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Titolo:
Diagenesis and crystal caskets: Echinoderm Mg calcite transformation, Dry Canyon, New Mexico, USA
Autore:
Dickson, JAD;
Indirizzi:
Univ Cambridge, Dept Earth Sci, Cambridge CB2 3EQ, England Univ CambridgeCambridge England CB2 3EQ Sci, Cambridge CB2 3EQ, England
Titolo Testata:
JOURNAL OF SEDIMENTARY RESEARCH
fascicolo: 5, volume: 71, anno: 2001,
parte:, A
pagine: 764 - 777
SICI:
1527-1404(200109)71:5<764:DACCEM>2.0.ZU;2-9
Fonte:
ISI
Lingua:
ENG
Soggetto:
PENNSYLVANIAN HOLDER FORMATION; SACRAMENTO MOUNTAINS; MAGNESIAN CALCITE; CARBONATE ROCKS; DOLOMITE; RECRYSTALLIZATION; PETROGRAPHY; SKELETON; CEMENT; GROWTH;
Tipo documento:
Article
Natura:
Periodico
Settore Disciplinare:
Physical, Chemical & Earth Sciences
Citazioni:
55
Recensione:
Indirizzi per estratti:
Indirizzo: Dickson, JAD Univ Cambridge, Dept Earth Sci, Downing St, Cambridge CB2 3EQ, England Univ Cambridge Downing St Cambridge England CB2 3EQ England
Citazione:
J.A.D. Dickson, "Diagenesis and crystal caskets: Echinoderm Mg calcite transformation, Dry Canyon, New Mexico, USA", J SED RES, 71(5), 2001, pp. 764-777

Abstract

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 bad their pore system (55% volume) filled by magnesian ferroan calcite cement before the stereom transformed (type 1) into small (< 1 to 3 mum dolomite crystals set in a single calcite crystal. Both dolomite and calcite contain round pores < 1 mum in diameter This stereom is texturally similar to E. esculentus stereom heated at 300 degreesC 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 mum) with sharp though irregular margins set in a single calcite crystal. Both dolomite and calcitecontain angular pores up to 4 mum in diameter. The stereom microstructure is largely destroyed by type 2 transformation. The dolomite is poorly ordered and calcian Ca0.55Mg0.44CO3, whereas the calcite is nearly stoichiometric Ca0.99Mg0.01CO3. Stereom that has undergone type I 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 with11 mole % MgCO3.. The preservation of primary growth banding similar to 3 mum wide and dolomite crystals <1 to 3 mum diameter in ossicles that underwent type I transformation indicates that lateral transfer of Mn2+ and probably other ions was only a few microns. Dolomite crystals > 20 mum 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 echinodermswas probably triggered, or accelerated, by rising temperature caused by burial. The bulk composition of stereom that has undergone type I 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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Documento generato il 02/10/20 alle ore 01:28:14