crystallizationofinterstitialliquidandlatent heat buffering in …eprints.esc.cam.ac.uk/3083/1/namur...

Crystallization of Interstitial Liquid and LatentHeat Buffering in Solidifying Gabbros:Skaergaard Intrusion, Greenland

OLIVIER NAMUR1*, MADELEINE C. S. HUMPHREYS2y ANDMARIAN B. HOLNESS1

1DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF CAMBRIDGE, DOWNING STREET, CAMBRIDGE CB2 3EQ, UK2DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF OXFORD, SOUTH PARKS ROAD, OXFORD OX1 3AN, UK

RECEIVED MAY 2, 2013; ACCEPTED MAY 8, 2014

New compositional profiles across plagioclase grains from theLayered Series (LS), Marginal Border Series (MBS) and UpperBorder Series (UBS) of the Skaergaard intrusion are used to under-stand the mechanisms of cumulate rock solidification and the fate ofthe interstitial liquid. The data show that plagioclase crystals dis-play three types of compositional profile over the whole intrusion, asfollows. (1) Grains with normal zoning, which dominate the MBSand UBS.These are interpreted as having formed at the top of a crys-tal mush and then buried in the cumulate pile. Crystallization ofthe interstitial melt resulted in liquid differentiation and producednormally zoned rims on plagioclase cores. (2) Unzoned crystals,which dominate the upper part of the LS, also crystallized at the topof the mush and were then buried in mush with a low interstitialliquid fraction or one experiencing convective movements that keptthe liquid to a constant composition. (3) Crystals with a mantle ofdecreasing An content followed by a rim of constant composition.Grains showing this complex zoning mostly occur in the lower partsof the LS. Depending on the stratigraphic position within the intru-sion, the composition of the rim can be An56, An51 or An40. In themain magma body, these compositions (An contents) correspond tothose of plagioclase primocrysts (e.g. cores) at the appearance of cu-mulus clinopyroxene (An56), Fe^Ti oxides (An51) and apatite(An40). Compositional buffering of plagioclase rims is interpretedas being a consequence of enhanced release of latent heat of crystal-lization at the appearance of new interstitial phases in the crystalmush.When a new phase saturates, the latent heat contribution tothe global enthalpy budget of the system becomes sufficiently high tokeep the interstitial melt at its liquidus temperature for a period oftime that could exceed thousands of years. Under these conditions,

equilibrium, adcumulus growth together with diffusion and possiblyadvection of chemical components result in the formation of plagio-clase rims of constant composition (An content). Efficient thermalbuffering of the mush liquid depends on the porosity (i.e. fraction ofliquid within the mush) and the degree of compositional homogeneityof the mush. In a heterogeneous and highly porous mush, saturationof the new phase occurs in the coldest part of the mush and theenhanced latent heat release at the saturation of this phase is quicklydissipated to the whole volume of liquid, including the warmest partthat is not yet saturated in a new phase. As a consequence, no thermalbuffering occurs and interstitial crystallization produces grains withnormal zoning. The distribution of the various types of plagioclasegrains throughout the Skaergaard intrusion can therefore be used toinfer the spatial variability in the physical properties of the crystalmush, such as the residual porosity, both at an intrusion-wide scaleand at a millimetre- to centimetre-scale.

KEY WORDS: plagioclase; thermal buffering; thermodynamics; zoning;cumulate; layered intrusion; Skaergaard

I NTRODUCTIONFormation of cumulate rocks starts with the nucleation andgrowth of liquidus crystals (primocrysts), which are thencaptured in a crystal mush. When a new phase saturatesin the mush liquid, it may form only a few nuclei onwhich further growth results in the formation of large poi-kilitic grains locally enclosing the primocrysts. In gabbroic

*Corresponding author. Present address: Institute of Mineralogy,University of Hannover, Callinstrasse 3, 30167 Hannover, Germany.Telephone: þ44 (0) 1223 333400. Fax: þ44 (0) 1223 333450. E-mail:[email protected] address: Department of Earth Sciences, University ofDurham, Science Labs, DurhamDH13LE, UK

�TheAuthor 2014. Published by Oxford University Press.This is an Open Access article distributed under the terms of theCreative Commons Attribution License (http://creativecommons.org/licenses/by/3.0/), which permits unrestricted reuse, distribution,and reproduction in any medium, provided the original work is prop-erly cited.

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rocks this is especially evident for pyroxenes (clinopyrox-ene and orthopyroxene) and for Fe^Ti oxides (Wageret al., 1960; McBirney & Hunter, 1995; Holness et al., 2007;Namur et al., 2010). Depending on the amount ofinterstitial material in cumulate rocks, cumulates may beclassified into three end-member types: adcumulate withless than 7% of interstitial material, mesocumulate with7^25% of interstitial material and orthocumulate with425% of interstitial material (Wager et al., 1960). The ini-tial porosity of a cumulate rock is estimated to be in therange 40^60 vol. % (Irvine, 1980; Shirley, 1986; Philpottset al., 1998; Jerram et al., 2003), which is significantlyhigher than the proportion calculated from the bulk chem-istry of the interstitial material in most cumulates (e.g.Tegner et al., 2009; Namur & Charlier, 2012). It is thereforeevident that at least some interstitial liquid is mobileduring the solidification of the crystal mush. Estimatingthe physical behaviour (i.e. mobility) of the interstitialmelt upon cooling is, however, hampered by the lack of ac-curate constraints on some fundamental physical proper-ties of the crystal matrix (e.g. viscosity), the thickness ofthe crystal mush and the rate of crystal accumulation atthe top of the crystal mush (Kerr & Tait, 1986; Namur &Charlier, 2012).Away to place constraints on thebehaviour of the intersti-

tial liquid within the crystal mush is to investigate the com-position of the interstitial phases in cumulate rocks.However, very little consideration has been given to suchinterstitial phases in the past (e.g. Meurer & Claeson, 2002;Toplis et al., 2008; Humphreys, 2009, 2011; Holness et al.,2011; Namur &Charlier, 2012), probably because they gener-ally equilibrate quickly with the continuously evolving inter-stitial melt. Their composition therefore does not help indeciphering the entire evolution of the melt. However,better constraints can be obtained from chemical zoning inplagioclase as this is not significantly altered by subsolidusdiffusion on the time-scale of solidification, at least for theCaAl-NaSi exchange (e.g. An-content; Grove et al.,1984).If the mush liquid follows the same differentiation path

as that of the main magma body (Meurer & Claeson,2002), intra-grain chemical variations will be identical tothose observed in primocryst cores from the level of primo-cryst accumulation to the top of the cumulate pile(Humphreys, 2009, 2011). However, processes such as phys-ical compaction of the crystal mush (McKenzie, 1984,2011; Tegner et al., 2009), compositionally driven convection(Tait et al., 1984; Toplis et al., 2008) or separation of Fe-richand Si-rich immiscible melts (Holness et al., 2011;Humphreys, 2011) may also affect the composition, abun-dance, and evolution of interstitial mush liquid. Such pro-cesses involving the migration of interstitial liquid mayproduce grains with normal or reverse zoning in meso- toorthocumulates, or unzoned grains in adcumulates (Topliset al., 2008; Humphreys, 2009; Morse, 2012).

In this study, we present a new and comprehensive set ofplagioclasemajor element compositions in Skaergaard sam-ples from the floor, roof and wall cumulates.These samplesare characterized by various liquidus assemblages and arange of crystallized interstitial liquid fractions.We describethree types of plagioclase grains: normally zoned, unzonedand with rims buffered to constant anorthite compositions.Plagioclase grains with normal zoning are interpreted as re-sulting from normal differentiation of the interstitial melt[i.e. orthocumulus growth according to the model ofWageret al. (1960)] and unzoned plagioclase grains are interpretedas having formed in a crystal mush depleted in mush liquidor during convective movements of liquid [i.e. they corres-pond to adcumulus growth according to the model ofWager et al. (1960)]. However, the growth of rims bufferedto a constant composition in the third type of plagioclasegrains cannot be explained by these processes.We demon-strate that the previously neglected process of thermal buf-fering, caused by peaks in latent heat released by thecrystallization of new liquidus phases (Holness et al., 2007;Morse, 2011), is critically important in controlling the solidi-fication of the crystal mush. Thermal buffering, coupledwith chemical diffusion of chemical species in the liquidphase, results in efficient adcumulus growth of thick rims ofconstant composition on plagioclase primocrysts. Ournovel observations link compositional zoning in plagioclasegrains to the enthalpy budget of fractionating magma in alayered intrusion, providing a link between the products ofcrystallization and the evolution of the physical propertiesof the mushy layer during solidification.Themineral zoningpatternswedescribearealso observed

in other layered intrusions (Meurer & Boudreau, 1996;Namur&Charlier, 2012) and in crystal mush fragments car-ried by erupted lavas (Hansen & Gronvold, 2000; Costaet al., 2012), indicating that the thermal buffering processmay be of general significance during solidification of anycrystal mush, andmay have important general implicationsfor reconstructing fractionation and differentiation paths inmushes andmagmachambers.

