Wed 18th November @ 12 pm, Rm 312.222

Eclogitic xenoliths from kimberlite are occasionally diamond-bearing, and are often interpreted as having an origin as subducted oceanic crust [1]. The existence of diamonds in these rocks constrains equilibrium temperatures and pressures of some eclogites to the upper mantle. However the additional critical parameter controlling the stability of diamonds, oxygen fugacity (ƒO2), is unknown in eclogitic assemblages.

A series of piston-cylinder experiments were conducted using model carbonate and kyanite bearing eclogite assemblages to determine the ƒO2 of the limiting reaction for graphite vs. diamond:

CaMg(CO3)2 + 2SiO2 = CaMgSi2O6 + 2C + 2O2  [2]

as a function of pressure (P=3.5-6 GPa) and temperature (T=900-1300oC).

The oxygen fugacity in the experiments was determined using Fe-Ir alloy fO2 sensors [3] and a newly developed Fe-Pd-based redox sensor [4]. The experimental data allowed calibration of two redox reactions (involving garnet-clinopyroxene [5] and garnet-kyanite as oxybarometers to determine ƒO2 of eclogitic rocks. Both reactions can be used to evaluate the ƒO2 of UHP metamorphic eclogites and eclogite xenoliths from kimberlite. The accuracy of the ƒO2 thus calculated is highly dependent on precision of the garnet Fe3+/ΣFe measurements, which were obtained using the flank method [6] and the synchrotron based Fe K-edge XANES method [7]. Both reactions were calibrated and used to estimate ƒO2 of diamond, kyanite and coesite bearing eclogite xenoliths from Udachnaya kimberlite pipe, Yakutia, Russia. The relatively high ƒO2 of diamond stability in eclogite relative to peridotite at the upper mantle PT conditions may explain the higher abundance of diamonds in eclogite xenoliths [8] and constrains the mechanism of transport of carbon to the deep mantle.

 [1] Jacob (2004) Lithos 77, 295-316 [2] Luth (1993) Science 261, 66-68 [3] Woodland & O’Neill (1997) GCA, 61(20), 4359-4366 [4] Vasilyev et al. (2014) IMA 2014 abstracts, 40 [5] Stagno et al. (2015) CMP 169, 16. [6] Höfer, Brey (2007) Am.Min. 92, 873-885 [7] Berry et al. (2010) Chem.Geol. 278, 31-37 [8] Cartigny (2005) Elements 1, 79-84