| Summary: | 碩士 === 國立成功大學 === 地球科學系碩博士班 === 93 === Compositional variations of clinopyroxene megacrysts occurred in continental alkali basalts provide constraints on magma evolutions in deep interior of the earth. In this study, clinopyroxene megacrysts sampled from Penghu islands in Taiwan Strait were analyzed for major and trace element concentration as well as Sr and Nd isotopic ratios to investigate the evolution history of the basaltic magma forming Penghu islands. The Mg and Si contents of the analyzed megacrysts decrease with increasing Ca, Fe, Al, Ti and Na contents, consistent with the results modeled from pMELTS, indicating that these clinopyroxene megacrysts were formed by isobaric crystallization. Samples from Wang-an island contain high Al and Nd concentrations representing products of late-stage crystallization. Based on the thermobarometer of Putirka et al. (1996), alkali basalts will not crystallize clinopyroxene until 9 kbar, consistent with the depth of Moho in the sampling locality.
The Sr and Nd isotopic compositions of the clinopyroxene megacrysts and their host lavas indicate derivation from a common source. However, the REE partitioning coefficients and Fe-Mg exchange coefficient further indicate that megacrysts and their host lavas were not in chemical equilibrium. The low 143Nd/144Nd isotopic ratios accompanied by enrichments in LILE, Nb and Ta occurring in the rims of the analyzed clinopyroxene megacrysts and the cores of the analyzed plagioclase megacrysts might reflect the results of small degrees of partial melting from recycled oceanic materials.
The crystal grow rate of clinopyroxene megacrysts, calculated on the basis of heat balance, pMELTS and correlation between mineral volumes and crystal temperature, is in the order of 10-8 cm/sec. The time scale of megacryst grow is about 100~1 years, comparable to that required for high degree crystal fractionation and hydraulic fracture reactivation (~102 years), implying crustal stress might control the extent of crystal fractionation. The results from this study confirm that the geochemical features of clinopyroxene megacrysts and phenocrysts can reveal the evolution history of intraplate basalts.
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