显生宙全球海水化学成分演化及其对蒸发岩沉积的约束

通过搜集显生宙以来不同地质时期内海相碳酸盐岩鲕粒及胶结物矿物成分、钾盐矿床矿物种类及组合特征、蒸发岩盆地中石盐流体包裹体成分,并利用这些资料与人工海水模拟实验得到的石盐中Br分配特征的对比,得出海水成分在5.5亿年以来的显生宙期间,经历了五个阶段:其中晚元古代至寒武纪早期、二叠纪早期至中生代早期、新生代早期至现今,这些时期的原始海水组成特征系数m(SO_4~(2-))+m(HCO_3~-)/2m(Ca~(2+)),为Na-Mg-K-SO_4-Cl型海水,此期间沉积的钾盐矿床的钾镁盐矿物主要为钾盐镁矾、无水钾镁矾、杂卤石、硫酸镁石等含MgSO_4矿物,海相鲕粒和碳酸盐胶结物矿物成分为文石;而寒武纪早期至石炭纪、中生代早期至新生代早期,原始海水组成特征系数m(Ca~(2+))m(SO_4~(2-))+m(HCO_3~-)/2,为Na-Mg-KCa-Cl型海水,此期间沉积的钾镁盐矿物主要为光卤石和钾石盐,甚至含有溢晶石,海相鲕粒和碳酸盐胶结物矿物成分为方解石。根据石盐流体包裹体成分计算得出:显生宙期间,海水K+含量大部分时间变化幅度较小,为9.3～11.5mmol/kg H_2O(除了石炭纪和晚元古代),平均为10.55mmol/kg H_2O。Mg~(2+)含量在早寒武世≥67mmol/kg H_2O、晚志留世至中泥盆世31～41mmol/kg H_2O、晚古生代≥48mmol/kg H22O、晚白垩世34mmol/kg H_2O和现代55.1mmol/kg H_2O。Ca~+含量在晚元古代至古生代早期≤11mmol/kg H_2O、古生代早期至石炭纪22～35mmol/kg H_2O、石炭纪至中生代早期≤17mmol/kg H_2O、中生代早期至新生代早期19～39mmol/kg H_2O及新生代早期至今7～21mmol/kg H_2O。SO_4~(2-)含量在晚元古代至古生代早期≥23mmol/kg H_2O、古生代早期至石炭纪5～17mmol/kg H_2O、石炭纪至中生代早期13～22mmol/kg H_2O、中生代早期至新生代早期5～19mmol/kg H_2O及新生代早期至今12～29.2mmol/kg H_2O。海水Ca~(2+)与SO_4~(2-)含量的相对变化是控制海相钾盐矿床钾镁盐矿物类型的基本因素。同时,利用以上数据计算得到的显生宙各时期海水[m(Mg~(2+))+m(SO_4~(2-))]/[m(K~+)+m(Ca~(2+))]的变化与各时期海相蒸发岩系石盐层底部的Br含量变化具有同步性,进一步验证了显生宙期间海水成分是不断变化的,是约束海相蒸发岩钾盐矿物类型的主要因素。海水成分变化的控制因素为洋中脊热液和陆地水,其中洋中脊热液起主要作用,而控制这些因素变化的根本原因为板块构造运动。

The chemical evolution of seawater during the Phanerozoic: Constriants on evaporites deposition

Information on the mineralogies of ooids and marine carbonate cements, minerals and their assemblages of potash deposits, brine compositions of primary fluid inclusions in marine halite during the Phanerozoic were collected, in addition, artificial seawater simulation experiment about the distribution of Br partition coefficient was studied. The integrated information indicate that the composition of seawater has undergone five stages during the past 550 million years:the seawater was Na-Mg-K-SO₄-Cl type from Late Proterozoic to Early Cambrian, Early Permian to Early Mesozoic, and Early Cenozoic to the present, with m(SO₄^2-)+m(HCO₃^-)/2 in excess of m(Ca²⁺). The major potassium-magnesia minerals were MgSO₄-bearing minerals including kainite, langbeinite, polyhalite, kieserite, etc. and the dominant mineralogy of ooids and cements of carbonate was aragonite during these periods. The seawater was Na-Mg-K-Ca-Cl type during Early Cambrian to Carboniferous and Early Mesozoic to Early Cenozoic, with m(Ca²⁺) in excess of m(SO₄^2-)+m(HCO₃^-)/2. Potassium-magnesia minerals in the potash deposits mainly consisted of sylvite and carnallite, even tachyhydrite, and the dominant mineralogy of ooids and marine carbonate cements was calcite during these periods. Based on the calculation from the compositions of marine halite fluid inclusions, during the Phanerozoic, the K⁺ concentration has not changed much, ranging from 9.3 to 11.5mmol/kg H₂O (excluding Carboniferous and Late Proterozoic), 10.55mmol/kg H₂O in average. The Mg²⁺ concentrations were more than 67 mmol/kg H₂O during Early Cambrian, 31~41mmol/kg H₂O from Late Silurian to Middle Devonian, more than 48mmol/kg H₂O during Late Paleozoic, 34mmol/kg H₂O during Late Cretaceous, and 55.1mmol/kg H₂O at present. The Ca²⁺ concentrations were less than 11mmol/kg H₂O from Late Proterozoic to Early Paleozoic, 22~35mmol/kg H₂O from Early Paleozoic to Carboniferous, less than 17mmol/kg H₂O from Carboniferous to Early Mesozoic, 19~39 mmol/kg H₂O from Early Mesozoic to Early Cenozoic, and 7~21mmol/kg H₂O from Early Cenozoic to the present. The SO₄^2- concentrations were more than 23mmol/kg H₂O from Late Proterozoic to Early Paleozoic, 5~17mmol/kg H₂O from Early Paleozoic to Carboniferous, 13~22mmol/kg H₂O from Carboniferous to Early Mesozoic, 5~19mmol/kg H₂O from Early Mesozoic to Early Cenozoic, 12~29.2mmol/kg H₂O from Early Cenozoic to the present. The relative variation of concentrations of Ca²⁺ and SO₄^2- is the primary factor controlling the type of potassium-magnesia minerals in marine potash deposits. Meanwhile, the[m(Mg²⁺)+m(SO₄^2-)]/[m(K⁺)+m(Ca²⁺)] ratios which were calculated from the compositions of primary fluid inclusions in marine halite, is consistent with that of Br contents of the basal halite in marine evaporite sequences from different geological times. All above further support the conclusion that the composition of seawater has undergone secular changes during the Phanerozoic, and has been the major controlling factor constraining the type of potassium-magnesia minerals in marine evaporites. The controlling factors of the variations of seawater compositions are hydrothermal fluids from Mid-Ocean ridge and continental waters, and the former plays a major role. The underlying cause for the factors controlling the variations of seawater is plate tectonics.