alkalic rocks, whereas the other fivesamples fall in the area of the tholeiitic rocks of HakoneVolcano.

The core of phenocrysts in each sample started tocrystallize at equilibrium in the magma chamber. Itscomposition and crystallization temperature may alsoreflect fractional crystallization of the magma in themagma chamber. It may therefore be reasonable toassume that the temperatures estimated earlier corre-spond to the temperature in the magma chamber. In thefollowing discussion, the temperatures of the magmawill be represented by those temperatures of pheno-crysts listed in Table 8. It is notable that the estimatedtemperatures of the groundmass stage of crystallizationare higher than those of the phenocryst stage.

CHEMISTRY OF ROCKS ANDTEMPERATURE OF MAGMA

Major elements contained in rocks and glass used inthis study have been analyzed by three methods: XRFanalyses by the Birmingham University group (Matteyet al., this volume) and by Ishii; wet chemical analyses(gravimetric, flame photometric, and calorimetric) byHaramura; and electron probe microanalyses (EPMA)by Ishii. The analytic results and their CIPW norms areshown in Table 9. By wet chemical method, H. Hara-mura analyzed 8 samples, including 3 rocks in Table 9collected from the Palau-Kyushu and West Marianaridges (see Appendix). The Fe2O3/FeO ratio of these 8samples averages 1.06. Contents of Fe2O3 and FeO in

Table 9. The major-element analyses of rocks and glasses from Leg 59, Holes 448, 448A and 451.

Rock

Sample

Anal. No.

A814

448-59-1,61 cm

1

B815

448-59-1,115 cm

2

448A-62-170 cm

3

C86O

, 448A-62-1,70 cm

4

451-38-1,134 cm

5

D1050

451-38-1,137 cm

6

451-38-1.134 cm

7

451-38-1,134 cm

8C

E1130

451-46-1,24 cm

9

F1200

451-59-1,54 cm

10

SiO2

TiO2

A12O3

Fe 2 O 3 aFeOMnOMgOCaONa 2OK2OP2O5H 2 O +H 2 O -Total

Ni (ppm)Cr (ppm)

CIPW NormQOrAhAnWoEnFsMtIIAp

SI d

Analyst^Methodd

53.631.22

12.376.706.700.173.698.182.590.44n.d.n.d.n.d.

95.71

(150)b(0)

16.532.72

22.9021.76

8.629.605.28

10.152.420.00

18.97T.I.

EPMA

48.270.97

17.185.956.110.204.699.162.780.790.151.332.11

99.69

37

20

3.384.85

24.4433.315.38

12.135.278.961.910.36

23.78H.H.

Wet C.

49.200.93

19.454.695.260.154.01

10.672.480.210.121.131.92

100.22

2416

5.921.28

21.5942.524.65

10.284.667.001.82

0.29

24.78H.H.

WetC.

49.710.92

18.674.964.960.153.86

10.84(1.87)

0.210.20n.d.n.d.

96.35

n.d.n.d.

10.561.29

16.4243.52

4.579.983.917.461.810.48

25.12T.I.

XRF

49.990.68

19.764.454.920.173.87

10.522.160.700.131.830.92

100.10

2111

6.904.25

18.7743.30

3.949.904.676.631.330.31

24.72H.H.

WetC.

50.300.70

19.004.884.880.184.31

10.832.160.720.06n.d.n.d.

98.02

312

6.664.34

18.6540.83

5.6710.954.197.221.360.14

26.18J.T. &N.M.

XRF

49.590.63

19.034.574.570.163.65

10.45(1.65)0.710.20n.d.n.d.

95.21

n.d.n.d.

10.504.41

14.6644.55

3.569.554.076.961.260.49

24.84T.I.XRF

53.761.33

12.307.317.310.303.127.192.931.51

n.d.n.d.n.d.

97.06

(0)

(0)

12.919.19

25.5416.438.488.005.92

10.922.600.00

14.55T.I.

EPMA

50.600.77

15.805.275.270.176.709.582.520.620.23n.d.n.d.

97.53

523

5.323.76

21.8630.736.87

17.114.487.831.500.55

33.75J.T. &N.M.

XRF

57.200.65

17.703.403.400.153.038.603.272.130.14n.d.n.d.

99.67

211

9.4312.6327.7627.426.047.572.654.951.240.33

20.35J.T. &N.M.

XRF

<* Fe2θ3/FeO = 1.0 for EPMA and XRF data (see text).b Degree of reliability is relatively low.<j Glass inclusion in the plagioclase phenocryst of Rock D1050.d Abbreviations: SI = solidification index; Wet C. = wet chemical analysis; T.I. = T. Ishii; H.H. = H. Haramura; J.T. & N.M. = J. Tarney & N. Marsh

(Mattey et al., this volume).

702


other samples analyzed by XRF and EPMA are onlycalculated on the assumption that the Fe2O3/FeO ratiois 1.0. Because of the common occurrence of alterationproducts, migration of some elements is expected duringalteration in the seafloor environment. Nevertheless, itis remarkable that many rocks are quartz normative andhave high A12O3 contents and low Na2O/K2O ratios.

Results of analyses including glass plotted in Figure5 are used to classify the volcanic rocks according totheir chemical composition. In Figure 5A, the SiO2-(Na2O + K2O) diagram (Kuno, 1965), and in theAl2O3-(Na2O + K2O)-SiO2 (Kuno, 1960) diagram (Fig.5B), all rocks plotted exist in the areas of high aluminabasalt and the tholeiite. In Figure 5C, the MgO-FeO-(Na2O + K2O) diagram, the data fall on the tholeiitic/calc-alkalic boundary (estimated by Kuno [1954] in theIzu-Hakone region) and into the calc-alkalic field. Onthe other hand, in both SiO2-FeO*/MgO (Fig. 5D) andFeO*-FeO*/MgO (Fig. 5E) plots (Miyashiro, 1974),Rock F1200 lies in the calc-alkalic field, but all theothers fall in the tholeiitic field.

