Interaction of continental lithosphere and asthenospheric melts below the ' 45 ' at Penn State U niversity


Spinel lherzolite and pyroxenite inclusions from the Geronimo volcanic field, Arizona (and Dish Hill, California) record, in their constituent minerals, a chronology of diverse mantle depletion and enrichment events. Certain portions of the lithospheric mantle have remained relatively isolated for considerable periods of time(l-4 b.y.) while wall rock adjacent to conduits of basanite has been recently (< 02 b.y.) modified. Evidence exists for a widespread ancient (1-4 b.y.) partial melting residue, now recognizable as MORB-like mantle below the southwestern U.S.A. Trace element enrichment (03-09 b.y.) has increased the light rare earth element (LREE) and Sr content of many refractory peridotites without any mineralogical changes to the host rock. The fluids/melt responsible for this enrichment have a complex history involving heterogeneous mantle sources. In contrast, modal metasomatism of the mantle (< 02 b.y.) in aureoles around evolved derivatives of basanite has petrographically and chemically transformed this ancient partial melting residue. The metasomatic fluids responsible for such metasomatism have an asthenospheric mantle source identical to the host magma. It is proposed that modal metasomatism occurs in contact metamorphic aureoles that surround apophyses of basanitic silicate melt in the lithospheric mantle. The gradient in CO;2/(CO2 + H2O) ratio that must surround such veins in the upper mantle (< 20 kb) may encourage the development of enrichment fronts. Immediately adjacent to the vein, a wet zone with a relatively low CO2/(COj + H2O) ratio would allow a precipitation of mica ± amphibole. Beyond this a dry zone with a higher CO2/(CO2 + H2O) ratio would hasten chemical but not petrographic transformation of the wall rock. 1

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