Experimental recalibration of the Cr-in-clinopyroxene geobarometer: improved precision and reliability above 4.5 GPa

Abstract

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 calculates P (GPa) as a function of T (K), CaCr Tschermak activity in clinopyroxene $$\left( {a_{{{\text{CaCrTs}}}}^{{{\text{cpx}}}} } \right)$$ a CaCrTs cpx , and Cr/(Cr + Al) (Cr#) in clinopyroxene. Rearranging experimental results into a 2n polynomial using multiple linear regression found the following expression for pressure:$$P\left( {{\text{GPa}}} \right) = 11.03 + \left( { - T{ }\left( {\text{K}} \right){\text{ ln}}(a_{{{\text{CaCrTs}}}}^{{{\text{cpx}}}} ) \times 0.001088{ }} \right) + \left( {1.526 \times {\text{ln}}\left( {\frac{{{\text{Cr}}\#^{{{\text{cpx}}}} }}{{T{ }\left( {\text{K}} \right)}}} \right)} \right){ }$$ P GPa = 11.03 + - T K ln ( a CaCrTs cpx ) × 0.001088 + 1.526 × ln Cr # cpx T K where $${\text{Cr}}\#^{{{\text{cpx}}}} = \left( {\frac{{{\text{Cr}}}}{{{\text{Cr}} + {\text{Al}}}}} \right)$$ Cr # cpx = Cr Cr + Al , $$a_{{{\text{CaCrTs}}}}^{{{\text{cpx}}}} = {\text{Cr}} - 0.81 \cdot {\text{Cr}}\#^{{{\text{cpx}}}} \cdot \left( {{\text{Na}} + {\text{K}}} \right),$$ a CaCrTs cpx = 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.

Multi‐Stage Evolution of the South Australian Craton: Petrological Constraints on the Architecture, Lithology, and Geochemistry of the Lithospheric Mantle

“…EPMA data acquired for this study were filtered to exclude oxide totals and cation normalized totals outside of the range: <98 wt% and >102 wt%, <3.96 and >4.04 (calculated based on six oxygens in Cr diopside) or <8.95 and >8.05 (calculated based on 12 oxygens in pyrope garnet). Additional filtering was applied to PT estimates on pyrope garnet and Cr diopside following the protocols outlined in Sudholz, Yaxley, Jaques, and Brey (2021), and Sudholz, Yaxley, Jaques, and Chen (2021). These protocols represent the most up‐to‐date compositional filters and data quality protocols for single grain clinopyroxene and garnet geothermobarometry and returned similar results to alternate filtering methods by Ziberna et al.…”

“…Equilibration PT of Cr diopside mantle xenocrysts were determined using the single-grain Cr-in-clinopyroxene geobarometer of Sudholz, Yaxley, Jaques, and Brey (2021) and enstatite-in-clinopyroxene single-grain geothermometer of Nimis and Taylor (2000). The geothermobarometer set reproduces equilibration conditions over a very wide range of mantle PT conditions and has a maximum estimated uncertainty of 4 kbar and 𝐴𝐴 35 • C .…”

“…Gaul et al (2003) interpreted these trends to indicate that the upper mantle layer was originally highly depleted and then metasomatically refertilized by infiltration of melts from the lithosphere-asthenosphere boundary or shallow asthenosphere. We have adapted methods for lithospheric mapping by taking advantage of recent improvements in single grain geothermobarometry (Sudholz, Yaxley, Jaques, & Brey, 2021, Sudholz, Yaxley, Jaques, & Chen, 2021) which now provide reliable estimates of equilibration conditions over a very wide PT range, and paleogeothermal gradient modeling (Mather et al, 2011). The following study also benefits from the addition of newly analyzed Cr diopside and pyrope from a larger range of kimberlite provinces and updated MT and seismic topography data sets from the AusLAMP data set.…”

Composition and P-T conditions of the lithospheric mantle beneath the Azimuth 125º Lineament, Northern and Southeastern Brazil: constraints from peridotite xenoliths enclosed in diamond-bearing kimberlites

“…The equilibrium temperatures calculated for the samples from Rondônia are higher relative to those from Minas Gerais based on the two-pyroxene thermometer of Brey and Koehler (1990) ( 1130 ºC vs. 879 and 1097 ºC) , the garnet-clinopyroxene thermometers of Powell (1985) (1063-1279 ºC vs 880-1160 ºC) and Ellis and Green (1979) (1063-1279 ºC vs 903-1170 ºC) and also the single clinopyroxene thermometer of Nimis and Taylor (2000) (1032-1227 ºC vs 831-989 ºC). The new calibration of Sudholz et al (2021) for the single clinopyroxene barometer provided a higher pressure to the garnet dunite VV-36 (50 kbar) enclosed in the Carolina-1 kimberlite. However, the pressure calculated for this sample using the garnet-orthopyroxene barometers of Brey and Koehler (1990) and Nickel and Green (1985) is otherwise in the same interval calculated for the peridotites enclosed in the Canastra-1 kimberlite (38-44 kbar).…”

“…P-T diagrams showing the stability fields (pink) generated by the intersections of the geothermometers and geobarometers used in PTQuick software (Sikamov and Dolivo-Dobrovolsky 2009) for the peridotite samples enclosed in Canastra-1 kimberlite. T BK90c : Two pyroxene Brey and Koehler (1990); T NG10 : Nimis and Grütter (2010); T EG79 :Ellis and Green (1979); T P85 :Powell (1985); T NT-00-T :Nimis and Taylor (2000); P BK90 : Garnet-orthopyroxeneBrey and Koehler (1990); P NT100-P :Nimis and Taylor (2000); P S21 :Sudholz et al (2021).…”