Cooling rate-induced differentiation in anhydrous and hydrous basalts at 500 MPa: Implications for the storage and transport of magmas in dikes

Abstract

Two sets of cooling experiments were run at 500 MPa conditions for one anhydrous and one hydrous (H 2O = 1.3 wt.%) starting basaltic melts: a) five cooling rates (15, 9.4, 3, 2.1, and 0.5 °C/min) between 1250 and 1000 °C, and b) a 0.5 °C/min cooling rate from 1250 to 1191, 1167, 1100, 1090, 1075, 1050, 1025 and 1000 °C final temperatures. Cooling rate plays a major role in the differentiation of run products. At the lower cooling rate, glasses of tephri-phonolitic and trachy-andesitic composition have been detected. At comparable cooling rate, the dry glasses show a larger compositional variability and degree of differentiation than the hydrous products. The amount of crystallizing solid phases is always larger in the dry products. It is strongly controlled by both cooling rate and water content and massive crystallization occurs only at lower cooling rates. At a constant cooling rate, massive crystallization is observed at lower temperatures. Clinopyroxene, plagioclase and oxide occur in the anhydrous products, whereas plagioclase crystallization is suppressed in the hydrous ones. The lack of plagioclase results from the faster crystallization kinetics for Fe- and Mg-bearing phases than for tectosilicates. Textural coarsening occurs at high cooling rate and, for a constant cooling rate, at higher temperatures. The textural and compositional variability observed at the margin of dikes may not mirror flow differentiation processes but could be due to cooling rate variations. Early homogeneous magma batches subjected to cooling rate-induced differentiation may also produce heterogeneous rocks similar to that originated by magma mingling. Cooling rate-related differentiation influences the physical properties (viscosity and density) of magmas. Dry or H 2O-poor magmas resulting from low cooling rate differentiation are not allowed to rise within dikes. Viscosity variations induced by cooling rate may be responsible for flow localization within conduits. The effects of cooling rate should be incorporated in fluid-mechanical models of magma ascent.