Partial crystallization of picritic melt and its applications for the genesis of high-Ti and low-Ti basalts

Abstract:

Geochemical and petrological studies have revealed the existence of high-Ti and low-Ti basalts in large igneous provinces (LIPs). However the originate of these high-Ti and low-Ti magmas are still under debate. Several different mechanisms have been proposed: (1) the high-Ti basalts are formed by the melting of mantle plume containing recycled oceanic crust (Spandler et al., 2008) while low-Ti basalts are formed by the melting of subcontinental lithospheric mantle (Xiao et al., 2004); (2) both high-Ti and low-Ti basalts are from mantle plume source, but the production of high-Ti basalts are associated with the thick lithosphere while the low-Ti basalts are controlled by the thin lithosphere (Arndt et al., 1993); (3) they are derived from the different degrees of melting, with high-Ti basalts representing low degree of partial melting of mantle plume (Xu et al., 2004). The low Mg# (below 0.7) of high-Ti and low-Ti basalts provides that they are far away from direct melting of mantle peridotite. In addition, seismic data indicate unusually high seismic velocities bodies beneath the LIPs which explained by the fractionated cumulates from picritic magmas (Farnetani et al., 1996). Therefore, we believed that the crystallization differentiation process might play a more significant role in the genesis of high-Ti and low-Ti basalts.

In order to investigate the generation of high-Ti and low-Ti basalts, a series of high pressure and high temperature partial crystallization experiments were performed at pressures of 1.5, 3.0 and 5.0 GPa and a temperature range of 1200-1700℃. The starting material is picrate glass with relative high TiO₂ (2.7 wt %), which is synthesized according to the chemical composition of primary magmas of Emeishan LIP (Xu et al., 2001). The experimental results show that: (1) At a given pressure, the TiO₂ content is decreased with increasing melt fraction; (2) At a given melt fraction, the TiO₂ content of melts is increased with increasing pressure. On the basis of our experimental results, we propose that the high-Ti and low-Ti basalts are inherited from the same picritic parental magmas. The high-Ti basalts are generated at relatively high pressure while the low-Ti basalts are generated at relatively low pressure.