THE SKAERGAARD LAYEREDINTRUSIONThe Eocene Skaergaard intrusion of East Greenland occu-pies a box-shaped, fault-bounded chamber, approximately8 km�11km� 4 km (Nielsen, 2004), at the contact be-tween Precambrian gneisses and a thick overlying se-quence of Eocene plateau lavas (Fig. 1a). The intrusion isdivided into three main units: the Layered Series (LS),which crystallized on the floor, the Upper Border Series(UBS), which crystallized from the roof, and theMarginal Border Series (MBS), which grew inwards fromthe vertical walls (Fig. 1b). The Skaergaard intrusion crys-tallized from a single, homogeneous, convecting magma

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body and the three series (LS, MBS, US) crystallized sim-ultaneously. These series follow roughly parallel trends ofdifferentiation into the centre of the intrusion, convergingat the Sandwich Horizon (Wager & Deer, 1939; Hoover,1989a; Salmonsen & Tegner, 2013). This is confirmed bythe identical An content of plagioclase at the appearanceof any cumulus phase in the three series (Hoover, 1989a;

Thy et al., 2009a; Salmonsen & Tegner, 2013). The compos-ition of other phases such as olivine and clinopyroxenecannot so easily be used to compare the three series be-cause these minerals equilibrate rapidly with the intersti-tial melt.The stratigraphy of the Layered Series is subdivided,

based on changes in the primocryst assemblage, into

UZaUZb

MZ MZ

UZa UZb

UZc

UBS

MBS

Forbindelsesgletscher

Uttental Sund

Basistoppen

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KraemerIsland

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erie

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UZc

Skaergaardintrusion

(a)

(b)

Core90-10

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LZb

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ofile

LZc

2000 &SKG11profiles

Hammer-pas

Kilen

MZ'

LZb'LZc'

UZ' Ham

mer

pas

Kile

n

+Ap

+Ol

-Ol+FeTi

+Cpx

Pl,Ol

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Mellemø

Ivnarmiut

Sk Mini-Peninsula

SMP

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Peninsula

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680800m N

314500m W

6813

3135

Sk Mini-Peninsula(SMP)

KraemerIsland (S)KR-03

KraemerIsland (N)KR-01 & KR-02

SkPenin(SP)

SK

G11

Fig. 1. (a) Simplified geological map of the Skaergaard intrusion showing the locations of the sampling traverses (SP, Skaergaard Peninsula;SMP, Skaergaard Mini-Peninsula; KR-01, KR-02, KR-03, Kraemer Island) and drill cores investigated for this study. LZ, Lower Zone; MZ,Middle Zone; UZ, Upper Zone; MBS, Marginal Border Series; UBS, Upper Border Series. Map in the upper right corner shows the locationof the map area in Greenland. (b) Schematic cross-section of the Skaergaard intrusion modified fromWager & Brown (1968) showing the spa-tial relationships between the Layered Series, Upper Border Series and Marginal Border Series. Subdivisions of each series and sequence of crys-tallization in the Layered Series are also shown. *, Notation of the zones and sub-zones of the MBS; ‘, notation of the zones and sub-zones ofthe UBS.

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plagioclaseþ olivine (Hidden Zone, HZ and Lower ZoneA, LZa); plagþolþaugite (LZb); plagþolþaugþFe^Ti oxides (LZc); plagþaugþox (þ pigeonite immedi-ately afterwards; Middle Zone, MZ); plagþolþaugþox(Upper Zone A, UZa); plagþolþaugþoxþ apatite(UZb); plagþolþaugþoxþ apþ ferro-hedenbergite (in-verted from b-ferrobustamite; UZc), where plag is plagio-clase, ol is olivine, aug is augite, ox is Fe^Ti oxides, ap isapatite (Fig. 1b). Continuous fractional crystallization isalso recorded by the compositions of the primocryst min-erals, which change systematically with increasing strati-graphic height (plagioclase: An72^29, Maaloe, 1976; Tegner,1997; Jang & Naslund, 2001; Tegner & Cawthorn, 2010;Morse, 2012; olivine: Fo70^0; clinopyroxene Mg#77^0;Wager & Brown,1968; McBirney, 1989; Thy et al., 2009a).The Marginal Border Series is subdivided in an analo-

gous manner into HZ*, LZa*, LZb*, LZc*, MZ*, UZa*and UZb* (Hoover, 1989a), as is the Upper Border Series(LZa’, LZb’, LZc’, MZ’, UZa’, UZb’ and UZc’; Salmonsen& Tegner, 2013). Again, primocryst compositions evolvecontinuously within this stratigraphic framework (e.g.Naslund, 1984; Hoover, 1989a; Salmonsen & Tegner, 2013).Primocrysts are generally slightly more primitive at theouter margins of the MBS than at the base of the LS (e.g.Fo74, cpx-Mg#80), but are more Fe-rich in general(Hoover, 1989a). Bulk-rock incompatible element concen-trations (e.g. P, Rb, U) suggest that the proportion of soli-dified trapped interstitial mush liquid was higher in theUBS (430% in most zones; see below) and MBS (430%in most zones) than in the corresponding sub-zones of theLS, especially after the appearance of cumulus Fe^Tioxides (510%; Wager & Brown, 1968; Hoover, 1989a;Tegner et al., 2009, 2011; Namur et al., 2013). This may beattributed to minimal compaction at the vertical wallsand roof compared with the floor of the intrusion (Tegneret al., 2009, 2011).

SAMPL ING AND ANALYTICALMETHODSSamplesSamples used in this study include suites collected duringthree field seasons (2000, 2008 and 2011) and materialfrom drill cores. The chosen suite of samples covers mostof the stratigraphic units of the Layered Series (48 sam-ples), the Marginal Border Series (15 samples) and theUpper Border Series (four samples; see Fig. 1 for locationsand Supplementary Dataset 1 for details; supplementarydata are available for downloading at http://www.petrology.oxfordjournals.org).

Analytical methodsPlagioclase compositions were obtained with a CamecaSX-100 electron microprobe (EPMA) at the University of

Cambridge (Supplementary Datasets 2^4). Some datawere acquired as single points, but most were acquired assingle 30^1831 mm long traverses either from one crystalmargin to another (rim^core^rim) or, more frequently,from core to rim along a straight line, with 4^80 singlepoints, spaced at 5^61 mm, in each profile. This datasettotals more than 9640 microprobe analyses. Peak countingtimes were 20 s for major elements and 40 s for minor elem-ents. Most data were acquired with a 15 kV and 10 nAbeam with a spot size of 1 mm. Some profiles were also per-formed with a 25 kV and 100 nA beam to obtain accuratedata for minor elements. Analyses that had a total outsidethe range 98·5^101wt % were excluded. The followingstandards were used for Ka X-ray line calibration: diop-side for Si and Ca; periclase for Mg; rutile for Ti; corun-dum for Al; fayalite for Fe; jadeite for Na; K-feldspar forK. Raw data were corrected with the CATZAF software.

SELECT ING AND F ITT INGPLAGIOCLASE PROF ILESThree types of plagioclase core^rim compositional profileshave been reported from the Skaergaard intrusion(Maaloe, 1976; Toplis et al., 2008; Humphreys, 2009, 2011;Morse, 2012), as follows.

(1) Cores surrounded by a mantle of decreasing anorthitecontent [An, with An¼Ca/(CaþNa) in molar frac-tions] followed by a rim of constant composition(Figs 2a, b and 3a). They are mostly found in meso-to orthocumulates of the Lower Zone of the LayeredSeries (Toplis et al., 2008). Some grains, however,show a slight decrease in An content in the external(outer) part of the rim (Toplis et al., 2008;Humphreys, 2009).

(2) Cores surrounded by rims of continuously decreasingAn content (Figs 2c^e and 3a) in the Layered Series(Maaloe, 1976; Humphreys, 2009, 2011) and in theMarginal Border Series (Humphreys, 2011).

(3) Unzoned plagioclase grains (Figs 2f and 3a) in theMiddle and Upper Zones of the Layered Series(Maaloe, 1976; Toplis et al., 2008).

Reverse zoning has also been reported in plagioclasefrom the Layered Series and has been attributed to crystal-lization in supercooled conditions (Maaloe,1976), to a pro-cess of compaction-driven dissolution and reprecipitation(Humphreys, 2009) or to crystallization from an Fe-richimmiscible melt (Humphreys, 2011). It is worth noting thatreverse zoning has not been observed in the plagioclasecompositional profiles acquired in this study, which indi-cates that this type of zoning might be rather uncommonin the Skaergaard intrusion and potentially confined tothe margins of pockets of Fe-rich liquid.


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500 μm500 μmPlag

Cpx

Ol

500 μmPlag core

Plagrim

GranophyrePlagrim

(a)

500 μm

PlagCpx

Ol

Plag Plag rims Other phases Granophyre

500 μm

130 μm

(b)

(c) (d)

(e) (f)

EPMA

Granop

hyre

Plagrim

Plagcore

Maficphases

MaficphasesPlag

core

Plagrim

Fig. 2. Photomicrographs and line drawings illustrating the types of plagioclase grains observed in the Skaergaard intrusion. (a)Photomicrograph of sample 458252 (LZa). The presence of large plagioclase rims of uniform colour (buffered composition) should be noted.Cross-polarized transmitted light. (b) Line drawing of (a) showing the distribution of plagioclase buffered rims. Rims have been traced onhigh-resolution photographs with various orientations of the microscope stage. (c) Photomicrograph of sample 90-22-421 (UZb) showing thepresence of a large granophyre pocket at the junction between plagioclase grains. The plagioclase rims (normal zoning down to An25) at thecontact with the granophyre pocket should be noted. Cross-polarized transmitted light. (d) Line drawing of (c) showing the distribution ofplagioclase rims and granophyre pockets. (e) Normally zoned plagioclase grain in contact with a granophyre pocket. Profiles investigated byelectron microprobe are shown for reference (EPMA profile). SK11-C2-4 (LZa*). Cross-polarized transmitted light. (f) Photomicrograph ofsample SKG11-36 (LZc) showing unzoned plagioclase grains. The absence of evidence for evolved interstitial phases should be noted. Cross-polarized transmitted light.


