CLINOPYROXENE GEOBAROMETRY OF MAGMATIC ROCKS PART 1 - AN EXPANDED STRUCTURAL GEOBAROMETER FOR ANHYDROUS AND HYDROUS, BASIC AND ULTRABASIC SYSTEMS

Crystal-structure modeling of experimental Ca-rich clinopyroxenes [Ca + Na > 0.5 apfu; Mg/(Mg + Fe2+) > 0.7] coexisting with basic and ultra basic melts was utilized for calibration of geobarometers based on uni t-cell volume (V-cell) vs M1-site volume (V-M1). The clinopyroxene dat abase includes over one hundred experiments from literature and sixtee n previously unpublished experiments on basanite and picro-basalt star ting materials. The coexisting melts span a wide range of petrological ly 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 f(O2) co nditions (from CCO-buffered to air-buffered) and mineral assemblages ( Cpx +/- Opx +/- Pig +/- Ol +/- Plag +/- Lc +/- Ne +/- Spl +/- Amp +/- Iln). As previously found for near-liquidus products of basaltic melts , the experimental clinopyroxenes follow two distinct trends: (i) at a given P, V-cell is linearly and negatively correlated with V-M1. This corresponds with the extent of Tschermak-type substitutions, which de pends strongly on a(SiO2) and a(CaO); (ii) for a fixed melt compositio n, V-cell and V-M1 decrease linearly as P increases, due to a combinat ion of M-1, M-2 and T site exchanges. Despite the chemical complexity of these relationships, P could be modeled as a linear function of V-c ell and V-M1. A simplified solution for anhydrous magmas reproduced th e experimental pressures with an uncertainty of 1.75 kbar (= 1 sigma; max. dev. = 5.5 kbar; N = 135). An expanded T-dependent solution capab le of recovering the measured pressures of both anhydrous and hydrous experiments with an uncertainty of 1.70 kbar (=1 sigma; max. dev. = 5. 4 kbar; N = 157) was obtained by correcting unit-cell and M1-site volu mes for thermal expansivity and compressibility. The corrected formula tion is more resistant to the effects of temperature variations and is therefore recommended. Nevertheless, it requires an independent, accu rate estimate of crystallization T. Underestimating T by 20 degrees C propagates into a 1-kbar increase of calculated P. The applicability o f the T-dependent formulation was tested on hydrous ultramafic to gabb roic rocks of the southern Adamello batholith for which P-T evolution could independently be constrained by field observation, petrography a nd experimentally determined phase relations. The pressure estimates o btained 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 implemente d as MS-DOS(R) and UNIX(R) software programs (CpxBar) designed to perm it retrieval of the pressure of crystallization directly from a chemic al analysis or from uncorrected unit-cell and M1-site volume X-ray dat a.