Copyright (c) 2013 University of Iowa All rights reserved. http://ir.uiowa.edu/geology_pubs Recent documents in Department of Geoscience Publications en-us Tue, 21 May 2013 15:05:33 PDT 3600 http://ir.uiowa.edu/geology_pubs/3 http://ir.uiowa.edu/geology_pubs/3 Mon, 20 Dec 2010 11:31:06 PST [I] Oxygen-isotope ratios of olivine and plagioclase phenocrysts in basalts and basaltic andesites from the Central American arc vary systematically with location, from a minimum 81S0olivinc value of 4.6 (below the range typical of terrestrial basalts) in Nicaragua near the center of the arc to a maximum 81S0olivinc value of 5.7 (above the typical range) in Guatemala near the northwest end of the arc. These oxygen-isotope variations correlate with major and trace element abundances and with Sr and Nd isotope compositions of host lavas, defining trends that suggest variations in 81S0 reflect slab contributions to the mantle sources of these lavas. These trends can be explained by a model in which both a low-81S0, water-rich component and a high-81S0, water-poor component are extracted from the subducting Cocos slab and flux melting in the overlying mantle wedge. The first of these components dominates slab fluxes beneath the center of the arc and is the principal control on the extent of melting of the mantle wedge (which is highest in the center of the arc); the second component dominates slab fluxes beneath the northwestern margin of the arc. Fluxes of both components are small or negligible beneath the southeastern margin of the arc. We suggest that the low-81S0 component is a solute-rich aqueous fluid produced by dehydration of hydrothermally altered rocks deep within the Cocos slab \Eerhaps serpentinites produced in deep normal faults offshore of Nicaragua) and that the high-81 0 component is a partial melt of subducted sediment on top of the plate.

]]> M K. Reagan et al. http://ir.uiowa.edu/geology_pubs/2 http://ir.uiowa.edu/geology_pubs/2 Thu, 16 Dec 2010 09:27:01 PST [I] We report 23RU_230Th_226Ra_21Ilpb_2IOpo, 232Th_32SRa and 235UJl1Pa measurements for a suite of 14 geologically and geochemically well-characterized basaltic sanlples from the Samoan volcanoes Vailulu'u, Malumalu, and Savai'i. Maximum eruption ages based on the presence of parent-daughter disequilibria indicate that Vailulu 'u is magmatically productive with young lavas «8 Ka) resurfacing both its sunmlit crater and lower flanks. 311lpb and 21 'To measurements indicate that several flows have erupted within its sunmrit crater in the past 100 years, with the newest observed flow being erupted in November of 2004. For lavas which have eruption ages that are demonstrably young, relative to the half-lives of 23( lTh, 231Pa and 226Ra we interpret their 23SU _DoTh n'U_23IPa and 230Th - 22(iRa disequilibria in ternlS of the magn~atic proce~ses occurring beneath the Samoan Islands. c"31IThP"U) > i indicates that g~et is required as a residual phase in the magma sources for all these lavas. The large range of c'''Ul''l"Th) and e"'Thj232Th) is attributed to long-ternl source variation. The Samoan basalts are all a1lcaline basalts and show significant 23°Th and Cl'Pa excesses but limited variabilil);', indicating that they have been derived by small but similar extents of melting. Their ("'ThP"U), C"PaF"U) and SmlNd fractionation are consistent with correlations anl0ng other ocean island basalt suites (particularly Hawaii) which show that eOThP3RU) and c" 31 PaP35U) of many OIBS can be explained by sinlple tinle-independent models. Interpretation of the""'Ra data requires tinle-dependent melting models. Both chromatographic porous flow and dynanlic melting of a garnet peridotite source can adequately explain the combined U-Th-Ra and U-Pa data for these Sanl0an basalts. Several young sanlples from the Vailulu'u sunmlit crater also exhibit significant 2 IOPb deficits that r~flect eith~r shallow magmati<; .Erocesses. or continuous magma degassing. In both cases, decadal residence tmles are mferred from these -, "Pb deficits. The young coeval volcamsm on Malun13lu and Vailulu'u suggests the SanlOa hot spot is currently migrating to the northeast due to dynanlic interaction with the Tonga slab.

]]> M K. Reagan et al. http://ir.uiowa.edu/geology_pubs/1 http://ir.uiowa.edu/geology_pubs/1 Mon, 22 Nov 2010 12:59:12 PST Dome lavas from the 2004 eruption of Mount St. Helens show elevated Li contents in plagioclase phenocrysts at the onset of dome growth in October 2004. These cannot be explained by variations in plagioclase-melt partitioning, but require elevated Li contents in coexisting melt, a fact confi rmed by measurements of Li contents as high as 207 μg/g in coexisting melt inclusions. Similar Li enrichment has been observed in material erupted prior to and during the climactic May 1980 eruption, and is likewise best explained via pre-eruptive transfer of an exsolved alkali-rich vapor phase derived from deeper within the magma transport system. Unlike 1980, however, high Li samples from 2004 show no evidence of excess (210Pb)/(226Ra), implying that measurable Li enrichments may occur despite signifi cant differences in the timing and/or extent of magmatic degassing. Diffusion modeling shows that Li enrichment occurred within ~1 yr before eruption, and that magma remained Li enriched until immediately before eruption and cooling. This short fl ux time and the very high Li contents in ash produced by phreatomagmatic activity prior to the onset of dome extrusion suggest that vapor transfer and accumulation were associated with initiation of the current eruption. Overall, observation of a high Li signature in both 1980 and 2004 dacites indicates that Li enrichment may be a relatively common phenomenon, and may prove useful for petrologic monitoring of Mount St. Helens and other silicic volcanoes. Lithium diffusion is also suffi ciently rapid to constrain vapor transfer on similar time scales to short-lived radionuclides.

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