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Most profiles were acquired from plagioclase grains incontact with another plagioclase grain because it has beensuggested that the most evolved compositions may occurat plagioclase^plagioclase contacts (Toplis et al., 2008).However, several profiles were also acquired from plagio-clase grains in contact with other phases (olivine, clinopyr-oxene, magnetite, ilmenite). We did not observe anyobvious compositional contrast in plagioclase rim compos-itions as a function of the adjacent phase. However, wenote that Humphreys (2011) observed that the compos-itional profiles in plagioclase may change as a function ofthe modal mineralogy (e.g. proportion of Fe^Ti oxides vspyroxenes) and texture (e.g. degree of igneous lamination)of the gabbros in the Skaergaard MZ.We adopted a mathematical approach to describe and

determine the relative size, which may be dependent onthe cutting geometry, and composition of the different

zones (Fig. 3b).We fitted an error function to each plagio-clase profile (An vs distance), starting from a standarderror function that could be elongated along the y-axis(An content) and the x-axis (distance), and rotated by1808. This function, here called ERFAn, was calculatedusing the equation

ERFAn ¼ a erf bþ x=2ð Þ½ � þ c ð1Þ

where a, b and c are constant parameters to be regressedand x is the length (i.e. distance in mm) of the profile. Theparameters a, b and c were regressed iteratively by mini-mizing the chi-squared (w2) value of the distribution withthe maximum expected analytical error of the electronmicroprobe, here considered as being 1% An, and chan-ging the parameters a, b and c to increase the quality ofthe fit. Simulations were stopped when (1) the lowest pos-sible value of w2 was reached or (2) after 10 000 iterations.

40

45

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4042444648505254

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45

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434445464748495051

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40

45

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49.249.449.649.850

50.250.450.6

Distance (μm)

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CoreCore

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Rim

Cor

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Cor

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Initialguess

Thirdderivative

ERF

Type I Type II Type III

Initialguess

Thirdderivative

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ent (

%)

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%)

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0 50 100 150 200 250 300 350 0 50 100 150 200 250 300 350 0 50 100 150 200 250 300 350

Fig. 3. (a) Theoretical types of plagioclase zoning (An vs distance) reported in the Skaergaard intrusion. Type 1: core of constant An contentfollowed by a mantle of decreasing An content and a rim of constant An content; Type II: core of constant An content followed by a rim ofdecreasing An content; Type III: unzoned grain. (b) Illustration of similar plagioclase grains showing the natural variability of plagioclase com-position in different zones and the effect of analytical uncertainty in electron microprobe measurements. It should be noted that determiningthe zones of the plagioclase profiles (core, mantle and rim) is somewhat ambiguous, as indicated by the grey vertical bars. (c) Best fit (continu-ous line) of an error function (ERF) on the plagioclase profiles.The dashed line represents the initial guess of the error function (see text for de-tails). The vertical bars represent the boundaries between the zones of the plagioclase grains (core, mantle and rim) and are defined on thebasis of the third derivative of the error function. The third derivative of the error function is shown for reference.


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To identify the zones (core, mantle, rim) of the An^distance profiles (Fig. 3c) we used the minimum values ofthe third derivative of ERFAn (d

3An/dx3). This allows usto define the three types of profiles described above(Fig. 3, Table 1): (1) Type I profile [two minima in (d3An/dx3)] with a core of constant composition followed by amantle of decreasing or increasing An content and a rimof constant composition; (2) Type II profile [one minimumin (d3An/dx3)] with a core of constant composition fol-lowed by a rim of decreasing or increasing An content;there is no mantle in this type of zoning profile; (3) TypeIII [no minima in (d3An/dx3)] profile with no zoning;there is neither a mantle nor rim in this type of zoning pro-file. Profiles were also considered as unzoned when the dif-ference between the highest and lowest An values ofERFAn is lower than 2% An.The global distribution of An content in each sample

was assessed using two types of histogram using (1) theKernel density distribution and (2) a Gaussian mixturemodel (Rudge, 2008; see Appendix for details on the calcu-lation procedure). The two types of histograms werefound to give similar results.

PLAGIOCLASE COMPOSIT IONLayered SeriesPlagioclase primocryst (core) compositions (Fig. 4a) in theLS evolve from c. An70Or1·0 at the base of LZa to c.An35Or3·5 at the top of UZb, in agreement with previousstudies (Wager & Brown, 1968; McBirney, 1989;Humphreys, 2011).Type I,Type II andType III compositional profiles were

identified in plagioclase crystals from the LS (Fig. 5;Table 1). Single grains are generally characterized by asingle type of zoning profile (Type I, II or III), althoughType I and Type II are locally observed on different facesof the same crystal. Most samples contain plagioclase crys-tals of different types (Type I, II and III; Fig. 5; Table 2),but a single type generally dominates within each sample.Quantifying the relative proportions of the three types isnot straightforward and cannot be done accurately

without a detailed analysis or chemical mapping on thescale of an entire thin section. The results of an attempt todo this are shown in Fig. 5. We used high-resolutionimages of a subset of samples to produce a map of allplagioclase grains in each thin section.We then identified,using an optical microscope, the grains belonging to eachtype of profile (Type I, II and III) and used the softwareImageJ to calculate the best-fit ellipses for each grain andestimate the relative proportion (vol. %) of each type ofzoning profile. For all other samples, we report only thenumber of grains of each type analysed for this study(Table 2). Although we performed a detailed petrographicinvestigation of all the samples, the relative proportions ofthe three types of grains (Type I, II or III) that we mea-sured may not be fully representative of their actual distri-bution in each sample.Type I profiles (Tables 1 and 2) are very abundant from

LZa to LZb, where they dominate the distribution ofzoning types (See Figs 5 and 6 and the sketch in Fig. 2).Type I profiles are also common from MZ to the lowerpart of UZa (core 4An42), but they were not observedeither in LZc or in samples from the upper part of UZa(core5An42) to UZb (Fig. 6). Whereas the An content ofthe plagioclase cores decreases upwards in the stratig-raphy, rim compositions fall into distinct and well-definedgroups within the LS (Figs 7a^c, 8a^c and 9). In LZa sam-ples, the rims have a constant composition of An54^58(mode An56; mean An57·0; n¼ 608; where n is the numberof analyzed points) regardless of the stratigraphic positionof the sample (Fig. 9), whereas the rims have a constantcomposition of An49^53 (mode An50; mean An50·8;n¼ 305) in LZb and An40^43 (mode An41; mean An41·4;n¼ 294) from MZ to the lower part of UZa (core4An42;Fig. 9). The global distribution of the An content in thesesamples (i.e. the distribution of all analysed compositions)is highly bimodal, with the first mode correspondingto the core composition and the second mode correspond-ing to the composition of the rim (Fig. 8a^c). In somegrains, the outer part of the rim may, however, show a de-crease in An content (see also fig. 5 in Humphreys, 2009).In some samples from LZa, Type I grains have two

Table 1: Types of plagioclase profiles observed in Skaergaard

Type of profile Third derivative Core Mantle Rim Layered* Marginal Border Upper Border

ERF function Series Border Series

Type I 2 minima Constant An Decreasing An Constant An LZa–LZb and MZ–UZa LZa*–LZc* LZa’ and MZ’–UZa’

Type II 1 minima Constant An – Decreasing An LZa–LZb and MZ–UZb LZa*–UZb* LZb’ and MZ’–UZa’

Type III 0 minima or �An52 Constant An – – LZc–UZb LZa* –

*Units where the various types of zoning profiles are highly abundant.


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clearly separate rims of constant composition. Such grainshave a distinct core surrounded by an inner mantle ofdecreasing An content, an inner rim of An55^58, a secondmantle and then an outer rim of An50^51 (Fig. 10). Itshould also be noted that, in contrast to Ca and Na (i.e.

An content), the concentration of some minor elements,such as Ti and to a lesser extent Fe, continuously changes(generally decreases outwards or first increases and thendecreases) within the rims of Type I grains (Humphreys,2009, 2011).

30 35 40 45 50 55 60 65 70 750.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0LZaLZb

LZcMZUZaUZb

UZc

Plagioclase An (%)

Pla

gioc

lase

Or (

%)

(a)

30 35 40 45 50 55 60 65 70 750.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0LZa*LZb*

LZc*MZ*UZa*UZb*

UZc

*

Plagioclase An (%)

Pla

gioc

lase

Or (

%)

(b)

1 Std Dev

Fig. 4. Orthoclase (Or) vs anorthite (An) content in plagioclase cores from (a) the Layered Series and (b) the Marginal Border Series.


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458218 - LZa - F:0.68 - n:793Type I: 81.6%; Type II: 9.7%;

Type III: 8.7%

118601 - LZa/LZb - F:0.67 - n:131Type I: 67.2%; Type II: 16.7%;

Type III: 16.1%

458224 - LZb - F:0.67 - n:354Type I: 70.2%; Type II: 13.3%;

Type III: 16.5%

SKG11-40 - LZc - F:0.36 - n:149Type II: 18.4%;Type III: 81.6%

90-10-453.7 - MZ - F:0.25 - n:138Type I: 19.6%; Type II: 10.7%;

Type III: 69.7%

90-22-824.2 - UZa - F:0.06 - n:139Type I: 26.5%; Type II: 16.2%;

Type III: 57.3%

90-22-67.6 - UZb - F:0.02 - n:139Type II: 69.1%;Type III: 30.9%

Optically zoned with rim of constant compositione.g. Type I

Optically zoned, no rim of constant compositione.g. Type II

Optically unzonede.g. Type III

Fig. 5. Digitized texture maps of plagioclase crystals for selected samples of the Skaergaard Layered Series. Three types of plagioclase crystalswith contrasted zoning types are shown; their relative proportions were determined using ImageJ software (see text for details). n, number ofcrystals; F, fraction of residual melt.


