On the effect of Cr2O3 addition on the crystallization of trachybasaltic melt

Even small variations in minor element composition can affect the crystallization history of a magma, altering the final mineral assemblage and/or microstructural evolution during crystallization. We discuss the results of two sets of crystallization experiments performed in a piston cylinder apparatus at 400 MPa and at fO2 close to NNO+2, using two synthetic trachybasaltic glasses, one with no Cr2O3 added (Glass 1) and one doped with 0.4 wt.% Cr2O3 (Glass 2). Experiments were carried out under both anhydrous (0 wt.% H2O) and hydrous conditions (2 wt% H2O). Experiments were initially heated at 1300 C for 30 minutes to ensure complete melting of the glass and then cooled at a rate of 80C/min to the final resting temperature (Trest = 1150C, 1100C and 1050C). Dwell time at Trest ranged between 5 minutes and 8 hours. The commonest crystal phases were always clinopyroxene (Cpx, up to 50 vol%) and spinel (up to 5 vol%, specifically chromite Chr and/or titanomagnetite Tmt) In all samples, some spinel grains are found isolated in the melt, while others are touching a Cpx. The ratio of isolated spinel to spinel in contact depends on the crystallization history of the sample, which varies according to the starting composition. Experiments performed with Glass 1 consists of Tmt grains always in contact with a Cpx grain, decorating its edges and tips. More than 90% of all Tmt grains following clear crystallographic orientation relationships (CORs) with Cpx, implying heterogeneous nucleation of Tmt on Cpx. In experiments performed with Glass 2 the amount of isolated Spinel crystals is higher. These grains are constituted by a Chr core and a Tmt rim. Even when these oxide grains are in contact with (or inside) a Cpx crystal, they rarely (<10 % of touching oxide grains) share CORs with Cpx, suggesting that the oxides were the first phases to crystallize, and were subsequently engulfed by Cpx. The rare oxide crystals sharing CORs with touching Cpx are located in the centre of the dendritic Cpx grains, suggesting that Cpx nucleated heterogeneously on the earlier-formed oxide. We evidence that even small variations in Cr2O3 content can affect the crystallization history of a magma, thereby affecting the final mineral assemblage and the microstructural evolution during crystallization. In particular, by altering the mechanism of crystal nucleation, we speculate that the nucleation of spinel stimulated by subtle changes in the chemistry of magmas (or variations of the fO2 conditions) may result in substantial modification of magma rheology. Funded by the Austrian Science Fund (FWF): P 33227-N