Archaean phosphates: a case study of transformation processes in apatite from the Barberton greenstone belt

Łukasz Birski,J Götze,M Koch-Müller,R Wirth,A Kuras,K Simon,A Wudarska,A Lepland,A Hofmann,E Słaby

doi:10.1007/s00410-019-1560-z

Abstract

Multiple tools have been used to determine the sensitivity of phosphates from the early Archaean Barberton greenstone belt to transformation. The assessment of the degree of transformation is crucial for verifying data about the parameters of the paleo-environment. From the obtained results, three generations of phosphates can be distinguished. Group A is observed in cherts and banded iron formation BIF early-generation fluor-hydroxyapatite that precipitated from seawater. It is characterized by flat rare earth element (REE) patterns with a positive Eu anomaly and high Y/Ho ratio in the range of 54–70. Apatites in this group lack any visible indicators of secondary alterations at the micro- and nanoscales. Fourier transform infrared spectra indicate that these apatites are relatively rich in water, and, due to cationic substitution, their OH-stretching regions exhibit complex ordering and numerous component bands. The characteristics observed in the cherts and silicified felsic volcaniclastics of group B imply advanced metasomatic alteration. They exhibit light and heavy REE depletion and an absence of water in the halogen site. Nanoscale investigations reveal cracks, pores, nanofluid inclusions and nanochannel-like structures, as well as inclusions. Group C is represented by igneous-derived apatites that partially reflect their igneous origin. The phosphates are predominantly fluorapatite with typical magmatic apatite REE distribution patterns. Imaging at the micro- and nanoscales indicates that they partially preserve the signature of igneous origin. It seems that some of the analyzed apatite partially preserved their primordial features; therefore, they might be used for the reconstruction of Archaean abiotic systems.

Highlights

The calcium phosphate mineral apatite, with the general formula Ca10(PO4)6(F,Cl,OH)2, occurs in a wide range of geologic and biologic settings on Earth
Three generations of apatite occurring in the Barberton greenstone belt (BGB) can be designated as follows: A
The rare earth element (REE) distribution patterns are flat with positive Eu anomalies and without a negative Ce anomaly, and the preserved Y–Ho ratios of 54–70 are comparable to those of modern seawater (Bau and Dulski 1994)

Summary

Introduction

The calcium phosphate mineral apatite, with the general formula Ca10(PO4)6(F,Cl,OH), occurs in a wide range of geologic and biologic settings on Earth. The Barberton greenstone belt (BGB) in southern Africa (3.55–3.22 Ga), one of the oldest well-preserved volcanic and sedimentary sequences on Earth, has been metamorphosed to relatively low grades. The carbonaceous material in these rocks has been interpreted as being of biological origin and may contain the oldest recognized microfossils (e.g., Walsh 1992; Walsh and Lowe 1999; Westall et al 2001; Javaux 2006; Hickman-Lewis et al 2018). These unique features make the rocks of the BGB ideal candidates to investigate apatite formation conditions during the early Archean. Because apatites/phosphates show some sensitivity to transformation, we try on the basis of their composition and structure to determine the degree of their susceptibility to secondary changes

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