Significance of basic and ultramafic rock inclusions in the basalts of Canary Islands
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Ultramafic rock
Peridotite
Xenolith
Fractional crystallization (geology)
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The properties of garnets in peridotite xenoliths from kimberlites in China are compared with garnets occurring as megacrysts (disaggregated peridotites) in alkali basalts. The pyropes in xenoliths from kimberlites in the Shandong and Liaoning provinces are distinctly purple, whereas the megacryst pyropes in basalts are considerably darker. Kelyphitic reaction rims are common on pyropes in peridotite xenoliths from kimberlites, but are rare on pyropes in basalts. The classification of pyropes according to the scheme of Dawson and Stephens (1975) demonstrates that pyropes in peridotite xenoliths from kimberlites are mainly Group-9 garnets, whereas those in basalts typically belong to Group 3 (calcic pyrope-almandine) and Group 1 (titanian pyrope). In China, pyropes in peridotite xenoliths of kimberlites, as compared with those in basalts, are richer in Cr2O3, Cr/(Cr + Al), knorringite, uvarovite, and Cr-component, but lower in Fe2O3 + FeO, Fe/(Fe + Mg), grossularite, and almandine molecules. Unlike the situation in basalts, there is a distinct negative correlation between Cr2O3 and Al2O3 contents of the pyropes in peridotite xenoliths from kimberlites. The formation pressures of pyropes in the peridotite xenoliths mainly plot within the diamond stability field (i.e., more than 4GPa). These garnets may be useful as an indicator mineral for diamond exploration.
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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.
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The 27 mantle xenoliths have been collected in eastern China.The content of CaO,Au and PGE of 21 peridotite xenoliths were determined.The relationship of gold with CaO for the 27-peridotite xenoliths suggests that the gold content of the xenoliths relates to partial melting degree of mantle.The peridotite xenoliths that their chondrite-normalized PGE patterns are smooth have higher golden contents.The peridotite xenoliths that their chondrite-normalized normalized PGE patterns are decline have lower gold content.
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Brucite
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