Abstract
Abstract Precambrian carbonates record secular variations in the style of CaCO3 nucleation and growth, yet the geochemical conditions recorded by some enigmatic textures remain poorly quantified. Here, we performed CaCO3 nucleation experiments in synthetic seawater in order to constrain the mineralization pathways of synsedimentary calcite microspar cement, a prolific component of Proterozoic carbonates. We found that dissolved PO4 above ∼12 μmol/L (µM) inhibits the nucleation of aragonite and calcite and permits the formation of an amorphous Ca-Mg carbonate (ACMC) precursor once CaCO3 supersaturation (Ωcal) is ≥ 45. Depending on seawater Mg/Ca, ACMC then rapidly recrystallizes to monohydrocalcite and/or calcite. This precipitation mechanism is consistent with sedimentological, petrographic, and geochemical characteristics of Proterozoic synsedimentary calcite microspar cement, and it suggests that kinetic interactions among common seawater ions may open nontraditional CaCO3 mineralization pathways and sustain high CaCO3 supersaturation.
Highlights
Before the advent of skeletal biomineralization, CaCO3 production is thought to have been strongly influenced by inorganic processes
We examined CaCO3 nucleation in the presence of PO4 with two types of experiments conducted in synthetic Tonian seawater at 22 ± 1.5 °C and stirred at 250 rpm: (1) degassing experiments, and (2) constant composition experiments (Fig. S1 in the Supplemental Material1)
Degassing and constant composition experiments showed that the Ωcal threshold at which CaCO3 nucleation occurs is strongly influenced by dissolved PO4 (Fig. 2; Tables S1C and S1D)
Summary
Before the advent of skeletal biomineralization, CaCO3 production is thought to have been strongly influenced by inorganic processes. Distinctive sedimentary and early diagenetic fabrics documented in Precambrian rocks record secular variations in marine carbonate chemistry (Grotzinger and James, 2000); the specific chemical controls on abiotic CaCO3 production are poorly constrained. A enigmatic product in this regard is synsedimentary calcite microspar cement, a widespread and abundant primary pore-filling component of Mesoproterozoic to Neoproterozoic (Tonian) carbonates (Fig. 1; James et al, 1998). This cement comprises mosaics of equant 5–15 μm calcite crystals that share identical cathodoluminescence characteristics and preserve evidence for initial spheroidal particle precipitation with polygonal overgrowths (Fairchild and Spiro, 1987; Pollock et al, 2006). Unlike other late Proterozoic carbonate components that clearly preserve evidence of former aragonite (consistent with relatively high Mg/Ca in Tonian seawater; Spear et al, 2014), the primary mineralogy of microspar is unconstrained
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
