Chemical and Isotopic Studies of Ultramafic Inclusions from the San Carlos Volcanic Field, Arizona: A Bearing on their Petrogenesis

Abstract

Compositions of the principal minerals and Pb, Nd, and Sr isotope analyses of clinopyroxene (cpx) separates are reported for Type I spinel peridotite xenoliths from the Peridot Mesa vent of the San Carlos Volcanic Field. The principal phases are in chemical equilibrium within each inclusion. Systematic changes in mineral composition accompany lithological changes from fertile lherzolites to infertile harzburgites. These changes are consistent with a fusion residue origin for the major element component of the xenoliths, as noted previously by Frey & Prinz (1978). Excess Fe is additionally present in some inclusions. Pyroxene equilibration temperatures calculated using the Wells (1977) geothermometer fall in the narrow range of 1022±34°C (1 s.d.). Equilibration pressures poorly limit corresponding depths to anywhere between 30 and 65 km within the lithospheric mantle. The geotherm is ‘advective’ and elevated by ∼500°C at the depth of sampling over a reference conductive shield geotherm. The high heat flow measured at the surface results from a combination of extension and magmatism, with the temperature perturbation extending into the lithospheric mantle. 143Nd/144Nd ratios (0·51251–0·51367) and 87Sr/86Sr ratios (0·70190–0·70504) in cpx demonstrate gross isotopic heterogeneity beneath the Peridot Mesa vent. This largely overlaps the oceanic mantle array, although four inclusions have εNd greater than mid-ocean ridge basalts (MORB). PM-228J with εNd = +20 is the most extreme yet reported for a spinel Iherzolite. Pb abundances in cpx (generally <0·03ppm) are far lower than previously reported values. 206Pb/204Pb ratios (17·5–19·1) overlap oceanic basalts and do not correlate with 87Sr/86Sr ratio. However, some of the inclusions exhibit MORB-like 206Pb/204Pb ratios but much higher 87Sr/86Sr ratios, which suggests a possible genetic link of detached lithospheric mantle with certain oceanic islands. Metasomatic trace element enrichment processes are most widespread in the infertile (Al-poor, Cr-rich) inclusions, as noted by Frey & Prinz (1978). This systematic relationship is a local feature of the mantle and suggests that some degree of melting occurs commensurately with incompatible element addition. In particular, anhydrous peridotite above its volatile-present solidus that was flushed with C-O-H fluids containing incompatible elements would melt and form an enriched infertile fusion residue. The ascending magmas responsible for forming Type II peridotite veins are the most probable source of the volatiles and may in some cases react to produce chemical gradients in the wall-rock. Prior metasomatism is also evident isotopically in some inclusions. Overall, the lithospheric mantle beneath Peridot Mesa has suffered a multi-stage history of enrichment, depletion and melting at various times since it became attached to the crust above.