Significant variability in 87Sr/86Sr and d88/86Sr in Hess Deep Rift lithologies due to hydrothermal alteration.

Hydrothermal alteration of seafloor basalts alters its elemental and isotopic composition. Studies on dredged basalts and ophiolite sequences using stable O, K, and radiogenic Sr ^[1,2,3] isotopes have documented the effect of seafloor alteration on such lithologies. Experimental studies of basalt-seawater interaction have also demonstrated exchange of Sr isotopic signatures between these two phases ^[4] while limited data for altered oceanic crust suggests incorporation of heavier Sr isotopes ^[5]. To further our understanding of the behaviour of Sr during seafloor alteration in natural settings, we measured ⁸⁷Sr/⁸⁶Sr and δ^88/86Sr in a suite of variably altered lithologies from Hess Deep Rift (HDR), which include basalt, norite, gabbro and troctolite.

Radiogenic Sr (⁸⁷Sr/⁸⁶Sr) was measured using TIMS (Thermo Scientific, Triton Plus), using internal normalization while stable Sr isotopes (δ^88/86Sr, reported relative to NIST SRM 987) were measured using a double spike (⁸⁴Sr-⁸⁷Sr) TIMS technique, both at the Centre for Earth Sciences, IISc, Bangalore. The δ^88/86Sr values of the HDR samples (0.308-0.810 ‰) are higher than the bulk silicate Earth (BSE) value (0.27 + 0.05 ‰) ^[6]; some samples show δ^88/86Sr values higher than modern-day seawater value (0.386 ‰) ^{[e.g., 7]}. The ⁸⁷Sr/⁸⁶Sr varies from ~0.703 in unaltered samples to ~0.709 in altered samples, the latter close to the modern-day seawater value. Overall, our data suggests incorporation of heavier isotopes of Sr in altered oceanic crustal samples; the heavier than seawater δ^88/86Sr values observed in some samples reflect formation of new mineral phases, consistent with high δ^88/86Sr observed in anhydrite formed in laboratory experiments of basalt-seawater interaction^[4].

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