galapagos plume source lithology : implications from ...dgeist/chapman/vidito_cv_poster.pdfgalapagos...

Galapagos Plume Source Lithology : Implications from Olivine Phenocryst CompositionsC. Vidito ,C. Herzberg and D.Geist1 Department of Earth and Planetary Sciences, Rutgers University, 610 Taylor Rd, Piscataway, NJ 08854-8066, USA.2 Department of Geological Sciences, University of Idaho 3022, Moscow, ID 83844, USA.

Ca (ppm) Mn (ppm)

Fe/Mn(ppm)

Ni (ppm)

Mg-Number Mg-Number

OlivinesPeridotite Derivative Magmas (13-20% MgO)


OlivinesPeridotite Primary Magmas (8-38% MgO)

Olivine Phenocryst CompositionFertile Peridotite Source(3.45% CaO)

Olivine Phenocryst CompositionFertile Peridotite Source(1964 ppm Ni)



Olivines PeridotitePrimary Magmas (8-38% MgO)

Olivines PeridotiteDerivative Magmas (8-20% MgO)



KR-4003

Fernandina


Ca (ppm) Mn (ppm)

Fe/Mn(ppm)

Ni (ppm)

Mg-Number Mg-Number


OlivinesPeridotiteDerivative Magmas (8-13% MgO)









KR-4003





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Ca (ppm) Mn (ppm)

Fe/Mn(ppm)

Mg-Number



Olivines Primary Magmas (8-38% MgO)


Olivines Primary Magmas (8-38% MgO)

Olivines Derivative Magmas (8-20% MgO)

KR-4003

Ca (ppm)


Mg-Number


Ni (ppm)

Mg-Number


Mn (ppm)

Fe/Mn(ppm)


San Carlos Standard

San Carlos Standard

San Carlos Standard

San CarlosStandard

San Carlos Standard

San Carlos Standard

San Carlos Standard

San CarlosStandard

San Carlos Standard

San Carlos Standard

San Carlos Standard

San CarlosStandard

Peridotite Pyroxenite

He/ He (R/Ra)0 10 20 30

43

92 W 40' 92 W 20' 92 W 91 W 40' 91 W 20' 91 W 90 W 40' 90 W 20' 90 W 89 W 40' 89 W 20' 2 S

1 S30'

1 S

0 S30'

0 N

0 N30'

1 N

1 N30'

2 N

FloreanaSan Cristobal

Santa Cruz Cerro AzulFernandina

Volcan Darwin

Volcan Ecuador

Roca Redonda

Wolf Is.Darwin Is.Genovesa*

1 1 2

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A petrologic map of the Galapagos Archipelago with the sample locations covered in this study. The spatial relationshipa of the different sample populations from Santa Cruz are shown by the different map symbols. Areas of peridotite and pyroxenite source magmatism are highlighted green and blue respectively. Samples from Genovesa were analyzed in this study and indicated a peridotite source, however the depleted geochemical signature of these lavas indicate that the source is predominantly MORB (Harpp et al., 2002). The 3He/4He diagram is positioned such that the latitude corresponds to the sampling locations of the related volcanoes (McBirney et al., 1985; Graham et al., 1993; White et al., 1993; Kurz and Geist, 1999; Blichert-Toft and White, 2001; Naumann et al., 2002; Geist et al., 2002; Geist et al., 2005; Teasdale et al., 2005; Geist et al., 2006; Saal et al., 2007; Kurz et al., 2009; Kent et al., 2010). There is an apparent correlation between source lithology and 3He/4He ration in that peridotite source magmas tend to be higher in 3He/4He than pyroxenite source lavas in the Galapagos.

The use of olivine phenocrysts allows us to map the source lithology of the present day Galapagos plume where primitive basalts are present. Calculated compositions of olivine phenocrysts crystallized from primary and derivative magmas of peridotite source magmas can be used to descriminate between peridotite and pyroxenite sources (Herzberg, 2011). Magmas of pyroxenite sources typically crystallize olivine with higher Ni and Fe/Mn and lower Ca and Mn than olivine that crystallizes from a peridotite source (Herzberg, 2011 ; Sobolev et al, 2007). The interpretation of olivine data can be complicated by the compositional effects of the fractionation of clinopyroxene (Geist et al, 1998) and other mineral phases. The plots below are representative of peridotite and pyroxenite source basalts covered in our study. In total over 3400 analyses from 42 samples and 11 volcanoes have been collected.

The 87Sr/86Sr and 206Pb/204Pb maps above (Harpp and White, 2001) show a "horse shoe" shaped area of isotopic enrichment which correlates strongly with the region of the archipelago we attribute to a lower mantle peridotite source. Other isotopes and trace element ratios show a similar pattern. The coupling of isotopic depletion with a pyroxenite source may be due to the subduction of oceanic crust into to a pre-existing peridotite region in the lower mantle, and both were brought up by the Galapagos plume. That is, the peridotite and pyroxenite lithologies may have had independent histories.

We have established that Galapagos plume is lithologically heterogeneous and that there exists a correlation between source lithology and isotopic data within this archipelago. More work is needed to further test the strength of the apparent correlations and to more accurately map the lithological structure of the plume. Samples from Wolf, Pinta and Marchena are needed to better constrain how far north the pyroxenite signature is found. Older samples from San Cristobal and samples from Alcedo, Sierra Negra, Santa Fe, Santiago and Espanola would allow for a more accurate determination of the boundary between peridotite and pyroxenite sources in the center of the archipelago.

