Schmelze/Gesteins-Wechselwirkung und fraktionierte Kristallisation in der unteren Kruste am Hess Deep Rift, East Pacific Rise - Eine kombinierte analytische und experimentelle Studie
Mineralogie, Petrologie und Geochemie
Zusammenfassung der Projektergebnisse
At the Hess Deep Rift (HDR) in the equatorial Pacific at the EPR, IODP Expedition 345 drilled, for the first time in the history of IODP, coherent cores of the deeper parts of the lower, plutonic crust from a fast-spreading ridge. The drilled cores show spectacular modal and/or grain size layering present in >50% of the recovered core, validating for the first time the use of the ophiolite model for interpreting EPR crust. A significant first-order observation from this expedition is that orthopyroxene was found as an abundant cumulus phase in many of the layered primitive gabbroic rocks. This was unexpected, since experiments on the liquid line of descent of MORB show that orthopyroxene never crystallizes as early phase. On the other hand, orthopyroxene is expected in relatively evolved gabbroic rocks from a shallow crustal level, where orthopyroxene crystallized as relatively late phase at the expense of olivine. Such gabbronorites have been also drilled at HDR: during Leg 147 sampled at Site 895. This project aims to understand the exact mechanism of the magmatic evolution of the lower crust at HDR, with focus on the role of orthopyroxene formation in the deep, primitive gabbros. This projects follows a multimethodical approach: Theme (1) focusses on the petrology/geochemistry of the natural rocks by using microanalytical tools to be applied to the minerals in the IODP samples. Theme (2) is an experimental project with the approach to simulate both melt/rock interaction as well as equilibrium crystallization and near-perfect fractional crystallization in a system corresponding to HDR bulk crust average. Focus of theme (1) was to produce a broad microanalytical data base. With respect to EPMA, 64 samples (45 with orthopyroxene) of the main lithologies (olivine gabbro, troctolite, gabbronorite) have been analyzed both from Expedition 345 and Leg 147, resulting in the largest mineral data set ever acquired for gabbros from modern fast-spreading ridges. These data reveal a general differentiation trend for Hess Deep gabbros, similar to trends from other key localities for oceanic gabbros from fast-spreading ridges (only two further sites exist), compatible with trends from experimental studies and from petrological modeling, implying that fractional crystallization from a primitive, parental MORB is the main magmatic process during crust accretion at a recent fast-spreading mid-ocean ridge. The detailed textural analysis of the rocks revealed a strong correlation between textural appearance and grade of differentiation, a characteristic relationships which was shown in this project for the first time. A strong variation in Cr2O3 content at similar Mg# implies that prismatic orthopyroxene within the deep, primitive gabbros drilled by Expedition 345 are formed by local interaction between melt and peridotite as suggested by the model of Coogan et al. (2002). On the other hand the data show that those orthopyroxenes in the shallow gabbros drilled by Leg 147 are clearly the product of late crystallization of orthopyroxene as the expense of olivine, as envisaged from petrological modeling. These results are in general supported from the trace element chemistry of the minerals obtained by LA-ICPMS. For the deep, primitive gabbros from Expedition 345, detailed analyses on both interstitial and prismatic clino- and orthopyroxenes in the same sample imply, that interstitial clinopyroxene was formed by interaction with an evolved melt, while interstitial orthopyroxene highly enriched in Cr might have crystallized from a special "primary melt" which has been compositionally buffered via melt-rock interaction with a mantle peridotite. From the different approaches to experimentally simulate the stabilization of orthopyroxene in primitive olivine gabbros within theme (2), the experiments on the interaction between a primitive MORB melt (HDR bulk crust composition) and mantle peridotite was most successful, verifying the hypothesis of Coogan et al., 2002, that orthopyroxene in primitive gabbros can be formed by reaction between uprising, Moho-crossing parental MORB melts and mantle rocks. The melt/rock interaction experiments show that interaction between primitive MORB melt and peridotite can increase the melt SiO2 content effectively without significantly changing its MgO content, resulting in an evolving trend to orthopyroxene saturation. This experimental study demonstrates that melt-peridotite interaction at crust/mantle boundary may be a common scenario responsible for the formation of heterogeneous MORB melts, in which some are close to orthopyroxene-saturation composition with the potential of generating orthopyroxene-bearing cumulates in the lowermost crust, as observed at Hess Deep.