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Table2:

Majorelementplagioclasezoning

Sam

ple

FUnit

P2O5

(wt%)1

Trapped

liquid

fraction2

Grains

Single

analyses

Contact

phases

3

Typ

eI

grains

Typ

eII

grains

Typ

eIII

grains

Core

904

Man

tle

Ran

ge

Rim

Typ

eI

Rim

Typ

eII

Core

Man

tle

Rim

nn

nn

nAn%

An%

An%

An%

Or%

Or%

Or%

LayeredSeries

Field

samples

458242

0·75

LZa

0·14

0·38

8305

Plag,cp

x,ox

8–

–68–69

69–56

56–58

–0·8–1·3

1·1–1·7

1·4–2·0

458214

0·71

LZa

0·14

0·41

6214

Plag,ol

51

–64–65

65–55

55–58

65–57

1·1–2·0

1·5–1·7

1·7–2·8

458218

0·68

LZa

0·14

0·41

493

Plag,cp

x4

––

62–63

63–55

55–56

–1·6–2·0

1·7–2·1

2·1–3·0

458224

0·67

LZb

0·11

0·31

6184

Plag,ol,cp

x5

1–

61–62

62–49

49–52

62–49

1·6–2·2

1·8–2·8

2·0–2·8

458231

0·57

LZb

0·10

0·24

135

Plag

1–

–55–56

56–50

50–51

–2·2–2·6

2·4–2·7

2 ·7–3·0

458205

0·46

LZb

0·10

0·21

6135

Plag,cp

x,gr

42

–56–57

57–49

49–50

57–48

1·7–2·1

2·0–2·3

2·3–2·8

SKG11-23

0·40

LZb

––

5179

Plag,cp

x1

4–

54–55

55–50

49–50

55–50

1·5–2·2

–1·8–2·4

SKG11-27

0·39

LZb

––

8218

Plag,cp

x,ox

71

–53–56

56–50

50–51

56–50

1·7–2·1

1·9–2·3

2·1–2·6

SKG11-28

0·38

LZb

––

378

Cpx

3–

–54–55

55–50

50–51

–2·1–2·3

2·1–2·4

2·1–2·6

SKG11-29

0·38

LZb

––

8124

Plag,cp

x4

22

53–55

55–49

49–51

55–51

1·6–2·2

1·7–2·1

2·0–2·3

458279

0·38

LZb–L

Zc

0·06

0·11

227

Plag

11

–52–53

53–49

49–50

53–50

1·8–2·0

1·8–2·2

2·2–2·4

SKG11-36

0·37

LZc

––

5206

Plag,cp

x,ox

––

549–51

––

–1 ·6–2·9

––

SKG11-40

0·36

LZc

––

340

Plag,cp

x,ox

––

350–52

––

–1·6–1·8

––

SKG11-43

0·35

LZc

––

577

Plag,cp

x,ox

––

551–52

––

–1·6–2·2

––

458286

0·35

LZc

0·02

0·04

3166

Plag,cp

x–

–3

50–52

––

–1·1–2·1

––

458287

0·33

LZc

0·02

0·04

2176

Plag,cp

x–

–3

50–52

––

–1·5–1·8

––

1966

drillco

re

1045’

0·84

LZa

––

3118

Plag,ol,cp

x2

1–

68–70

70–57

57–58

70–57

1·1–1·5

1·2–1·9

1·5–2·3

937’1’’

0·82

LZa

––

4101

Plag,cp

x2

2–

68–72

72–56

56–58

72–58

0·8–1·5

1·1–1·8

1·5–2·2

118678

0·78

LZa

––

12405

Plag,ol,cp

x,ox

83

163–65

65–55

55–57

65–55

1·2–1·9

1·6–2·1

1·5–2·5

661’7’’

0·77

LZa

––

3117

Plag

3–

–65–67

67–55

56–58

–1·1–1·7

1·3–1·7

1·3–1·9

118653

0 ·74

LZa

––

5192

Plag,ol

41

–63–65

65–55

5565–55

1·4–1·7

1·6–2·3

1·9–2·7

316’

0·71

LZa

––

5121

Plag,cp

x,ox

41

–62–64

64–54

54–55

64–54

1·3–1·9

1·7–2·3

1·5–2·3

319’6·5’’

0·71

LZa

––

3121

Plag,ol

3–

–62–63

63–54

54–55

–1·5–2·1

1·7–2·0

1·7–2·7

118601

0·67

LZa–LZb

––

269

Plag,ol

–2

–59–62

–54–55

–1·5–1·9

–1·8–2·4

118605

0·67

LZa–LZb

––

379

Plag,ol

11

159–61

61–55

55–58

61–55

1·5–1·8

–1·4–2·4

(continued

)


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Table2:

Continued

Sam

ple

FUnit

P2O5

(wt%)1

Trapped

liquid

fraction2

Grains

Single

analyses

Contact

phases

3

Typ

eI

grains

Typ

eII

grains

Typ

eIII

grains

Core

904

Man

tle

Ran

ge

Rim

Typ

eI

Rim

Typ

eII

Core

Man

tle

Rim

nn

nn

nAn%

An%

An%

An%

Or%

Or%

Or%

Platinova

90-10co

re

90-10-456

0·25

MZ

––

441

Plag,ox

–1

348–50

––

50–46

2·1–2·8

––

90-10-453.7

0·25

MZ

––

9167

Plag,cp

x7

2–

47–49

49–41

41–42

49–41

1·7–2·9

2·0–3·4

1·6–3·7

90-10-444.8

0·24

MZ

––

4138

Plag,cp

x,ox

1–

347–49

49–42

42–41

–2·3–2·8

2·5–2·9

2·8–3·1

90-10-444.35

0·24

MZ

––

273

Plag,cp

x2

––

48–49

49–40

40–41

–2·3–2·9

2·5–2·9

2·8–3·1

90-10-440.5

0·23

MZ

––

357

Plag,cp

x2

1–

46–48

48–40

40–41

48–40

2·7–3·4

2·8–4·1

2·7–4·6

90-10-431.45

0·22

MZ

––

2109

Plag,cp

x1

1–

45–47

47–40

40–42

47–40

2·5–3·2

2·8–3·2

3·1–3·9

90-10-212

0·14

UZa

––

6131

Plag,cp

x,ox

41

145–46

46–39

39–40

46–40

2·0–2·9

2·3–3·0

2·1–3·1

90-10-210.15

0·14

UZa

––

374

Cpx

2–

145–46

46–39

39–41

–2·1–2·8

2·5–3·0

2·4–3·5

90-10-524

0·32

MZ

––

231

Plag

11

–47–48

48–40

40–41

48–40

1·9–2·3

2·2–2·6

2·1–2·8

Platinova

90-22co

re

90-22-824.2

0·19

UZa

0·06

0·06

14330

Plag,cp

x,ox

8–

646–47

47–40

40–41

–1·9–2·6

2·3–2·6

2·1–2·9

90-22-689

0·15

UZa

0·05

0·04

5103

Cpx,

ol,ox

4–

143–45

45–40

40–41

–1·7–2·4

2·0–2·4

2·4–3·1

90-22-585.8

0·13

UZa

0·08

0·04

6124

Plag,cp

x–

6–

40–42

––

42–28

2·3–3·4

–3·0–4·1

90-22-491.84

0·10

UZa

0·08

0·04

8143

Plag,cp

x–

26

39–42

––

42–26

2·5–3·0

––

90-22-481.8

0 ·10

UZa

0·07

0·04

13301

Plag,cp

x,gr,

ox

112

–39–42

42–35

35–33

42–17

2·6–3·3

2·6–4·1

2·3–4·6

90-22-481.2

0·10

UZa

0·07

0·04

8132

Plag,cp

x,gr

–5

340–41

––

41–22

2·5–3·2

–2·6–4·1

90-22-480

0·10

UZa

0·07

0·04

10157

Plag,cp

x,gr

–7

340–42

––

42–24

2·1–3·5

–2·9–4·6

90-22-479.2

0·10

UZb

––

7155

Plag,gr,

cpx

–5

239–42

––

42–25

2·3–3·6

–2·8–5·4

90-22-461.8

0·09

UZb

––

12266

Plag,cp

x,ox

–5

740–42

––

42–33

2·5–3·4

––

90-22-421

0·09

UZb

––

10233

Plag,gr,

ol,cp

x–

10–

38–40

––

40–25

2·3–4·1

–2·9–6·5

90-22-323.8

0·06

UZb

––

568

Plag,gr,

ol

–4

137–39

––

39–30

3·1–3·7

–3·1–4·4

90-22-210.9

0·04

UZb

––

7133

Plag,ol

–5

236–39

––

39–24

2·7–4·1

–2·4–4·6

90-22-107.7

0·03

UZb

––

473

Plag,gr,

cpx

–4

–34–35

––

35–10

2·4–3·3

–2·1–4·2

90-22-67.6

0·02

UZb

––

8117

Plag,cp

x,ol

–8

–35–37

––

37–7

1·8–2·9

–3·0–6·7

(continued

)


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Table2:

