Thermal and redox conditions of formation of upper-mantle xenoliths in Cenozoic basalts of northwestern Spitsbergen

Abstract

Upper-mantle xenoliths in Cenozoic basalts of northwestern Spitsbergen are rocks of peridotite (spinel lherzolites) and pyroxenite (amphibole-containing garnet and garnet-free clinopyroxenites, garnet clinopyroxenites, and garnet and garnet-free websterites) series. The upper-mantle section in the depth range 50–100 km is composed of spinel peridotites; at depths of 80–100 km pyroxenites (probably, dikes or sills) appear. The equilibrium conditions of parageneses are as follows: in the peridotites—730–1180 ºC, 13–27 kbar, and oxygen fugacity of –1.5 to +0.3 log. un.; in the pyroxenites—1100–1310 ºC, 22–33 kbar. The pyroxenite minerals have been found to contain exsolved structures, such as orthopyroxene lamellae in clinopyroxene and, vice versa, clinopyroxene lamella in orthopyroxene. The formation temperatures of unexsolved phases in orthopyroxene and clinopyroxene are nearly 100–150 ºC higher than the temperatures of the lamellae–matrix equilibrium and the equilibrium of minerals in the rock. The normal distribution of cations in the spinel structure and the equilibrium distribution of Fe2+ between the M1 and M2 sublattices in the orthopyroxenes point to the high rate of xenolith ascent from the rock crystallization zone to the surface. All studied Spitsbergen rock-forming minerals from mantle xenoliths contain volatiles in their structure: OH–, crystal hydrate water H2Ocryst, and molecules with characteristic CH and CO groups. The first two components are predominant, and the total content of water (OH- + H2Ocryst) increases in the series olivine → garnet → orthopyroxene → clinopyroxene. The presence of these volatiles in the nominally anhydrous minerals (NAM) crystallized at high temperatures and pressures in the peridotites and pyroxenites testifies to the high strength of the volatile–mineral bond. The possibility of preservation of volatiles is confirmed by the results of comprehensive thermal and mass-spectral analyses of olivines and clinopyroxene, whose structures retain these components up to 1300 ºC. The composition of hypothetic C–O–H fluid in equilibrium (in the presence of free carbon) with the underlying mantle rocks varies from aqueous (>80% H2O) to aqueous–carbonic (~60% H2O). The fluid becomes essentially aqueous when the oxygen activity in the system decreases. However, there is no strict dependence of the redox conditions on the depth of formation of xenoliths.