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

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. Thermobarometric estimates based on phase equilibria and CO₂ inclusions indicate different pressure - temperature equilibration histories for xenoliths from different tectonic settings. In spinel peridotite xenoliths from all tectonic settings, modal metasomatic processes are suggested by widespread pyrometamorphic textures in some cases associated with new phases such as amphibole or jadeitic Cr-endiopside. Chemical effects of metasomatism include variable enrichment of incompatible elements. The metasomatic agents can be envisaged as strongly alkaline H₂O-CO₂-rich silicate melts. In the case of Cape Verde, the widespread development of jadeitic Cr-endiopside is better accounted for by assuming a carbonatite melt as metasomatizing agent which progressively converted harzburgitic rocks to lherzolitic rocks. fO₂ values recorded in samples relatively unaffected by metasomatism indicate a pristine redox state around QFM for xenoliths from continental within-plate settings and below QFM for both oceanic and rift settings. -from Authors