Discriminating assimilants and decoupling deep- vs. shallow-level crystal records at Mount Adams using <sup>238</sup>U-<sup>230</sup>Th disequilibria and Os isotopes

Jicha, B. R.; Johnson, C. M.; Hildreth, W.; Beard, B. L.; Hart, G. L.; Shirey, S. B.; Singer, B. S.

Abstract:

A suite of 23 basaltic to dacitic lavas erupted over the last 350??kyr from the Mount Adams volcanic field has been analyzed for U-Th isotope compositions to evaluate the roles of mantle versus crustal components during magma genesis. All of the lavas have (²³⁰Th/²³⁸U) > 1 and span a large range in (²³⁰Th/²³²Th) ratios, and most basalts have higher (²³⁰Th/²³²Th) ratios than andesites and dacites. Several of the lavas contain antecrysts (crystals of pre-existing material), yet internal U-Th mineral isochrons from six of seven lavas are indistinguishable from their eruption ages. This indicates a relatively brief period of time between crystal growth and eruption for most of the phenocrysts (olivine, clinopyroxene, plagioclase, magnetite) prior to eruption. One isochron gave a crystallization age that is ~ 20-25??ka older than its corresponding eruptive age, and is interpreted to reflect mixing of older and juvenile crystals or a protracted period of magma storage in the crust. Much of the eruptive volume since 350??ka consists of lavas that have small to moderate ²³⁰Th excesses (2-16%), which are likely inherited from melting of a garnet-bearing intraplate ("OIB-like") mantle source. Following melt generation and subsequent migration through the upper mantle, most Mt. Adams magmas interacted with young, mafic lower crust, as indicated by ¹⁸⁷Os/¹⁸⁸Os ratios that are substantially more radiogenic than the mantle or those expected via mixing of subducted material and the mantle wedge. Moreover, Os-Th isotope variations suggest that unusually large ²³⁰Th excesses (25-48%) and high ¹⁸⁷Os/¹⁸⁸Os ratios in some peripheral lavas reflect assimilation of small degree partial melts of pre-Quaternary basement that had residual garnet or Al-rich clinopyroxene. Despite the isotopic evidence for lower crustal assimilation, these processes are not generally recorded in the erupted phenocrysts, indicating that the crystal record of the deep-level ‘cryptic‘ processes has been decoupled from shallow-level crystallization. ?? 2008 Elsevier B.V.

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Article

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70035828

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Jicha, B. R.; Johnson, C. M.; Hildreth, W.; Beard, B. L.; Hart, G. L.; Shirey, S. B.; Singer, B. S.

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1-2

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English

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Earth and Planetary Science Letters

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Discriminating assimilants and decoupling deep- vs. shallow-level crystal records at Mount Adams using ²³⁸U-²³⁰Th disequilibria and Os isotopes; 2009; Article; Journal; Earth and Planetary Science Letters; Jicha, B. R.; Johnson, C. M.; Hildreth, W.; Beard, B. L.; Hart, G. L.; Shirey, S. B.; Singer, B. S.