Petrochemistry of the Mantle beneath Thailand: Evidence from Peridotite Xenoliths

Spinel-lherzolites are most abundant among ultramafic xenoliths in Late Cenozoic alkali basalts, which were erupted on Thailand's two lithospheric fragments, namely the Shan-Thai and Indochina microcontinents. Major-element compositions of the Cr-diopside—group xenoliths (Type I) vary considerably—e.g., 0.92 to 5.80 wt% A1₂O₃, 32.7 to 43.2 wt% MgO—covering a range from fertile to depleted compositions relative to primary mantle models. The extra-fertile mantle is evidenced in subchondritic Ca/Al ratios of whole-rocks and the occurrence of high modal orthopyroxene xenolith (websterite), suggesting metasomatism of mantle rocks. Interaction of the xenoliths with host basalts is observed as the formation of spongy rinds and symplectite on clinopyroxene and orthopyroxene, respectively. This results in a decrease of A1₂O₃ and Na₂O with increasing CaO and MgO in the primary clinopyroxene, and locally the formation of K-Na-rich, feldspathic glass within the spongy rinds. Subsolidus re-equilibration also produced compositional gradients observed in pyroxenes, particularly in A1₂O₃ contents, because of the slower diffusivity of Al compared to that of other cations. Olivines have limited compositional variations (Fo_88.5_91.5), in contrast to spinels (Cr# = 5.3 to 52.9). This large range in the Cr#s of spinels is consistent with variable degrees of partial melting among the xenoliths.

A distinction between the mantle beneath the Shan-Thai and the Indochina microcontinents is observed in the oxidation states of the xenoliths and the Mg#s of coexisting pyroxenes and olivines, suggesting an influx of oxidized fluids or melts through the mantle of Indochina. However, both microcontinents were equilibrated at similar P-T, within a range of temperatures from 1003 to 1132°C, with the core temperatures being ～50°C higher than the rim temperatures, and pressures of ~8 to 20 kbar. The calculated temperatures reported previously (Promprated, 1998) and extended with this work, based on different geothermometers, also reveal a heating event, corresponding to the ascent of hot asthenosphere. This asthenospheric upwelling is the most likely process responsible for the absence of garnet lherzolites, due to thermal erosion at the base of lithosphere, and extensive basaltic volcanism in Thailand.