Sulfur and strontium isotopic geochemistry of the crust-mantle transition of the Oman Ophiolite: records of fluid circulation

The Samail ophiolite in Oman was sampled by scientific drilling targeting crucial sections of the oceanic crust and mantle during the Oman Drilling Project- OmanDP [1]. Drillhole CM1A aimed at characterizing the transition from the lower crust to the mantle Moho Transition Zone (MTZ), where both magmatic and hydrothermal exchanges took place. Four magmatic sequences were defined: SI- Layered Gabbro, with thin wehrlite and dunite layers (1.5-160.2 m); SII- fully serpentinized Dunite (160.2-250.0 m); SIII- Dunite with rodingitized gabbro (250.0-311.0 m) and; SIV- Mantle, harzburgite with opx-dunite levels (311.0-404.2 m).

We present a sulfur and Sr isotope profile to characterize the sulfur cycling during hydrothermal alteration within the MTZ (SI-SIII). Acid Volatile Sulfides (AVS), Cr-Reducible Sulfur (CRS) and acid-soluble sulfate (SO₄) were sequentially extracted and analyzed for δ³⁴S on the same whole-rock powders analyzed for Sr isotopes.

The crust-mantle transition records extreme and often decoupled variations in sulfur (δ³⁴S=-25.8 to +56.9‰) and ⁸⁷Sr/⁸⁶Sr (0.703088-0.711688) signatures. Total extracted sulfur from sulfide (TS=AVS+CRS) contents increase gradually from the top to the bottom of SI from ca ~65-2820 ppm, to maximum of 5043 ppm in a Cpx-Pl-dunite layer ca. 16 m above SII. Sulfide assemblages comprises magmatic pyrrhotite+pentlandite+chalcopyrite and secondary pyrrhotite (in Fe-serpentine pseudomorphs)+bornite+cubanite+millerite+sphalerite±haezlewoodite. Excluding one dunite layer with δ³⁴S_AVS=+11.4‰, the δ³⁴S_AVS,CRS (-0.6 to +3.3‰) for SI are close to slightly elevated relative to mantle values. Scarce sulfates have identical δ³⁴S relative to coexisting sulfides implying formation via abiotic oxidation of precursor sulfides. Despite widespread background alteration, olivine gabbros preserve primitive ⁸⁷Sr/⁸⁶Sr ratios (0.703088-0.703332) whereas serpentinised ultramafic layers have significantly more radiogenic signatures (0.707817-0.711688), close to or above Cretaceous seawater (⁸⁷Sr/⁸⁶Sr=0.70745). Gradual enrichment in sulfides by magmatic processes in SI, towards the MTZ, was followed by hydrothermal alteration with minor incorporation of seawater sulfate, leading to highly decoupled Sr-³⁴S enrichment in the ultramafic layers due to their Sr-depleted nature. Narrow pegmatoid dikelets (amphibole+zoisite+prehnite+titanite) within SI have low TS (<80 ppm), mildly radiogenic ⁸⁷Sr/⁸⁶Sr (<0.704923) and a fracture-hosted, higher fS₂sulfide assemblage (pyrite+Co-pentlandite+siegenite) with δ³⁴S_CRS down to -25.8‰ implying low-T (<110 C), open-system bacterial sulfate reduction (BSR) processes.

The Dunite Sequence-SII has decreasing TS towards its interior (2-1253 ppm), consistent with extensive desulfurization producing an assemblage (awaruite+pentlandite+Co-pentlandite+magnetite, coexisting with brucite), during extremely low oxygen and sulfur fugacities typical of early serpentinization stages. SIII is highly heterogenous and S-depleted (3-623 ppm), with a heazlewoodite-bearing assemblage and lower ⁸⁷Sr/⁸⁶Sr (0.703952) relative to SII dunites (0.707065). The MTZ upper limit (SII) marks the onset of large shifts in S-isotopic composition, tendentially increasing downward throughout SII (δ³⁴S_CRS=-2.5, +15.6‰; δ³⁴S_SO4=+19.2, +32.4‰) and SIII (δ³⁴S_CRS=+1.4, +56.9‰; δ³⁴S_SO4=+19.4, +36.5‰). The occurrence of both sulfides and sulfates with δ³⁴S above Cretaceous seawater sulfate (~18‰) can be explained by input of fluids at the top of SII which composition progressed towards extreme heavy values via closed system BSR during multi-staged serpentinization events.

AJ acknowledges WWU International Visiting Scholars and EU-H2020 Marie Sklodowska-Curie #894599 Fellowships, FCT-project UIDB/GEO/50019/2020 

[1] Kelemen PB, Matter JM, Teagle DAH, Coggon JA, OmanDP Science Team (2020) Proceedings of the OmanDP: College Station, TX (IODP).