We examined the mineralogical, chemical and isotopic compositions of secondary fluid inclusions in olivine-rich rocks from two active serpentinization systems: the Von Damm hydrothermal field (Mid-Cayman Rise) and the Zambales ophiolite (Philippines). Peridotite, troctolite and gabbroic rocks in these systems contain abundant CH 4 -rich secondary inclusions in olivine, with less abundant inclusions in plagioclase and clinopyroxene. Olivine-hosted secondary inclusions are chiefly composed of CH 4 and minor H 2 , in addition to secondary minerals including serpentine, brucite, magnetite and carbonates. Secondary inclusions in plagioclase are dominated by CH 4 with variable amounts of H 2 and H 2 O, while those in clinopyroxene contain only CH 4 . We determined hydrocarbon abundances and stable carbon isotope compositions by crushing whole rocks and analysing the released volatiles using isotope ratio monitoring—gas chromatography mass spectrometry. Bulk rock gas analyses yielded appreciable quantities of CH 4 and C 2 H 6 in samples from Cayman (4–313 nmol g −1 CH 4 and 0.02–0.99 nmol g −1 C 2 H 6 ), with lesser amounts in samples from Zambales (2–37 nmol g −1 CH 4 and 0.004–0.082 nmol g −1 C 2 H 6 ). Mafic and ultramafic rocks at Cayman exhibit δ 13 C CH 4 values of −16.7‰more »
The potential for aqueous fluid-rock and silicate melt-rock interactions to re-equilibrate hydrogen in peridotite nominally anhydrous minerals
Hydrogen is a rapidly diffusing monovalent cation in nominally anhydrous minerals (NAMs, such as olivine, orthopyroxene, and clinopyroxene), which is potentially re-equilibrated during silicate melt-rock and aqueous fluid-rock interactions in massif and abyssal peridotites. We apply a 3D numerical diffusion modeling technique to provide first-order timescales of complete hydrogen re-equilibration in olivine, clinopyroxene, and orthopyroxene over the temperature range 600-1200°C. Model crystals are 1-3 mm along the c-axis and utilize H+ diffusion coefficients appropriate for Fe-bearing systems. Two sets of models were run with different boundary compositions: 1) “low-H models” are constrained by mineral-melt equilibrium partitioning with a basaltic melt that has 0.75 wt% H2O and 2) “high-H models,” which utilize the upper end of the estimated range of mantle water solubility for each phase. Both sets of models yield re-equilibration timescales that are identical and are fast for all phases at a given temperature. These timescales have strong log-linear trends as a function of temperature (R2 from 0.97 to 0.99) that can be used to calculate expected re-equilibration time at a given temperature and grain size. At the high end of the model temperatures (1000-1200°C), H+ completely re-equilibrates in olivine, orthopyroxene, and clinopyroxene within minutes to hours, consistent with more »
- Publication Date:
- NSF-PAR ID:
- 10192866
- Journal Name:
- The American mineralogist
- ISSN:
- 0003-004X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Publisher's Version of Record
- https://doi.org/10.2138/am-2020-7435
