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Small-Scale Disequilibrium in a Magmatic Inclusion and its More Silicic Host Public Deposited

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  • Basaltic andesite inclusions and their host dacite from the Purico-Chascon complex in northern Chile are isotopically distinct. Textural characteristics of the inclusions are typical of those resulting from magma mingling. Serial sectioning across the interface of an inclusion and its host dacite, complemented by microdrill sampling and detailed microprobe work, has enabled us to examine the scale of mixing and chemical (isotopic and trace element) disequilibrium. The results of this work show that (1) the composition of the inclusion is relatively homogeneous; (2) the dacite host is generally higher in ⁸⁷Sr/⁸⁶Sr and lower in ¹⁴³Nd/¹⁴⁴Nd than the enclave, but it is heterogeneous on a small scale, and probably a “hybrid”; (3) the isotopic composition in the marginal zone, apparently on both host and inclusion sides of the weakly chilled interface, actually shows the highest ⁸⁷Sr/⁸⁶Sr and lowest ¹⁴³Nd/¹⁴⁴Nd; (4) large plagioclase crystals in the inclusions and host are xenocrystic, with higher ⁸⁷Sr/⁸⁶Sr than any of the other samples. These observations are reconciled with a model of magma evolution in a crustal magma chamber. In such a scenario the mafic magma is overlain by a cap of rhyolite - a partial melt of the dacitic ignimbrites which now underlie the Purico-Chascon complex. Overturn of such a magma system gives rise to a hybrid dacite containing discrete mafic inclusions.
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  • Davidson, J. P., S. L. de Silva, P. Holden, and A. N. Halliday (1990), Small-Scale Disequilibrium in a Magmatic Inclusion and its More Silicic Host, Journal of Geophysical Research, 95(B11), 17,661–17,675, doi:10.1029/JB095iB11p17661.
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  • 95
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  • B11
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  • The Turner Foundation of the University of Michigan (J.P.D) and the Royal Society of London (S.d.S) supported the field work. Isotopic analysis at the University of Michigan was supported by NSF grant EAR 8720564. S.d.S is a visiting postdoctoral fellow at the Lunar and Planetary Institute, Houston, supported by NASA grant NAGW-1167, and would like to thank Vincent Yang at the Johnson Space Center for his help and advice with microprobe work. This is Lunar and Planetary Institute contribution number 733, and a contribution to IGCP project 249 "Andean Magmatism and its Tectonic Setting".
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