Chemical Equilibrium between Garnet and Clinopyroxene in Eclogitic Xenoliths from Kimberlites

E. Jagoutz Max-Planck-Inst. f. Chemie, Abt. Kosmochemie, Saarstr.23, D-55128 Mainz, Germany

E. Zinner Earth & Planetary Sciences, Washington Univ., St. Louis, Mo, USA

D. E. Jacob Mineralog.-Petrolog. Inst., Univ. Göttingen, D-37077 Göttingen, Germany

From Sm-Nd isotopic studies on garnet-clinopyroxene pairs we know that in some cases these minerals are not in chemical equilibrium (Jagoutz, 1986). In this study we selected eclogites with equilibrated and some with unequilibrated minerals. Trace element abundances were measured by ion microprobe in the constitutent garnet (Gnt) and clinopyroxene (Cpx) of 5 samples. A large variation in REE abundances is observed and reproduces the total variation of REE concentrations for comparable eclogites reported in the literature. LREE mainly reside in Cpx while HREE are concentrated in Gnt. The ratios of REE between Gnt and Cpx vary over two orders of magnitude. Trace element-poor Gnt frequently has small positive Eu anomalies indicating that these Gnts are replacing plagioclase. This evidence indicates that the eclogites may be metamorphosed gabbros (Jagoutz et al., 1984). Textural evidence for the exsolution of Gnt from Cpx is commonly observed in eclogites and garnet pyroxenites. Gnts formed by this reaction are in isotopic disequilibrium and have low HREE as demonstrated by one of our samples. These new data indicate that most eclogites record a prograde path of metamorphism in their isotopes and trace elements. The common deficit of LREE is a strong indication of an open chemical system. For example, a partial melt may have been lost during the dehydration process. Consequently, most eclogites are dry and chemically residual. Nonetheless, we still observe characteristic trace element patterns and isotopic heterogeneity which can only be explained as a record of the premetamorphic history. Although some of these eclogites contain diamonds indicating a minimum pressure of 40 kb and 800°C, the metamorphism was not sufficient to completely equilibrate the minerals. LREE are not soluble in Gnts and HREE are not soluble in Cpx. The Gnts might not be equilibrated to mantle conditions because the surrounding Cpx does not transport the REE suitable for Gnts due to limited solubility. The CPX on the other hand might be "locked" by the Gnts for the same reason. This interlocking diffusion system might actually be the reason that eclogites do not equilibrate and contain premetamorphic isotopic and trace element relicts.


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