The evolution of spinel lherzolite xenoliths and the nature of the mantle at Kilbourne Hole, New Mexico

摘要

In peridotites, olivine, clinopyroxene, and orthopyroxene are complex solid solutions with wide stability fields. Depending mostly on bulk composition and pressure, these minerals may be accompanied by plagio-clase (low pressure), spinel (moderate pressure), or garnet (high pressure), resulting in 4-phase and rarer 5-phase assemblages. Although a particular mineral assemblage is stable over a range of P-T, the compositions of the individual minerals vary with changing P-T conditions. Application of standard geothermobarometers to olivine-clinopyroxene-orthopyroxene-spinel peridotites is problematic. An alternative approach is to use a bulk rock composition to calculate equilibrium phase diagrams to determine the conditions under which a particular assemblage is stable. This requires consideration of the 7-com-ponent system SiO_2--Al_2O_3-Cr_2O_3-FeO-MgO-CaO-Na_2O, internally consistent thermodynamic data for end members, and reliable mixing models for all mineral solutions. Experimental studies in simpler systems, and solution models from the literature, permit derivation of multicomponent thermodynamic mixing models for the key minerals. The models, when applied to xenoliths from Kilbourne Hole, constrain P and T of equilibration and areless sensitive to mineral compositional variations, or uncertainty in activity models, than standard thermoba-rometry. Our modeling provides the first tightly constrained pressure estimates for Kilbourne Hole, placing the xenoliths in the spinel stability field at depths (30-45 km) that correspond to the uppermost mantle beneath the Rio Grande Rift. The fine-grained equigranular lherzolite, porphyroclastic lherzolite, and some harzburgite-dunite specimens equilibrated at average conditions of 11.5 Kbar-930°C, 12 Kbar-990°C, and 13 Kbar-l,080°C, respectively. The mantle beneath the Rio Grande Rift is layered; the fine-grained equigranular lherzolite derives from relatively shallow depth (35 km average), and the porphyroclastic lherzolite from slightly deeper levels. Lying 5-10 km beneath both lherzolites, the harzburgite-dunite represents a depth where melt extraction has significantly altered mantle chemistry and where local thermodynamic equilibrium has not been maintained.