Continued

Sam

ple

FUnit

P2O5

(wt%)1

Trapped

liquid

fraction2

Grains

Single

analyses

Contact

phases

3

Typ

eI

grains

Typ

eII

grains

Typ

eIII

grains

Core

904

Man

tle

Ran

ge

Rim

Typ

eI

Rim

Typ

eII

Core

Man

tle

Rim

nn

nn

nAn%

An%

An%

An%

Or%

Or%

Or%

MarginalBorderSeries

Field

samples

KR-02-4

0·84

LZa*

449

Plag,cp

x–

4–

69–73

––

73–60

0·6–0·8

–0·7–1·4

KR-02-37

0·81

LZa*

0·12

0·41

8150

Plag,cp

x5

3–

66–69

69–55

55–58

69–55

1·3–1·7

1·4–1·9

1·6–2·6

SP60

0·71

LZa*

0·12

0·35

9255

Plag

72

–65–68

68–55

55–58

68–36

1·0–1·6

1·3–1·8

1·6–2·4

SP46

0·68

LZa*–L

Zb*

0·11

0·33

11310

Plag,cp

x7

4–

59–62

62–49

49–51

62–30

1·3–1·8

1·4–2·2

2·0–3·1

KR-02-21

0·60

LZb*

0·14

0·45

439

Plag

–4

–59–60

––

60–30

1·5–2·0

–1·9–3·5

SK11-C14-C3

0·57

LZb*

0·12

0·35

7190

Plag,gr

–7

–54–57

––

57–0

2·1–3·0

–0·5–7·2

SP8

0·55

LZb*

0·13

0·48

655

Plag,cp

x,ol

–6

–55–57

––

57–43

1·5–2·0

–1·7–3·2

SK11-C1-6

0·45

LZb*

0·17

0·41

8208

Plag,cp

x–

8–

57–58

––

58–16

1·3–2·6

–2·4–6·7

SK11-C2-4

0·39

LZb*

0·12

0·34

8242

Plag,gr,

cpx

–8

–50–54

––

54–16

1·8–2·7

–2·0–7·0

SP16

0·35

LZc*

0·09

0·28

9145

Plag,cp

x2

7–

51–52

52–41

41–40

52–37

2·4–2·8

1·3–2·1

2·6–4·4

SP20

0·25

MZ*

––

10171

Plag,cp

x,ox

–8

249–51

––

51–18

1·8–2·5

–2·0–4·9

SP26

0·15

UZa*

––

489

Plag,gr

–4

–42–45

––

45–18

2·0–3·1

–2·5–7·0

SP29

0·06

UZb*

––

217

Plag

–2

–37–38

––

38–18

2·7–3·0

–2·9–4·2

SK11-C11-3

0·05

UZb*

––

4147

Plag,gr

–4

–39–40

––

40–16

2·3–2·9

–0·6–5·5

SK11-C12-1

0·03

UZb*

––

6179

Plag,cp

x,gr

–6

–38–40

––

40–3

1·9–3·6

–0·3–8·2

UpperBorderSeries

Field

samples

SK08-159

0·68

MZ’

0·27

0·33

5162

Plag,cp

x,gr

23

–45–47

47–40

40–39

29–3

2·8–3·6

2·8–7·2

0·5–7·8

SK08-140

0·55

Uza’

0·29

0·31

5187

Plag,ol,gr

14

–45–47

47–41

41–40

20–37

2·7–3·8

2·5–3·1

3–11·2

SK08-119

0·21

LZa’

0·08

0·21

5101

Plag,cp

x,ol

5–

–65–67

67–55

55–57

1·1–1·9

1·5–2·2

1·6–2·8

SK08-124

0·17

LZb’

0·18

0·43

5203

Plag,cp

x–

5–

56–58

––

24–43

1·6–2·2

–2·1–4·9

1Datafrom

Teg

ner

etal.(2009),Nam

uret

al.(2013),Salmonsen&

Teg

ner

(2013)

andunpublished

data.

2Calcu

latedfrom

theP2O5bulk-rock

content(see

Appen

dix

fordetails).

3Plag,plagioclase;

ol,olivine;

cpx,

clinopyroxene;

ox,

Fe–Tioxides;gr,

granophyre.

4Compositional

rangefor90%

ofthemeasuremen

tscentred

onthemean.


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The orthoclase content in compositional profiles fromType I plagioclase grains evolves in the opposite way tothe An content (Fig. 11a), but the width of the rim withconstant Or content may locally be different from that ofthe An rim. This difference may result from faster subsoli-dus diffusion of K:Na compared with Ca:Na, especially inplagioclase with low An content (Giletti & Shanahan,1997).Type II plagioclase grains (Fig. 7d and e) are observed

from LZa to LZb and from MZ to UZb (Fig. 6; Table 2).The most evolved composition of the rims (i.e. the averageof the outermost three measurements) changes signifi-cantly from one zone to another in the LS. It is generally

in the range An55^58 (mean An56·4; n¼11; Table 2; Fig. 6)in LZa, from An48 to An51 (mean An50·3; n¼ 9; Fig. 5e) inLZb and from An40 to An41 (mean An40·8; n¼ 7) fromMZ to the lower part of UZa (core 4An42). It is worthnoting that these compositions are identical to the compos-itions of the rims observed in Type I grains (Fig. 6). Fromthe top of UZa (core5An42) to UZb, the most externalvalues from the various profiles span a very large rangefrom An42 to An5 (mean An29·2; n¼ 63; Fig. 6; Table 2).The global distribution of the plagioclase An content insingle samples is unimodal (generally with a long negativetail), or slightly bimodal when Type II and Type I grainscoexist in the same sample (Fig. 8d and e). The orthoclase

10

20

30

40

50

60

70

Type IIIType IIType I

LZaLZb

LZcMZUZaUZb LZa*LZb*

LZc*MZ*UZa*

UZb

*

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Fraction of residual melt (F)0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Fraction of residual melt (F)

Pla

gioc

lase

An-

cont

ent (

%)

10

20

30

40

50

60

70

Pla

gioc

lase

An-

cont

ent (

%)

Pla

gioc

lase

An-

cont

ent (

%)

Pla

gioc

lase

An-

cont

ent (

%)

(b)

(c)

)d()a(

(e)(f)

Core compositionsLayered Series

Core compositionsMarginal Border Series

Rim compositions

Rim compositions

LZaLZbLZc

MZUZaUZb LZa*LZb*LZc*

MZ*UZa*UZb*

10

20

30

40

50

60

70

10

20

30

40

50

60

70

Type IType IIType III

Type IIIType IIType I

Fig. 6. Plagioclase compositional variations in the Skaergaard Layered Series and Marginal Border Series. (a) Stratigraphic distribution of thetypes of zoning profile (Type I, II and III) in the Layered Series. Continuous lines, highly abundant; dashed lines, locally present. (b) Mode(symbol) and range of core compositions in single samples from the Layered Series. (See text for details on the procedure to discriminate be-tween core, mantle and rim.) Data are symbol- and shading-coded as a function of the dominant type of zoning profile (Type I, II and III) ineach sample. (c) Mode and range of rim (or mantleþ rim) compositions in the Layered Series. (d) Stratigraphic distribution of the types ofzoning profiles (Type I, II and III) in the Marginal Border Series. (e) Mode (symbol) and range of core compositions in single samples fromthe Marginal Border Series. (f) Mode and range of rim (or mantleþ rim) compositions in the Marginal Border Series. Diamonds are singlemicroprobe analyses from Humphreys (2011).


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0 50 100 150 200 250 300 350 400 45054

55

56

57

58

59

60

61

62

63

64

0 100 200 300 400 500 600 700 800 90050.0

50.5

51.0

51.5

52.0

52.5

53.0

53.5

54.0

54.5

0 100 200 300 400 500 600 700 800 90040

41

42

43

44

45

46

47

48

49

Pla

gioc

lase

An

(%)

Pla

gioc

lase

An

(%)

Pla

gioc

lase

An

(%)

Distance (μm)

Distance (μm)

Distance (μm)

Core

Mantle

Rim

Core

Man

tle

Rim

Core Rim

Man

tle

Best erf curveIndividual analysis

LZa458218

LZbSKG11-28

MZ90-10-431.45

(a)

(b)

(c)

0 100 200 300 400 500 600 700 800 900 100051.0

51.5

52.0

52.5

53.0

53.5

54.0

54.5

55.0

Pla

gioc

lase

An

(%)

Distance (μm)

Core

Rim

LZbSKG11-23

(d)

0 50 100 150 200 250 300 350 400 450 50015

20

25

30

35

40

Pla

gioc

lase

An

(%)

Distance (μm)

Core

Rim

UZc90-22-67.6(e)

0 200 400 600 800 1000 120049.5

50.0

50.5

51.0

51.5

52.0

52.5

53.0

53.5

54.0

Pla

gioc

lase

An

(%)

Distance (μm)

Core

Rim?

LZc458287

(f)

Plagioclase

Plagioclase

Clinopyroxene Granophyre

Clinopyroxene Magnetite

1 StdDev

Fig. 7. Examples of plagioclase profiles in the Layered Series. (a) Type I profile, 458218, LZa. It should be noted that the rim is buffered to aconstant composition of An55. (b) Type I profile, SK11-28, LZb. The rim is buffered to a constant composition of An51. (c) Type I profile, 90-10-431.45, MZ. The rim is buffered to a constant composition of An41. (d) Type II profile, SKG11-23, LZb. It should be noted that the mostevolved composition of the rim is An51. (e) Type II profile, 90-22-67.6, UZc. (f) Type III profile, 458287, LZc.


