Interstitial carbonates in pillowed metabasaltic rocks from the Pilbara Craton, Western Australia: A vestige of Archean seawater chemistry and seawater-rock interactions

Abstract

Calcites hosted in the interpillow void spaces of extremely well preserved, 3.47–3.12 Ga pillow lavas of the Archean Pilbara Craton, Australia, provide new geochemical insights into the composition of Archean seawater and its interaction with basaltic crust. We present a comprehensive dataset of major and trace elements, radiogenic 147Sm-143Nd, 87Rb-87Sr, and stable C-O isotopes for these calcites.Based on their elemental composition and Post-Archean Australian Shale-normalized rare earth element and yttrium (REYPAAS) patterns, two types of calcites can be distinguished. Type I (n = 9) are coarse-grained calcites with elevated Mn concentrations (478–14,790 ppm) and high REY concentrations that match the basaltic host rock compositions. Petrographic textures in combination with geochemical data indicate precipitation from boiling seawater trapped between pillows during basalt eruption. Small light-REYPAAS depletions, only slightly super-chondritic Y/Ho (34.5–39.1), and low δ18O(VSMOW2) (9.25–16.66 ‰) suggest relatively high fluid-rock interactions of boiling seawater with the basaltic host rock. These characteristics are similar to those of interstitial carbonates found in modern mid-ocean ridge basalts. Type II carbonates (n = 6) are fine-grained, low-Mn (13.7–420 ppm) calcites that exhibit comparably high Sr concentrations (825–2,516 ppm) and anoxic modern seawater-like REYPAAS patterns, strong super-chondritic Y/Ho (39.1–55.8) and δ18O(VSMOW2) values of 9.10–13.18 ‰. These features hint to direct calcite precipitation from seawater within the degassing space with little fluid-rock interaction.Almost all (Type I and Type II) of the calcites investigated here, combined with their respective host rocks, yield well-defined Sm-Nd isochron ages for the Apex (3,466 ± 23 Ma), Euro (3,365 ± 43 Ma), Honeyeater (3,187 ± 33 Ma), and Bradley (3,131 ± 24 Ma) formations. Only calcites from the Mt. Ada Basalt and their associated host rocks show a ∼ 100 Ma younger Sm-Nd isochron age (3,361 ± 22 Ma) compared to the U-Pb zircon age of formation.Highly radiogenic initial Sr isotope compositions of the high-Mn calcites (Type I) and low Sr concentrations, suggest Sr remobilization for these calcites. In contrast, the low-Mn calcite (Type II) exhibit higher Sr concentrations and lower 87Sr/86Sr(i) of 0.7010–0.7011 at 3.34 Ga and 0.70137 at 3.12 Ga, suggesting increasing influence of crustal weathering on the composition of Paleoarchean seawater through time, and progressive decoupling from the Archean mantle (87Sr/86Sr(i) = 0.7003–0.7007).