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Primitive layered gabbros from fast-spreading lower oceanic crust

Abstract

Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the mantle and the sea floor. Despite the importance of these rocks, sampling them in situ is extremely challenging owing to the overlying dykes and lavas. This means that models for understanding the formation of the lower crust are based largely on geophysical studies1 and ancient analogues (ophiolites)2,3,4,5 that did not form at typical mid-ocean ridges. Here we describe cored intervals of primitive, modally layered gabbroic rocks from the lower plutonic crust formed at a fast-spreading ridge, sampled by the Integrated Ocean Drilling Program at the Hess Deep rift. Centimetre-scale, modally layered rocks, some of which have a strong layering-parallel foliation, confirm a long-held belief that such rocks are a key constituent of the lower oceanic crust formed at fast-spreading ridges3,6. Geochemical analysis of these primitive lower plutonic rocks—in combination with previous geochemical data for shallow-level plutonic rocks, sheeted dykes and lavas—provides the most completely constrained estimate of the bulk composition of fast-spreading oceanic crust so far. Simple crystallization models using this bulk crustal composition as the parental melt accurately predict the bulk composition of both the lavas and the plutonic rocks. However, the recovered plutonic rocks show early crystallization of orthopyroxene, which is not predicted by current models of melt extraction from the mantle7 and mid-ocean-ridge basalt differentiation8,9. The simplest explanation of this observation is that compositionally diverse melts are extracted from the mantle and partly crystallize before mixing to produce the more homogeneous magmas that erupt.

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Figure 1: Tectonic setting of the HDR and the location of IODP Site U1415.
Figure 2: Typical gabbroic rocks at Site U1415.
Figure 3: Variations in whole-rock CaO and Al2O3 with MgO for different parts of the crust at the HDR.

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Acknowledgements

This research used samples and data provided by the IODP. We thank the USIO staff and the Siem Offshore crew for recovering hard rock core in an unsedimented environment at >4,850-m water depth, and for their invaluable assistance during the expedition. We gratefully acknowledge the contributions of the drilling proposal proponents and the leaders (C. MacLeod and D. Teagle) and participants of the site survey cruise (JC21) in making IODP Expedition 345 possible. We thank E. Klein and H. Dick for reviews.

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All authors were shipboard participants on IODP Expedition 345, contributed to the shipboard data collection, and discussed the results and their implications. K.M.G. wrote the first draft of the manuscript, K.M.G. and J.E.S. were co-chief scientists on the expedition, and A.K. was the staff scientist.

Corresponding author

Correspondence to Kathryn M. Gillis.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Summary lithostratigraphic columns of the gabbroic rocks recovered at IODP holes.

a, U1415I; b, U1415J; c, U1415P. Columns show recovery, lithological units, major rock types, dip of magmatic foliations and well-constrained magnetic remanence inclination values (mean and 1 s.d. listed). Lithological units were identified on the basis of similarities in rock types, magmatic textures and foliations. Palaeomagnetic remanence directions and the dip of the magmatic foliations and layers (not shown) for units II and III in holes U1415J and U1415P are most easily interpreted as blocks that probably formed by slumping and were rotated relative to each other. Ghost cores (G cores) are intervals drilled during hole cleaning operations. In a, Unit II refers to the Unit II layer gabbro series. d, Map showing the relative locations of holes U1415I, U1415J and U1415P; microbathymetry from ref. 45.

Extended Data Figure 2 Core images showing examples of simple, centimetre-scale modal layering and a moderate-to-strong magmatic foliation.

a, 345-U1415J-5R-2, piece 1, 2.0–17.5 cm; b, 345-U1415J-8R-2, piece 9, 105.5–121.0 cm.

Extended Data Table 1 Bulk compositions of crustal sections used to calculate the bulk-crustal composition and the bulk composition of the HDR crust and plutonic section

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Gillis, K., Snow, J., Klaus, A. et al. Primitive layered gabbros from fast-spreading lower oceanic crust. Nature 505, 204–207 (2014). https://doi.org/10.1038/nature12778

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