The experimental examination of silicate/silicate-melt systems: Derivation of two pressure sensitive expressions, and modelling of basaltic intrusion of the deep crust.

摘要

The equilibrium between anorthite, quartz, and the Ca-tschermak's component in clinopyroxene: CaAl2SiOCpx6+SiOQtz 2=CaAl2Si2OPl 8SCAn can be used as a granulite geobarometer, and has been calibrated using data from partial melting experiments of natural assemblages, and from phase equilibrium experiments on the end-member CMAS system. Linear least squares regression of the experimental data resulted in two empirical barometric expressions. Application of these expressions demonstrates that the barometers are applicable over a wide range of pressures, temperatures, and bulk compositions. The exchange of Na₂O between clinopyroxene and melt was also used as a geobarometer. Distribution of Na₂O between clinopyroxene and melt was examined using 76 experiments over the P-T-X range: 7–30 kbar; 700–1450°C; and granite to primitive basalts. Linear least squares regression resulted in two empirical expressions that are accurate to within an average error of ±1.8 kbar.; Basaltic intrusion into the continental crust was simulated with melting experiments on layered charges of high-alumina olivine tholeiite and metapelite, at 11 kbar. Rapid diffusion of oxides between the metapelite and basalt resulted in spectacular phase changes: garnet crystallized across the basalt layer; cumulate assemblages resembled rocks from underplating localities; and glasses were silicic, even at 1150°C. Calculations based on these results and comparison with natural occurrences suggest that high-temperature felsic melts with 4 wt% H₂O can form and segregate at ∼1 cm/y. Diffusion couples of alkali basalt (Alk) and the Auburn Metagreywacke (AMG) were also run. Hybrid Alk-AMG glasses straddle the total alkali boundary that separates basaltic to dacitic lavas from their alkali-rich equivalents (i.e. trachybasalts to trachytes). Comparison of hybrid Alk-AMG glasses' and natural rocks' major element compositions indicates that major element chemistry cannot be used to identify the protoliths of natural hybrid intermediate rocks. Major element compositions of basalt-side hybrid cumulates are also not diagnostic of contamination, but the presence of orthopyroxene sometimes is. Qualitative inferences of trace element behavior suggests that Sr, Rb, Ba, and Nd are the trace elements most likely to enable identification of a hybrid rock's protoliths.