PETROGRAPHY AND GEOCHEMISTRY OF ECLOGITES FROM THE MIR KIMBERLITE, YAKUTIA, RUSSIA

Diamond-bearing eclogites are an important component of the xenoliths that occur in the Mir kimberlite, Siberian platform, Russia. We have s tudied 16 of these eclogite xenoliths, which are characterized by coar se-grained, equigranular garnet and omphacite. On the basis of composi tional variations in garnet and clinopyroxene, this suite of eclogites can be divided into at least two groups: a high-Ca group and a low-Ca group. The high-Ca group consists of high-Ca garnets in equilibrium w ith pyroxenes that have high Ca-ratios [Ca/(Ca+Fe+Mg)] and high jadeit e contents. These high-Ca group samples have high modal% garnet, and g arnet grains often are zoned. Garnet patches along rims and along amph ibole- and phlogopite-filled veins have higher Mg and lower Ca content s compared to homogeneous cores. The low-Ca group consists of eclogite s with low-Ca garnets in equilibrium with pyroxenes with a low Ca-rati o, but variable jadeite contents. These low-Ca group samples typically have low modal% of garnet, and garnets are rarely compositionally zon ed. Three samples have mineralogic compositions and modes transitional to the high- and low-Ca groups. We have arbitrarily designated these samples as the intermediate-Ca group. The rare-earth-element (REE) con tents of garnet and clinopyroxene have been determined by ion micropro be. Garnets from the low-Ca group have low LREE contents and typically have [Dy/Yb](n) < 1. The high-Ca group garnets have higher LREE conte nts and typically have [Dy/Yb](n) > 1. Garnets from the intermediate-C a group have REE contents between the high- and low-Ca groups. Clinopy roxenes from the low-Ca group have convex-upward REE patterns with rel atively high REE contents (ten times chondrite), whereas those from th e high-Ca group have similar convex-upward shapes, but lower REE conte nts, approximately chondritic. Reconstructed bulk-rock REE patterns fo r the low-Ca group eclogites are relatively flat at approximately ten times chondrite. In contrast, the high-Ca group samples typically have LREE-depleted patterns and lower REE contents. The delta(18)O values measured for garnet separates range from 7.2 to 3.1 parts per thousand . Although there is a broad overlap of delta(18)O between the low-Ca a nd high-Ca groups, the low-Ca group samples range from mantle-like to high delta(18)O values (4.9 to 7.2 parts per thousand), and the high-C a group garnets range from mantle-like to low delta(18)O values (5.3 t o 3.1 parts per thousand). The oxygen isotopic compositions of two of the five high-Ca group samples and four of the eight low-Ca group eclo gites are consistent with seawater alteration of basaltic crust, with the low-Ca group eclogites representative of low-temperature alteratio n, and the high-Ca group samples representative of high-temperature hy drothermal seawater alteration. We interpret the differences between t he low- and high-Ca group samples to be primarily a result of differen ces in the protoliths of these samples. The high-Ca group eclogites ar e interpreted to have protoliths similar to the mid to lower sections of an ophiolite complex. This section of oceanic crust would be domina ted by rocks which have a significant cumulate component and would hav e experienced high-temperature seawater alteration. Such cumulate rock s probably would be LREE-depleted, and can be Ca-rich because of plagi oclase or clinopyroxene accumulation. The protoliths of the low-Ca gro up eclogites are interpreted to be the upper section of an ophiolite c omplex. This section of oceanic crust would consist mainly of extrusiv e basalts that would have been altered by seawater at low temperatures . These basaltic lavas would probably have relatively flat REE pattern s, as seen for the low-Ca group eclogites.