Minor-element chemistry determined by XRF (Mat-tey et al., this volume) is also available for the samplesfrom the Palau-Kyushu and West Mariana ridges. Theseselected data are shown in Table 10 with compiledchemical values of calc-alkalic, island-arc tholeiitic, andabyssal tholeiitic series (Jakes and Gill, 1970). It is clearthat none of the samples from the Palau-Kyushu andWest Mariana ridges are abyssal tholeiites. According tothe criteria of Jakes and Gill, data of Rb and Ba con-tents, as well as Rb/Sr and Na2O/K2O ratios (Table 10),strongly suggest that F1200 is of calc-alkalic affinity,whereas D1050, El 130, and the others from the Palau-Kyushu and West Mariana ridges are of island-arctholeiitic affinities. However, Sr contents are very highin the calc-alkalic and tholeiitic rocks from the WestMariana Ridge.

Temperatures of magma deduced in the previous sec-tion (Table 8) are plotted with respect to major-elementanalyses (Fig. 6) and to the solidification index (SI) ofbulk rock; the solidification index is 100 × Mg/(MgO+ FeO* + Na2O + K2O) (Kuno, 1954) and can be usedas an indicator of degree of differentiation of magma(Fig. 7). The data of previously studied igneous rocksare also shown in Figure 7.

The following remarkable features are shown inFigure 7; (1) the temperatures of magmas from thePalau-Kyushu Ridge are higher than those from theWest Mariana Ridge, with similar SI values, (2) tholeii-tic magmas from Hakone Volcano have certain temper-ature ranges, the highest one of which is still lower thanthose of some tholeiitic magmas from the other area; (3)Rock F1200 is very close to the calc-alkalic magma ofthe Hakone Volcano; and (4) the other five samplesfrom the Palau-Kyushu and West Mariana ridges are in-cluded in the high- and low-temperature regions of thetholeiitic magma. It is expected that Rock F1200 is ofcalc-alkalic affinity.

As shown in Figures 6 and 7, magmas with a similarchemical composition of nonvolatile components have avery wide temperature range. According to the data, it is

difficult to judge which components have significantefects on the temperature of magmas. Oxygen pressuredoes not significantly lower their temperature (Aramakiand Katsura, 1973). Differences in volatile content, par-ticularly water, appear to be the controlling factor indetermining these temperature differences. It is sug-gested that water content in the high-temperaturemagma is lower than that in the low-temperature one.Temperature differences (Fig. 7) suggest that water con-tent of the Palau-Kyushu Ridge magma was lower thanthat of the West Mariana Ridge magma and that watercontent of the F1200 magma was as high as thecalc-alkalic magma of Hakone Volcano.

MAGNETIC PROPERTY ANDPALEOENVIRONMENT

Changes in saturation magnetization (Js) with tem-perature {T) were studied with five of the samples; theglass sample (A814) was excluded. The method of meas-urement is the same as that described by Kobayashi etal. (1979).

Two types of samples can be distinguished by theirthermomagnetic behavior: a thermally reversible typeand an irreversible type. Samples from the West Mari-ana Ridge (Fig. 8A) are thermally reversible and thosefrom the Palau-Kyushu Ridge (Fig. 8B) irreversible.Their Curie temperatures are estimated to be 440 °C(B815), 280°C (C860), 35O°C (D1050), 560°C (El 130),and 505°C (F1200) by heating experiments.

As previously mentioned, the exsolution lamellae ofilmenite in titanomagnetite can only be observed bymicroscope and microprobe in Rock El 130.The occur-rence and magnetic properties of titanomagnetite in thevolcanic clasts of the West Mariana Ridge can be inter-preted to indicate that the clasts have suffered fromhigh-temperature oxidation. It is plausible that thesevolcanic rocks were erupted and cooled in a subaerialenvironment.

The thermomagnetic behavior of Rocks B815, takenfrom the interior of a massive lava, and C860, from in-terior of a massive dike from the Palau-Kyushu Ridge,indicates that the former has undergone intensive low-temperature oxidation, whereas the latter has under-gone relatively weak low-temperature oxidation. It isexpected that the degree of oxidation depends on thegeological environments in which magmas were settled.It may be reasonable to assume that magma of B815 waserupted and cooled on the seafloor and was intensivelyoxidized by seawater, whereas magma of C860 intrudedinto the previously settled pyroclastic sediment in theseafloor so that the effect of seawater was relativelyweak. Magnetic properties seem to be a useful tool withwhich to investigate the geologic paleoenvironment inwhich igneous activity occured.

DISCUSSION AND CONCLUSIONThe volcanic sequences at Sites 448 and 451 were

formed by basaltic igneous activities that differ signifi-cantly from typical oceanic igneous activity formingLayer 2 of the oceanic crust. Abundance of pyroclasticrocks, high vesicularity of extrusive units, and wide

703

T. ISHII

FeO*

20r

15

10

20

15

10

20

15

10

B.

Si0 o = 47.51-50.00

10

SiO2 = 50.01-52.50

10

SiO2 = 51.51-55.00

Na2O + K2O10

Na2O + K2O Mgo

1 2 3FeOVMgO

1 2 3FeOVMgO

Figure 5. Various diagrams to classify volcanic rocks according to their major chemistry. A. SiO2-(Na2O + K2O) diagramB. Al2O3-(Na2O + K2O)-SiO2 diagram. C. MgO-FeO*-(Na2O + K2O) diagram. D. SiO2O-FeO*/MgO diagram. E. FeO*-FeOVMgO diagram. (Symbols are the same as in Fig. 4. FeO* = total Fe as FeO.)

704


Table 10. Selected minor-element analyses.

ppm

NiCrRbSrBa

Ratio

K/RbRb/SrNa2θ/K2<

Site 451,

D1050

31211

493176

5410.022

3 3.09

(Mattey et al.,

West Mariana Ridge

E1130

5238

539186

6390.0154.06

F1200

21143

520446

4110.0831.54

this volume)

Site 448(rangea)

1-3011-690-21

137-21222-99

604-26810.006-0.146

1.27-24.80

Site 451(range*5)

3-137-232-11

482-621114-186

541-11360.004-0.022

1.54-9.68

(Jakes and Gill,

Calc-AlkalicSeries

185630

380270

400-5000.05-0.10

2-3

Island-ArcTholeiitic

Series

0-300-503-10

100-20050-150

10000.01-0.05

4-6

1970)

AbyssalTholeiitic

Series

30-200200-4000.2-5.070-150

6-30

10000.02

10-15

a Excluding analysis of Sample 448A-36-4, 130 cm.b Excluding analyses of Rock F1200 and Sample 451-102,CC, 11

1200

1100

1000h

900

1200

Wt.10 20 " 40

(TiO2, Na2O + K2O, MgO, FeO\ CaO, AI2O3)50 60

Wt. % (SiO2)

Δ TiO2

Nao0 + K o 0

O FeO*

• CaO

V AI 2 O 3

• SiO2

D MgO

Figure 6. Temperatures of magmas and major-element analyses.

distribution of orthopyroxene phenocrysts suggest thatthe volcanic rocks are of island-arc affinity.

