Experimental evidence for Nd-Sr decoupling during low-temperature (20-170oC) hydrothermal alteration of olivine and clinopyroxene
Abstract
Serpentinization of abyssal peridotites represents a major reaction front between the hydrosphere and the mantle. While several studies have investigated the phase equilibria relationships that describe seawater - peridotite interaction at high temperature hydrothermal conditions (~400oC), there is limited data on the elemental mass exchange between seawater and ultramafic lithologies at temperatures similar to those expected at the flanks of hydrothermal vent sites. To better constrain seawater - peridotite elemental exchange alteration processes at low-temperatures, a series of experiments were conducted involving natural mantle olivine (Fo=90) and clinopyroxene coexisting with synthetic seawater enriched in elemental or isotopically enriched Sr, Ba, Nd, Sm, Gd, Dy, Yb, Pb, and U. The experiments were performed at temperatures from ambient to 170oC (at saturation vapor pressure), ranging from 15 minutes to 8 weeks and at water/rock mass ratios ~20. Our data shows strong decoupling between alkaline earth elements (Sr, Ba) and rare earth elements (REE). Overall, the REE are quantitatively removed from the solution to the mineral surface while Sr and Ba invariably remain in solution. In detail, we find that the rate of REE removal is proportional to temperature and inversely proportional to particle size distribution. For example at the 350-200um olivine grain size experiments 60% of REE removal occurred in 7 days at ambient temperature and in 6 hours at 100oC. No difference was observed on the removal rates between clinopyroxene and olivine. Additionally, we observe a fractionation of REE in solution where the HREE were removed at a faster rate than the LREE. The calculated apparent kDs for the experiments that approached steady state are similar to Fe-hydroxide scavenging experiments, and importantly show the tetrad effect in REE. We note in the experiments run with clinopyroxene and isotopically enriched seawater at 170oC, results indicate a simultaneous REE precipitation-dissolution process at the seawater/mineral interface, suggesting a bi-directional exchange between the rock and the solution. The decoupling between REE, and Sr implies that during low-temperature peridotite - seawater reaction, Nd may be preferentially deposited on the peridotite mineral surface while Sr, in the absence of carbonate precipitation, may not. The implications on the estimates of integrated water/rock mass ratios using bulk rock Sr-Nd isotopes in serpentinites will be discussed.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V41C2507F
- Keywords:
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- 1034 GEOCHEMISTRY / Hydrothermal systems;
- 3017 MARINE GEOLOGY AND GEOPHYSICS / Hydrothermal systems;
- 3616 MINERALOGY AND PETROLOGY / Hydrothermal systems