Stable and radiogenic strontium isotope fractionation during hydrothermal seawater-basalt interaction

Abstract

The fluid-rock interactions occurring in hydrothermal systems at or near mid-oceanic ridges (MOR) were studied experimentally by reacting crystalline and glassy basalt with seawater at 250 and 290 °C while monitoring the fluid phase Sr isotopic evolution (87 Sr/ 86 Sr and δ 88/86 Sr). The results indicate that seawater Sr was incorporated into anhydrite during the early stages of seawater-basalt interaction. Fluid 87 Sr/ 86 Sr values trend towards the basaltic signature as non-stoichiometric basalt dissolution became the dominant process. This suggests that the interplay between fast Sr incorporation into secondary sulfates versus slow and continuous Sr liberation due to basalt dissolution at intermediate temperatures could partly explain previously identified discrepancies between MOR heat budget constraints and the marine 87 Sr/ 86 Sr budget. Late-stage anhydrite re-dissolution, likely caused by the fluid phase becoming more reducing through further basalt dissolution, as well as by quenching of the 2 experiments, represents a potential explanation for the low amounts of anhydrite found in naturally altered oceanic basalt samples. Relatively strong decreases in fluid δ 88/86 Sr values in experiments with crystalline basalt suggest that isotopically light Sr was preferentially released due to non-stoichiometric dissolution. A slight preference of anhydrite for isotopically heavy Sr (Anhydrite−Liquid 88/86 = 0.33 ± 0.020 ‰) is indicated by the data, suggesting that changes in MOR spreading rates and Sr removal could be recorded in the isotope compositions of authigenic, sedimentary Sr phases. Such insights will help to constrain the influence of hydrothermal systems on the oceanic stable Sr cycle.

Sr isotopes in arcs revisited: tracking slab dehydration using δ88/86Sr and 87Sr/86Sr systematics of arc lavas

“…Both have significantly higher δ 88/86 Sr yet similar 87 Sr/ 86 Sr to seawater, which is consistent with isotopic fractionation during precipitation, which enriches the solid phase in heavier isotopes. This is supported by an experimental study that found that anhydrite precipitation drove the composition of the liquid to isotopically lighter compositions, thus indicating that anhydrite has higher δ 88/86 Sr than liquid it precipitated from (Voigt et al, 2018). The extreme Sr isotope composition of the Messinian evaporites makes δ 88/86 Sr a potentially powerful tracer of the involvement of these evaporites in the source of the Aegean arc magmas.…”

“…The AOC composites show remarkably little variation in δ 88/86 Sr and only the most radiogenic 0-110 VCL composite sample has δ 88/86 Sr that is significantly heavier than unaltered MORB; all other AOC composites overlap with our data for N-MORB ( Figure 5b). The 0-110 m VCL composite sample plots towards the composition of 150-170 Ma seawater (McArthur et al, 2001;Vollstaedt et al, 2014) and has similar δ 88/86 Sr to the basaltic material altered by seawater in the experimental study of Voigt et al (2018). These authors found that alteration by seawater preferentially removes the lighter Sr isotopes from the basaltic host due to non-stoichiometric dissolution of crystalline basalt, but our results indicate that, at site 801, a positive shift in δ 88/86 Sr is only observed in the most strongly altered fractions of the upper oceanic crust.…”

“…(McArthur et al, 2001;Vollstaedt et al, 2014). The range in δ 88/86 Sr of altered basalt from seawater reaction experiments (Voigt et al, 2018) is shown as dashed vertical lines as no radiogenic Sr data are reported for the reacted solid phases in that study. The composition of modern, unaltered MORB is shown as black triangles; filled symbols are from this study, open symbols are literature data (Charlier et al, 2012;Amsellem et al, 2018).…”

“…Carbonate-rich sediments are isotopically lighter while seawater is isotopically heavier than mantle values (Krabbenhöft et al, 2010;Pearce et al, 2015;Stevenson et al, 2016). An experimental study found that the Sr isotope composition of basalt becomes heavier during interaction with seawater at 250-290 °C (Voigt et al, 2018), which suggests that altered oceanic crust may have higher δ 88/86 Sr than pristine MORB. Hence, combining radiogenic and mass-dependent Sr isotope variations can contribute to disentangling the contributions of these components to the Sr budget of arc magmas.…”

Application of B, Mg, Li, and Sr Isotopes in Acid‐Sulfate Vent Fluids and Volcanic Rocks as Tracers for Fluid‐Rock Interaction in Back‐Arc Hydrothermal Systems

“…Geochemistry, Geophysics, Geosystems et al, 1988, Voigt et al, 2018. In contrast, the dissolution of anhydrite and admixing with seawater can directly influence the concentration of Sr and its isotopic composition.…”