Significance of trace elements in syntaxial quartz cement, Haushi Group sandstones, Sultanate of Oman

Trace element contents and distributions in authigenic quartz cement in deeply buried (2500–4000 m) Haushi Group sandstones from wells in Oman have been investigated in order to determine the factors that control trace element uptake during precipitation.Scanning electron microscope-cathodoluminescence images show well developed growth zones within the quartz cement, which correlate with chemical zonations observed in electron microprobe Al distribution maps. The most abundant trace elements are Al (50–3000 μg g−1), Li (1–100 μg g−1), Na (1–40 μg g−1), and Ge (0.3–5 μg g−1) with a strong linear correlation between Li and Al and a weaker one between Ge and Al. The molar concentration of Li (+ Na) accounts only for ~ 15% of the charge compensation for Al3+ substitution of Si4+. Though H was not measured in this study, these data indicate a major role of H in charge balancing Al3+. The samples belong to the same stratigraphic unit and have similar petrography, but show considerable variability in absolute trace element concentrations between different wells. This variability does not correlate with either sample depth or temperature and shows no regional pattern, but seems to reflect petrophysical and tectonic differences within the sedimentary basin.Petrographic observations of the cogenetic mineral assemblages and hydrochemical modelling indicate that a change from the equilibrium assemblage quartz–kaolinite (–dolomite) to quartz–illite (–dolomite) reflects a decrease in the CO2 concentration and concurrent variations of the Al concentration. It is concluded that changes in the CO2 concentrations are responsible for fluctuations in fluid Al concentrations and thus likely also in the investigated quartz cements.

Research Highlights
► Fluid's CO2 influences fluid's Al and therewith Al uptake into quartz.
► Fluid's CO2 influences paragenetic mineral assemblages.
► Trace element fluctuations reflect chronological changes of fluid composition.
► Variable trace element concentrations reflect presence/absence of geochemical connectivity in a stratigraphic unit.