A suite of 15 spinel-peridotite-xenoliths of the Atsagin-Dush volcano, Mongolia have been investigated with an excimer-laser based O-isotope-microprobe. The Atsagin-Dush suite contains fertile lherzolites (CaO>3.0%, Al2O3>3.5%) as well as extremely depleted (CaO<1%, Al2O3<1%) harzburgites. Half the samples are enriched in incompatible elements which are hosted mainly in phenocryst-glass aggregates (melt pockets) and clinopyroxenes, no metasomatic minerals have been found. Because the clearest metasomatic overprint should be observed in melt pockets three samples with large melt pockets have been investigated with the laser-microprobe. d18O values of these melt pockets range from 5.64 to 5.86 (V-SMOW) which is within the range of clinopyroxenes (5.44-6.01), being in line with the formation of melt pockets by metasomatism induced melting of mainly clinopyroxene (Ionov et al., 1994). Metasomatic fluids/melts have therefore not left a characteristic oxygen isotope signature.
d18O values of olivine are high in two extremely depleted harzburgites (5.64 and 5.83), whereas six olivines in slightly depleted samples give lower d18O values from 5.28 to 5.48. The same trend is obtained from calculated bulk rock d18O values. Since extracted melts may be isotopically heavier than restites (Rosenbaum et al., 1994), observed relationships are in contrast to what is expected theoretically. Averages of in-situ measurements on mineral separates of all samples for olivine (5.41±0.18), enstatite (5.90±0.13), diopside (5.71±0.18) are slightly higher than averages of a larger collection of spinel peridotites reported by Mattey et al. (1994) (olivine: 5.14±0.12, enstatite: 5.72±0.14, diopside: 5.54±0.18). Probably 18O enriched water from subducted oceanic crust was involved in melting and depletion of Atsagin-Dush peridotites. Oxygen isotopic composition of spinels range from 4.41 to 5.38 which is mainly controlled by Cr and Al contents.
The spatial resolution of in-situ analyses with excimer-lasers allows to look at mineral heterogeneities. Highest d18O values occur close to the rims of large enstatite crystals, whereas rims of large olivines have low d18O values. Major element profiles of orthopyroxene show depletion in Al and Cr in some 100µm broad zones which are probably caused by a cooling event before xenoliths were collected by the host basalt. Some of the variation of d18O values in olivine and enstatite might be caused by this cooling event too. Some small clinopyroxenes have very low d18O values from 5.2 to 5.5, these clinopyroxenes are often accompanied by spinel and small olivine crystals (<1mm) with relatively low d18O values (4.9 to 5.2). We speculate that these ol-cpx-sp clusters have been formed from garnets as the Atsagin-Dush peridotites crosses the garnet-spinel transition.
Ionov, D.A., Hofmann, A.W. & Shimizu, N., J. Petrol. 35, 753-785 (1994).
Mattey, D., Lowry, D. & Macpherson, C., Earth Planet. Sci. Lett. 128, 231-241 (1994).
Rosenbaum, J.M., Walker, D. & Kyser, K., Geochim. Cosmochim. Acta 58, 4767-4777 (1994).