DEVELOPMENT OF LARGE-SCALE ISOTOPIC ZONING BY CRYSTAL-MELT SEPARATION IN MAGMAS UNDERGOING REACTIVE BULK ASSIMILATION

Isotopically zoned plutons and eruptions are common in , even characteristic of, large continental magmatic systems. This zoning is most often manifested as a pluton core or early erupted magma chamber roof that is relatively enriched in radiogenic isotopes. Reactive bulk assimilation, in which fragments of country rock incorporated into a magma undergo partial melting, disaggregation, and mixing, provides a mechanism to produce isotopic zoning without calling upon either selective contamination or sequential emplacement of isotopically distinct batches of magma. In a 400 Ma biotite gneiss, biotite, having high Rb/Sr will be very strongly enriched in 87Sr. Plagioclase, with low Rb/Sr will retain 87Sr/86Sr close to the original protolith (e.g. graywacke) value. During dehydration melting, biotite is consumed and most of the radiogenic Sr from the biotite will enter the melt. The amount of less radiogenic, plagioclase-hosted Sr incorporated into the melt will vary according to the exact stoichiometry of the melt-forming reaction. However, as long as the restite contains some plagioclase, there will be a net transfer of radiogenic Sr to the melt phase and a net retention of non-radiogenic Sr by the restite. Disaggregation and mixing of the xenolith will (in most cases) lead to a net increase in radiogenic Sr in magma as a whole. However, the net increase results from mixing of two isotopically distinct components, a radiogenic melt and relatively non-radiogenic crystals. Once the system is homogenized, this inequality is retained. This can be expressed as:

For the original magma: 87Sro liquid = 87Sro crystals

For the assimilated material: 87Sra liquid > 87Sra crystals

For the physically homogenized system: 87Sro liquid + 87Sra liquid > 87Sro crystals + 87Sra crystals

Thus, any separation of crystals and melt in the physically homogenized system must lead to the effective fractionation of radiogenic isotopes. All that is required for zoning to develop is that the non-radiogenic portion of the magma chamber (e.g. the margin or floor) be enriched in crystals relative to the radiogenic portion (e.g. the core or roof). It goes without saying that non-isotopic chemical heterogeneities can also develop in this fashion.