Experimental Peridotite - melt reaction at one atmosphere: A textural and chemical study.

Sieve-textured clinopyroxene and spinel are common in mantle xenoliths and have been interpreted to be the result of partial melting, mantle metasomatism and host magma - xenolith reaction during transport. In this paper we test the latter hypothesis with a series of reduced and oxidized experiments at 1200 and 1156 C at one atmosphere using a synthetic leucitite melt and discs of natural peridotite. The experiments were performed at atmospheric pressure so that large sample volumes could be used; this allowed us to use natural peridotite with a grain size of several millimeters which facilitates sample preparation and analysis. The peridotite used in the reaction couples was cored from a 30 cm diameter anhydrous lherzolite xenolith from Meerfelder Maar in the West Eifel volcanic field, Germany (see Witt-Eickschen et al., 1998) for a detailed description of the xenoliths from this locality). The melt used was a synthetic version of a leucitite from the Rockeskyller Kopf volcano, also in the West Eifel. Our results show that sieve texture development on clinopyroxene and spinel in mantle xenoliths is the result of a multistage reaction process. In the first step, orthopyroxene undergoes incongruent dissolution to produce a silica- and alkali-rich melt together with olivine. As this melt migrates along grain boundaries it causes incongruent dissolution of clinopyroxene and spinel. The incongruent dissolution mechanism involves complete dissolution of the clinopyroxene or spinel followed by nucleation and growth of a secondary clinopyroxene or spinel once the reacting melt is saturated. The reaction of orthopyroxene, clinopyroxene and spinel with infiltrated host magma results in a range of melt compositions that are very similar to those observed in nature that have been interpreted to be due to very small degrees of partial melting.