P-T AND COMPOSITIONAL CONTROLS ON HYDRATION AND DEHYDRATION OF OROGENIC LHERZOLITES

Abstract

The natural processes, examined in upper mantle-derived xenolith suites, orogenic lherzolites and primary magmas, all confirm the presence of significant and variable quantities of water in minerals or melts. It has been well known that the P-T location of the mantle solidus is extremely sensitive to pargasitic amphibole stability (e.g., Green, 1973). It is hardly known, however, how the amphibole acts under the subsolidus mantle conditions. Recently, Niida and Green (1999) discussed on chemical and modal variations of pargasitic amphibole in mantle peridotites. In this paper, crystallization and breakdown reaction of pargasitic amphiboles, generated through the subsolidus P-T regime, will be focused on orogenic lherzolites, to understand P-T controls and bulk-rock composition effects during hydration and dehydration in the upper mantle. DEHYDRATION A significant dehydration of pargasitic amphibole-bearing lherzolites occurs just on the solidus, where the breakdown reaction of amphibole becomes active, producing a melt phase. The amphiboles are all eliminated at the amphibole stability limit, approximately 15°C higher than the solidus. It is evident from melting experiments (Wallace and Green, 1991) that the location of P-T of amphibole breakdown is controlled by the bulk rock chemical composition. The upper stability limit of amphiboles in a fertile peridotite (Hawaiian pyrolite: 1150-1170°C at 25 kbar) is 150°C higher than those in a depleted peridotite (Tinaquillo lherzolite: 1025°C at 20-25 kbar). The P limit of the stability expands slightly in fertile peridotites. We also expect a breakdown reaction of pargasitic amphibole occurring continuously throughout the subsolidus region, based on the phase relationships combined with the modal abundance of coexisting phases (Niida and Green, 1999). In the plagioclase lherzolite field, tremolite component in amphiboles decreases with increasing T and pargasite component increases with increase in both T and P. In the spinel lherzolite field pargasite components is replaced by richterite towards higher P. The P-T effects on amphibole composition are more evident in the garnet lherzolite field, where there is decrease in Tschermak’s component and further increase in richterite. The modal and mineral compositional data suggest that enstatite is decreasing, garnet is increasing, and jadeite solid solution in clinopyroxene is increasing over the same P intervals, suggesting the reaction: 3pa + (CaAl2SiO6)cpx + 10en -> ri + jd + (2gr+3py) + 4fo + 2H2O. The pargasite breakdown also occurs as part of the spinel lherzolite to the garnet lherzolite transition as: 3pa + sp + di + en -> ri + jd + (2gr+3py) + 5fo + 2H2O. The high P limit is expressed as a richterite breakdown reaction, e.g., ri + 2(CaAl2SiO6)cpx -> 2jd + gr + en + 2fo + H2O, or more plausibly to include a pargasitic amphibole breakdown reaction: (3pa+ri) + (5MgAl2SiO6 + 8en) -> 5jd + 7(gr0.33py0.67) + 8fo + 4H2O. HYDRATION When H2O-rich fluids and/or melts migrate through the upper mantle lherzolites, a considerable amount of amphiboles crystallize in the subsolidus P-T regime. The modal proportion and the chemical composition of amphiboles crystallized are strongly controlled by P-T and the bulk lherzolite compositions (Niida and Green, 1999). We infer that a significant hydration of lherzolite occurs in lower T fields and that the modal amphibole increases with decreasing pressure at a constant T. We also infer that the maximum proportion of amphibole attains at P-T close to the transition of the plagioclase lherzolite and the spinel lherzolite. In the case of MORB pyrolite composition, more than 30% pargasitic amphiboles have been confirmed as maximum modal proportion at T=925°C and P=10 kbar, whereas less than 10% as minimum at T=1000°C and P=27 kbar. The Na content of amphibole is positively correlated with P-T, in harmony with the modal variation of amphibole. Again, in a fertile lherzolite, we expect slightly more abundant amphiboles as hydration products. During hydration of mantle lherzolites, pargasitic amphibole act as a tank of H2O and alkali (Na + K). It is plausible, however, that P-T and the bulk rock compositions strictly constrain the capacity of storage in terms of modal proportions.