This conclusion is supported by major and minorgeochemical data: abundance of normative quartz, highcontent of A12O3, Rb, Sr, and Ba, low content of Ni andCr, and low ratio of Na2O/K2O are obvious chemicalevidences of the volcanic activities of the island-arctholeiite and/or calc-alkalic affinities.

Occurrence of groundmass orthopyroxene is a min-eralogically reliable criterion to distinguish the calc-alkalic rock from tholeiite (Kuno, 1950, 1954). Unfor-tunately, it is difficult to apply this criterion to alteredrocks and glassy samples with low crystallinity ofgroundmass. In this chapter, groundmass ortho-pyroxene has not been found with the microscope ormicroprobe.

Occurrences of titanomagnetite phenocrysts and in-tensively resolved plagioclase phenocrysts with honey-

1100r-

1000h

40

Sl =

V F

O

V

o

o

•

1

•OT

o

D

Δ°

* SL

V +AK

OO

o

oo

SM

O

o

TAS

A

i 1

s u ^ -

o

o

o

Legend

Hakone Volcano

O : Tholeiitic rock• : Calc—alkalic rock

(phenocryst stage)

Other tholeiitic rocksT F : FunagataV O : OshimaT Ak : Akita-KomagataT AS : Ashio

B : BushveldSL : Skaergaard lower zoneSMSU

Leg 59

D • O

Skaergaard middle zoneSkaergaard upper zone

Site 448V A Δ : Site 451 (See Fig. 4)

30 20

MgO

10

MgO + FeO* + Na 2 O + K 2 O

Figure 7. Temperatures of magmas and solidification indexes (SI) (seeIshii, 1978).

combed structure, followed by overgrowth of moresodic plagioclase, may be other criteria for identifyingcalc-alkalic rocks but are not sufficiently reliable.

On the bases of the geochemical criteria—high Rband Ba contents, high Rb/Sr ratio, and low Na2O/K2Oratio—one sample (F1200) can be distinguished as acalc-alkalic rock from the other rocks, which are island-arc tholeiites (Table 10).

Furthermore, Rock F1200 can be independently iden-tified as a calc-alkalic rock by new criteria: the com-parative studies of crystallization temperature of or-thopyroxene phenocrysts and its X¥e value (Fig. 4) aswell as of the temperature of magma (obtained bypyroxene geothermometers) and its solidification index(Fig. 7).

It may be reasonable to state that the present rocksare classified into the following three groups: (1) high-temperature island-arc tholeiite (A814, B815, andC860), (2) low-temperature island-arc tholeiite (D1050and El 130) and (3) calc-alkalic andesite (F1200). The

705

T. ISHII

600

600

r°c

Figure 8. A. Thermomagnetism curves, thermally irreversible type(C860); B. Thermomagnetism curves, thermally reversible type(El 130).

water content of magma increases in this order. A prob-lem then arises as to the cause of varied water contentsin the island-arc magma. Further investigation of thecrustal structure and the evolutionary history of the tworemnant arcs may solve the problem.

ACKNOWLEDGMENTS

The author is very grateful to the late Professor Hisashi Kuno forhis consultation on the pyroxene problem, to Professor K. Kobayashi(University of Tokyo) for his critical reading of the manuscript, and toMr. H. Haramura (University of Tokyo) for his wet chemicalanalyses. Thanks are also extended to Professors I. Kushiro and H.Takeda, and to Mrs. T. Furuta, K. Fujioka, and H. Tokuyama(University of Tokyo) for their discussions and encouragements dur-ing the research and to Miss K. Tabata for her help in preparation ofthe manuscript.

REFERENCES

Aoki, K., 1960. Early stage basalts from the Nasu volcanic zone. / .Japan Assoc. Mineral. Petrol. Econ. Geol., 45:54-65.

Aramaki, S., and Katsura, T., 1973. Petrology and liquidus tempera-ture of the magma of the 1970 eruption of Akita-Komagatakevolcano, northeastern Japan. J. Japan Assoc. Mineral. Petrol.Econ. Geol., 68:101-124.

Boyd, F. R., and Schairer, J. F., 1964. The system MgSiO3-CaMgSi2O6. / . Petrol., 5:275-309.

Brown, G. M.( 1968. Experimental studies on inversion relations innatural pigeonitic pyroxenes. Carnegie Inst. Washington year-book, 66:347-353.

Hilde, T. W. C , Uyeda, S., and Kroenke, L., 1977. Evolution of thewestern Pacific and its margins. Tectonophysics, 30:145-165.

Ishii, T., 1974. Pyroxene geothermometry and its application to thepetrological study of the Hakone Volcano [Ph.D. dissert.]. Univer-sity of Tokyo.

, 1975. The relations between temperature and compositionof pigeonite in some lavas and their application to geother-mometry. Mineral. J., 8:48-57.

., 1976. Crystallization temperature and composition of py-roxenes from tholeiitic magmas, Hakone Volcano, Japan. Geol.Soc. Am. 1976 Annual Meetings, 8:936-937. (Abstract)

., 1978. Temperature and water content of tholeiitic magmasfrom Hakone Volcano and Bushveld intrusion. Intern. Geophys.Conf. "Western Pacific" and "Magma Genesis" (Tokyo), pp.256-257. (Abstract)

Ishii, T., Miyamoto, M., and Takeda, H., 1976. Pyroxene geother-mometry and crystallization, subsolidus equilibration tempera-tures of lunar and achondritic pyroxenes. Lunar Science VII:Houston (Lunar Science Institute), pp. 408-410.

