Distinguishing primary and secondary inclusion assemblages in Jack Hills zircons

•Mineral inclusions in Jack Hills zircons are classified by relation to alteration features.•Distinct modal mineralogies are seen in assemblages intersecting cracks, filling cracks, and isolated from cracks.•Inclusions isolated from cracks do not differ in mineralogy from magmatically zoned to possibly altered regions; are likely primary.•Distinct primary assemblages in different zircon age populations indicate provenance shifts.

Detrital igneous zircons from Jack Hills, Western Australia, range in age from ~ 3.0 to nearly 4.4 Ga and contain an inclusion assemblage dominated by quartz and muscovite, cited as evidence of their derivation from peraluminous granitoids. However, some phosphate inclusions in these zircons are known to be secondary from their post-depositional U–Pb ages and manifest mineralization along cracks. We undertook a survey of mineral inclusions in 4.3–3.0 Ga Jack Hills zircons with particular emphasis on their relationship to possible alteration features (e.g., cracks, disturbed internal zonation, and visual turbidity). Mineral inclusions revealed at polished surfaces show variations in modal mineralogy, mostly corresponding to their relationship with cracks. Muscovite is common both on and away from cracks, although the chemistry of muscovite inclusions shows little relationship with other potential alteration features. Inclusions filling cracks (secondary) and inclusions isolated from cracks differ in their modal mineralogy, although both suites are rich in muscovite and quartz. The higher incidence of crack-intersecting inclusions among younger zircons may reflect effects of the (generally larger) inclusion size among younger zircons. Mismatches between the isolated and crack-intersecting populations indicate selective loss of certain phases (e.g., feldspar, apatite) and over-representation of quartz and muscovite along cracks likely due to the effects of larger inclusion size and varying degrees of overpressure following zircon cooling and decompression. Inclusions not associated with cracks in magmatically zoned versus regions with disturbed zoning have similar phase proportions. This indicates only minor inclusion replacement away from cracks (i.e., the isolated assemblage is likely primary). This holds true also for inclusions within visually turbid versus clear volumes of zircon. Phase proportions within the inclusion assemblages differ with age indicating a provenance shift toward fewer mafic phases and apatite in < 3.6 Ga relative to Hadean granitoid sources.