Home – Home – Jornals – Russian Geology and Geophysics 2009 number 12

We have studied a large (12 × 22 × 30 cm) spinel lherzolite xenolith with undeformed margins in alkali basalt (basanite) from the eroded crater of Late Cenozoic Shavaryn Tsaram-1 volcano in western Mongolia. The xenolith was sampled along its median transversal profile, at every 15-20 mm for bulk chemistry of lherzolite and basalt (ICP MS) and at 4-10 mm for the chemistry of olivine, orthopyroxene, clinopyroxene, and Cr-spinel minerals, and of material filling cracks (LA ICP MS). Incompatible elements (especially LREE) are distributed unevenly over the xenolith, both in lherzolite and in its constituent minerals, as well as in crack-filling material, with abnormal LREE enrichment in some specimens. Judging by the measured trace-element spectra compared with the model patterns, incompatible elements reside in different amounts as interstitial impurity in cracks inside and between mineral grains in lherzolite, also being a substitutional impurity in the lherzolite constituent minerals. Experimental acid leaching of specimens from sites of high crack density showed (La/Yb)_n ratios in the crack fill to be much higher than in the basalt host and more so in bulk lherzolite (180 against 33 and 1.5-3.6, respectively). The proportional contents of P and Ca in the leaching solution, especially in that from the xenolith's center, mark the presence of an apatite microphase, which can be a LREE repository.
The observed patterns of LREE and other incompatible elements in the xenolith and in the host alkali basalt fit a model implying that mobile elements residing as interstitial impurity came with fluids which were released from rising basaltic magma and percolated into the xenolith along cracks.