Origin of calc-alkaline series lavas at Medicine Lake Volcano by fractionation,assimilation and mixing |
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| Authors: | Timothy L Grove David C Gerlach Thomas W Sando |
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| Institution: | (1) Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, 02139 Cambridge, Massachusetts, USA |
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| Abstract: | The results of experimental studies and examination of variations in major elements, trace elements and Sr isotopes indicate
that fractionation, assimilation and magma mixing combined to produce the lavas at Medicine Lake Highland. Some characteristics
of the compositional differences among the members of the calc-alkalic association (basalt-andesite-dacite-rhyolite) can be
produced by fractional crystallization, and a fractionation model reproduces the major element trends. Other variations are
inconsistent with a fractionation origin. Elevated incompatible element abundances (K and Rb) observed in lavas intermediate
between basalt and rhyolite can be produced through assimilation of a crustal component. An accompanying increase in 87Sr/86Sr from ∼ 0.07030 in basalt to ∼0.7040 in rhyolite is also consistent with crustal assimilation. The compatible trace element
contents (Ni and Sr) of intermediate lavas can not be produced by fractional crystallization, and suggest a magma-mixing origin
for some lavas. Unusual phenocryst assemblages and textural criteria in these lavas provide additional evidence for magma
mixing.
A phase diagram constructed from the low pressure melting experiments identifies a distributary reaction point, where olivine+augite
react to pigeonite. Parental basalts reach this point at low pressures and undergo iron-enrichment at constant SiO2 content. The resulting liquid line of descent is characteristic of the tholeiitic trend. Calc-alkalic differentiation trends
circumvent the distributary reaction point by three processes: fractionation at elevated pH2O, assimilation and magma mixing. |
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