Dissolution - reprecipitation reactions as a mechanism for magma contamination: An example from interaction of partially melted sanidine megacrysts and clinopyroxene phenocrysts in nephelinite from Graulei, West Eifel Volcanic Field, Germany

Abstract

Sanidine – diopside – aegirine-augite enclaves in a nephelinite from the West Eifel Volcanic Field in Germany provide new insights into the process of xenolith assimilation. The enclaves originated as sanidine megacrysts which were entrained by the rising nephelinite magma and partly melted at ~1190 °C and 0.5 GPa. The partly melted sanidine captured clinopyroxene phenocrysts (Type 1 cpx) from the nephelinite. These captured phenocrysts underwent dissolution and reprecipitation to form sieve-textured to euhedral near end member diopside crystals (Type 3 cpx) that grew epitaxially on the original phenocryst. The components of the dissolved phenocryst that were not taken up by the diopside were enriched in the potassic melt that formed the enclave, and during quenching formed aegirine-augite crystals (Type 4 cpx) that locally overgrow the Type 3 diopside. Only enclaves > ~ 5 mm preserve the diopside – aegirine-augite – sanidine assemblage. Small Type 2 clinopyroxene-and leucite-rich clusters in the nephelinite are the remnants of assimilation of mm-sized sanidine crystals. The mechanism of phenocryst capture by molten enclaves shows that the assimilation process is more complex than simple dispersal of melt and crystals from disaggregated xenoliths and that crystal transfer from the magma to partly molten xenoliths may play a significant role in the process.