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Journal of Sedimentary Research

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Diagenesis and Crystal Caskets: Echinoderm Mg Calcite Transformation, Dry Canyon, New Mexico, U.S.A.

¹ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, U.K.; jadd1{at}esc.cam.ac.uk

The stereom of living Echinus esculentus skeletons is composed of Mg calcite (interambulacral plates 6 mole % MgCO₃; spines 4 mole % MgCO₃) and is compared with the stereom of Pennsylvanian echinoderms from the Holder Formation, Dry Canyon. Some Dry Canyon echinoderms had their pore system (55% volume) filled by magnesian ferroan calcite cement before the stereom transformed (type 1) into small (< 1 to 3 µm) dolomite crystals set in a single calcite crystal. Both dolomite and calcite contain round pores < 1 µm in diameter. This stereom is texturally similar to E. esculentus stereom heated at 300°C for several hours. The bulk composition of this Dry Canyon stereom (dolomite + calcite) is 11 mole % MgCO₃, Sr = 1600 ppm, Fe = 890 ppm, Na = 780 ppm, and S = 2430 ppm. Other Dry Canyon echinoderms had their pore system filled with Mg calcite cement before both stereom and cement were transformed (type 2) into dolomite crystals (1 to 20 µm) with sharp though irregular margins set in a single calcite crystal. Both dolomite and calcite contain angular pores up to 4 µm in diameter. The stereom microstructure is largely destroyed by type 2 transformation. The dolomite is poorly ordered and calcian Ca_0.55Mg_0.44CO₃, whereas the calcite is nearly stoichiometric Ca_0.99Mg_0.01CO₃.

Stereom that has undergone type 1 transformation and some stereom with Mg calcite cement that has undergone type 2 transformation have the same bulk composition: Mg calcite with 11 mole % MgCO₃. The proportions and compositions of dolomite and calcite in these two materials are the same as that modeled stoichiometrically for closed-system transformation of Mg calcite with 11 mole % MgCO₃. The preservation of primary growth banding 3 µm wide and dolomite crystals <1 to 3 µm diameter in ossicles that underwent type 1 transformation indicates that lateral transfer of Mn₂₊ and probably other ions was only a few microns. Dolomite crystals > 20 µm diameter in ossicles that underwent type 2 transformation indicates ionic transfer over tens of microns. The margins of some (type 2) ossicles set in a clay matrix are Fe-rich and contain barite crystals; the Fe₂₊ and Ba₂₊ were transferred from the surrounding clay hundreds of microns into these ossicles.

Type 1 transformation was catalyzed by internal water from the stereom; the presence of a ferroan calcite crystal casket confined alteration to the skeletal Mg calcite. Type 2 transformation involved both stereom and pore-filling Mg calcite cement. The unstable dolomite crystals formed by both transformation processes were protected from further change by the crystal caskets in which they are encased. The transformation of Dry Canyon echinoderms was probably triggered, or accelerated, by rising temperature caused by burial.

The bulk composition of stereom that has undergone type 1 transformation can be used as a proxy for the original, major-element chemistry of echinoderm skeletons. Minor amounts of Mg calcite cement that occur in Dry Canon (type 1) ossicles indicate the chemistry of porewater close to the sediment-water interface.

Fossil echinoderms are preserved in many ways, and it is suggested that their textural variations can be used to map the distribution of diagenetic reactions. The stereom is destroyed in most fossil echinoderms, and even in those where the stereom shape is preserved its internal structure is changed. Echinoderm Mg calcite does not transform with perfect textural preservation.

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