Boehm, Florian; Eisenhauer, Anton; Fietzke, Jan; Rausch, Svenja; Kluegel, Andreas; Bach, Wolfgang (2011): Calcium isotope fractionation during dolomite formation. Mineralogical Society, London, United Kingdom, In: Anonymous, Goldschmidt 2011 abstract volume, 75 (3), 544, georefid:2012-064475

Dolomite is an important component of the global calcium cycle, being a source of Ca to the oceans [1]. Calcium released to the oceans by dolomitization of chalk and limestones can potentially be quantified using marine Ca isotope records [2]. However, little is known about Ca isotope fractionation and the Ca isotopic composition of dolomitic rocks. Theoretical calculations point to a -2 ppm depletion in delta (super 44/40) Ca of dolomite relative to calcite at 25 degrees C [3]. Ordovician dolostones interbedded with limestones were reported to be depleted in delta (super 44/40) Ca by -0.6 ppm relative to the limestones [4]. We have investigated dolomites from an ODP core (Site 183-1140) drilled at the Northern Kerguelen Plateau (46.3 degrees S 68.5 degrees E, 2394 mbsl). The core penetrated 235 m of nannofossil ooze and chalk of early Oligocene to middle Miocene age, and 88 m of pillow basalts forming the basement for the sediments. The basalts erupted at about 34 Ma (latest Eocene) [5]. An interbedded chalk layer was found in the basalt, about 40 m below the top of the basement. The chalk was partly dolomitized at the contact with the basalt. We measured oxygen, carbon, calcium and radiogenic strontium isotopes of bulk chalk and dolomite samples. The (super 87) Sr/ (super 86) Sr ratios indicate an age of dolomitization of about 10 Ma (late Miocene). Dolomitization obviously occurred about 20 Ma after eruption of the basalts, in a deep burial setting. Oxygen isotope values of chalk and dolomite demonstrate that dolomitization occurred at slightly elevated temperatures (10-20 degrees C). The calcium isotopes of the dolomite are enriched in delta (super 44/40) Ca by about +0.5 ppm compared to the chalk. This is in contrast to the depletion of delta (super 44/40) Ca in dolomite reported in the literature. On the other hand, dolomite veins in ocean crust basalts (DSDP/ODP Sites 37-332 and 129-801) are depleted in delta (super 44/40) Ca compared to calcite veins of similar age. Fluid composition, diagenetic history and kinetic isotope fractionation have to be considered when interpreting Ca isotope values of dolomite.
West: 68.2930 East: 68.2930 North: -46.1536 South: -46.1536
Expedition: 183
Site: 183-1140
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