Ishii, T., and Takeda, H., 1974. Inversion, decomposition and ex-solution phenomena of terrestrial and extraterrestrial pigeonites.Mem. Geol. Soc. Japan, 11:19-36.

Ishii, T., Takeda, H., and Yanai, K., 1979. Pyroxene geother-mometry applied to a three-pyroxene achondrite from Allan Hills,Antarctica and ordinary chondrites. Mineral. J., 9:460-481.

Jakes, P., and Gill, J., 1970. Rare earth elements and the arc tholeiiticseries. Earth Planet. Sci. Lett., 9:17-28.

Karig, D. E., 1975. Basin genesis in the Philippine Sea. In Karig,D. E., Ingle, J. C , Jr., et al., Init. Repts. DSDP, 31: Washington(U.S. Govt. Printing Office), 857-879.

Kobayashi, K., Steiner, M., Faller, A., et al., 1979. Magneticmineralogy of basalts from Leg 49. In Luyendyk, B. P., Cann, J.R., et al., Init. Repts. DSDP, 49: Washington (U.S. Govt. Prin-ting Office), 793-805.

Kuno, H., 1936. Petrological notes on some pyroxene-andesites fromHakone volcano, with special reference to some types with pigeon-ite phenocrysts. Japan J. Geol. Geogr., 13:107-140.

, 1950. Petrology of Hakone Volcano and the adjacentareas, Japan. Bull. Geol. Soc. Am., 61:957-1020.

, 1954. Volcanoes and Volcanic Rocks: Tokyo (Iwanami)., 1960. High-alumina basalt. J. Petrol., 1:121-145., 1965. Fractionation trends of basalt magmas in lava flows.

/. Petrol., 6:302-321._, 1969. Pigeonite-bearing andesite and associated dacite

from Ashio, Japan. Am. J. Sci., 267:257-268.Kushiro, I., 1972. Determination of liquidus relations in synthetic

silicate systems with electron probe analysis: the system forsterite-diopside-silica at 1 atmosphere. Am. Mineral., 57:1260-1271.

Miyashiro, A., 1974. Volcanic rock series in island arcs and activecontinental margins. Am. J. Sci., 274:321-355.

Nakamura, Y., and Kushiro, I., 1970a. Compositional relations of co-existing orthopyroxene, pigeonite, and augite in a tholeiitic ande-site from Hakone volcano. Contrib. Mineral. Petrol., 26:265-275.

, 1970b. Equilibrium relations of hypersthene, pigeonite,and augite in crystallizing magmas: microprobe study of apigeonite andesite from Weiselberg, Germany. Am. Mineral.,55:1999-2015.

Uyeda, S., and Ben-Avraham, Z., 1972. Origin and development ofthe Philippine Sea. Nature, 240:176-178.

706


Wood, B. J., and Banno, S., 1973. Garnet-orthopyroxene and ortho-pyroxene-clinopyroxene relationships in simple and complexsystems. Contrib. Mineral. Petrol., 42:109-124.

Yang, H. Y., 1973. Crystallization of iron-free pigeonite in the systemanorthite-diopside-enstatite-silica at atmospheric pressure. Am. J.

Sci., 273:488-497.

APPENDIX

Data on Major-Element Compositions of DSDP Leg 59 TholeiiticBasalt, Calc-Alkalic Andesite, and Glass Obtained by Wet Chemical

Analysis, XRF, and EPMA MethodsTeruaki Ishii, Ocean Research Institute, University of Tokyo,

Minamidai, Nakano-ku, Tokyo 164, Japanand

Hiroshi Haramura, Geological Institute, Faculty of Science,University of Tokyo, Hongo, Tokyo 113, Japan

During the DSDP Leg 59 cruise, various igneous rocks (lava, in-trusive rocks, volcaniclastic breccias and tuffs) were obtained fromSites 447 through 451, along the South Philippine Sea transect; theseinclude oceanic igneous rocks from the West Philippine Basin (Site447) and the Parece Vela Basin (Sites 449 and 450), and island-arc ig-neous rocks from the Palau-Kyushu Ridge (Site 448) and West Mari-ana Ridge (Site 451). Major-element analyses of rocks and glasses aswell as microprobe analyses of some minerals will be presented in thisAppendix.

The major elements contained in some rocks and glasses obtainedduring Leg 59 have been analyzed by three methods: wet chemicalanalyses by Haramura, XRF analyses, and electron microanalyses(EPMA) by Ishii (preceding chapter). The analytic results and theirCIPW norms are shown in Tables 1 through 8 and Figure 1 (the oxide[wt. %]-solidification index [SI] diagram). The microprobe analysesreported in the preceding chapter include groundmass compositions ofrocks from the West Mariana Ridge and roughly estimated bulk com-positions of hornblende gabbro from the Palau-Kyushu Ridge. To ob-tain reliable composition with polished thin sections, these sectionswere moved rapidly over a wide area under the electron beam duringanalysis.

The hornblende gabbro consists of green hornblende, augite,plagioclase, and opaques. Plagioclase-olivine-phyric basalts from thelowermost part of Hole 447A contain plagioclase aggregates and Al-rich pyroxene and plagioclase xenocrysts. Microprobe analyses ofthose particular minerals in the above rocks are presented in Tables 9and 10 and plotted in Figures 2 and 3.

ACKNOWLEDGMENTS

The author is grateful to Mr. K. Fujioka and Dr. R. Matsumotofor their help on the XRF analyses, to Dr. M. Nomura, Mr. A.Uchiyama, and Mr. K. Kutsukake for the computer programs theycontributed to the study, and to Miss S. Washida for her help inpreparation of the manuscript. I thank Professor K. Kobayashi for hisencouragements during the research.

Table 1. The major-element analyses obtained by wet chemical analysis of the rocks from Leg 59, Holes447A, 449, and 450.a

Anal. No.

