Influence of fO 2 and cooling rate on the kinetics and energetics of Fe-rich basalt crystallization

Abstract

Cooling rate and fO 2 are each varied over several orders of magnitude in a matrix of 1-atm constant-rate cooling experiments using synthetic basalt. The modes and compositions of olivine, pyroxene and titanomagnetite are sensitive to oxygen fugacity (ranging from QFM − 4 to QFM + 5 log units), whereas their textures respond to the degree of undercooling as modulated by cooling rate (ranging from 2.8 to 231 °C h − 1 ). Key results pertain to the trends in mineral composition, melt differentiation, and crystal texture development: (1) The degree of pyroxene compositional zoning in a given experiment increases with decreasing cooling rate. For a given cooling rate, the Ti/Al ratio and MgO crystal–melt partitioning coefficients for pyroxene are inversely correlated with fO 2, and the Mg contents of Ca-rich pyroxene and titanomagnetite increase with increasing fO 2. Both trends are inferred to result from the fO 2 control over melt Fe 3+/Fe 2+ ratio. (2) Reducing conditions lead to greater crystal contents than do oxidizing conditions, yet for any given cooling rate the compositions of matrix glasses are progressively more evolved as fO 2 increases. The liquid lines of descent followed by residual melts are tholeiitic at reducing conditions and calc-alkaline at oxidizing conditions. The greater viscosities of residual liquids evolving toward silica enrichment at high fO 2 may be responsible for declining solidification efficiency with increasing fO 2. (3) Up to three distinct crystal populations of titanomagnetite, olivine, and pyroxene crystals, identified in each charge using qualitative morphologic criteria, are quantified in terms of volume fraction and the surface area per unit volume, S v P [mm − 1 ]. The presence of distinct populations suggests that nucleation of given mineral phases occurs episodically. The balance between thermodynamic driving force and kinetic inhibiting factors is optimized for the greatest number of pyroxene nucleation events at intermediate cooling rates. Anhedral crystals possess large surface area to volume ( S v P) ratios, and these ratios are relatively insensitive to cooling rate. In contrast, the S v P ratios of euhedral crystal populations, as well as the weighted population averages, steadily decline with decreasing cooling rate. These trends are consistent with the energetics of 2D and 3D nucleation processes.