Origin of anhedral Sr-rich calcite in deep-water mixed sediment, north-east Australia



    1. Ottawa-Carleton Geoscience Centre, and Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S 5B6
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At burial depths of 800-1000 m, within the epicontinental Queensland Trough of north-east Australia (ODP Site 823), microcrystalline inter- and intraskeletal mosaics of anhedral (loaf-shaped, rounded) calcite have Sr2+ values ranging from below microprobe detection limits (<150 ppm) to 8100 ppm. Host rocks are well lithified, fine-grained mixed sediment to clayey wackestone and packstone of Middle and Late Miocene age. Petrography demonstrates that calcite precipitation has spanned shallow to deep burial, overlapping formation of framboidal pyrite in the upper 50 m; shallow-burial dolomitization (<300 m); and dedolomitization during sediment consolidation and incipient chemical compaction at greater (>400–500 m) depths. Petrographic observations illustrate that the calcite microfabric formed through coalescing crystal growth resulting from one or a combination of displacive growth in clay, porphyroid neomorphism of aragonite/vaterite, and clay replacement by calcite. Sr2+ mean concentrations in calcite between depths of 800 and 1000 m are similar to an expected equilibrium pore-water concentration, using a Dsr of 0.06, and may indicate active calcite precipitation. However, Sr2+ variation (2000–5000 ppm) within and among crystals, and concentrations that range well above predicted equilibrium values for a given depth, illustrate either variable Sr2+ retention during recrystallization of shelf-derived aragonite (and authigenic local vaterite) or relative uptake of Sr2+ during calcite precipitation with burial. Within the context of calcite formation during burial to 1 km, diagenetic attributes that affect the latter process include increased concentrations of pore-water Sr2+ with depth associated with aragonite recrystallization/dissolution; upward migration of Sr-rich pore water; and increased DSr related to local variation in precipitation/recrystallization rates, differential crystal sector growth rates and/or microvariation in aragonite distribution.