Recognizing the Geochemical Consequences of Zircon Alteration at Jack Hills, Western Australia

Abstract:

Detrital zircons from Jack Hills, Western Australia are the best known >4 Ga terrestrial mineral record. As their ca. 3 Ga host quartzite experienced several documented Precambrian metamorphic events, it is unsurprising that various alteration features have been suggested or identified in the zircons. Using these grains to understand early Earth conditions requires knowledge to what extent they have been altered. We have used 200 ion microprobe δ¹⁸O and 67 trace element analyses of these zircons, together with backscattered electron and cathodoluminescence imaging, to assess differences between spots falling on crack-bearing vs. uncracked surfaces. Analyses on uncracked surfaces were further subdivided into those within magmatic zonations versus those in altered regions. We have combined the results with data from previous studies of Jack Hills zircons to identify alteration signatures and their mechanisms. Analyses on cracks overlap the field of uncracked regions in terms of most variables, but also display trends toward distinct compositions. Regions with altered zonationare often intermediate between magmatic and cracked regions but also overlap magmatic regions in other variables. Clear alteration signals include high P (>1000 ppm), high LREE/HREE (Nd/Yb (N) >0.01), high Ti (>900°C apparent T^xlln), high Fe (>1000 ppm), and low HREE/MREE (Yb/Gd (N) <10). Cracks and altered regions are more likely to show high Th/U, low Ce/Ce*, and low δ¹⁸O, but the considerable overlap with magmatic regions limits the diagnostic usefulness of these variables. Crack-associated alteration is likely due to the extensive Fe-Ti oxide and xenotime mineralization seen in the zircons. Many but not all CL-dark regions are consistent with solid-state recrystallization. Bright regions cross-cutting magmatic zonation are evident in some zircons, but we have not yet identified a diagnostic signature of this alteration mode.Mineral inclusions falling on and away from cracks show distinct modal mineralogy, but inclusions falling in altered vs. magmatic regions do not appear to differ and are likely primary. Identifying alteration signatures and likely mechanisms will help guide future work on the Jack Hills zircons, in particular in distinguishing systems robust against alteration.