Sample

Siθ2TiO2

A12O3

Fe2O3

FeOMnOMgOCaONa2θK2O

P2O5H2O +H 2 O -C O 2

Total

CIPW NormQOrAbAnNeWoEnFsEnFsFoFaMt11

Ap

Ni (ppm)Cr (ppm)

1

447A-19-3,85 cm

46.801.14

16.326.954.190.156.06

11.702.400.610.112.301.37

—

100.10

2.113.74

21.0633.14

0.010.989.330.226.320.150.00.0

10.452.250.26

93256

West Philippine Sea Basin

2

447A-23-1,39 cm

49.601.07

15.112.437.190.178.47

11.792.130.050.071.360.80—

100.24

0.630.30

18.3732.14

0.011.297.223.33

14.296.600.00.03.592.070.17

74226

3

447A-24-2,99 cm

46.800.83

17.292.405.970.149.58

11.872.280.060.052.400.70—

100.37

0.00.36

19.8337.80

0.09.356.462.133.951.309.893.593.581.620.12

130275

4

447A-27-1,21 cm

46.230.84

18.865.203.220.157.17

11.762.310.410.043.160.92—

100.27

0.02.52

20.3241.46

0.07.906.680.199.000.262.020.067.841.660.10

125260

5b

447A-36-5,9 cm

41.250.79

16.485.513.070.134.53

16.602.051.020.140.941.506.1

100.11

0.06.584.65

35.777.74

14.2812.320.020.00.00.00.08.721.640.35

96336

6

449-15-2,1 cm

46.120.92

16.816.433.370.196.12

12.032.570.370.152.602.59—

100.27

0.822.30

22.8734.96

0.011.189.660.06.370.00.00.09.271.840.37

102208

Parece Vela Basin

7

449-17-2,98 cm

46.321.05

18.066.633.960.154.84

11.712.790.350.271.622.03—

99.78

1.572.15

24.5637.16

0.08.957.570.214.970.140.00.0

10.002.070.65

80212

8

450-36-2,125 cm

46.501.42

17.507.233.550.174.84

11.352.870.810.252.301.32—

100.11

0.704.96

25.1733.66

0.09.618.300.0

4.190.00.00.0

8.172.800.60

62260

9

450-36-3,134 cm

47.091.34

16.316.004.370.186.08

12.142.660.320.201.981.41—

100.08

1.281.96

23.2832.70

0.011.799.610.766.050.470.00.0

9.002.630.48

84268

a Analyst: H. Haramura.b Intensively altered.

707

T. ISHII

Table 2. The major-element analyses obtained by wet chemical analysis of rocks from Leg 59,Holes 448, 448A, and 451.a

Anal. No.

Sample

Siθ2TiO 2

A12O3

F e 2 O 3

FeOMnOMgOCaONa2OK2O

P2O5H2O +H 2 O -

Total

CIPW NormQOrAbAnNeWoEnFsEnFsFoFaMt11Ap

Ni (ppm)Cr (ppm)

1

448-59-1,115 cm

48.270.97

17.185.956.110.204.699.162.780.790.151.332.11

99.69

3.384.85

24.4433.31

0.05.383.491.528.643.750.00.0

8.961.910.36

3720

2

448A-16-3,50 cm

48.311.06

17.465.696.450.223.89

10.012.810.710.211.621.93

100.37

3.384.33

24.5634.01

0.06.623.932.356.073.640.00.08.522.080.50

2416

Palau-Kyushu Ridge

3

448A-50-2,2 cm

49.211.29

14.076.976.710.205.388.182.420.230.171.793.55

100.17

10.261.43

21.5928.31

0.05.563.771.35

10.363.710.00.0

10.662.580.42

2418

4

448A-62-1,5 cm

49.541.13

18.385.694.780.253.889.412.910.420.201.242.39

100.22

6.692.57

25.4937.11

0.04.122.940.817.061.960.00.08.542.220.48

2412

5

448A-62-1,70 cm

49.200.93

19.454.695.260.154.01

10.672.480.210.121.131.92

100.22

5.921.28

21.5942.52

0.04.652.991.367.293.300.00.07.001.820.29

2416

West Mariana Ridge

6

451-38-1,134 cm

49.990.68

19.764.454.920.173.87

10.522.160.700.131.830.92

100.10

6.904.25

18.7743.30

0.03.942.501.187.403.490.00.06.631.330.31

2111

7

451-59-1,60 cm

55.010.72

18.784.172.670.161.779.263.262.550.281.500.12

100.25

7.7015.2827.9729.48

0.05.684.470.570.00.00.00.06.131.390.66

168

8

451-69-2,85 cm

57.340.62

17.702.484.220.172.057.123.931.230.322.330.59

100.10

11.777.48

34.2227.81

0.02.671.321.293.933.840.00.0

3.701.210.76

85

a Analyst: H. Haruyama.

Table 3. The major-element analyses obtained by XRF method of therocks from Leg 59, Holes 449 and 45O.a

Anal. No.

Sample

1

449-15-2,59 cm

Parece Vela Basin

West (Site 449)

2

449-17-1,85 cm

3

449-17-1,103 cm

4

449-17-2,98 cm

East (Site 450)

5

450-36-3,104 cm

6

450-36-3,117 cm

SiO2TiO2

A12O3

Fe2θ3bFeOMnOMgOCaONa2OK2OP2O5H 2O + cH 2 O - c

Total

CIPW NormQOrAbAnNeWoEnFsEnFsFoFaMt11

Ap

45.870.98

18.0910.67

0.0

0.213.26

12.322.140.460.270.00.0

94.27

6.042.88

19.2140.73

0.08.147.030.01.580.00.00.00.00.480.66

47.050.94

18.2310.09

0.0

0.174.14

12.822.080.390.270.00.0

96.18

5.682.40

18.3040.81

0.08.677.490.03.230.00.00.00.00.380.65

46.100.99

18.0010.410.00.143.55

12.952.070.410.340.00.0

94.96

5.762.55

18.4440.66

0.09.027.800.01.510.00.00.00.00.320.83

47.001.00

17.2010.83

0.0

0.134.64

12.042.230.370.350.00.0

95.79

5.802.28

19.7037.40

0.08.137.020.05.040.00.00.00.00.290.85

48.541.33

15.789.310.0

0.155.94

12.401.990.440.220.00.0

96.10

6.622.71

17.5234.16

0.010.088.720.0

6.680.00.00.00.00.330.53

47.611.22

16.089.760.0

0.156.01

12.302.160.460.270.00.0

96,02

4.632.83

19.0334.18

0.09.908.560.0

7.030.00.00.00.0

0.330.65

a Analyst: T. Ishii.b Total Fe as Fe2θ3c Not determined.

