Thermobarometric evolution and metasomatic processes of upper mantle in different tectonic settings: evidence from spinel peridotite xenoliths
Siena, Franca; Coltortl, Massimo
European Journal of Mineralogy Volume 5 Number 6 (1993), p. 1073 - 1090
published: Dec 1, 1993
manuscript accepted: Jul 5, 1993
manuscript received: Dec 29, 1992
ArtNo. ESP147050506021, Price: 29.00 €
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. Melboune (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 Iherzolites to harzburgites (and minor dunites), showing a decreasing abundance of clinopyroxene and orthopyroxene, which is correlated with the Cr/Cr + AI ratio in coexisting spinels. Accordingly, the incompatible element abundances of unmetasomatized samples vary from 1 -3 times chondritic in Iherzolites to about 0. 1 times chondritic in the most depleted harzburgites. Reractoy lithologies characterize suboceanic mantle. Thermobarometric estimates based on phase equilibria and CO2 inclusions indicate different pressure - temperature equilibration histories for xenoliths from different tectonic settings: I) in both continental (Sardinia) and oceanic (Cny 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-nothem Italy and Mt. Melboune-Antarctica), the temperature bimodality recorded in single xenolith samples, suggests patial 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, LEE, and b (up to 20-30 times chondritic). The metasomatic agents - of deep lithospheric/asthenospheric origin - can be envisaged as strongly alkaline H20-C02- rich silicate melts, on the basis of their inherent physical ability to iniltrate (dihedral angle « 60°) and the composition of stabilization products, such as amphibole and pyrometamophic glasses. The latter vary from strongly silica-undersaturated to saturated or oversaturated in relation to the extent of incongruent melting of othopyroxene. 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 Iherzolitic rocks. fO2 values recorded in samples relatively unaffected by metasomatism indicate a pristine redox state around QFM for xenoliths from continental within-plate settings (Sardinia), and below QFM for both oceanic (Canary and Cape Verde Islands: QFM 0 I -2) and rift (Mt. Lessini and Antarctica: QFM +0.5 I -2) settings. Metasomatic processes generally cause oxidation (up to QFM +3) of the upper mantle, due to pervasive infiltration of volatilerich alkali silicate melts.