Chronothermometry of peridotitic and pyroxenitic xenoliths: Implications for the thermal evolution of the Arabian lithosphere

Abstract

The thermal history of the subcontinental lithosphere beneath Israel has been evaluated by surface heat flow and the chemical and isotopic compositions of mineral phases in xenoliths of spinel lherzolite, garnet pyroxenite and garnet granulite, brought to the surface by Cretaceous and Neogene-Quaternary alkali basalts. The spinel lherzolites show large variations in chemical and isotopic compositions: Al Si = 0.030 to 0.095 , 147Sm 144Nd = 0.11 to 0.24 , 143Nd 144Nd = 0.51289 to 0.51316 and 87Sr 86Sr = 0.70270 to 0.70321 . The isotopically more depleted peridotites lie within the N-MORB field while those with more enriched isotopic compositions are similar to Phanerozoic alkali basalts from the studied area. Xenoliths of garnet pyroxenite from Kerem Maharal volcano (Mt. Carmel) have 143Nd 144Nd of 0.51290 to 0.51292, and 87Sr 86Sr of 0.70296 to 0.70313, within the range of Cretaceous alkali basalts from Israel. These pyroxenites were probably segregated from an alkali basalt magma. The garnet pyroxenite BS-701 from Bat Shlomo volcano (Mt. Carmel) has 143Nd 144Nd of 0.51360, and 87Sr 86Sr of 0.70264, values that are similar to those of some highly depleted peridotites from Arabia. It is postulated that the uppermost mantle beneath the Arabian plate underwent a major melting event during the formation of the Arabian continent in the late Proterozoic. This melting was responsible for the variations in the major and trace element compositions of the peridotites. Pyroxenites, such as BS-701, were probably segregated from magmas extracted from the peridotites. The enrichment of some peridotites in LREE could have occurred during the late Proterozoic magmatic events and/or later during the episodes of Phanerozoic basaltic magmatism. The mineral phases in the xenoliths show homogeneous major element distribution and Neodymium isotopic equilibrium at (or close to) eruption time. Equilibrium temperatures of the peridotites and the pyroxenites range from 900–1100°C, and 990–1190°C, respectively. These temperatures are elevated 300–400°C above the conductive steady state geotherm that characterized the Arabian lithosphere during most of the Phanerozoic. We suggest that the lithosphere beneath Israel was affected by transient heating events which increased the temperatures and reset the chemical and isotopic systems in the xenoliths. The surface low heat flow and the inter-mineral Neodymium isotopic equilibrium in the xenoliths imply that the heating of the lithospheric rocks occurred shortly before (~10–15 Ma) the eruption of the host basalts. Elevated equilibrium temperatures are recorded by other lithospheric xenoliths from the Arabian plate. The thermal perturbations are probably caused by thinning of the Arabian lithosphere in times of extension.