Trace element and OH content of quartz grains in the Amazon river: Potential application in provenance analysis

Abstract

Quartz grains in sediment carried by the Amazon River and five of its major tributaries were analyzed by FTIR-spectroscopy and secondary ion mass spectrometry to evaluate their potential for provenance analysis. Additionally, heavy mineral analysis was performed in the same samples to support sediment discrimination and provenance interpretation. Average defect water contents in quartz grains carried by branches with headwaters draining the Andes are higher (Solimões River 8.8 wt ppm, Madeira River 7.2 wt ppm) than in branches exclusively draining cratonic areas (Xingu River 6.5 wt ppm, Tapajós River 4.6 wt ppm, Negro River 4.7 wt ppm); values in one Amazon mainstem sample are intermediate with prevalence of the Andean signal (7.7 wt ppm). The average defect water content correlates to the fraction of grains that are defect-water rich (> 10 wt ppm), ranging from 30 to 40 % for the Solimões, Madeira, and Amazon Rivers down to 10–20 % for the Tapajós and Negro Rivers. The average molecular-water signal exhibits a much clearer contrast than the OH-defect signal. Grains from cratonic tributaries (Negro, Tapajós and Xingu rivers) exhibit significantly weaker molecular water signals than grains from Andean tributaries (Solimões and Madeira rivers); slightly weaker than the latter is the signal from grains in the Amazon mainstem, reflecting a mixture of Solimões and Negro rivers quartz in the Amazon mainstem sample.

In contrast, trace-element concentrations widely overlap among samples, preventing any robust discrimination about the sources of quartz grains. Trace metals show a fair correlation between Li and Al in all samples; a weak correlation between Al and Ti (indicative of igneous origin) was observed in Negro, Xingu and Amazon River sands, but not in Solimões, Madeira and Tapajós sands. Boron correlates with Li in Tapajós river and Amazon mainstem samples and with Al in Madeira and Tapajós samples.

Heavy-minerals are mostly amphibole, epidote, augitic clinopyroxene and hypersthene in Solimões River sands largely derived from the Andes, and andalusite, amphibole and epidote in Madeira River sands. Cratonic tributaries contribute durable tourmaline and zircon with andalusite (Negro River) or staurolite and topaz (Tapajós River).

The higher content of OH defects and molecular water in quartz grains derived from the Andes than in those from cratonic areas indicates that this property can provide useful complementary information to discriminate the source of quartz grains, one of the thorniest tasks in provenance analysis.