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

The crystal chemical response of basalt clinopyroxene to increasing pr essure was investigated by means of crystal-structure simulation (a pr ocedure that enables modeling of the structural parameters of a clinop yroxene 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 magma tic environment. The general internal consistency of the simulation da ta permitted construction of an empirical geobarometer based on the re lationship of cell volume (Vc) vs M1-site volume (VM1). The straightfo rward geobarometric formulation in the absence of direct X-ray analysi s 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 for mula 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 clinop yroxenes crystallized from basaltic melts in the absence of garnet (ex cepting high-Al2O3 basalts). It is therefore suitable for many natural clinopyroxenes occurring as mega- or phenocrysts or forming well-pres erved cumulate pyroxenites. If the above restrictions are not wholly s atisfied, the Vc vs VM1 plot can also be used qualitatively to deduce the relative pressure conditions of clinopyroxenes forming from simila r batches of magma. The structural simulation of experimental data als o provided insight into the influence of minor chemical changes of the parental magma on the crystal chemistry of clinopyroxene at high pres sure. Within the considered compositional space at given P-T, a(CaO) a nd a(SiO2) in the melt have opposite effects on M2- and T-site cation populations. As a result, under similar physical conditions, clinopyro xenes 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.