Journal Article

Clinopyroxene geobarometry of magmatic rocks Part 1: An expanded structural geobarometer for anhydrous and hydrous, basic and ultrabasic systems

Nimis, P., Ulmer, P.

1998

Overview

Crystal-structure modeling of experimental Ca-rich clinopyroxenes [Ca + Na > 0.5 apfu; Mg/(Mg + Fe super(2+)) > 0.7] coexisting with basic and ultrabasic melts was utilized for calibration of geobarometers based on unit-cell volume (V sub(cell)) vs M1-site volume (V sub(M1)). The clinopyroxene database includes over one hundred experiments from literature and sixteen previously unpublished experiments on basanite and picrobasalt starting materials. The coexisting melts span a wide range of petrologically relevant anhydrous and hydrous compositions (from quartz-normative basalt to nephelinite, excluding high-Al basalts and melts coexisting with garnet or melilite) at pressure conditions pertinent to the earth's crust and uppermost mantle (P= 0-24 kbar) in a variety of fO 2 conditions (from CCO-buffered to air-buffered) and mineral assemblages (Cpx plus or minus Opx plus or minus Pig plus or minus Ol plus or minus Plag plus or minus Lc plus or minus Ne plus or minus Spl plus or minus Amp plus or minus Ilm). As previously found for near-liquidus products of basaltic melts, the experimental clinopyroxenes follow two distinct trends: (i) at a given P, V sub(cell) is linearly and negatively correlated with V sub(M1). This corresponds with the extent of Tschermak-type substitutions, which depends strongly on aSiO and a sub(CaO); (ii) for a fixed melt composition, V sub(cell) and V sub(M1) decrease linearly as P increases, due to a combination of M sub(1), M sub(2) and T site exchanges. Despite the chemical complexity of these relationships, P could be modeled as a linear function of V sub(cell) and V sub(M1). A simplified solution for anhydrous magmas reproduced the experimental pressures with an uncertainty of 1.75 kbar (=1; max. dev. = 5.5 kbar; N = 135). An expanded T-dependent solution capable of recovering the measured pressures of both anhydrous and hydrous experiments with an uncertainty of 1.70 kbar (=1; max. dev. = 5.4 kbar; N = 157) was obtained by correcting unit-cell and M1-site volumes for thermal expansivity and compressibility. The corrected formulation is more resistant to the effects of temperature variations and is therefore recommended. Nevertheless, it requires an independent, accurate estimate of crystallization T. Underestimating T by 20 degree C propagates into a 1-kbar increase of calculated P. The applicability of the T-dependent formulation was tested on hydrous ultramafic to gabbroic rocks of the southern Adamello batholith for which P-T evolution could independently be constrained by field observation, petrography and experimentally determined phase relations. The pressure estimates obtained by clinopyroxene structural geobarometry closely matched those predicted by phase equilibria of a picrobasaltic melt parental to the investigated magmatic rocks. To facilitate application of the present geobarometers, both anhydrous and corrected solutions were implemented as MS-DOS super( registered ) and UNIX super( registered ) software programs (CpxBar) designed to permit retrieval of the pressure of crystallization directly from a chemical analysis or from uncorrected unit-cell and M1-site volume X-ray data.

• Publisher: Springer Nature B.V
• Subject: Mineralogy / Petrology