708


Table 4. The major-element analyses obtained by XRF method of the rocks from Leg 59,Holes 448A and 45l. a

Anal. No.

Sample

SiO2

TiO2

A12O3

Fe 2O 3bFeOMnOMgOCaONa 2OK 2 O

P2O5H 2 O + cH 2 O ~ c

Total

CIPW Norm

QOrAbAnNeWoEnFsEnFsFoFaMt11Ap

Palau-Kyushu Ridge

1

448A-62-1,70 cm

49.710.92

18.6710.470.00.153.86

10.841.870.210.200.00.0

96.90

12.271.28

16.3343.27

0.03.422.950.06.970.00.00.00.00.330.48

2

448A-62-2,9 cm

48.300.92

18.4910.660.00.124.08

10.871.840.130.200.00.0

95.61

11.120.80

16.2843.73

0.03.533.050.07.580.00.00.00.00.270.48

3

451-38-1,134 cm

49.590.63

19.039.640.00.163.65

10.451.650.710.200.00.0

95.71

12.294.38

14.5944.32

0.02.862.470.07.030.00.00.00.00.360.48

West Mariana Ridge

4

451-38-2,12 cm

51.680.76

17.2110.730.00.173.278.892.151.040.290.00.0

96.19

14.366.39

18.9135.59

0.02.602.250.06.220.00.00.00.00.380.70

5

451-58-2,132 cm

55.610.67

18.336.740.00.131.898.842.622.410.310.00.0

97.55

12.8914.6022.7231.920.03.803.280.01.540.00.00.00.00.290.74

6

451-89-2,54 cm

64.550.85

14.126.730.00.120.805.212.411.920.560.00.0

97.27

33.3511.6620.9622.66

0.00.00.00.02.050.00.00.00.00.261.33

a Analyst: T. Ishii.b Total Fe as Fe 2 θ3.c Not determined.

709

T. ISHII

Table 5. The major-element analyses obtained by EPMA of the glass from Leg 59, Hole 447A.a

Anal. No.

Sample

SiO2

TiO2

AI2O3,F e 2 O 3

b

FeOMnOMgOCaONa2θK 2 OP 2 0 5 cH 2 O + cH 2 O - c

Total

CIPW Norm


id

447A-17-1,79 cm

46.851.05

15.921.238.220.158.39

11.972.680.090.00.00.0

96.61

0.00.55

21.6132.24

1.0112.207.214.380.00.0

10.106.771.852.060.0

2

447A-22-1,112 cm

49.891.04

14.861.298.610.168.23

12.222.150.060.00.00.0

98.57

0.00.36

18.4631.160.0

12.677.324.77

11.407.421.461.041.902.000.0

West

3

447A-22-2,34 cm

49.011.05

14.541.308.660.178.26

12.222.170.040.00.00.0

97.48

0.00.24

18.8430.590.0

13.207.624.989.546.232.771.991.932.050.0

Philippine Sea

4

447A-32-1,69 cm

50.440.84

15.091.157.640.138.90

13.121.840.040.00.00.0

99.27

0.00.24

15.6833.040.0

13.588.314.51

13.327.230.490.291.681.610.0

Basin

5

447A-35-3,70 cm

50.370.92

15.411.228.130.138.67

12.762.070.040.00.00.0

99.76

0.00.24

17.5632.720.0

12.837.664.51

11.096.532.031.321.771.750.0

6

447A-36-2,35 cm

50.150.90

15.411.218.050.158.70

12.612.090.040.00.00.0

99.37

0.00.24

17.8032.750.0

12.617.564.39

11.006.392.271.451.771.720.0

7e

447A-36-3,90 cm

48.440.88

14.931.278.440.238.06

11.452.250.640.00.00.0

97.89

0.03.86

19.4529.37

0.011.977.054.326.043.705.193.511.881.710.0

a Analyst: T. Ishii.b Fe2θ3/FeO = 0.15.c Not determined." Degree of reliability is relatively low.e Glass in the plagioclase aggregate.

710


Table 6. The major-element analyses obtained by EPMA of the glass fromLeg 59, Holes 449 and 45O.a

Anal. No.

Sample

SiO2

TiO2

A12O3

Fe2θ3 b

FeOMnOMgOCaONa 2OK 2 OP 2 O 5 cH 2 O + cH 2 O - c

Total

CIPW Norm


1

449-15-2,1 cm

48.591.01

15.801.268.400.157.72

11.582.510.190.00.00.0

97.25

0.01.15

21.8432.170.0

11.236.424.335.853.945.263.911.881.970.0

Parece Vela Basin

2

449-15-2,59 cm

48.770.99

16.251.258.320.177.65

11.622.560.190.00.00.0

97.77

0.01.15

22.1533.02

0.010.836.174.195.303.595.624.201.851.920.0

3

450-36-3,134 cm

49.541.52

14.871.308.690.167.75

11.792.760.100.00.00.0

98.51

0.00.60

23.7128.31

0.012.977.464.925.593.694.583.331.912.930.0

4

450-36-3,134 cm

49.321.55

14.991.308.660.167.71

11.733.010.080.00.00.0

98.58

0.00.48

25.8327.540.0

13.157.584.972.521.656.574.751.912.990.0

5

450-36-3,134 cm

49.051.52

14.741.288.560.147.51

11.651.280.160.00.00.0

95.90

4.500.99

11.2935.45

0.010.365.953.9513.569.010.00.01.943.010.0

a Analyst: T. Ishii.b Fe2θ3/FeO 0.15.c Not determined.

711

Table 7. The major-element analyses obtained by EPMA of the glass from Leg 59, Holes 448 and 448A.'

Anal. No.