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48 49 50 51 52 53 54 55

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Plagioclase An (%)

Pro

babi

lity

dens

ity

LZc458287

52 54 56 58 60 62 64 6650

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Plagioclase An (%)

Pro

babi

lity

dens

ity Gaussian mixture modelKernel density estimate

LZa458218

48 49 50 51 52 53 54 55 56 57 58

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

Plagioclase An (%)

Pro

babi

lityd

ensi

ty

LZbSK11-28

38 40 42 44 46 48 50 52

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Plagioclase An (%)

Pro

babi

lity

dens

ity

MZ90-10-431.45

45 50 55 60

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

Pro

babi

lity

dens

ity

Plagioclase An (%)

LZbSKG11-23

5 10 15 20 25 30 35 40

0.05

0.10

0.15

0.20

0.25

Pro

babi

lity

dens

ity

Plagioclase An (%)

UZc90-22-67.6

Type I dominant; n=93

Rim

Cor

e

Type I dominant;n=78

Rim C

ore


Rim

Cor

e

Type II dominantMinor Type I;

n=179

Typ

e I

Rim

Cor

e

Type II dominant; n=117

Cor

e

Type III dominant;n=176

Cor

e

(a)

(b)

(c)

(d)

(e)

(f)

Fig. 8. Global distribution (density probability function) of An content in selected samples from the Layered Series (see representative com-positional profiles for the same sample in Fig. 7). For simplicity, the density function can be understood as the relative proportion of each com-position (0^1) calculated from the number of observations (n). Reported data include compositions from the cores, the mantles (if present)and the rims. The global distribution has been calculated in two ways: a Kernel density distribution and a mixture of Gaussian curves (seeAppendix for details on the calculation procedure). These two ways to represent the global distributions are more statistically robust thansimple histograms for which the size of the bins has to be selected arbitrarily. (a^c) Samples dominated by grains withType I compositional pro-files. The well-defined bimodal distribution, with the first mode corresponding to the core compositions and the second mode correspondingto the rim compositions, should be noted. (d, e) Samples dominated byType II compositional profiles. It should be noted that the global distri-bution of An is unimodal (possibly with a long negative tail) or slightly bimodal (whenType I andType II profiles coexist in a given sample).(f) Sample dominated byType III compositional profiles. The unimodal distribution of An should be noted.


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content inType II plagioclase grains evolves in the oppos-ite direction to the An content (Fig. 11b), but starts to de-crease when the plagioclase An content becomes lowerthan 22^25.Unzoned (�An lower than 2%) Type III plagioclase

grains are mostly found from LZc to UZb (Table 1; Figs 6and 7f). They show a unimodal distribution of plagioclaseAn contents and minor variation in Or content (Fig. 8f).

Marginal Border SeriesPlagioclase core compositions in the MBS evolve continu-ously from c. An70 and Or1 in LZa* to c. An35 and Or3 inUZb*, in a similar way to plagioclase in the LS (Fig. 4b).The MBS is dominated by Type II plagioclase grains

(Tables 1 and 2; Fig. 6). In these samples, the core compos-ition continuously evolves as a function of the stratigraphicposition (Fig. 6), whereas the rims of single grains shownormal zoning (e.g. decreasing An content; Fig. 12a). Theoutermost edge of the rims spans a very large range ofcompositions: An29^69 (mean An50; n¼17) in LZa*(Fig. 6), An3^60 (mean An36; n¼ 42) in LZb*, An18^46(mean An37; n¼ 21) in the interval between LZc* and thelower part of UZa*, and An2^40 (mean An23; n¼12) inthe upper part of UZa* and UZb*. The global distribu-tion of An content in these samples is generally unimodal(Fig. 12b).Type I plagioclase grains have been observed in three

samples from the external part of the MBS (two samplesin LZa* and one sample in LZb*; Tables 1 and 2). SimilartoType I plagioclase in the LS, they have a rim with a con-stant composition of An55^58 (mean An57·5; mode An57;n¼100; Figs 9 and 12c, d) in LZa* and An49^52 (meanAn50·2; mode An50; n¼ 42) in LZb* (Figs 6 and 12e, f).

These values are identical to those of rims of the Type Igrains in the LS (LZa and LZb).Type III (unzoned) plagioclase grains are a very minor

feature in the MBS and we measured only a singleunzoned grain in SP-20 (MZ*; An43^45; Or2·8^3·1).

Upper Border SeriesPlagioclase core compositions in the UBS evolve continu-ously from c. An67 and Or1 in LZa’ to c. An45 and Or3 inUZa’ (Table 2). The UBS is dominated byType I andTypeII plagioclase grains (Tables 1 and 2). We did not observeanyType III plagioclase grains.Type I plagioclase grains were observed in LZa’, MZ’

and in the lower part of UZa’ (core4An42). The rim hascomposition An55^57 (mean An56·5; mode An56; n¼ 23) inLZa’ and An39^41 (mean An40·3; mode An40; n¼ 43) inMZ’ and UZa’, identical to the values observed in the LSand MBS (Fig. 9).

UZa MZ

LZc LZa

Core An: 70-58LZb

Core An: 58-50Core An: 50-40

Layered SeriesMarginal Border SeriesUpper Border Series

35

40

45

50

55

60

65

0.1 0.2 0.3 0.4 0.60.5 0.7 0.8 0.9

Fraction of residual melt (F)

Pla

gioc

lase

An

(%)

Fig. 9. Composition of rims of Type I grains as a function ofstratigraphic position (fraction of remaining liquid; Nielsen, 2004).Error bars correspond to the compositional range of rims observedin single samples. Ranges of core compositions are shown forreference.

0 50 100 150 200 250 30045

50

55

60

65

70

Distance (µm)

Pla

gioc

lase

An

(%)

0 50 100 150 200 250 300 350 400 45045

50

55

60

65

70

Distance (µm)

Pla

gioc

lase

An

(%)

CoreMantle 1Rim 1

Man

tle 2

Rim

2

118653LZa

Core

Man

tle 1

Rim 1

Man

tle 2

Rim

2

458214LZa

Plagioclase

Plagioclase

Fig. 10. Examples of Type I plagioclase grains in LZa showing a coresurrounded by two different mantles of decreasing An content andtwo rims buffered to a constant composition (rim 1: An55^58; rim 2:An50^51).


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Type II plagioclase grains were observed in LZb’, MZ’and UZa’. The outermost edge of the rims spans a largerange of compositions from An24 to An43 (mean An34;n¼ 5) in LZb’ and An3 to An38 (mean An28; n¼ 8) in theinterval between MZ’and the lower part of UZa’.

Summary of the key compositionalobservations in plagioclase profilesThree main types of plagioclase compositional profiles areobserved in the Skaergaard intrusion. Type I grains showa core of constant composition, surrounded by a mantle ofdecreasing An content and a rim of constant composition.They are abundant in the Layered Series LZa and LZb,and are also common in MZ and UZa. They are absent

from LZc and UZb.The core composition evolves continu-ously as a function of the stratigraphic position but therims show only three compositions: An56�2 in LZa (andequivalent units in the MBS and UBS), An51�1 in LZband An41�1 in MZ^UZa (Fig. 13). Type II grains also havea core of constant composition surrounded by a rim ofdecreasing An content. In most of the LS (LZa^LZb), therims show relatively restricted ranges of compositions andgenerally do not evolve to An contents lower than thoseobserved in the rims of Type I grains (Fig. 13). In contrast,in UZa as well as in the MBS and UBS, Type II grainsare very abundant and the rims locally show very evolvedcompositions down to An5^10 (Fig. 13). Type III grains aremostly observed in the LS from LZc to UZb and areunzoned.

0 50 100 150 200 250 300 350 400 45050

52

54

56

58

60

62

1.5

2.0

2.5

0 50 100 150 200 250 300 35052

54

56

58

60

62

64

1.6

1.7

1.8

1.9

2.0

2.1

2.2

2.3

Distance (μm)

Pla

gioc

lase

(An

%) Plagioclase (O

r %)

Distance (μm)

Pla

gioc

lase

(An

%) Plagioclase (O

r %)

OrthoclaseAnorthite

Orthoclase erf functionAnorthite erf function

458224LZb

118605LZb

Core

Core

MantleRim

Rim

(a)

(b)

Plagioclase

Olivine

Fig. 11. Comparison of An content vs distance and Or vs distance compositional profiles in plagioclase grains. (a) Type I profile, 458224, LZb.(b) Type II profile, 118605, LZb. Different error functions have been fitted to the An profile and the Or profile. The boundaries between thezones (core, mantle and rims) are based on the An error function.


