Experimental recalibration of the Cr-in-clinopyroxene geobarometer: improved precision and reliability above 4.5 GPa
Authors: Sudholz Z. J. , Yaxley G. M. , Jaques A. L. , Brey G. P. 
Published: January 2021

The pressure dependence of the exchange of Cr between clinopyroxene and garnet in peridotite is applicable as a geobarometer for mantle-derived Cr-diopside xenocrysts and xenoliths. The most widely used calibration (Nimis and Taylor Contrib Miner Petrol 139: 541–554, 2000; herein NT00) performs well at pressures below 4.5 GPa, but has been shown to consistently underestimate pressures above 4.5 GPa. We have experimentally re-examined this exchange reaction over an extended pressure, temperature, and compositional range using multi-anvil, belt, and piston cylinder apparatuses. Twenty-nine experiments were completed between 3–7 GPa, and 1100–1400 °C in a variety of compositionally complex lherzolitic systems. These experiments are used in conjunction with several published experimental datasets to present a modified calibration of the widely-used NT00 Cr-in-clinopyroxene (Cr-in-cpx) single crystal geobarometer. Our updated calibration calculatesP(GPa) as a function ofT(K), CaCr Tschermak activity in clinopyroxene$$\left( {a_{{{\text{CaCrTs}}}}^{{{\text{cpx}}}} } \right)$$aCaCrTscpx, and Cr/(Cr + Al) (Cr#) in clinopyroxene. Rearranging experimental results into a 2npolynomial using multiple linear regression found the following expression for pressure:

$$P\left( {{\text{GPa}}} \right) = 11.03 + \left( { - T{ }\left( {\text{K}} \right){\text{ ln}}(a_{{{\text{CaCrTs}}}}^{{{\text{cpx}}}} ) \times 0.001088{ }} \right) + \left( {1.526 \times {\text{ln}}\left( {\frac{{{\text{Cr}}\#^{{{\text{cpx}}}} }}{{T{ }\left( {\text{K}} \right)}}} \right)} \right){ }$$PGPa=11.03+-TKln(aCaCrTscpx)×0.001088+1.526×lnCr#cpxTK

where$${\text{Cr}}\#^{{{\text{cpx}}}} = \left( {\frac{{{\text{Cr}}}}{{{\text{Cr}} + {\text{Al}}}}} \right)$$Cr#cpx=CrCr+Al,$$a_{{{\text{CaCrTs}}}}^{{{\text{cpx}}}} = {\text{Cr}} - 0.81 \cdot {\text{Cr}}\#^{{{\text{cpx}}}} \cdot \left( {{\text{Na}} + {\text{K}}} \right),$$aCaCrTscpx=Cr-0.81·Cr#cpx·Na+K,with all mineral components calculated assuming six oxygen anions per formula unit in clinopyroxene.

Temperature (K) may be calculated through a variety of geothermometers, however, we recommend the NT00 single crystal, enstatite-in-clinopyroxene (en-in-cpx) geothermometer. The pressure uncertainty of our updated calibration has been propagated by incorporating all analytical and experimental uncertainties. We have found that pressure estimates below 4 GPa, between 4–6 GPa and above 6 GPa have associated uncertainties of 0.31, 0.35, and 0.41 GPa, respectively. Pressures calculated using our calibration of the Cr-in-cpx geobarometer are in good agreement between 2–7 GPa, and 900–1400 °C with those estimated from widely-used two-phase geobarometers based on the solubility of alumina in orthopyroxene coexisting with garnet. Application of our updated calibration to suites of well-equilibrated garnet lherzolite and garnet pyroxenite xenoliths and xenocrysts from the Diavik-Ekati kimberlite and the Argyle lamproite pipes confirm the accuracy and precision of our modified geobarometer, and show that PT estimates using our revised geobarometer result in systematically steeper paleogeotherms and higher estimates of the lithosphere‒asthenosphere boundary compared with the original NT00 calibration.