Magma Reservoir Formation and Evolution at a Slow-Spreading Center (Atlantis Bank, Southwest Indian Ridge)

Marine Boulanger, Lydéric France, Jeremy R.L. Deans, Carlotta Ferrando, C. Johan Lissenberg, Anette von der Handt

Research output: Contribution to journalArticlepeer-review

2 Scopus citations

Abstract

Several ODP-IODP expeditions drilled oceanic core complexes interpreted as exhumed portions of lower crust close to the ridge axis, and provide the community with invaluable sampling opportunity for further constraining magmatic processes involved in the formation of the slow-spreading lower oceanic crust. ODP Hole 735B presents the most primitive lithologies sampled at Atlantis Bank oceanic core complex (Southwest Indian Ridge) in a ∼250 m thick section that was previously interpreted as a single crustal intrusion. We combined detailed structural and petrographic constraints with whole rock and in situ mineral analyses of this section in order to precisely determine the processes of emplacement, crystallization, and melt migration within the lower crust. The lower half of the unit is comprised of alternating olivine gabbros and troctolites showing intrusive contacts, magmatic fabrics, and crystal-plastic fabrics. Such structures and primitive lithologies are lacking in the upper half, rather uniform, gabbroic sequence. Whole rock compositions highlight the cumulative character of both lower and upper units and a great compositional variability in the lower sequence, whereas the upper sequence is homogeneous and differentiates up-section. In situ analyses of mineral phases document magma emplacement processes and provide evidence for ubiquitous reactive porous flow (RPF) during differentiation. We show that the whole section, and related geochemical unit, constitutes a single magmatic reservoir, in which the lower unit is formed by stacked primitive sills formed by repeated recharge of primitive melts and melt-present deformation. Recharge led to partial assimilation of the crystallizing primitive cumulates, and hybridization with their interstitial melts. Hybrid melts were progressively collected in the overlying mushy part of the reservoir (upper unit), whereas the sills’ residual hybrid melts differentiated by RPF processes under a predominantly crystallization regime. Similarly, hybrid melts’ evolution in the upper unit was governed by upward RPF, and progressive differentiation and accumulation of evolved melts at the top of the reservoir. Our results provide the community with the first integrated model for magma reservoir formation in the lower slow-spreading oceanic crust that can potentially be applied to other magmatic lower crust sections.

Original languageEnglish (US)
Article number554598
JournalFrontiers in Earth Science
Volume8
DOIs
StatePublished - Sep 15 2020

Bibliographical note

Funding Information:
This research used samples and data provided by the Ocean Drilling Program (ODP). ODP was sponsored by the US National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc. We acknowledge IODP France funded by CNRS for participating in the funding of this project. The assistance of the staff at the Kochi IODP core repository is gratefully acknowledged. We also express our warm thanks to Jean-Luc Devidal (LMV Clermont-Ferrand) and Olivier Bruguier (Geosciences Montpellier) for their assistance during EPMA and LA-ICP-MS measurements, respectively. This study greatly benefited from thorough reviews by H. J. B. Dick, J. D.

Funding Information:
This research was supported by CNRS-INSU and IODP-France. MB’s PhD project was funded by the French Ministry of Research (MESRI). JL was supported by the UK Natural Environment Research Council (NERC Grant NE/I001670/1).

Funding Information:
This research used samples and data provided by the Ocean Drilling Program (ODP). ODP was sponsored by the US National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc. We acknowledge IODP France funded by CNRS for participating in the funding of this project. The assistance of the staff at the Kochi IODP core repository is gratefully acknowledged. We also express our warm thanks to Jean-Luc Devidal (LMV Clermont-Ferrand) and Olivier Bruguier (Geosciences Montpellier) for their assistance during EPMA and LA-ICP-MS measurements, respectively. This study greatly benefited from thorough reviews by H. J. B. Dick, J. D. Blundy, M. R. Perfit, and the editor F. J. Spera, whom we gratefully thank. The authors also thank J. H. Bastin, M. Cannat, K. V. Cashman, H. J. B. Dick, M. Godard, C. J. MacLeod, and A. Sanfilippo for constructive discussions. This is CRPG contribution number 2740.

Publisher Copyright:
© Copyright © 2020 Boulanger, France, Deans, Ferrando, Lissenberg and von der Handt.

Keywords

  • Atlantis Bank
  • assimilation-fractional crystallization
  • crystal mush
  • lower oceanic crust
  • magma intrusion
  • reactive porous flow

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