Metasomatism-induced Melting in Mantle Xenoliths from Mongolia

Mantle xenoliths from two locations in Mongolia contain patchesof glass-phenocryst aggregates (‘melt pockets’)up to 1 cm in diameter, including one ‘composite’xenolith, which shows a complete transition from unaltered spinelIherzolite to a zone containing melt pockets surrounded by acpx and spinel-free peridotite matrix. We have analyzed majorelements by wet chemistry, X-ray fluorescence (XRF), and electronmicroprobe, trace elements by ion microprobe and inductivelycoupled plasma mass spectrometry (ICP-MS) techniques, and Srand Nd isotopes by mass spectrometry, to elucidate the originof these melt pockets. Petrographic and chemical evidence shows that the melt pocketswere formed neither by infiltration of the host basalt nor bydehydration melting of hydrous phases, such as amphibole. Instead,melting was induced by the interaction of a metasomatic fluidwith clinopyroxene and spinel. The reaction produced melts ofvariable composition, with SiO₂ ranging from 52 to 68% and MgOfrom 4.5 to 0.5%. The melts contain euhedral grains of olivine,clinopyroxene, and spinel, and a large number of (now empty)vugs. The melt shows no sign of having invaded the Iherzolitematrix surrounding the pockets. There is some evidence for fractionalcrystallization, but some of the major element chemical trends,such as the negative correlation between Na₂O and SiO₂, cannotbe accounted for by such a mechanism. The glasses and clinopyroxenephenocrysts are very rich in light rare earth elements (LREE)and Sr, and completely dominate the bulk contents of these andsome other incompatible elements in the rocks with melt pockets.The invading fluid introduced high concentrations of LREE, Th,U, Pb, and Sr, but was relatively depleted in Ba, Rb, Nb, Ta,Zr, Hf, and Ti, and had unusually high Zr/Hf and Nb/Ta ratios. Ion microprobe analyses of fresh glass directly adjacent toclinopyroxene microphenocrysts yield a series of cpx-melt partitioncoefficients for REE and several other trace elements. D_Yb (cpx-melt)varies between 0–3 and 1.6 and is positively correlatedwith the A1₂O₃+SiO₂ and Na₂O contents of the glass, and negativelycorrelated with MgO, FeO, and CaO contents. These correlationsare consistent with qualitative predictions from considerationsof silicate melt structure. The clinopyroxenes in the unaltered zones of the composite xenolithshow evidence of an earlier phase of metasomatism which enrichedCe, La, and Sr, but did not affect the other REE. Clinopyroxenesfrom these zones have high _Nd values of + 14 and +19, indicatinga history of low Nd/Sm ratios. At the same time, ⁸⁷Sr/⁸⁶Sr ratiosare high (>0.704), indicating infiltration of relativelyradiogenic Sr during the early stage of metasomatism. Ion microprobetraverses show no zoning of La/Nd ratios. Therefore, there wasenough time to equilibrate the metasomatic effects in the graininteriors, and we estimate the time required for this equilibrationto be of the order of 10⁵ years. In sharp contrast, the second, or main, metasomatic event thatcaused the formation of the melt pockets must have been extremelyshort-lived and probably lasted only hours or days before thexenolith was captured by the magma and erupted at the surface.This short duration is required by the preservation of freshglass and by the lack of equilibration of the melt pockets withtheir surrounding matrix. The isotopic compositions of Sr and Nd are identical betweenmelt pockets and host basalts in both localities. Therefore,we conclude that the metasomatic fluids were probably derivedfrom the same source rocks as the host basalts. We speculatethat the xenoliths originally resided in an upper-mantle regionwhich was intruded by a partially molten diapir. Volatiles wereexpelled from the unmelted margin of the diapir and invadedthe adjacent upper-mantle peridotites. The fluid infiltrationtriggered formation of the melt pockets, whereupon the materialwas picked up by rapidly ascending magma and erupted at thesurface. The fluids appear to have been poor in water, as nohydrous minerals are present among phenocryst or quench phasesin melt pockets. The major component of the fluid may have beenCO₂ or liquid carbonate.