Projektbezogene Publikationen (Auswahl)
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Geochemistry of Fast-spreading Lower Crust: Results from IODP Expedition 345 at the Hess Deep Rift. Goldschmidt Conference, Florence.
Godard, M.; Meyer, R.; Saha, A.; Gillis, K.M.; Snow, J.E.; Klaus, A. & IODP Expedition 345 Shipboard Scientific Party
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Hypersolidus deformation and melt flow in the lower ocean crust: Preliminary observations from IODP Leg 345. AGU Fall Meeting, San Francisco
Snow, J.E.; Koepke, J.; Falloon, T.; Abe, N.; Hoshide, T.; Akizawa, N.; Jean, M.; Maeda, J.; Cheadle, M. & IODP Expedition 345 Shipboard Scientific Party
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IODP Expedition 345: Orthopyroxene omnipresent in Layered Gabbros from the Hess Deep, EPR. AGU Fall Meeting, San Francisco
Koepke, J.; Abe, N.; Akizawa, N.; Falloon, T.; Hoshide, T.; Jean, M.; Maeda, J.; Snow, J.E. & IODP Expedition 345 Scientific Party
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Layered gabbro recovered by IODP from fast-spreading lower oceanic crust at Hess Deep, East Pacific Rise. Joint annual meeting of the DMG, GV and Sediment, Tübingen
Koepke, J.; Mengel, K. & the Shipboard Scientific Party
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Gabbro glacier or “sheeted sills”? New constraints on the accretion mode of oceanic crust at fast-spreading oceanic ridges. Annual Meeting Deutsche Mineralogische Gesellschaft, Jena
Koepke, J.; Mueller, T.; Linsler, S.; Wolff, E.P.; Schuth, S. & Garbe-Schönberg, D.
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Geochemistry of Fast-Spreading Lower Oceanic Crust: Results from Drilling at the Hess Deep Rift (ODP Leg 147 and IODP Expedition 345; East Pacific Rise). AGU Fall Meeting, San Francisco
Godard, M.; Fallon, T.; Gillis, K.M.; Akizawa, N.; Alden de Brito, A.; Koepke, J.; Marks, N.; Meyer, R.; Saha, A. & Garbe-Schönberg, D.
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Orthopyroxene in primitive Layered Gabbros from Hess Deep (EPR) drilled by IODP. Goldschmidt Conference, Sacramento
Koepke, J.; Meyer, R.; Snow, J.E. & IODP Expedition 345 Shipboard Scientific Party
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Petrogenesis of orthopyroxene and primitive orthopyroxene–bearing gabbros at Hess Deep. AGU Fall Meeting, San Francisco
Jean, M.M.; Koepke J.; Meyer; R.; Falloon; T. & Snow; J.E.
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Petrological characteristics of Opx-bearing primitive gabbros from the East Pacific Rise and the Oman ophiolite. EGU General Assembly, Vienna
Python, M.; Akizawa, N.; Godard, M.; Ildefonse, B.; Koepke, J. & IODP Expedition 345 science party
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Petrogenesis of primitive orthopyroxene–bearing gabbros at Hess Deep, IODP Exp345. Goldschmidt Conference, Prague
Jean, M.; Koepke, J.; Meyer, R.; Snow, J.E.; Falloon, T. & Danyushevsky, L.
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Trace Element Budget along a Section of Fast-spread Oceanic Crust: Results from Drilling at the Hess Deep Rift (ODP Leg 147 and IODP Expedition 345). Goldschmidt Conference, Prague
Godard, M.; Fallon, T.; Gillis, K.M.; Akizawa, N.; Koepke, J.; Marks, N.; Saha, A.; Garbe-Schönberg, D. & Meyer, R.
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Orthopyroxene in primitive layered gabbros from Hess Deep (EPR): Formation by mantle/melt interaction or fractional crystallization? European Mineralogical Conference EMC, Rimini
Koepke J.; Zhang C.; Namur O.; Meyer R. & Feig S.T.
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Electron microprobe technique for the determination of iron oxidation state in silicate glasses. American Mineralogist, 103(9), 1445-1454.
Zhang, Chao; Almeev, Renat R.; Hughes, Ery C.; Borisov, Alexander A.; Wolff, Eric P.; Höfer, Heidi E.; Botcharnikov, Roman E. & Koepke, Jürgen
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MORB modified by melt-peridotite interaction at crust-mantle boundary: An experimental perspective. AGU Fall Meeting, Washington
Zhang, C.; Koepke, J. & Namur, O.
