GEOTHERMAL FIELD, AND HENGILL VOLCANO, ICELAND: IMPLICATIONS FOR THE IDENTIFICATION AND INTERPRETATION OF HYDROTHERMAL DEPOSITS ON MARS. S. R. Black1,2, B. M. Hynek1,2, L. J. McHenry3, C. Glenister3, B. I. Cameron3, T. M. McCollom1, J. Ludyan3, 1 Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, 1234 Innovation Drive, Boulder, CO 80303; 2 Dept. of Geological Sciences, University of Colorado Boulder, Campus Box 600 UCB, Boulder, CO 80303; 3 Dept. of Geosciences, University of Wisconsin-Milwaukee

In the Central American arc, southeastern Guatemala hosts the most diverse volcanism. Large stratovolcanoes at the volcanic front (VF) form as a result of subduction of the oceanic Cocos plate beneath the continental Caribbean plate. Behind the volcanic front (BVF) volcanism, however, has undergone a fundamental change in eruptive style during the Quaternary from older, polygenetic central volcanism to younger, monogenetic cinder cone volcanism. Magmas that traverse the 40-45-km-thick crust in southeastern Guatemala are highly susceptible to crustal contamination. Consequently, mineral chemical data, whole-rock oxygen isotope, and light element geochemistry are used to investigate the relationship between edifice type and the magnitude of crustal contamination.The lack of systematic variation between compositions of phenocryst phases and host rocks strongly suggests that open system processes were operating. Moreover, phenocryst core compositions are generally out of equilibrium with host rock compositions. Olivine from BVF cinder cones deviate only slightly from the equilibrium line in comparison to the older behind the volcanic front (OBVF) central volcanoes and VF stratovolcanoes, suggesting less assimilation of crustal lithologies. Steep arrays on the delta(18)O-SiO(2) diagram cannot be explained by crystal fractionation and favor the incorporation of (18)O-enriched crustal rocks. Higher delta(18)O values in the OBVF central volcanoes and VF stratovolcanoes support the idea that larger, shallow magma bodies experienced greater amounts of crustal contamination. Regional extension in the Ipala Graben of southeastern Guatemala likely promoted short residence times in crustal reservoirs and small degrees of crustal assimilation for the BVF cinder cone magmas.

The major and trace element geochemistry of lavas erupted from four volcanic front (VF) stratovolcanoes in southeastern Guatemala show differences in the relative importance of flux and decompression melting in a continental are setting. The VF stratovolcanoes exhibit a wide compositional range from basalt to dacite, although modern Pacaya erupts basaltic lavas. The VF basalts have relatively low MgO contents and plot outside the field of primary arc magmas defined by melting experiments on hydrous peridotite. After subtracting the effects of the fractionation, assimilation, and alteration of some VF lavas, separate partial melting and mixing trends were identified for Agua-Pacaya and Tecuamburro-Moyuta.The distinct chemical signatures of the hemipelagic and carbonate sediments subducted off Guatemala provide constraints on material transfer processes that occurred between the slab and mantle wedge. Model fluids and melts from the subducted slab were calculated using recently published mineral-aqueous fluid partition coefficients. Wide separation of the model fluid and melt compositions on a U/La versus Ba/Th diagram creates diagnostic mixing curves with an enriched mid-ocean ridge basalt source. Fluid from mature ocean crust has high U/La, fluid from carbonate sediment has high Ba/Th, and fluid and melt from hemipelagic sediments have both high U/La and Ba/Th. In a simple single-stage model, a mantle metasomatized by fluid originating largely from the oceanic crust with only minor sediment fluid contributions best explains the overall large ion lithophile element composition of the VF lavas. (Th/Rb)(N) ratios of similar to 1 in the VF lavas from southeastern Guatemala require a component of sediment melting. Therefore, a more realistic two-stage model to describe the Guatemalan arc data involves an initial hemipelagic sediment melt input to the wedge followed by minor fluid additions from the oceanic crust or sediments. Correlation between measures of slab input and extent of melting in the older VF lavas from Tecuamburro and Moyuta favors flux-dominated melting near the base of the mantle wedge. In sharp contrast, the lack of a relationship between slab additions and melting in younger lavas from Agua and Pacaya volcanoes implies a significant role for decompression melting closer to the top of the wedge. In this melting scenario, the rate of crustal extension determines the extent of melting. (C) 2002 Elsevier Science B.V. All rights reserved.