INSIGHTS INTO ANCIENT MAGMA CHAMBER EVOLUTION - ISOTOPIC EVIDENCE

A wealth of information concerning magma chamber complexities has been gathered in recent years based primarily on the observation of chemical and isotopic zonation in phenocrysts from Recent volcanic rocks. In particular, considerable data now exists for the Sr isotopic zonation of feldspar phenocrysts in modern tephra. Fewer isotopic zonation data exist for other minerals in young volcanics and there is an almost total absence of data for minerals in ancient tephra, making it difficult to assess the level of complexity of magma chamber evolution in ancient systems. To test the potential for intracrystal isotopic variation in highly altered ash beds we determined initial Sr isotopic compositions of apatite from an extensive ~ 450 Ma K-bentonite collected in the southeastern USA. Although single crystals have not yet been measured, groups of a small number of crystals from a vertical transect in the bentonite show similar isotopic variations that are seen in phenocrysts from recent tephra. Single-crystal work will allow for more detailed comparisons with Quaternary systems.

There have been far fewer studies investigating the isotopic variability in minerals from plutonic rocks than in volcanic rocks, despite the potential insights that might be gained from such studies. There is an increasing body of Hf isotopic data from zircon which suggests that mineral isotopic zonation might be as common in plutons as in tephra, but because of the possibility of zircon xenocrysts and the large isobaric corrections that must be made during in situ Hf isotope ratio measurements it would be beneficial to have additional isotopic data from other minerals. We thus investigated the utility of measuring both Sr and Nd isotopic ratios in apatite from a series of ~ 300 Ma Appalachian granitoids. Although many of the plutons show only a small range in isotopic composition between core and rim, several plutons demonstrate very large differences. It is thus possible that isotopic homogeneity of minerals in granitic plutons is the exception rather than the rule.