The Utility of Clinopyroxene in Diamond ExplorationTuesday, November 20, 2018 - 10:40 to 10:59 Theatre 1
Clinopyroxene single-crystal thermobarometry is an essential tool in the identification and evaluation of prospective kimberlites. The paleogeothermal gradient preserved by clinopyroxene xenocrysts elucidates the thermal structure of the underlying lithospheric mantle; indicates the depth to and thickness of the “diamond window”.
The widely used clinopyroxene thermometer-barometer of Nimis and Taylor (2000) requires that clinopyroxene equilibrated with both garnet and orthopyroxene. With the rare exception of wehrlites, equilibration with orthopyroxene is nearly a given for the majority of chrome-diopside clinopyroxene xenocrysts. Demonstrating equilibration with garnet, however, is a major obstacle for clinopyroxene-based thermobarometry. The most commonly used method for clinopyroxene discrimination is an Al2O3-Cr2O3 diagram proposed by Ramsay and Thompkins in 1994 supplemented with an additional MgO-Al2O3 from Nimis (1998) and an additional 1-dimensional filter based on chemical composition. Despite the aggressiveness of the filtering method, single-clinopyroxene pressure-temperature results have large scatter that can obscure the true paleogeothermal gradient. This is especially true of areas where the lithospheric mantle has undergone chemical modification by melt/fluid influx. Using a database of clinopyroxenes derived from kimberlite-borne mantle-derived lherzolites, we have developed a simple and effective discrimination plot that identifies clinopyroxene from garnet lherzolites and simultaneously removes clinopyroxene from metaosomatised peridotites.
Calculated paleogeothermal gradients from clinopyroxene xenocrysts cut across model conductive geotherms which can complicate the interpretation of thermobarometry data. Grütter (2009) presented a solution to the problem by way of relative reference geotherms. He used xenocryst data from three Canadian locations with different thermal structures as references in comparison to the dataset under investigation. Taking a cue from this earlier work, we have developed a new set of relative reference geotherms that are based on single-clinopyroxene thermobarometry data for xenoliths from well-characterized regions – Somerset Island, Kaapvaal on-craton, and the Central Slave. A simple linear fit through the data produces sub-parallel clinopyroxene reference geotherms that are simpler to use and easier to visualize compared to the xenocrysts reference geotherms.
Using these two new and simple tools will greatly help maximize the utility of clinopyroxene data in large exploration databases.