Sample

SiO2

TiO 2

A12O3

F e 2 θ 3 b

FeOMnOMgOCaO

Na 2OK 2 OP 2 O 5 c

H 2 O ~ C

Total

CIPW NormQOrAbAnNeWo

EnFsEnFsFoFaMt

IIAp

Id

448-50-3,80 cm

56.500.45

13.431.026.830.202.176.363.790.690.00.00.0

92.74

14.374.40

34.5818.970.0

6.282.234.213.606.790.00.0

1.590.920.0

2

448-51-2,88 cm

52.531.19

13.311.73

11.520.234.218.872.660.340.00.00.0

96.60

6.892.08

23.3024.20

0.0

8.923.325.767.53

13.060.00.0

2.602.340.0

3

448-51-3,90 cm

52.321.19

13.471.71

11.410.224.178.962.620.350.00.00.0

96.42

6.812.14

22.9924.85

0.0

8.873.315.737.46

12.920.00.0

2.572.340.0

4

448-51-3,134 cm

52.171.19

13.041.69

11.270.214.338.952.540.340.00.00.0

95.73

7.292.10

22.4524.21

0.0

9.263.555.847.71

12.670.00.0

2.562.360.0

5

448-51-4,3 cm

52.631.15

13.411.70

11.320.254.339.092.620.350.00.00.0

96.88

6.772.13

22.8824.56

0.0

9.183.505.827.63

12.680.00.0

2.542.250.0

6

448-52-1,95 cm

51.291.18

13.431.68

11.220.204.388.892.530.340.00.00.0

95.14

6.252.11

22.5025.52

0.0

8.703.375.458.10

13.090.00.02.562.360.0

7

448-53-2,34 cm

50.641.14

13.261.81

12.050.214.548.962.610.320.00.00.0

95.54

4.261.98

23.1124.62

0.09.153.455.858.38

14.180.00.02.752.270.0

Palau-Kyushu Ridge

8

448-59-1,61 cm

53.631.22

12.371.68

11.220.173.698.182.590.440.00.00.0

95.21

10.662.73

23.0221.87

0.08.663.065.826.60

12.500.00.02.562.430.0

9

448-59-3,56 cm

53.641.22

12.961.70

11.320.253.588.032.590.440.00.00.0

95.73

10.432.72

22.8923.44

0.0

7.592.605.206.71

13.430.00.0

2.572.420.0

10

448A-16-2,76 cm

54.791.22

12.921.64

10.920.243.688.242.670.430.00.00.0

96.78

11.122.63

23.3422.73

0.0

8.142.915.436.56

12.240.00.0

2.462.390.0

11

448A-16-2,106 cm

54.861.19

13.181.62

10.800.243.618.242.620.450.00.00.0

96.81

11.392.75

22.9023.63

0.0

7.772.755.206.53

12.330.00.02.432.330.0

12

448A-17-1,23 cm

53.931.18

13.131.63

10.850.163.768.182.690.440.00.00.0

95.98

10.092.71

23.7123.39

0.07.892.875.186.88

12.420.00.02.462.330.0

13

448A-17-1,120 cm

54.421.10

14.301.51

10.040.163.398.712.730.390.00.00.0

96.77

10.342.38

23.8726.47

0.0

7.592.725.056.00

11.130.00.0

2.262.160.0

14

448A-17-1,120 cm

54.811.20

13.181.65

11.000.193.778.252.500.420.00.00.0

96.97

11.652.56

21.8124.23

0.0

7.502.714.966.97

12.780.00.02.472.350.0

15

448A-20-4,35 cm

53.871.19

12.501.69

11.270.223.778.262.450.410.00.00.0

95.63

11.272.53

21.6822.90

0.0

8.332.955.586.87

12.970.00.0

2.562.360.0

16

448A-50-2,102 cm

45.701.98

11.822.49

16.600.365.568.801.240.260.00.00.0

94.81

2.551.62

11.0727.34

0.0

7.812.795.20

11.8122.03

0.00.0

3.813.970.0

a Analyst: T. Ishii.b F e 2 θ 3 / F e O = 0.15.c Not determined." Roughly estimated analysis of hornblende gabbro.


Table 8. The major-element analyses by EPMA of the groundmass (see text)in the rock from Leg 59, Hole 451. a

Anal. No.

Sample

SiO2

TiO2

A12O3

Fβ2θ3 b

FeOMnOMgOCaONa 2OK 2 O

P 2 θ 5

c

H 2 O + cH 2 O " C

Total

CIPW Norm


1

451-38-2,12 cm

58.950.86

18.100.966.390.162.206.964.041.630.00.00.0

100.25

8.289.61

34.1026.370.03.371.232.214.237.590.00.01.391.630.0

West Mariana Ridge

2

451-58-2,132 cm

66.310.53

16.050.412.750.040.694.044.113.380.00.00.0

98.31

19.3420.3235.3715.J30.01.990.631.431.122.550.00.00.601.020.0

3

451-59-1,60 cm

64.050.48

17.350.352.360.030.454.714.153.200.00.00.0

97.17

16.7619.4636.1419.820.01.760.471.380.682.010.00.00.520.940.0

4

451-69-2,85 cm

61.870.48

14.500.614.060.181.124.044.631.710.00.00.0

93.20

17.6810.8442.0314.730.02.830.882.052.114.910.00.00.950.980.0

5

451-89-2,54 cm

63.870.91

15.950.765.100.110.735.353.622.040.00.00.0

98.44

20.5312.2531.1121.580.02.250.471.931.385.630.00.01.121.760.0

a Analyst: T. Ishii.b Fe2θ3/FeO = 0.15.c Not determined.

713

T. ISHII

(wt.%)

80

70

5 60to

50

4020

CO

O

< 1 5

10

09

o"– 5

0n

O^ 5

Site 447 West Philippine Basin * : ^ c h e m i c a l a"a|Ys is ( b u l k )

+ : EPM A analysis (glass)

.#+

(wt.%)

15

10

O 10to

CJ

+ -ft*

• ÷• ll ll

• + -H- -H.+

50 40 30 20 10 50 40 30 20 10

Figure 1. Oxide (wt. %)-solidification index (SI) diagram of chemical analyses of rocks and glasses from Site 447 in the West Philippine Sea (A),Site 448 on the Palau-Kyushu Ridge (B), Site 449 and 450 in the Parece Vera Basin (C), and Site 451 on the West Mariana Ridge (D).

714


(wt.%) Site 448 Palau-Kyushu Ridge80

Wet chemical analysis (bulk)×RF analysis (bulk)EPMA analysis (glass)

O 60 -

4020

coOS* 15<

10

co 15O

CM03

"– 10+

o

u. 5

0CO

OCM _» 5

015

O 103)

LL

5

0

B.

