Hydrothermal alteration in the Lowermost Ocean Crust: Unique constraints from Oman Drilling Project Hole CM1A

Abstract

The hydrothermal alteration of the ocean crust at mid-ocean ridges and on ridge flanks contributes to global geochemical budgets by modifying the chemical and isotopic compositions of seawater and the lithosphere. In this research study, layered gabbros from the lowermost ocean crust preserved in the Cretaceous Samail ophiolite of Oman are investigated to characterise and quantify the types, modes and extents of hydrothermal fluid-rock interactions between fast-spreading lower ocean crust and seawater. Oman Drilling Project Hole CM1A recovered a 160 m-thick section of layered gabbros directly above the crust-mantle transition zone in Wadi Zeeb in the Wadi Tayin massif. A multi-stage hydrothermal alteration occurred predominantly from greenschist (550-350 °C), to prehnite – actinolite (350-200 °C) and zeolite (200-100°C) facies conditions. The Layered Gabbro Sequence is moderately to extensively altered by pervasive wall-rock alteration and cross-cut by five vein generations: 1) chlorite + serpentine and chlorite ± amphibole microcracks and veins; 2) epidote and clinozoisite veins; 3) prehnite veins and microcracks; 4) prehnite + calcite veins; 5) calcite veins and microcracks. Alteration halos have secondary mineral assemblages dominated by either chlorite or prehnite. The highest extents of alteration are within and around major zones of prehnite-rich cataclasis and hydrothermal brecciation. The great majority of this alteration occurred whilst the Wadi Tayin Massif remained submerged in the Tethyan ocean.

Hydrothermal fluid-rock exchanges in the lowermost ocean crust are quantified through mass balance calculations using whole rock geochemical data. Gabbroic rocks display generally slight to substantial (10-200% change) additions of Al, Mg, Ca, H2O, C, Li, Co, Zn, Cs, Pb, Ta and Th relative to initial compositions, owing to extensive background and halo alterations. Prehnite-rich cataclasites exhibit moderate to extensive gains of CaO, Al2O3, H2O, CO2, Li, Cs, LREE, Ta, Th and U, but moderate losses of FeO, MnO, MgO, Na2O, K2O, Cu, Rb, Sr and Ba. This suggests enduring and intense fluid-rock exchanges with seawater-derived hydrothermal fluids downwelling in the lowermost ocean crust through such fault zones at 320-200 °C. The contributions of alteration in 1 km-thick lower ocean crust to global geochemical cycle were estimated from elemental mass changes, assuming a production rate of 3 km2/yr. Calculated annual chemical fluxes are more significant to the lowermost crust than to the oceans, with important contributions for Si, Al, Mg, Ca, H2O, C, Li, Co and Zn by faults, veins and halo alterations.

The lowermost ocean crust hydrothermal alteration during oceanic spreading produced substantial modifications of initial gabbroic rocks 87Sr/86Sr up to altered 0.7055 – 0.7065 values. The final δ18O signatures of altered rocks were variably 18O-enriched or 18O-depleted compared to fresh magmatic value, with -1 to +1.6 ‰ exhibited by epidote minerals. These isotopic signatures reveal extensive exchange between rocks and evolved seawater-derived hydrothermal fluids at least between 400-150 °C. Seawater recharge conduits are likely represented by prehnite-rich veins and fault zones that form at 320-200 °C, whereas possible pathways for discharge of hot hydrothermal fluids at 350-400 °C correspond to epidote vein networks.

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Identifiers

ORCID for Francesca Claudia Rotondo: orcid.org/0000-0002-3922-9254

ORCID for Damon Teagle: orcid.org/0000-0002-4416-8409

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