科研之友
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摘要
New bulk rock geochemical data on an extensive suite of samples from the Marion and Prince Edward Islands, located in the Southern Ocean at similar to 37 degrees 45%26apos;E, 46 degrees 55%26apos;S, show highly coherent major and trace element variations. Rock types are dominated by alkali basalt, with lesser hawaiite and minor mugearite and benmoreite. Prince Edward Island has in addition rare trachytes exposed on the plateau, and xenoliths of comenditic rhyolite pumice were found in a basalt flow of the 1980 eruption on Marion Island. Lavas from Prince Edward Island differ from those erupted on Marion Island by having subtly distinct incompatible trace element ratios (e.g. Zr/Nb = 9.1 vs 6.7; Ba/Nb = 7.8 vs 6.7; Ce/Pb = 31 vs 36), and lower Sr-87/Sr-86 (0.70302) and higher Nd-143/Nd-144 (0.51300) isotope ratios than found on Marion Island (0.70336-0.70349; 0.51292-0.51295). The two islands are indistinguishable in terms of Pb isotope ratios (Pb-206/Pb-204 = 18.61-18.75, Pb-207/Pb-204 = 15.54-15.56). %26lt;br%26gt;Quantitative modelling suggests that the range in rock types found on each island can be ascribed to extensive low pressure crystal fractionation of phenocryst phases. For example, hawaiite can be related to the more primitive alkali basalts by some 24% fractional crystallisation of olivine, clinopyroxene, plagioclase and minor FeTi-oxides, whereas the more evolved mugearites and benmoreites require, respectively a further 36% and 24% fractional crystallisation to account for their felsic compositions. The trachytes of Prince Edward Island appear to represent the residual 28% magma following extensive crustal fractionation of dominantly clinopyroxene, plagioclase and FeTi-oxide with minor apatite. None of the sampled lavas have compositions characteristic of true primary magmas, but the least evolved, with Mg# similar to 63, appear to have experienced as much as 18% olivine fractionation from a primary magma with similar to 15 wt.% MgO. The rhyolite pumice is distinct in composition from Bouvet rhyolite or South Sandwich Island pumice and may belong to an earlier, submarine eruption at or near Marion Island. %26lt;br%26gt;The enriched incompatible element ratios (Zr/Nb = 6-9; La/Yb-n = 10.6; Y/Nb = 0.73) and radiogenic Sr-87/Sr-86 and unradiogenic Nd-143/Nd-144 isotope ratios show that the lavas erupted on Marion and Prince Edward Islands resulted from melting an enriched mantle source, consistent with derivation from an upwelling deep mantle plume. Pb isotope ratios (Delta 8/4Pb similar to 33) confirm the absence of any DUPAL signature in the source region of these magmas.
- 出版日期2012-4-15