, 1

,1

,1

,

1 , 1 ,

-

:

:

r

r

f•

-

I . I .

i

•

•«#

••

+*I . I ,

-

+

+

•

-

50 40 30SI

20 10

15

(wt.%)Oσi

O

oCN

+o

CM

z

oCN

ro

OCN

CNOI -

o"5

CN

a.

O

10

5

0

15

10

5

010

5

0432104321032101

01

I _L I

1 1

1 1

-

1 , 1

1

•Yt*

# +

I

• _|LTr-|4t-

+

t

1 .

-

1 1

1 1

-

-

1

50 40 30 20SI

10

Figure 1. (Continued).

715

T. ISHII

• : Wet chemical analysis (bulk)* : ×RF analysis (bulk)

(wt.%) Sites 449 and 450 Parece Vela Basin + : EPMA analysis (glass)80

7 0 -

CΛ 60

5 0 -

4020

CO

O-T>1 15<

10

u- 10|-

OLL 5

0

CO

CM ^<D

LL

o 1 0

CUu.5

0

c.

-

I 1

r

-

1 , , , , 1 . , , , 1 . ,

I

++H-

i

i

* * * *

•

••

i

* •

.

•i

* *

i i i .

-

50 40 30 20 10

(wt.%)

15

10

0

15

o ioo

5

0°̂ 10

50

**• ** *

•

* * . •

+ 44+ W i ** , * *i40 30

SI20 10


716


(wt.%) site 451 West Mariana Ridge80

70

CN

S 60

50

10

« 1 5

oCM

£ 10

10|r

5

0

Wet chemical analysis (bulk)XRF analysis (bulk)EPMA analysis (groundmass)

-

-

1 . 1

'-

... •

1.

,1

.1

..

r

r

i , i

*

•

i

*

i

*

i

*+ +

-

+ *

-

i

50 40 30 20 10

(wt.%)15

10

O 10o

oCNCO

oCN

V

CN

O\-

inO

CND-

n

0432104321032101

01

50

«• t*+ +

+ + I

4 + 4 + ~

# * ? *

40 30 20SI

10


717

T. ISHII

Table 9. The pyroxene and hornblende analyses from Leg 59.a

Anal. No.

SiO 2

A1 2O 3

TiO 2

CT2O3

FeO BMnOMgOCaONa 2OK2O

Total

O = 6.000SiAlAl

TiCrFeMnMgCaNaKZWXY

Atomic %CaMgFe

Plagioclase-phyricbasalt (G740),

Sample 447A-36-5, 7 cm

Al-augite

Groundmass

G804

47.826.391.670.285.980.08

12.7322.880.59

—

98.42

1.8060.1940.0900.0470.0080.1890.0030.7160.9260.043

—2.0002.022

50.639.110.3

G805

48.495.541.520.816.540.11

13.1522.920.50

—

99.58

1.8160.1840.0600.0430.0240.2050.0030.7340.9200.036

—2.0002.025

49.539.511.0

Xenoc.

G904

49.1311.880.460.088.640.22

13.5214.14

1.61—

99.68

1.7940.2060.3050.0130.0020.2640.0070.7360.5530.114

—2.0001.994

35.647.417.0

Hornblende gabbro0803), Sample

448-50-3, 80 cm

Augite

Phc.

1811

50.400.420.020.00

11.540.77

11.0223.83

0.24—

98.24

1.9630.0190.0000.0010.0000.3760.0250.6400.9940.018

1.9822.054

49.531.818.7

Hornb.

Phc.

1810

49.443.090.280.00

21.830.779.94

10.710.620.13

96.81

1.9700.0300.1150.0080.0000.7280.0260.5900.4570.0480.024

—

25.833.341.0

a Abbreviations: Hornb. = hornblende, Xenoc. = xenocryst,and Phc. = phenocryst.

b FeO* = total Fe as FeO.

CaMg CaFe

Mg Fe

CaMg CaFe

Mg Fe

Figure 2. A. Ca-Mg-Fe plot of analyzed pyroxene in Rock G740(plagioclase-phyric basalt, Sample 447A-36-5, 7 cm). B. Ca-Mg-Fepot of analyzed pyroxene and hornblende in Rock 1803 (horn-blende gabbro, Sample 448-50-3, 80 cm).

Table 10. The plagioclase analyses from Leg 59.a

Anal. No.

SiO2

A12O3

TiO2

Cr2θ3FeO*BMnOMgOCaONa2OK2O

Total

0 = 6.000SiAlTiCrFeMnMgCaNaK

Atomic %NaCaK

Plagioclase-phyricbasalt (G740);

Sample 447A-36-5,7 ( :m

Plagioclase

Xenoc.

G806

43.1135.25

0.000.030.310.000.21

19.430.810.00

99.15

1.5151.4600.0000.0010.0090.0000.0110.7320.0550.000

7.0192.99

0.00

Phc.

G807

45.8833.49

0.000.010.350.020.26

17.831.710.01

99.56

1.5961.3740.0000.0000.0100.0010.0130.6650.1150.001

14.7885.16

0.06

PI. aggre-gate (H7389),

Sample 447A-36-3,90 cm

PI.

Phc.

H808

46.7133.53

0.000.000.300.020.28

17.092.090.01

100.03

1.6131.3650.0000.0000.0090.0010.0140.6320.1400.002

18.1181.83

0.06

Hofnb.gabbro (1803)

Sample 448-50-3,80 cm

PI.

Phc.

1813

51.6229.94

0.020.010.660.000.12

13.464.240.01

100.17

1.7651.2070.0010.0000.0190.0000.0060.4930.2810.017

36.1063.34

0.56

a Abbreviations: Hornb. = hornblende, PI. = plagioclase, Xenoc.xenocryst, and Phc. phenocryst.

b FeO = total Fe as FeO.

G740- -H7389

Figure 3. K-Na-Ca plot of analyzed plagioclase in Rocks G740, 1803,and H7389 (plagioclase aggregate, Sample 447A-36-3, 90 cm).(Lines show compositional ranges of plagioclase.)

718

31. pyroxene geothermometry of basalts and an … · group 6, which will be called two-pyroxene...

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