Hoskin and three co-authors reported in early 2000 that numerical reconstructions of processes that may have operated during igneous petrogenesis often model the behavior of important trace elements. The geochemistry of these trace elements may be controlled by accessory mineral saturation and fractionation. Determination of the saturation point of accessory minerals in granitoid rocks is ambiguous because assumptions about crystal morphology and melt compositions do not always hold.
An integrated approach to identifying accessory mineral saturation involving petrography, whole-rock geochemical trends, saturation calculations and mineral chemistry changes is demonstrated here for a compositionally zoned pluton. Within and between whole-rock samples of the Boggy Plain zoned pluton, eastern Australia, the rare earth element (REE)-enriched accessory minerals zircon, apatite and titanite exhibit compositional variations that are related to saturation in the bulk magma, localized saturation in intercumulus melt pools and fractionation of other mineral phases. Apatite is identified as having been an early crystallizing phase over nearly the whole duration of magma cooling, with zircon (and allanite) only saturating in more felsic zones.
Titanite and monazite did not saturate in the bulk magma at any stage of differentiation. Although some trace elements (P, Ca, Sc, Nb, Hf, Ta) in zircon exhibit compositional variation progressing from mafic to more felsic whole-rock samples, normalized REE patterns and abundances (except Ce) do not vary with progressive differentiation. This is interpreted to be a result of limitations to both simple 'xenotime' and complex xenotime-type coupled substitutions. The authors' data indicate that zircon REE characteristics are not as useful as those of other REE-rich accessory minerals as a petrogenetic indicator.
The authors concluded that:
1) Changes in the trace element chemistry of accessory zircon, apatite and titanite relate to changes in the bulk chemistry of the BPZP magma and the effect of localized saturation in trapped intercumulus melts. The REE chemistry of apatite and titanite in the adamellite and aplite reflects fractionation of the LREE from the magma by allanite. The increasing abundances of some trace elements in accessory phases (e.g. P, Sc, Nb in zircon; Mn, Ba, Hf in apatite; Mn, Y, Nb in titanite) from progressively more felsic samples reflects enrichment of the bulk magma in these elements. Large abundance ranges for individual trace elements in a single mineral population are a result of localized saturation in intercumulus melt pools.
2) An integrated approach to identifying saturation points of accessory phases, involving 'inverse' modelling, saturation calculations and mineral chemistry, shows that these phases were saturated in the BPZP: inner granodiorite-apatite; outer and inner adamellite-allanite, apatite and zircon (plus hornblende and biotite); aplite-allanite, apatite, zircon (plus biotite).
3) Zircons from the BPZP have chondrite-normalized REE patterns that show no variation with progressive whole-rock Si saturation. This is interpreted to be a result of simple 'xenotime' and complex 'xenotime-type' substitutions where REE not charge balanced by substituted P are charge balanced by Mg, Al and Fe in interstitial lattice sites.
4) Changing apatite REE abundances and patterns, and successful back-calculation using mineral-melt Kd values to a 'melt' composition, for early-crystallized apatite indicate its potential use as a tracer of processes occurring during magma crystallization. The remarkable similarity both between BPZP zircon REE patterns and to published examples from a range of rock types indicates that zircon REE characteristics are generally not useful as an indicator of magmatic processes.
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