查看更多>>摘要：Isotopic disequilibrium is not as well constrained as equilibrium, hindering interpretation of isotopic variations. Kinetic isotope effects, a subset of disequilibrium, are common in nature and have been assumed to be distinct from equilibrium and mass independent isotope effects based on underdeveloped criteria. Using basic physical principles, we provide needed mechanistic constraints on mass-dependent kinetic isotope effects for the triple oxygen isotope system. We find some kinetic isotope effects yield large isotopic variations, exceeding equilibrium, which could be mistaken for mass independent relationships. Meanwhile, other kinetic isotope effects are found to have triple oxygen isotope relationships that could be mistaken for equilibrium isotope effects. Comparison against prior case studies of thermal decomposition of calcite (CaCO3) and brucite (Mg(OH)(2)) further tests our results. Although oxygen is the focus here, our approach applies to any system with more than two isotopes.
查看更多>>摘要：Crustal anatexis in collisional orogens has great bearing on geochemical differentiation of the continental crust. However, it is often uncertain what kind of crustal rocks were partially melted for felsic magmatism at convergent plate boundaries. To address this issue, a combined study of in-situ monazite U-Th-Pb ages, in-situ monazite and allanite Sm-Nd isotopes and whole-rock Sm-Nd isotopes was carried out for Higher Himalayan metamorphic rocks and leucogranites in the Himalayan orogen. Metapelite, metagreywacke and granitic gneiss are the dominant constituents of the Higher Himalayan Crystallines, and they experienced upper amphibolite to granulite facies metamorphism at ca. 26-13 Ma. Although the three rock types show consistently negative (epsilon Nd)(t) values at t = 20 Ma, their Nd isotope compositions become less and less enriched from metapelite through metagreywacke to granitic gneiss. The metapelite has the lowest (epsilon Nd)(t) values of -19.9 to -15.7, the metagreywacke has intermediate (epsilon Nd)(t) values of -17.4 to -12.7, and the granitic gneiss has the highest (epsilon Nd)(t) values of -14.1 to -7.7. Leucogranitic magmatism occurred at ages from ca. 26 to 7 Ma, coeval with anatectic metamorphism of the three rock types during the Oligocene-Miocene. Relict zircon U-Pb age distributions, whole-rock trace element patterns, initial Nd isotope compositions and two-stage Nd model ages for the Higher Himalayan leucogranites are comparable with those for the Higher Himalayan metamorphic rocks, confirming that the metapelite, metagreywacke and granitic gneiss would have their compositional counterparts at structurally deeper positions to serve as the crustal sources of the leucogranites. This is also verified by forward phase equilibrium modelling for partial melting of the metamorphic rocks with respect to the differences in their composition and fertility. In addition, the leucogranites show a decreasing trend in (epsilon Nd)(t) values with their ages, indicating that the dominant crustal sources would gradually change from the least fertile granitic gneiss through the intermediately fertile metagreywacke to the most fertile metapelite during the protracted crustal anatexis from the late Oligocene to the Miocene. Therefore, the deep crust in the Himalayan orogen would consist of the metamorphic rocks with similar compositions to the shallow crust. The combined geochronological and geochemical study of accessory minerals and host rocks can provide the genetic link between the crustal sources and their melting products in collisional orogens. Nevertheless, the Nd isotope variation in these leucogranites may also be related to incongruent melting of the crustal sources, which has a potential to result in the Nd isotope disequilibrium between melt and residue.
NSTL
Elsevier