References: Blichert-Toft, J. and White, W.M., 2001, Hf isotope geochemistry of the Galapagos Islands: Geochemistry Geophysics Geosystems, v. 2, (200GCOO0138). Geist, D., White, W.M., Albarede, F., Harpp, K., Reynolds, R., Blichert-Toft, J., and Kurz, M.D., 2002, Volcanic evolution in the Galapagos: The dissected shield of Volcan Ecuador: Geochemistry Geophysics Geosystems, v. 3, (2002GC000355). Geist, D.J., Fornari, D.J., Kurz, M.D., Harpp, K.S., Adam Soule, S., Perfit, M.R., and Koleszar, A.M., 2006, Submarine Fernandina: Magmatism at v. 7, (2006GC001290). Geist, D.J., Naumann, T.R., and Larson, P., 1998, Evolution of Galapagos magmas: Mantle and crustal fractionation without assimilation: Journal of Petrology, v. 39, p. 953 971. Geist, D.J., Naumann, T.R., Standish, J.J., Kurz, M.D., Harpp, K.S., White, W.M., and Fornari, D.J., 2005, Wolf volcano, Galapagos Archipelago: Melting and magmatic evolution at the margins of a mantle plume: Journal of Petrology, v. 46, p. 2197 2224. Graham, D.W., Christie, D.M., Harpp, K.S., and Lupton, J.E., 1993, Mantle Plume Helium in Submarine Basalts from the Galapagos Platform: Science, v. 262, p. 2023-2026. Harpp K. S., White W. M., 2000, Tracing a mantle plume: isotopic and trace element variations of the Galapagos Seamounts: Geochemistry Geophysics Geosystems, v. 2, (2000GC000137). Herzberg, C., 2011, Identification of source lithology in the Hawaiian and Canary Islands: Implications for origins: Journal of Petrology, v. 52, p. 113-146. Herzberg, C., and P. D. Asimow, 2008, Petrology of some oceanic island basalts: PRIMELT2.XLS software for primary magma calculation: Geochemistry Geophysics Geosystems, v. 9, (2008GC09001). Kent, D.V., Wang, H., and Rochette, P. , 2010, Equatorial paleosecular variation of the geomagnetic field from 0 to 3 Ma lavas from the Galapagos Islands: Physics of the Earth and Planetary Interiors, v. 183, p. 404-412. Kurz, M.D., and Geist, D., 1999, Dynamics of the Galapagos hotspot from helium isotope geochemistry: Geochimica Et Cosmochimica Acta, v. 63, p. 4139-4156. Kurz, M.D., Curtice, J., Fornari, D., Geist, D and Moreira, M., 2009, Primitive neon from the center of the Galapagos hotspot: Earth and Planetary Science Letters, v. 286, p. 23-34 McBirney, A.R., 1993, Differentiated rocks of the Galapagos hotspot: Geological Society, London, Special Publications, v. 76, p. 61-69. Naumann, T., Geist, D., and Kurz, M., 2002, Petrology and geochemistry of Volcan Cerro Azul: Petrologic diversity among the western Galapagos volcanoes: Journal of Petrology, v. 43, p. 859-883. Raquin A., Moreira, M. A., 2009, Atmospheric 38AR/36AR in the Mantle: Implications for the Nature of the Terrestrial Parent Bodies: Earth Planetary Science Letters, 287, p. 551-558. Reynolds R. W., Geist D. J. , 1995, Petrology of Lavas from Sierra Negra Volcano, Isabela Island, Galapagos Archipelago: Journal of Geophysical Research, B100, p. 24537-24553. Saal, A.E., Kurz, M.D., Hart, S.R., Blusztajn, J.S., Blichert-Toft, J., Liang, Y., and Geist, D.J., 2007, The role of lithospheric gabbros on the composition of Galapagos lavas, Earth and Planetary Science Letters, v. 257, p. 391-406. Teasdale, R., Geist, D., Kurz, M., and Harpp, K., 2005, 1998 eruption at Volcan Cerro Azul, Galapagos Islands: I. Syn-eruptive petrogenesis: Bulletin of Volcanology, v. 67, p. 170-185. White, W.M., McBirney, A.R., and Duncan, R.A., 1993, Petrology and Geochemistry of the Galapagos Islands - Portrait of a pathological mantle plume: Journal of Geophysical Research-Solid Earth, v. 98, p. 19533-19563.

Olivine analyses from Santa Cruz. Much like Volcan Ecuador a pyroxenite source is indicated by the composition of the olivines, but as the symbols indicate the samples analyzed from this island separate into 3 distinct populations which are parallel to one another. This may indicate that these basalts formed from the melting of pyroxenites which were compositionally dissimilar (Herzberg, 2011) or that varying proportions of peridotite melts mixed with pyroxenite melts (Sobolev et al., 2007).

Olivine analyses from Volcan Ecuador. The pyroxenite source of these basalts is indicated by the low Ca and low Mn and elevated Ni and Fe/Mn. Unlike Fernandina, the deviation of these olivine analyses from the peridotite source fields cannot be accounted for by the effects of Cpx fractionation, the pyroxenite source is inferred.

Olivine analyses from Fernandina. The compositional effects of variable Ol and Cpx fractionation on olivine phenocrysts are indicated by the green lines. The primary magma composition from which the green lines were calculated is from a Primelt II (Herzberg and Asimow, 2008) solution of a Fernandina sample. A peridotite source is indicated in all Fernandina samples. Most deviations from calculated olivine compositions from a peridotite source are explained by varying degrees of Cpx fractionation.

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FernandinaPeridotite Source

Volcan EcuadorPyroxenite Source

Santa CruzVariable Pyroxenite Source


Source Lithology Map

CONCLUSIONS

ACKNOWLEDGMENTSThe authors of this poster would like to thank Karen Harpp, Dennis Kent and Terry Naumann for providing samples for this project.

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