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0 200 400 600 800 1000 1200 1400

35

40

45

50

0 200 400 600 800 1000 1200

58

60

62

64

66

0 100 200 300 400 500 600

46

48

50

52

54

56

58

60

62

Pla

gioc

lase

An

(%)

Distance (μm)

CoreRim

Best erf curveIndividual analysis

MZ*Sp-20

LZa*KR-02-37

LZa*-LZb*SP-46

Pla

gioc

lase

An

(%)

Pla

gioc

lase

An

(%)

Distance (μm)

Distance (μm)

CoreRim

Mantle

Core

Rim

Mantle

(a)

(c)

(e)

Magnetite

Plagioclase

Plagioclase

20 25 30 35 40 45 50 55

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

50 52 54 56 58 60 62 64 66 68 70 72

0.02

0.04

0.06

0.08

0.10

0.12

0.14

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0.18

30 35 40 45 50 55 60 65

0.02

0.04

0.06

0.08

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0.12

0.14

0.16

0.18

Plagioclase An (%)

Gaussian mixture modelKernel density estimate

Insignificant gaussian

LZa*-LZb*SP-46

LZa*KR-02-37

MZ*SP-20

Pro

babi

lity

dens

ityP

roba

bilit

y de

nsity

Pro

babi

lity

dens

ity

Plagioclase An (%)

Plagioclase An (%)

Type II dominant;n=171

Cor

e


Cor

e

Rim


Cor

e

Rim

(f)

(d)

(b)

Fig. 12. Examples of plagioclase profiles (An vs distance) in the Marginal Border Series. (a) SP-20, MZ*,Type II. (c) KR-02-37, LZa*,Type I.It should be noted that the rim is buffered to a composition of An58. (e) SP-46, LZa*^LZb*, Type I. The rim is buffered to a composition ofAn49. (b, d, f) Histograms showing the global distribution (density probability function) of An content in the same three samples (SP-20; KR-02-37; SP-46). The significance of the Gaussian curves (e.g. Insignificant Gaussian) was determined with SiZer plots (Rudge, 2008; seeAppendix for details).


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MECHANISMS OF CRYSTAL MUSHSOL IDI F ICAT ION: INS IGHTSFROM PLAGIOCLASESolidification of the Skaergaard intrusion involved highlyeffective fractional crystallization of an original ferrobasal-tic magma (Hoover,1989b; Nielsen, 2004).This is indicatedby the continuous evolution of primocryst compositionsthroughout the stratigraphy, and the close match betweenthe roof, sidewall and floor cumulate series (e.g. Wager &Brown, 1968; Naslund, 1984; Hoover, 1989a; McBirney,1989; Salmonsen & Tegner, 2013; Fig. 4). However, the pre-cise liquid line of descent of the magma in the chamber isstill poorly understood (e.g. Hunter & Sparks, 1987;McBirney & Naslund, 1990; Toplis & Carroll, 1996; Thyet al., 2009a; Tegner & Cawthorn, 2010). Large plagioclasecores of constant composition are envisaged as beingformed at the top of the crystal mush (i.e. the interface be-tween the mush and the main magma body; McBirney &Noyes, 1979) where strong liquid convection allows thecompatible elements to be brought to the crystal^liquidinterface whereas the incompatible elements are removed(Morse, 1986, 2008a). The activation energy barrier thathas to be overcome for the nuclei to grow and reach a crit-ical size (i.e. leading to the appearance of a new crystal)is relatively small given the presence of pre-existing sur-faces for nucleation. Crystal growth is also efficient, even

at relatively low degrees of undercooling (Brandeis &Jaupart, 1986; Pupier et al., 2008).The evolution of the interstitial liquid within the crystal

mush is much less well understood than the evolution ofthe liquid in the main magma body. When the crystal isburied in the crystal mush owing to the overlying accumu-lation of new crystals, differentiation and crystallization ofthe interstitial liquid should result in continuous normalzoning in the outer parts of plagioclase primocrysts,matching their intrusion-scale stratigraphic variation.Although clinopyroxene and olivine should show equiva-lent variations, these are subject to more rapid diffusiveequilibration and hence do not provide such a usefulrecord in the final solidified cumulate.Mush processes that disrupt fractional crystallization

(e.g. infiltration metasomatism, replenishment of intersti-tial liquid during convection, or fluid flow and dissol-ution^reprecipitation caused by compaction) disturb thiscontinuous chemical evolution and could potentially pro-duce rocks with normal zoning that do not match the liqui-dus trend observed in crystal cores, or grains with reversezoning or, in the extreme, adcumulate rocks with essen-tially unzoned mineral grains. Some of the plagioclasecompositional profiles that we observe in the Skaergaardintrusion clearly deviate from the continuous chemicalevolution expected for simple crystallization of a continu-ously evolving melt, and therefore show that more

+Cpx

+FeT

i Ox

-Ol

+Ol

+Ap

HZ-LZaLZbLZcMZUZaUZb

Plagioclase coreRims of Type I grainsCompositional range in rims from Type II grains (LS)Compositional range in rims from Type II grains (UBS & MBS)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

15

25

35

45

55

65

Fraction of residual liquid (F)

Pla

gioc

lase

An-

cont

ent (

%)

Fig. 13. Schematic representation of the key compositional variations of plagioclase crystals in Skaergaard.The evolution of core and rim com-positions (An%) in each unit of the Skaergaard is shown as a function of the residual melt fraction (F) in the magma chamber. The order ofmineral appearance is also shown for reference.


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complicated processes must have occurred during the so-lidification of the interstitial liquid. In particular, we needto account for the formation of wide rims of constant com-position in Type I grains. It is also crucial to explain whythese crystals are more common in the lower parts of theLS, and less common in the UBS and the MBS.Importantly, we must also explain why the rim compos-ition of Type I plagioclase grains is buffered to An56�2 inLZa (þ LZa* and LZa’) independently of the strati-graphic position (Fig. 9), An51�1 in LZb (þ LZb* andLZb’) and An41�1 from LZc to the lower part of UZa(þ LZc*^UZa* and LZc’^UZa’). It is also of interest tonote that the evolution of some minor elements in intersti-tial plagioclase starts to diverge to that observed in plagio-clase cores when the An-content reaches An56�2 (seefig. 9 in Humphreys, 2011).A critical observation is that the rim compositions of

Type I grains (An56�2; An51�1; An41�1) do not showrandom patterns but correspond to significant points inthe fractionation path of the bulk magma. In detail,An56�2 is the same within error as the core plagioclasecomposition at the first appearance of primocryst clinopyr-oxene in the LS (An56^59; McBirney, 1989), MBS (An57^59;Hoover, 1989a; Namur et al., 2013) and UBS (An57^59;Salmonsen & Tegner, 2013). Additionally, An51�1 corres-ponds to the plagioclase primocryst composition at thefirst appearance of Fe^Ti oxides in the three series (An50^52) and An41�1 corresponds to that of the first appearanceof cumulus apatite (An40^41).Detailed examination of Ti zoning in interstitial plagio-

clase shows that new interstitial phases saturate in themush liquid at an An content similar to that for the ap-pearance of new primocryst phases in the main magmabody (Humphreys, 2009). Our new data therefore suggestthat the interstitial plagioclase major element compositioncan locally become buffered each time the interstitial meltadds a new phase to its liquidus assemblage.

ORIGIN OF COMPOSIT IONALBUFFER ING OF PLAGIOCLASER IMSBased on the primary controls on plagioclase composition,there are several possible causes for the compositional buf-fering (An content) of rims in Type I plagioclase. Theseinclude maintaining the interstitial melt to a constant com-position or changing pH2O, pressure or temperature inthe crystal mush.Despite the control of pressure on plagioclase compos-

ition (Longhi et al., 1993; Putirka, 2005) we rule out signifi-cant pressure changes during crystallization of theSkaergaard intrusion because the confining pressure chan-ged only slowly during solidification, owing to progressiveburial caused by the continuous contemporaneous

eruption of flood basalts (Larsen & Tegner, 2006).Increasing the pH2O in the magma increases the An con-tent of equilibrium plagioclase (e.g. Panjasawatwong et al.,1995; Putirka, 2005). However, the production of plagio-clase rims of constant composition would require pH2O toincrease in such a way that it would exactly counteractthe evolution of liquid composition owing to crystallization(i.e. the formation of overgrowths on existing phases andthe crystallization of new interstitial crystals). An incom-mensurately larger increase in pH2O at any time wouldresult in the formation of reverse rims on plagioclasecores. Such a progressive increase in pH2O is not sup-ported by any petrographic (e.g. presence of hydrousphases) or geochemical (Humphreys, 2011) evidence, andis also inconsistent with the presence of rims with multiplebuffered components in some samples (Fig. 10). We there-fore conclude that the most likely cause of the crystalliza-tion of constant compositon plagioclase rims is thebuffering of the composition of the interstitial melt. Thiscould occur via interstitial liquid mobility or progressivecrystal dissolution, or by changes in the thermal regime ofcrystal mush solidification without addition of any newmelt component.

