矿物之间的元素和同位素平衡:地质测温和等时线定年的热力学和动力学控制

在特定的地质事件过程中,矿物等时线放射体系是否达到并且保持了平衡是变质岩Sm-Nd和Rb-Sr同位素年代学中的一个重要问题。在这个问题上矿物对O同位素测温与矿物等时线定年相似,因此两者之间可以相互制约。在岩浆岩和变质岩中,矿物中Sm-Nd、Sr和O之间的扩散速率在无水的条件下一般具有可比性,因此矿物之间O同位素的平衡状态可以用来对Sm-Nd和Rb-Sr定年的有效性进行检验。对大别-苏鲁造山带超高压变质岩的Sm-Nd和Rb-Sr等时线矿物进行O同位素测温,得到Sm-Nd等时线有时给出三叠纪年龄,有时给出非三叠纪年龄;对应的矿物O同位素分馏分别处于平衡和不平衡状态。对于引起非三叠纪等时线年龄的原因,一方面可以是由于榴辉岩相变质过程中同位素体系没有达到平衡,另一方面则可能角闪岩相退变质作用打破了平衡。等时线矿物中初始同位素比值的均一化速率主要受慢扩散矿物的影响,而矿物等时线时钟的启动主要受高母/子比值矿物控制。因此在变质作用过程中,只有当高母/子比值矿物同时具有快的放射成因同位素扩散速率,才可能得到有效的矿物等时线来用于变质年龄的测定。根据不同矿物中不同元素在扩散速率上的差异,能够定量估计大陆碰撞过程中榴辉岩相变质的持续时间。应用增量方法和离子孔隙度经验模型,不仅分别能够从理论上准确计算所有固体矿物的氧同位素分馏系数和获得不同矿物中元素的扩散参数,而且分别能够定量预测热力学平衡条件下共生矿物之间的¹⁸O富集顺序和相同条件下矿物中元素扩散速率的相对快慢。

Element and isotope equilibration between minerals: Thermodynamic and kinetic controls of geothermometry and isochron dating

With respect to mineral Sm-Nd and Rb-Sr isochron dating of igneous and metamorphic rocks, a critical premise is that the chronometric system of mineral isochrones has achieved isotopic equilibrium during a given metamorphic event and the equilibrium has not changed afterwards. An analogue to mineral chronometry is O isotope geothermometry, so that this provides a mutual constraint on validity of the two methods. Because the rates of Sm-Nd, Sr and O diffusion in igneous and metamorphic minerals are comparable under nominally anhydrous conditions, the state of O isotope equilibrium between high-T minerals can provide a test for the validity of mineral Sm-Nd and Rb-Sr chronometers. Applicability of this principle is illustrated for ultrahigh-pressure metamorphic rocks from the Dabie-Sulu orogenic belt in east-central China. When mineral Sm-Nd isochrones do not always give consistent Triassic ages for eclogite-facies metamorphism, mineral O isotope geothermometry do not yields concordant temperatures either. There is a direct correspondence in equilibrium or disequilibrium state between the O and Sm-Nd isotopic systems of metamorphic minerals. Some omphacite-garnet pairs from the eclogites exhibit O isotope equilibrium fractionations at eclogite-facies conditions and mineral Sm-Nd isochrones give meaningful Triassic ages. In contrast, some of the omphacite-garnet pairs show O isotope disequilibrium fractionations, and mineral Sm-Nd isochrones give geologically meaningless non-Triassic ages. While the mineral with relatively slower diffusivity of the element has exerted the primary control on the homogenization rate of initial isotope ratios among isochron minerals during retrograde metamorphism, the mineral with high parent/daughter ratio has exerted the principal control on the determined age. Valid mineral isochrones can be expected to date the timing of metamorphism only if the mineral with high parent/daughter ratio has a fast rate of radiogenic isotope diffusion during the metamorphism. Based on the differences of element diffusivity in different minerals, it is possible to quantitatively estimate the timescale of eclogite-facies metamorphism during continental collision. Oxygen isotope fractionation factors and element diffusion Arrhenius parameters for all solid minerals can be theoretically calculated by the increment method and the empirical model of ionic porosity, respectively. The results can be used to quantitatively estimate the relative ¹⁸O-enrichment in coexisting minerals at thermodynamical equilibrium and the relative rates of element diffusion in various minerals under the same conditions.