A CLINOPYROXENE GEOBAROMETER FOR BASALTIC SYSTEMS BASED ON CRYSTAL-STRUCTURE MODELING

The crystal chemical response of basalt clinopyroxene to increasing pressure was investigated by means of crystal-structure simulation (a procedure that enables modeling of the structural parameters of a clinopyroxene of known chemistry without requiring direct X-ray diffraction analysis) using available experimental chemical data. Pressure proved the main physical variable governing clinopyroxene behavior in a magmatic environment. The general internal consistency of the simulation data permitted construction of an empirical geobarometer based on the relationship of cell volume (Vc) vs M1-site volume (VM1). The straightforward geobarometric formulation in the absence of direct X-ray analysis is: P(kbar) = 698.443 + 4.985 . Al-T - 26.826 . Fe-M1(2+) - 3.764 . Fe3 + 53.989 Al-M1 + 3.948 . Ti + 14.651 . Cr - 700.431 . Ca - 666.629 . Na - 682.848 . Mg-M2 - 691.138 . Fe-M2(2+) - 688.384 . Mn - 6.267 . (Mg-M2)(2) - 4.144 . (Fe-M2(2+)) where: (Fe-M1(2+) . Mg-M2)/(Fe-M2(2+) . Mg-M1) = e**(0.238 . R(3+) + 0.289 . CNM - 2.315), CNM = Ca + Na + Mn, and R(3+) = Al-M1 + Fe3+ + Ti + Cr, with cations in atoms per formula unit. The geobarometer reproduces experimental pressures within +/- 2 kbar ( = 1 sigma; max. dev. less than or equal to 5 kbar; N = 29) in the range 0-24 kbar and is applicable to near-liquidus C2/c clinopyroxenes crystallized from basaltic melts in the absence of garnet (excepting high-Al2O3 basalts). It is therefore suitable for many natural clinopyroxenes occurring as mega- or phenocrysts or forming well-preserved cumulate pyroxenites. If the above restrictions are not wholly satisfied, the Vc vs VM1 plot can also be used qualitatively to deduce the relative pressure conditions of clinopyroxenes forming from similar batches of magma. The structural simulation of experimental data also provided insight into the influence of minor chemical changes of the parental magma on the crystal chemistry of clinopyroxene at high pressure. Within the considered compositional space at given P-T, a(CaO) and a(SiO2) in the melt have opposite effects on M2- and T-site cation populations. As a result, under similar physical conditions, clinopyroxenes from higher-CaO or more undersaturated basalts have higher VM2, VT and Vc and lower VM1. For basalts with normal contents of Al2O3 ( < 18 wt %), variations of major elements in the melt do not reduce the accuracy of the geobarometer.