Thermobarometric evolution and metasomatic processes of upper mantle in different tectonic settings: evidence from spinel peridotite xenoliths

Abstract

Petrological studies of spinel peridotite xenoliths provide information on the nature and physico-chemical evolution of the upper mantle and its variability on a regional scale, in both oceanic and continental environments. The xenolith populations studied in this work ― from Mt. Lessini and Sardinia (Italy), Mt. Melbourne (Antarctica), the Canary Islands and the Cape Verde Islands ― on the whole define a variably depleted sequence which is residual after partial melting; they range from lherzolites to harzburgites (and minor dunites), showing a decreasing abundance of clinopyroxene and orthopyroxene, which is correlated with the Cr/Cr + Al ratio in coexisting spinels. Accordingly, the incompatible element abundances of unmetasomatized samples vary from 1-3 times chondritic in lherzolites to about 0.1 times chondritic in the most depleted harzburgites. Refractory lithologies characterize suboceanic mantle. Thermobarometric estimates based on phase equilibria and CO 2 inclusions indicate different pressure-temperature equilibration histories for xenoliths from different tectonic settings: 1) in both continental (Sardinia) and oceanic (Canary and Cape Verde Islands) stable within-plate settings, complete phase equilibration is generally reached at the P-T conditions of the regional geothermal gradient; 2) in continental rift settings (Mt. Lessini-northern Italy and Mt. Melbourne-Antarctica), the temperature bimodality recorded in single xenolith samples, suggests partial reequilibration during decompressive upwelling of mantle material to shallower levels. In spinel peridotite xenoliths from all tectonic settings, modal metasomatic processes are suggested by widespread pyrometamorphic textures (spongy pyroxenes, glassy patches, and Cr-rich embayed spinels), in some cases associated with new phases such as amphibole or jadeitic Cr-endiopside. Chemical effects of metasomatism include variable enrichment of incompatible elements, such as LFSE, LREE, and Nb (up to 20-30 times chondritic). The metasomatic agents ― of deep lithospheric/asthenospheric origin ― can be envisaged as strongly alkaline H 2 O-CO 2 -rich silicate melts, on the basis of their inherent physical ability to infiltrate (dihedral angle << 60°) and the composition of stabilization products, such as amphibole and pyrometamorphic glasses