Influence of pyroxene and spinel on the kinetics of peridotite serpentinization
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Abstract Hydrothermal experiments were performed at 311°C and 3.0 kbar on natural olivine and peridotite to investigate the kinetics of serpentinization. The results show that the rates of reaction strongly depend on grain sizes of solid reactants, with smaller grain sizes resulting in faster kinetics. After 27 days of reaction, the reaction extent was 99% for peridotite with grain sizes of <30 μm, and the reaction extent was 28% for grain sizes of 100–177 μm. Compared to peridotite, olivine is serpentinized at much slower rates, e.g., 5.3% of reaction extent was achieved for olivine with grain sizes of 100–177 μm after 27 days, approximately five times lower than that reached during peridotite serpentinization. Such contrasting results are due to the presence of pyroxene and spinel, an interpretation which is supported by a marked increase in reaction extents for experiments with the addition of pyroxene and spinel. The reaction extent achieved in experiments with 3 wt % spinel greatly increased to 98% after 27 days, much higher than that achieved during olivine serpentinization. These results appear to be related to pyroxene and spinel releasing Al and Cr during serpentinization. As indicated by compositions of serpentine, orthopyroxene lost ~60% of Al at a reaction extent of 59%. Influence of Al and Cr is suggested by a dramatic increase in reaction extents with the addition of Al 2 O 3 and Cr 2 O 3 powders. Olivine in natural geological settings is commonly associated with pyroxene and spinel; consequently, serpentinization kinetics may be much faster than previously thought.Keywords:
Peridotite
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Abstract The proton microprobe has been used to determine contents of Ca, Ti, Ni, Mn and Zn in the olivine of 54 spinel lherzolite xenoliths from Australian and Chinese basalts. These data are compared with proton-probe data for Ni, Mn and Zn in the olivine of 180 garnet peridotite xenoliths from African and Siberian kimberlites. Fe, Mn, Ni and Zn contents are well-correlated; because the spinel lherzolite olivines have higher mean Fe contents than garnet peridotite olivines (average Fo 89.6 vs. Fo 90–92 ) they also have lower Ni and higher Mn contents. Zn and Fe are well-correlated in garnet peridotite olivine, but in spinel peridotites this relationship is perturbed by partitioning of Zn into spinel. None of these elements shows significant correlation with temperature. Consistent differences in trace-element contents of olivines in the two suites is interpreted as reflecting the greater degree of depletion of Archean garnet peridotites as compared to Phanerozoic spinel lherzolites. Ca and Ti contents of spinel-peridotite olivine are well correlated with one another, and with temperature as determined by several types of geothermometer. However, Ca contents are poorly correlated with pressure as determined by the Ca-in-olivine barometer of Köhler and Brey (1990). This reflects the strong T -dependence of this barometer: the uncertainty in pressure (calculated by this method) which is produced by the ±50°C uncertainty expected of any geothermometer is ca ± 8 kbar, corresponding to the entire width of the spinel-lherzolite field at 900–1200°C.
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Summary Lodran is a unique meteorite consisting of roughly equal amounts of metal, olivine, and pyroxene with minor amounts of sulphide, chromite, phosphide, chrome-diopside, and a new phase with a composition close to (K,Na)AlSi 5 O 12 . Zähringer reported planetary-type rare gases in both the metal and silicates, suggesting a primitive nature. The pyroxene composition is Fs 13.8 with little variation. Olivine composition averages Fa 12.6 , but varies at least ±20 % both among grains and zoned within single grains; only the Fe-rich olivine is in equilibrium with the pyroxene. The metal probably cooled rapidly (700 K/Myr) at high temperatures and slower (30 K/Myr) at lower temperatures. Two compositional populations of chromite are found. A model for the formation of Lodran includes three steps: Formation of large olivine, pyroxene, and metal grains, with the trapping of small olivine inclusions in pyroxene and pyroxene in olivine. Equilibration and recrystallization of olivine, pyroxene, and metal, loss of alkalis and Ca; this probably occurred in a parent-body setting. And incorporation of reducing materials and mild reheating sufficient to produce the zoning in the olivine but not enough to re-equilibrate the pyroxene. Phase compositions and rare-gas concentrations in ureilites are similar to those in Lodran. In some respects Lodran appears to be a metal-rich ureilite, but the higher Fe/(Fe+Mg) ratios in the latter (Fa 21 olivine) suggest origin on separate parent bodies. The Harvard University meteorite is a mesosiderite and not closely related to Lodran.
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