Stepped alteration of lower oceanic crust at Hess Deep from whole rock 87Sr/86Sr data and quantitative modal mineralogy

Variations in the ⁸⁷Sr/⁸⁶Sr ratios of oceanic rocks are controlled by an interplay between magmatic and seawater Sr reservoirs. Because the metamorphic mineral products as well as the compositions of reactive aqueous fluids change with decreasing temperatures of alteration, Sr isotopic evolution is necessarily nonlinear. This study investigates the Sr isotopic compositions of whole core channel samples from IODP Expedition 345 in the context of whole rock geochemistry and quantitative modal mineralogy from powder XRD analysis and the Rietveld method. The drilled layered gabbros, olivine gabbros, and troctolites were affected by pervasive high-T (>300°C) alteration following initial crystallization at the East Pacific Rise and subsequent, but localized, low-T (<300°C) alteration associated with faulting within the Hess Deep Rift. Results show that alteration to amphibole, talc, serpentine, and chlorite at T>350°C did not affect bulk rock ⁸⁷Sr/⁸⁶Sr. However, upon the addition of prehnite at T>300°C the rocks began to sequester seawater Sr and whole rock ⁸⁷Sr/⁸⁶Sr began to rise. ⁸⁷Sr/⁸⁶Sr in rocks affected primarily by high-T alteration correlates positively and systematically with LOI and total wt.% alteration minerals. This suggests a "rock-dominated" metamorphic environment in which the initial seawater was modified by the dissolution of magmatic plagioclase to higher Sr concentrations (~52 ppm) and lower ⁸⁷Sr/⁸⁶Sr (0.70337). Fluid/rock volume calculations suggest that most if not all of the 0.1-0.3 rock volumes of high-T fluid were retained in metamorphic minerals. Rocks additionally overprinted by low-T alteration show random correlations between ⁸⁷Sr/⁸⁶Sr and other alteration indices, suggesting more extensive Sr metasomatism during alteration at T<300°C. Moreover, the presence of Ca-thomsonite, corrensite, and chlorite-smectite among the low-T products—all of which host Sr reversibly—allow Sr ion exchange with circulating seawater to continue long after mineral precipitation has ceased. Results show (1) that prehnite replacement of plagioclase is essential to increasing the whole rock ⁸⁷Sr/⁸⁶Sr at T>300°C as it is the major metamorphic Sr host in these rocks and (2) that this replacement is independent of other alteration indices such as degree of serpentinization of olivine.