Application Of Clumped Isotope In Dolomite Research

The“dolomite problem”have been debated for a long time between researchers and the enormous ecnomic potential make it worthy of study.To have a better understanding of dolomite formation mechanism,this paper integrates clumped isotope thermometry,petrological,mineralogical and conventional geochemical approaches to study dolomite of the Upper Ordovician Wufeng Formation in upper Yangtze region and the Miocene-Pliocene Huangliu-Yinggehai Formations in Xisha region.Samples of the Wufeng and Longmaxi Formations of the Qiliao Section are observerd under optical microscope and scanning electron microscope and measured by X-ray diffraction analysis.The Wufeng and Longmaxi Formation are mainly comprised of black siliceous shale with high contents of organic matter.Abundant of different graptolite fossils can be observed from hand-samples.Stratified quartzs can be observed under optical microscope and pyrite framboids can be recognized under scanning electron microscope.The stratum at the top of the Wufeng Formation called the Guanyinqiao Bed is mainly comprised of black dolomite.No graptolite fossils can be found in the hand-samples.Micritic dolomite,radiolarian fossils and bitumen residual can be observed under optical microscope.Finely crystallined dolomite crystals can be recognized under scanning electron microscope.The X-ray diffraction analysis shows dolomite samples have relatively low cation degree of order（0.33-0.39）which suggests a relatively disordered arrangement of cations.The organic carbon measurement reveals total organic carbon（TOC）content of the Wufeng Formation black siliceous shale is 8%,the TOC content of Longmaxi Formation black siliceous shale is 10%and the TOC contents of black dolomite of the Guanyinqiao Bed are 3.69%-4.33%which indicate high-organic content dolomite.The major,minor and trace elements analysis results suggest black dolomite samples have relatively high contents of Mn and Zr and relatively low contents of Fe,Zn,Sr and Th elements.The rare earth elements（REEs）show enrichment in light rare earth elements（LREEs）and depleted in heavy rare earth elements（HREEs）.Besides,the Eu/Eu*value shows minor positively anomaly（1.07）and Ce/Ce*value shows minor negatively anomaly（0.95）which suggest the samples are not sufferd from high temperature fluid.The clumped isotope analysis suggestsδ¹³C（VPDB）values of dolomite samples are range from-3.17±0.04‰to-2.48±0.03‰andδ¹⁸O（VPDB）values are range from-11.28±0.04‰to-10.99±0.20‰.TheΔ₄₇ values are range from 0.418±0.027‰to 0.481±0.027‰suggest dolomite formation temperatures are range from 73°C to 122°C.Moreover,δ¹⁸O_w（VSMOW）values of dolomitization fluid source are range from-3.7±2.4‰to 1.2±2.3‰.Overall,the petrological,mineralogical and geochemical characters suggest the high temperature of the Guanyinqiao Bed dolomite is not related to magmatic hydrothermal event.The dolomite is formed in a medium-depth burial environment with high geothermal gradient.The required Mg²⁺ions needed for dolomitization are supposed mainly supplied by clay minerals.Samples of the Miocene-Pliocene Huangliu-Yinggehai Formations from the XK-1 well are observed under optical microscope,scanning electron microscope and measured by X-ray diffraction analysis.The upper Miocene Huangliu Formation is mainly comprised of bioclastic dolomite and bioclastic limestone.Micritic dolomite in molded pores with cloudy center and clear rim and algal dolomite with abundant molded pores can be observed under optical microscope.The Pliocene Yinggehai Formation is mainly composed of limestone and only small amounts of dolomitic limestone,bioclastic dolomite and reef dolomite can be found at the top of the 2^ndmember of the Yinggehai Formation.Finely crystallined dolomite crystals can be observed under scanning electron microscope.The X-ray diffraction analysis suggests moderate cation degree of order that ranges from 0.40-0.50 which suggest a relatively normal arrangement of cations.The minor and trace elements analysis suggest all samples have extremely low contents of Zn,Th and Zr and relatively low content of Sr element.The REEs pattern shows enrichment of LREEs and depleted in HREEs.The Eu/Eu*value shows minor positively anomaly（1.07）and Ce/Ce*value shows significant negatively anamoly（0.43）which suggest the samples are not suffered from high temperature fluid and are formed in relatively oxygen-rich environment.Clumped isotope analysis suggests theδ¹³C（VPDB）values of dolomite samples are range from1.83±0.05‰to 2.58±0.04‰andδ¹⁸O（VPDB）values are range from 0.45±0.10‰to2.98±0.10‰.TheΔ₄₇ values are range from 0.598±0.002‰to 0.657±0.002‰suggest dolomite formation temperatures are range from 16.9..°C to 35.3.°C.Furthermore,δ¹⁸O_w（VSMOW）values of dolomitization fluid source are range from-0.6±1.8‰to 1.8±1.5‰.Above all,the petrological,mineralogical and geochemical characters suggest dolomite of XK-1 well is formed in lagoonal environment.The dolomite of the Huangliu-Yinggehai Formation in the studied core is formed due to hypersaline water reflux and the dolomite maybe affected by the circulation of seawater driven by tide.The Mg²⁺ions are supplied by seawater and the relatively low formation temperature of dolomite are consistant with contemporary seawater temperature.Furthermore,similar characteristics can be found by compiling and comparing formation temperature data of low temperature Miocene-Pliocene dolomite from low to middle latitude area.The formation temperatures of all dolomites are range from16°C-45°C and all burial depths are no deeper than 600 m.The relatively low temperature and shallow burial depth suggest all dolomites are not sufferd from deep burial geothermal gradient and magmatic hydrothermal event.All formation temperatures of dolomites are robust to represent their original formation temperatures.By comparing with contemporary global temperature,the author suggests the formation temperatures of low-temperature dolomites from low to middle latitude area,buried in shallow depth and not suffered from high temperature are coincide with global climate changes.