IN-SITU PETROCHRONOLOGY AND TRACE ELEMENT GEOCHEMISTRY OF APATITE: A USEFUL DEEP-TIME INDICATOR OF DEPOSITIONAL ENVIRONMENT

A well-preserved microbialite reef complex from the c. 2.4 Ga Turee Creek Group in Western Australia, provides rare insight into life at the time of the Great Oxidation Event (GOE). However, interpreting environmental information from this deposit is difficult due to the combined effects of diagenesis, metamorphism and re-working. Here we investigate a peritidal phosphorite within the shallow water section of the reef complex to explore local environmental conditions. The phosphorite is heterogenous, and composed of in situ to ripped-up microbial sediment and mat fragments as well as peloids with variably preserved matrices [1]. Textures contain organic and dispersed carbonate-fluorapatite-rich, to clay-rich matrices with euhedral apatite grains and organic matter (OM) lining the rim[1]. Here we collected U-Pb isotopic and trace-element data during in situ Laser Ablation Split Stream (LASS) from samples across the breadth of the reef complex (n=168). Preliminary data showed a large spread of dates with no meaningful age or consistent rare earth element (REE) + Y patterns.

However, we were able to extract meaningful patterns by tying chemistry to spatial and textural differences. Euhedral apatite within clay-rich matrices that have arched, concave-down patterns and are dated to c. 1.8 Ga were interpreted to have been recrystallised during regional metamorphism. Microbial sediment fragments with OM-rich matrices and low clay have flat, HREE depleted patterns with positive Y and La anomalies, a mixed riverine-oceanic signature. Different grains within this texture type are dated to ~2.1-2.3 Ga and are considered the most meaningful age in this data set for the timing of deposition. One microbial sediment fragment with higher clay content, slight MREE enrichment and general REE enrichment compared to other microbial sediment fragments (suggestive of secondary processes) has an unrealistically old date (~3.5 Ga) that can be dismissed due to REE enrichment correlated with Th gain and U loss.

While interpretation is complicated by sample heterogeneity we show how spatially resolving geochemistry with texture can produce meaningful depositional environmental information from data affected by diagenesis and secondary recrystallization.

[1] Soares G.G., et al. (2019), Precambrian Research 320, 193-212.