A New Type of Mantle Metasomatism? Trace Element Composition of Feldspar-Bearing Peridotite Xenoliths in Basalts from Southern Siberia.

D. A. Ionov1, W. L. Griffin1, V. S. Prikhodko2 and S. Y. O'Reilly1, 1GEMOC, School of Earth Sciences, Macquarie University, 2109 NSW, Australia; dmitri.ionov@mq.edu.au, 2Institute of Tectonics and Geophysics, Far-Eastern Branch, Russian Academy of Sciences, 65 Kim Yu Chen str., Khabarovsk 680063, Russia.

Alkali-rich feldspar has been found in lherzolite xenoliths in Cenozoic alkali basalts in the Hamar-Daban range in southern Siberia (Ionov et al., 1995) and southern Far East of Russia (SE of Habarovsk). 'Anhydrous' spinel lherzolite is the most common xenolith rock type at both localities; about half of them contain interstitial feldspar and some have feldspar-rich veins. The interstitial feldspar occurs as fine-grained aggregates in reaction zones adjacent to corroded spinel and orthopyroxene in association with secondary olivine (apparently formed by the reaction: spl + opx + cpx + fluid Æ fs + ol). The feldspar-rich veins are thin, irregular and are cross-cutting or follow grain boundaries. Associated with feldspar are tiny grains of Ti-rich oxides (ilmenite, armalcolite, rutile) and Cr-spinel; no silicate glass was found. The feldspars are generally alkali-rich; their K2O content ranges from 0.3 to 11.2% and is much higher than in plagioclase from massif lherzolites (usually <0.1% K2O). The Ca-opx temperature estimates range from 950 to 1010°C for the Hamar-Daban samples and 900-950°C for the Habarovsk lherzolites.

Trace elements were determined in minerals by laser ablation ICP-MS and in whole-rocks by solution ICP-MS. Common for the xenoliths from both localities are high contents of Rb and Ba in the feldspar (reaching 200 times primitive mantle (PM) values) and the enrichment of whole-rocks in alkalies (Na, K, Rb) relative to unmetasomatised mantle lherzolites, but the abundances of other incompatible elements vary broadly. Most Hamar-Daban xenoliths and their clinopyroxenes (cpx) are depleted in light REE, Nb, Th; few are slightly enriched in LREE, Sr, Th and U. Their feldspars show REE patterns similar to those of coexisting cpx at somewhat lower levels, but have moderate to strong positive anomalies for Sr, Pb, Th, Ba, Rb. In contrast, the feldspar-oxide aggregates in xenoliths from the Habarovsk site show consistent enrichment in incompatible elements from heavy to light REE (40-200¥PM for La), Nb, Th, U, Ba, Rb. Their compositions in individual samples are heterogeneous; some analyses have yielded very high contents (100-500¥PM) of Nb, Th, U, Zr, Hf that may largely reside in the Ti-rich oxides (Haggerty, 1991). The contents of medium and light REE, Sr, Pb, Zr, Hf, Nb, Th, U are much higher in the feldspar-oxide aggregate than in the coexisting clinopyroxene. The cpx is moderately enriched in LREE and Th; this enrichment is stronger in the rims of cpx grains than in the cores indicating lack of chemical equilibrium.

The formation of the alkali feldspar (accompanied by Ti-rich oxides) appears to be a relatively recent phenomenon related to infiltration of an alkali-rich fluid into spinel peridotites. The absence of amphibole and mica in peridotite xenoliths at both localities may indicate low H2O contents in the fluid. The unusual mineralogical composition and trace element signatures of the feldspar-bearing xenoliths indicate a specific type of mantle metasomatism, distinct from those commonly attributed to H2O-rich fluids, carbonate melts or Fe-Ti-rich silicate melts.

Ionov D.A., S.Y. O'Reilly and I.V. Ashchepkov, 1995. Feldspar-bearing lherzolite xenoliths in alkali basalts from Hamar-Daban, southern Baikal region, Russia. Contrib. Mineral. Petrol. 122, 174-190.

Haggerty, S.E., 1991. Oxide mineralogy of the upper mantle. In: Oxide Minerals, Reviews in Mineralogy, Miner. Soc. Amer., vol. 25, 355-416.

Back to the GEMOC Abstract Titles Page