期刊
EARTH AND PLANETARY SCIENCE LETTERS
卷 277, 期 1-2, 页码 38-49出版社
ELSEVIER
DOI: 10.1016/j.epsl.2008.09.035
关键词
Mount Adams; Cascade arc; U-Th isotopes; Os isotopes; assimilation
资金
- NSF [EAR-9980512, EAR-0309853, -0337667, -0738007]
- Department of Geology and Geophysics
- University of Wisconsin-Madison
- Directorate For Geosciences
- Division Of Earth Sciences [0337667] Funding Source: National Science Foundation
- Division Of Earth Sciences
- Directorate For Geosciences [0738007] Funding Source: National Science Foundation
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 (Th-230/U-238) > 1 and span a large range in (Th-230/Th-232) ratios, and most basalts have higher (Th-230/Th-232) 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 similar to 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 Th-230 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 Os-187/Os-188 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 Th-230 excesses (25-48%) and high Os-187/(OS)-O-188 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. (c) 2008 Elsevier B.V. All rights reserved.
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