An electron microprobe, LAM-ICP-MS and single-crystal X-ray structure refinement study of the effects of pressure, melt-H2O concentration and fO2 on experimentally produced basaltic amphiboles

Abstract

Amphiboles were crystallized in sub-liquidus experiments at 0.5–2.0 GPa and 1000–1050 °C from hydrous nepheline basanite and olivine basalt starting compositions. The amphiboles and coexisting (quenched) melts were analysed for major, minor and trace elements by a combination of electron microprobe, laser ablation microprobe and inductively-coupled plasma mass-spectrometry (LAM ICP-MS). Individual amphiboles were also characterized by single-crystal X-ray structure refinement, and empirical estimates of dehydrogenation were obtained based on M1–M2 distances. The amphiboles display compositional variation that can be interpreted as crystal-chemical responses to: (1) increasing pressure, and (2) changes in oxygen fugacity ( f O 2 ) and the activity of H 2 O. As pressure increases, Al moves from the T1 tetrahedron (where it is replaced by Si) to the octahedral M2 site. This coupled substitution, which implies an increase in coordination number for Al, results in a decrease in the c and b unit-cell edges. The overall decrease in unit-cell volumes is kept small, however, by an increase in the B (Fe, Mg) content with increasing pressure, which in turn decreases the volume occupied by the B-cations but increases the sin β value. In this way, the entrance of minor K at the A site and Cl at the O3 site (K D s for both increase with pressure) is allowed, resulting in a slight lengthening of the a edge. The degree of dehydrogenation at O3 correlates inversely with the H 2 O concentration in coexisting melts. Generally, dehydrogenation is locally balanced by M1 Ti, with the Ti excess with respect to ½ O 2− ordered at the M2 site. In one sample, crystallized under more oxidizing conditions, O 2− is > 2Ti, and local charge balance requires the presence of Fe 3+ ordered at the M1 (and M3) sites. D amph/melt values measured for the high field strength elements Ti, Zr, Hf, Nb and Ta ( D HFSE ) correlate positively with O 2− and with [4] Al, suggesting that Ti, Zr, Hf, Nb and Ta (HFSE) are incorporated in both the M1 and the M2 sites. Partition coefficients for rare earth elements ( D REE ) correlate positively with [4] Al and negatively with [6] Al. Increased f O 2 results in increased Fe 3+ , [4] Al and D REE , but does not produce a noticeable increase in O 2 − or in D HFSE .