Magnesium-isotope and REE compositions of Lower Ordovician carbonates from eastern Laurentia: Implications for the origin of dolomites and limestones

Abstract Carbonates representing different depositional and diagenetic settings (near-surface to deep burial), including modern sabkha, were collected from the Lower Ordovician St. George Group in eastern Laurentia (western Newfoundland, Canada). Based on petrographic examination, three dolomite phases were identified: D1 dolomicrite (crystals ranging from 4 to 50 μm), D2 dolomite (50 to 150 μm) and D3 saddle dolomite (≥ 500 μm). They occur as replacements and cements, and exhibit dull (for D1 and D3) to zoned (for D2) luminescence. The occurrence of near-micritic dolomites (D1) suggests that dolomitization started at low temperatures under near-surface conditions during an early stage of diagenesis, whereas microthermometric studies of the D2 and D3 dolomites confirmed their formation at higher temperatures, on average, of 112 ± 19 °C and 153 ± 30 °C, respectively, under deeper burial conditions. The D1 dolomicrite yields an average δ 26 Mg value of − 1.92 ± 0.30‰ (DSM3), which is slightly more negative than those of the D2 dolomite with − 1.75 ± 0.34‰ (DSM3) and D3 saddle dolomite with − 1.58 ± 0.31‰ (DSM3). The slightly more positive δ 26 Mg values of the higher-temperature dolomites (D2 and D3) suggest insignificant kinetic fractionation and slight overprinting by 26 Mg-enriched diagenetic fluids recirculating in siliciclastic and 26 Mg-rich crustal rocks under closed to semi-closed diagenetic conditions. The Sr-isotope signatures support the formation of D1 dolomicrite in an early diagenetic stage and the REE compositions of the investigated dolomites favor their formation in semi-closed to closed diagenetic systems with fluid compositions evolving by circulation through crustal rocks with progressive burial. Furthermore, the Mg isotopes suggest that the lime mudstones of the St. George Group had an algal origin, whereas the D2 dolomite was sourced and subsequently altered mostly from the D1 dolomicrites of the succession.