Interstitial liquid mobilityAn important matter of debate concerning the crystalliza-tion of layered intrusions concerns the mobility of theinterstitial liquid within the crystal mush. If interstitialliquid cannot be removed from the mush, in situ crystalliza-tion of new interstitial phases (e.g. K-feldspar, apatite, bio-tite and zircon) will result, incorporating incompatibleelements in the evolving liquid consistent with a high‘trapped liquid’ fraction (e.g. Tegner et al., 2009). In con-trast, if liquid can be physically removed from the mushby compaction (Tegner et al., 2009; McKenzie, 2011), or ifrejected components can be efficiently removed from thesites of crystallization by convective flow (e.g. Toplis et al.,2008), adcumulates with low trapped liquid contents canresult. Detailed grain-size distribution analysis of cumulaterocks could potentially help to constrain whether adcumu-lates mostly form as a result of compaction or convectiveflow (Higgins, 1998). Silicate liquid immiscibility in thecrystal mush has also been proposed as a physical mechan-ism for changing the interstitial liquid composition, bypreferential loss of the low-density Si-rich conjugate(Holness et al., 2011).Compaction of the crystal mush should result in up-

wards liquid migration, bringing an evolved liquid intocontact with overlying crystals that are in equilibriumwith more primitive compositions (Irvine, 1980;Humphreys, 2011). This process may have a strong influ-ence on mineral compositions (Irvine, 1980). For theSkaergaard LS, this process should amplify any normalzoning, but it cannot account for the compositional buffer-ing that we see in Type I grains. Moreover, compaction


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cannot have been operative either at the roof of the magmachamber (UBS) or on the walls (MBS), where Type Iplagioclase grains are also locally present.Toplis et al. (2008) proposed that compositional convec-

tion played an important role in the Skaergaard floor cu-mulates, beginning when the mush liquid becamesaturated in magnetite. After this point on the liquid lineof descent, progressive fractionation results in decreasingliquid density, thus causing gravitational instability withinthe mush and overturn of the interstitial liquid.Compositional convection exchanges more evolved inter-stitial liquid with less evolved liquid from the mainmagma body (or the upper part of the mush), and cantherefore keep the melt to a constant composition; we callthis ‘chemical buffering’. This process has been suggestedas acting to buffer the plagioclase rims to An51 in cumulaterocks without liquidus Fe^Ti oxides (LZa and LZb), andto produce unzoned grains in cumulates with cumulus Fe^Ti oxides (LZc^UZc; Toplis et al., 2008). Although weagree that compositional convection could lead to effectivechemical buffering of the interstitial liquid composition atthe saturation of Fe^Ti oxides, and so may have contribu-ted to the formation of some Type I grains in theSkaergaard intrusion, it cannot explain the chemical vari-ability that we observe. This is because saturation of theliquid in interstitial clinopyroxene (at �An56^58) does notchange the liquid density significantly and there is there-fore no driving force for compositional convection in LZa.Although Toplis et al. (2008) did not describe any grainswith rims buffered at An56�2, our study shows that theyare actually a major component of rocks from LZa andequivalent units in the MBS and UBS (Fig. 6;Humphreys, 2011). Furthermore, it has been shown byHumphreys (2011) that the concentration of minor elem-ents such as Ti continuously changes within the rims ofType I grains. Effective compositional convection should,however, result in a buffered concentration of any elementin the plagioclase rims (Tait et al., 1984).A further argument against compositional convection is

that it is a gravity-driven process that cannot produce thesame results simultaneously on the roof, floor and walls ofthe magma chamber, as observed in the plagioclase datafrom the LS, MBS and UBS. Although Type I grains aremore common in the LS, they are also present in the wall-rocks (MBS) and at the roof (UBS) of the magmachamber.It has recently been shown that the interstitial liquid in

the Skaergaard crystal mush became immiscible duringprogressive fractionation (Holness et al., 2011). The coexist-ence of immiscible conjugate liquids is recorded by thepresence of two types of late-stage interstitial intergrowths:those containing quartz and K-feldspar are interpreted asforming from the Si-rich immiscible melt, whereas thoserich in Fe^Ti oxides, apatite and pyroxene are interpreted

as forming from the Fe-rich immiscible melt (Holnesset al., 2011; Humphreys, 2011). When the two immiscible li-quids are in thermodynamic equilibrium, both shouldcrystallize the same plagioclase composition which shouldthen evolve with further differentiation and form normallyzoned plagioclase rims. When the two immiscible liquidsare sufficiently separated that they lose chemical commu-nication they crystallize plagioclase of a distinctly differentcomposition (Humphreys, 2011). The rims of plagioclasegrains forming the walls of Si-rich melt pockets havenormal zoning, whereas those forming the walls of Fe-richmelt pockets have a constant composition or even reversezoning (Humphreys, 2011). The development of two immis-cible conjugate liquids, followed by relative movementand/or separation, is therefore not a viable mechanism toproduce plagioclase rims of constant composition, unlessthe relative proportion of the two melts is continuouslychanging with differentiation. However, there is no reasonto suspect that a change in the relative proportion of thetwo melts would buffer the plagioclase composition tothree specific values (An56�2, An51�1 and An41�1). In add-ition, the residual porosity in rocks from the lower part ofthe intrusion (HZ^LZa) is likely to have been very low atthe onset of immiscibility (510 vol. %; Humphreys, 2011),and immiscible melts are therefore unlikely to producethe large rims (up to 800 mm wide) of constant compos-ition observed in Type I plagioclase grains, which musthave started to form at higher porosity (see below).

Dissolution^reprecipitation in thecrystal mushAt low undercoolings, the grain-size distribution of a poly-disperse suspension within a liquid evolves under the influ-ence of interfacial energies. The higher free energy ofsmall crystals relative to larger crystals means that thelarge grains grow at the expense of the smaller. The sizeeffect on solubility equates to an effective superheating ofthe smaller crystals (Cabane et al., 2005; Simakin &Bindeman, 2008). This is described by the classical theoryof Ostwald ripening (Lifshitz & Slyozov,1961).This processhas negligible effect if the crystals are widely separated,particularly for grains larger than a few microns, butoccurs at a greater rate when the crystals are touching, asgrains may coalesce (Means & Park, 1994; Pupier et al.,2008; Schiavi et al., 2009) or if there is thermal cycling(Simakin & Bindeman, 2008; Mills et al., 2011; Mills &Glazner, 2013). Loss of small grains may also occur by pref-erential dissolution caused by stress resulting from the pres-sure of overlying material in the crystal pile (Higgins,2002; Boorman et al., 2004; Schmidt et al., 2012).Dissolution of small plagioclase grains can feed thegrowth of larger grains, with the potential to producecompositionally buffered rims and significantly changethe original crystal-size distribution (Pupier et al., 2008).An important argument against this being a significant


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process for the formation of the bufferedType I rims in theSkaergaard intrusion is that if dissolution results from thestress at the interface between crystals, there is no reasonto suspect that dissolution would preferentially occurwhen the plagioclase An content reaches An56�2, An51�1or An41�1.Interface-coupled dissolution of grain boundaries and

precipitation of a new phase through the circulation of afluid phase was recently proposed as a mechanism to pro-duce large plagioclase rims of constant composition(An64) by dissolution of large crystals (An80) in metamor-phosed leucograbbro (Svahnberg & Piazolo, 2013). Fluidcirculation results in a reaction front migrating into thehigh-An plagioclase crystals (Svahnberg & Piazolo, 2013)replacing them with a low-An plagioclase, therefore form-ing a rim around partially resorbed plagioclase grains.However, no clear petrographic or geochemical evidencehas been reported for a pervasive and intrusion-wide fluidphase at Skaergaard. Furthermore the buffering atAn56�2, An51�1 or An41�1 would apparently require thepercolation of three different fluids. We consider this pro-cess highly unlikely, especially in the lower part of the LSwhere the liquids are far too primitive to exsolve a freefluid phase.

Latent heat buffering in a crystal mushOverproduction of mineral phases during the crystallizationof a basaltic meltThe equilibrium proportions of the various liquidus phasesof a magma at a given pressure and temperature are fixedby the thermodynamic properties of the system. However,many examples from layered intrusions show that when anew cumulus phase appears its mode is initially higherthan expected from cotectic relationships (Morse, 1979a;Wiebe & Snyder, 1993; Tegner et al., 2009; Namur et al.,

2010; Table 3), declining to the expected cotectic proportionwithin a short stratigraphic distance. This overproductionof a given phase was described by Morse (1979b, 2011) andmay be attributed to the supersaturation of the relevantmineral component in the melt, which results in crystal-lization on metastable extensions of field boundaries(Morse, 1979b). This oversaturation of a new phase mayalso occur during crystallization from the interstitial melt(Morse, 2011).Thermodynamic calculations reveal that the crystal

productivity [x/T; where x is the amount of crystallization(g) for a given incremental change in temperature (T);e.g. 18C] at the appearance of a new phase might be up to10 times higher than under ‘normal’ conditions (Ghiorso,1997; Holness et al., 2007, 2009a; Morse, 2011).Crystallization of minerals from a silicate melt is respon-sible for the release of latent heat of crystallization(�Hfus), which together with the heat lost through thewalls and roof of the magma chamber controls the thermalstate of the system and its global cooling rate. Increasingx/T at the appearance of a new phase may thereforeresult in a non-linear evolution of �Hfus as a function oftemperature (Ghiorso, 1997; Holness et al., 2007, 2013).Such a non-linear evolution of �Hfus with temperaturewas observed by Lange et al. (1994) during the melting ofbasalt and ugandite. Results from this study therefore alsopoint to a non-linear evolution of �Hfus during the crystal-lization of a basaltic melt.In layered intrusions, this process may contribute to

keeping the top of the crystal mush close to the liquidustemperature for extended periods (see below). Morse(2011) suggested that this thermal buffering effect contrib-utes to the textural maturation of cumulate rocks.Thermal buffering owing to enhanced release of latentheat at the saturation of a new phase is therefore

Table 3: Mineral mode at the appearance of cumulus phases in layered intrusions

Augite Fe–Ti oxides Apatite

Appearance

F/H

Oversaturation

mode

Average

mode

Appearance

F/H

Oversaturation

mode

Average

mode

Appearance

F/H

Oversaturation

mode

Average

mode

Skaergaard1 0·668/175 27� 2 24� 2 0·385/742 39� 5 24� 4 0·097/1620 7·3� 1·3 4·4� 0·7Sept Iles2 0·72/-3375 29� 2 23� 3 0·86/–3680 34� 8 22� 6 0·52/–2575 7·2� 0·5 4·6� 1·2Kiglapait3 �0·16/– 40–60 20–30 �0·08/– 10–40 5–10 �0·05/– 8–12 1–5Newark Island4 –/1000 25–30 15–20 –/

crystallizationofinterstitialliquidandlatent heat buffering in …eprints.esc.cam.ac.uk/3083/1/namur...

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