Clumped isotope thermometry reveals diagenetic origin of the dolomite layer within late Ordovician black shale of the Guanyinqiao Bed (SW China)

The origin of widespread dolomite deposits in Paleozoic strata is a longstanding open question. Here we use a suite of geochemical methods to investigate a prominent Paleozoic dolomite occurrence in SW China. The upper Ordovician Wufeng Formation of the upper Yangtze Block (SW China) covers the time interval just before the Ordovician-Silurian Mass Extinction event, consisting of wide-spread black shale with an interlayer of bioclastic limestone, the Guanyinqiao Bed. In the western part of the Yangtze Block, in Shizhu county however, the Guanyinqiao Bed contains a particular organic-rich dolomite interlayer of which the formation mechanism is unclear. To distinguish between potential primary dolomite precipitation and late stage diagenetic/magmatic alteration, we studied this dolomite interlayer with petrological and geochemical techniques including microscopy, REE content and carbonate clumped isotope thermometry. The petrological observations show micritic dolomite with euhedral-subhedral shape and relatively low ordering (0.33 to 0.39). Conventional stable isotope analysis revealed negative carbon (δ¹³C of −3.17 ± 0.04‰ to −2.48 ± 0.03‰) and oxygen isotope compositions (δ¹⁸O of −11.28 ± 0.18‰ to −10.99 ± 0.20‰), and the organic carbon content ranged from 3.69% to 4.33%. The averaged clumped isotope temperature estimate revealed hot dolomite formation conditions of 87₋₁₀⁺¹¹ °C, in a presumable fluid source with isotopic composition of δ¹⁸O_w = −1.8_−1.5^+1.5 ‰. The elevated formation temperature estimates, the slightly depleted δ¹⁸O of dolomite (~−11.14‰) with respect to contemporary deposited limestone (~−10‰) and the δ¹⁸O_w which is similar to the assumed late Ordovician seawater points at diagenetic alteration of primary limestone at a burial depth of approximately 3000 m. The analyzed dolomite samples with low contents of Fe, Zn and small Eu anomaly show further evidence for a diagenetic fluid source and exclude magmatic/hydrothermal alteration. We suggest that at this burial depth the temperature pressure conditions triggered significant Mg release from the surrounding clay to the diagenetic fluid, providing ideal conditions to transform the primary limestone into diagenetic dolomite.