Ancient lithospheric lherzolite xenolith in alkali basalt from Baja California

Abstract

K, Rb, Sr contents and Sr isotopic composition are reported for (1) the coexisting silicate minerals of a spinel lherzolite xenolith, (2) the whole rock xenolith, and (3) the host alkali basalt from the Pleistocene-Recent San Quintin volcanic field in Baja California. The data also include major element chemistry of the four mineral phases of the xenolith. The olivine-spinel and pyroxene thermometers indicate that the last temperature of equilibration of the xenolith in the upper mantle was about 1100°C. K, Rb, and Sr abundances are extremely low in the minerals of the xenolith, in contrast with the general enrichment of Ca, Al, Fe and Na in the whole xenolith. Furthermore, the abundance levels and the 87Sr/ 86Sr ratios of the minerals are greatly lowered by surface washing of the minerals in 2N cold HCl for three minutes. It is suggested that this is due to grain boundary and surface contamination of the minerals which took place in the upper mantle by a vapor phase deposition, prior to the inclusion of the xenolith in the basalt. The source of the vapor phase must have a 87Sr/ 86Sr ratio greater than 0.7070, the highest measured ratio in the unwashed orthopyroxene. Sr in the host alkali basalt has a 87Sr/ 86Sr ratio of 0.7031, unrelated to the grain boundary material. The acid-washed minerals, the unwashed minerals and the acid-washed whole rock xenolith show a scatter on a 87Rb/ 86Sr versus 87Sr/ 86Sr diagram. However, the surface-washed minerals and the whole rock alone define a straight-line relationship with a positive slope, which corresponds to an age 3.4 ± 0.3AE(2σ) for the rock and an initial 87Sr/ 86Sr of 0.70057 ± 0.0004 (2σ). The age of 3.4 AE for the lherzolite is interpreted as its last involvement in a small degree of partial melting and the consequent extreme depletion of the large ion lithophile elements from the constituent minerals.