CALCIUM EXCHANGE BETWEEN OLIVINE AND CLINOPYROXENE CALIBRATED AS A GEOTHERMOBAROMETER FOR NATURAL PERIDOTITES FROM 2 TO 60 KB WITH APPLICATIONS

Abstract

The equilibrium Ca content of olivine coexisting with clinopyroxene was experimentally determined in natural lherzolitic compositions (Mg/(Mg + Fe)ol ≈ 0.9). The experiments from 2–60 kb and 900–1400°C cover most of the crystallization conditions of upper mantle peridotites.The Ca content of olivines in the experimental products was determined by electron microprobe in the WDS mode with a precision of ±5 rel% (1σ). In a few instances an ion microprobe was used (±2 rel%). Accuracy was tested with olivine standards analyzed for calcium by isotope dilution analysis.The solubility of Ca in olivine showed a non-linear change at around 1100°C. Therefore, we have fitted our experimental data to two equations, given here in the form of barometers: where Dca = Caol/Cacpx(Caol and Cacpx = atomic proportions of Ca in structural formulae of ol and cpx based on 4 and 6 oxygens, respectively). These equations reproduce experimental conditions to ±1.7 kb(1σ).∗This is the first barometer which can be applied successfully to spinel peridotites. Calculated P, T conditions for spinel peridotite nodules from southwestern USA plot within the spinel stability field and along predicted geotherms for this region. Application to garnet peridotites from southern Africa generally shows good agreement with the Al-opx/gt barometer, but some significant deviations also exist. They are most likely caused by short-lived thermal events immediately before eruption to the surface. These are recorded by the Ca contents of olivines but not in the associated clinopyroxenes, since the diffusion coefficient for Ca in olivine is 3 to 4 orders of magnitude higher than in clinopyroxene. The tendency is that olivines from “high-temperature” peridotites record heating events while olivines from peridotites plotting along a shield geotherm record cooling events. High Cr spinel peridotites from southern Africa plot in the P, T field of “low-temperature” peridotites from the same occurrences. Pressures calculated for spinel-bearing “fertile” harzburgites (with exsolved cpx) from southern Africa are excessively high, indicating intense cooling with reequilibration of olivines to lower temperatures. Careful investigation of zoning patterns of minerals, especially of Ca in olivine will help to constrain the magnitude of cooling or heating these rocks have experienced.Due to the high diffusion coefficient of Ca in olivine, Ca contents can also be used as a “speedometer” for geologically very brief thermal events such as heating of mantle xenoliths by their host magmas or cooling in meteorites.