Sulfide differentiation at the lower oceanic crust with high magma supply; fractional crystallization and melt-rock reaction (IODP Hole U1473A, Atlantis Bank)

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Author(s): Ciazela, Jakub; Pieterek, Bartosz; Boulanger, Marine; Dick, Henry; France, Lyderic; Grabowska, Marta; Koepke, Juergen; Kuhn, Thomas; Muszynski, Andrzej; Strauss, Harald; Wegorzewski, Anna
Author Affiliation(s): Primary:
Polish Academy of Sciences, Space Research Centre, Warsaw, Poland
Adam Mickiewicz University, Poland
University of Lorraine, France
Woods Hole Oceanographic Institution, United States
University of Plymouth, United Kingdom
Leibniz University Hannover, Germany
Federal Institute for Geosciences and Natural Resources, Germany
University of Münster, Germany
Volume Title: European Geosciences Union general assembly 2019
Source: Geophysical Research Abstracts, Vol.21; European Geosciences Union general assembly 2019, Vienna, Austria, April 7-12, 2019. Publisher: Copernicus GmbH on behalf of the European Geosciences Union (EGU), Katlenburg-Lindau, Germany. ISSN: 1029-7006
Note: In English
Summary: Magma evolution in the lower oceanic crust affects the metal contents in the derivative magmas and the formation of seafloor massive sulfides. At spreading segments with low magma supply, sulfide differentiate primarily by melt-mantle reaction, with subordinate role of fractional crystallization. At spreading segments with high magma supply, differentiation mechanisms of sulfides are yet not well constrained. The International Ocean Discovery Program Expedition 360 cored 810-m-long section of fully developed lower crust from Hole U1473A at the Atlantis Bank ocean core complex along the Southwest Indian Ridge (32°42'S, 57°17'E). We studied 101 samples spread throughout the hole to understand major mechanisms of sulfide differentiation at the lower crust accreted during high magma supply. Importantly, magmatic sulfides (pyrrhotite-chalcopyrite-pentlandite grains) occur in all the samples with hydrothermal pyrite in only two samples. Consequently, the mantle-derived isotopic signatures of δ34S (-0.1 to +0.6 ppm except for 1.5-4.8 in three samples and biogenic -24.9 in one sample), 87Sr/86 (0.70275-0.702796), and δ18O (4.2-5.8 ppm) indicate scarce hydrotheral alteration. The sulfides co-exist with oxides and apatites, and we observe correlations between the bulk-rock S, MnO, TiO2, Fe2O3; and P2O5 contents: All of them and Mg-numbers show upward-decreasing trends within two large-scale gabbroic bodies at 60-300 and 300-750 mbsf. This indicates predominant role of fractional crystallization in sul-fide differentiation. As sulfides and oxides fractionate early, they tend to accumulate at the lower portions of the gabbroic bodies enriched in Cu by ∼50% and in S by ∼100% with respect to the upper portions. These large-scale trends related to fractional crystallization are locally disrupted by narrow zones enriched in Cu by 200-450%, where sulfide differentiation was driven by melt-rock reaction. There, sulfides were attracted by boundaries between chemically contrasting units of early-stage coarse-grained gabbro and late-stage fine-grained gabbro. Based on the Platinum Group Element analysis of inherent sulfides, the populations of sulfides in late fine-grained gabbro and the contact zone are similar to each other and differ from the coarse-grained gabbro. We conclude that sulfides of the contact zone are crystallized from the focused late-stage magma (represented by the fine-grained gabbro) flowing through crystal mush (represented by the coarse-grained gabbro) due to local chemical disequilibrium at their interface. The chemical disequilibrium might be driven by FeO loss followed by a decrease in sulfur concentration at sulfide saturation level of the late-stage melt in contact with the crystal mush. The enhanced precipitation of sulfide grains at layer boundaries affects metal budget of the lower oceanic crust, which is thus not fully controlled by fractional crystallization. We estimate that ∼11% of the Cu budget in the lower oceanic crust could accumulate at layer boundaries and be missed in global mass balances as those are rarely sampled. Ciazela, J. et al. Thin crust and exposed mantle control sulfide differentiation in slow-spreading ridge magmas. Geology 45, 935-938 (2017). Ciazela, J. et al. Sulfide enrichment at an oceanic crust-mantle transition zone: Kane Megamullion (23°N, MAR). Geochim. Cosmochim. Acta 230, 155-189 (2018). [Copyright Author(s) 2019. CC Attribution 4.0 License:]
Year of Publication: 2019
Research Program: IODP Integrated Ocean Drilling Program
IODP2 International Ocean Discovery Program
Key Words: 07 Marine Geology and Oceanography; Atlantic Ocean; Atlantis Seamount; Cores; Expedition 360; IODP Site U1473; Indian Ocean; International Ocean Discovery Program; Marine sediments; Mineral composition; North Atlantic; Sediments; SloMo; Southwest Indian Ridge; Sulfides
Coordinates: S324222 S324222 E0571641 E0571641
Record ID: 2019061792
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from European Geosciences Union, Munich, Germany