Chemical and Isotopic Studies of Ultramafic Inclusions from the San Carlos Volcanic Field, Arizona: A Bearing on their Petrogenesis |
| |
| Authors: | GALER S J G; O'NIONS R K |
| |
| Institution: | 1Department of Earth Sciences, University of Cambridge, Downing Street Cambridge CB23EQ
2Scripps Institution of Oceanography, University of California at San Diego La Jolla, CA 92093-0212 |
| |
| Abstract: | Compositions of the principal minerals and Pb, Nd, and Sr isotopeanalyses of clinopyroxene (cpx) separates are reported for TypeI spinel peridotite xenoliths from the Peridot Mesa vent ofthe San Carlos Volcanic Field. The principal phases are in chemicalequilibrium within each inclusion. Systematic changes in mineralcomposition accompany lithological changes from fertile lherzolitesto infertile harzburgites. These changes are consistent witha fusion residue origin for the major element component of thexenoliths, as noted previously by Frey & Prinz (1978). ExcessFe is additionally present in some inclusions. Pyroxene equilibrationtemperatures calculated using the Wells (1977) geothermometerfall in the narrow range of 1022?34?C (1 s.d.). Equilibrationpressures poorly limit corresponding depths to anywhere between30 and 65 km within the lithospheric mantle. The geotherm is‘advective’ and elevated by  500?C at the depth ofsampling over a reference conductive shield geotherm. The highheat flow measured at the surface results from a combinationof extension and magmatism, with the temperature perturbationextending into the lithospheric mantle. 143Nd/144Nd ratios (0?51251–0?51367) and 87Sr/86Sr ratios(0?70190–0?70504) in cpx demonstrate gross isotopic heterogeneitybeneath the Peridot Mesa vent. This largely overlaps the oceanicmantle array, although four inclusions have  Nd greater thanmid-ocean ridge basalts (MORB). PM-228J with Nd = +20 is themost extreme yet reported for a spinel Iherzolite. Pb abundancesin cpx (generally <0?03ppm) are far lower than previouslyreported values. 206Pb/204Pb ratios (17?5–19?1) overlapoceanic basalts and do not correlate with 87Sr/86Sr ratio. However,some of the inclusions exhibit MORB-like 206Pb/204Pb ratiosbut much higher 87Sr/86Sr ratios, which suggests a possiblegenetic link of detached lithospheric mantle with certain oceanicislands. Metasomatic trace element enrichment processes are most widespreadin the infertile (Al-poor, Cr-rich) inclusions, as noted byFrey & Prinz (1978). This systematic relationship is a localfeature of the mantle and suggests that some degree of meltingoccurs commensurately with incompatible element addition. Inparticular, anhydrous peridotite above its volatile-presentsolidus that was flushed with C-O-H fluids containing incompatibleelements would melt and form an enriched infertile fusion residue.The ascending magmas responsible for forming Type II peridotiteveins are the most probable source of the volatiles and mayin 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 suffereda multi-stage history of enrichment, depletion and melting atvarious times since it became attached to the crust above. |
| |
| Keywords: | |
| 本文献已被 Oxford 等数据库收录！ |
|
