A profile of multiple sulfur, oxygen, strontium isotopes and related chalcophile elements through lower slow-spreading crust (IODP Expedition 360, Atlantis Bank, Southwest Indian Ridge)
Mineralogy, Petrology and Geochemistry
Final Report Abstract
While oceanic crust formed at fast-spreading rates exhibits a relatively uniform seismic stratigraphy and is regarded as layered and relatively homogeneous, oceanic crust generated at slow-spreading ridges is characterized by considerable heterogeneity. This discrepancy may affect how sulfur and metals are transported in melts and fluids from the upper mantle to the ocean floor. Long drilled sections through the crust obtained by the international oceanic drilling programs (ODP, IODP) along with related research improved our knowledge on the accretion, evolution and cooling of slow-spreading oceanic crust significantly. In this project, we worked on the 810-m-long drilled section of the slow-spread lower crust from IODP Hole U1473A in Atlantis Bank along the Southwest Indian Ridge recovered by Expedition 360. We aimed at quantifying, for the first time, the cycling of sulfur and related chalcophile elements in slow-spreading oceanic crust with moderate magma supply and thick crust. We studied 101 samples spread throughout the drilled core to understand major mechanisms of sulfide differentiation at the lower oceanic crust accreted during moderate magma supply. Importantly, magmatic sulfides (pyrrhotite, chalcopyrite, pentlandite assemblages) occur in all gabbroic samples. On the other hand clearly hydrothermal pyrites occur only in two samples, evidencing that most investigated sulifides are of primary, magmatic origin. The mantle-derived isotopic signatures of δ34S = −0.1 to +0.6‰ (except for +1.5–4.8‰ in three samples and biogenic −24.9‰ in one sample), 87Sr/86Sr = 0.7028–0.7032, and δ18O = 4.2–5.8‰ further indicate minor hydrothermal alteration. The sulfides coexist with Fe-Ti oxides and apatite, and the bulk-rock S, MnO, TiO2, Fe2O3, and P2O5 contents are positively correlated. These elements and whole-rock Mg/(Mg+Fe+2) decrease from the bottom to the top within two large-scale gabbroic bodies at 60–300 and 300–800 mbsf. These trends indicate that sulfide abundances are mostly controlled by fractional crystallization, although in some samples we observe an excess enrichment in sulfides relative to fractional crystallization likely caused by zone refining (mostly 615–730 mbsf) or melt-rock reaction (mostly 110–115 mbsf). Based on textural features, oxides crystallize first and are immediately followed by sulfides. The crystallization of sulfides after oxides is achieved by lowering the level of sulfide concentration at sulfide saturation (SCSS) due to loss of Fe and decreased oxygen fugacity of the melt after oxide precipitation. Because sulfides and oxides segregate early, they tend to accumulate at the lower parts of both gabbro bodies, which accordingly are enriched in Cu by ~50% and in S by ~100% relative to the upper portions. In addition, the deeper gabbro body (300–750 mbsf) is enriched in Cu, S, and other metals with respect to the shallow gabbro body (60–300 mbsf). Importantly, early stage of sulfide accumulation, favored in the deeper parts of the lower oceanic crust, may inhibit their leaching by hydrothermal fluids and formation of seafloor massive sulfide deposits.
Publications
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(2017). Distribution of sulfides across igneous layering at the slow-spread lower oceanic crust: Atlantis Bank, Southwest Indian Ridge. The Geosymposium of Young Researchers „Silesia 2017”, Zabrze, September 21-23, 2017. Abstracts, p. 76
Pieterek, B., Ciazela, J., Koepke, J., Dick, H.J.B., Duczmal-Czernikiewicz, A., Bender, M., Muszyński, A., and Szczepanik, M.
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(2017). Layer boundaries attract sulfides throughout the igneous layering of the lower oceanic crust: IODP Hole U1473A, Atlantis Bank, Southwest Indian Ridge. 24th Session of the Petrology Group of the Mineralogical Society of Poland, October 19-22, 2017. Mineralogia special papers 47, 36
Pieterek, B., Ciazela, J., Koepke, J., Strauss, H., Dick, H.J.B., Duczmal-Czernikiewicz, A., Bender, M., and Muszyński, A.
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(2017). Sulfide-rich gabbro within the footwall of an oceanic core complex: looking into the source of seafloor massive sulfides. 2nd GOOD Meeting. Postgraduate conference on the „Geology of Ore Deposits“, Hannover, Germany, March 8-11, 2017. Conference proceedings, p. 13
Bender, M., Ciazela, J., Koepke, J., Dick, H.J.B., Strauss, H., Pieterek, B., Muszynski, A., and Kuhn, T.
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(2017). Sulfide-rich interval in gabbros of the IODP drill core from site U1473 (Atlantis Bank, Southwest Indian Ridge, SWIR). Gemeinsames Kolloquium der Schwerpunktprogramme IODP/ICDP, Braunschweig, Germany, March 14-16, 2017. Conference proceedings, p. 47
Ciazela, J., Koepke, J., Strauss, H., Pieterek, B., Bender, M., Dick, H.J.B., Kuhn, T., and Muszynski, A.
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(2018). Insight from Atlantis Bank igneous layering: focused magma flow reacts with crystal mush and leaves sulfides at the interface. Post-cruise meeting of the IODP Expedition 360, Siracusa, Sicily, Italy, May 14-19, 2018
Pieterek, B., Ciazela, J., Boulanger, M., France, L., Koepke, J., Dick, H.J.B., and Muszynski, A.
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(2018). Local example of sulfide differentiation through melt-rock reaction at the lower oceanic crust (U1473A, Atlantis Bank, Southwest Indian Ridge). 25th Session of the Petrology Group of the Mineralogical Society of Poland, October 25–28, 2018. Mineralogia special papers 48, 73. [The Professor Ryszard Kryza Young Researcher Award for the best poster presentation]
Pieterek, B., Ciazela, J., Dick, H.J.B., and Muszyński, A.
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(2018). Sulfide-rich interval discovered deep in the lower crust (U1473, Atlantis Bank, SWIR). ECORD Newsletter 31, 23–25
Ciazela, J
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(2020). Sulfide-oxide precipitation and enrichment in metals at the slowspreading lower oceanic crust: A case from IODP Hole U1473A. International Conference on Ophiolites and the Oceanic Lithosphere: Results of the Oman Drilling Project and Related Research, Muscat, Oman, January 12–14, 2020
Ciazela, J., Koepke, J., Strauss, H., Pieterek, B., Kuhn, T., Dick, H.J.B., Wang, Z., Muszyński, A., and Abe, N.
