http://www.geo.mtu.edu/EHaz/ConvergentPlatesClass/macpherson/macpherson_etal_2006.pdf

Adakites without slab melting: High pressure differentiation of island arc magma, Mindanao, the Philippines

Calculation of oxygen isotope fractionation in anhydrous silicate minerals

Isotopic and trace element analysis of zircons can provide reliable and robust estimates of age, compositions of coexisting minerals and melts, and constraints on the genesis and protoliths of host rocks. Recent technological developments facilitate analysis of oxygen isotope ratios in zircon with high accuracy and precision by laser heating/gas-source mass-spectrometry and in situ from thin sections or grain mounts by ion microprobe/secondary ion mass-spectrometer. A large number of studies have shown that non-metamict zircons preserve their δ18O value from the time of crystallization; hence oxygen isotope ratio can be correlated with age (U-Pb) or trace element composition. The zircon δ18O record is generally preserved despite other minerals that have been reset by high-grade metamorphism or intense hydrothermal alteration. Thus the refractory nature and robust inheritance of zircon offers a potential means to sort out magmatic equilibration and reequilibration, and post-magmatic alteration, an eternal problem for igneous rocks. New processes and interpretations for igneous events have been proposed when the effects of post-magmatic exchange are fully recognized. Crustal recycling can be recognized from magmatic values of δ18O(zircon), and if source rocks are igneous and young at the time of melting, δ18O will often be the best geochemical signature.

Microanalytical techniques are increasingly useful for stable isotope analysis of silicates. Both the laser fluorination/mass-spectrometer and the ion microprobe/secondary ion mass-spectrometer offer significant advantages over conventional techniques for analysis of zircon.

### Laser fluorination

Accurate analysis of δ18O in refractory minerals such as zircon is optimized by use of a CO2 laser (λ = 10.6 μm). The best precision has been obtained when zircons are powdered before analysis. Accuracy and precision of ±0.05 to ±0.1‰ (1 standard deviation = 1sd) are reported for homogeneous samples of ~2 mg (~0.5 mm3) (Valley et al. 1994, …

/pdf/oxygen-isotopes-in-zircon-901f8pk8q1.pdf

Oxygen Isotopes in Zircon

This chapter summarizes the oxygen isotope geochemistry of terrestrial basalts and
their mantle sources, including the conceptual framework for interpreting such data and
the phenomenology of known variations. In particular, the first section outlines the
motivations for and first-order results of oxygen isotope studies of terrestrial and lunar
basalts over the last 30 years; the second section reviews oxygen isotopic fractionations
among phases relevant for studying basalts and mantle rocks; the third summarizes
variations in δ^(18)O of various crustal rocks that may contribute to the petrogenesis of
basalts either as subducted source components or lithospheric contaminants; and the
final and longest section describes observed oxygen isotope variations of major classes
of terrestrial basalts and related mantle nodules with an emphasis on data generated
within the last six years using laser-based fluorination techniques. In the interests of
brevity, I do not describe in detail methods for oxygen isotope analysis or changes in
δ^(18)O of volcanic rocks caused by sub-solidus alteration; however, these issues are
important practical considerations for anyone studying oxygen isotope compositions of
basalts and interested readers are directed to the following references: analytical
methods: Sharp (1990), Mattey and Macpherson (1993), and Valley et al. (1995); basalt
alteration: Muehlenbachs (1986), Alt (1993), and Staudigel et al. (1995).

/pdf/oxygen-isotope-variations-of-basaltic-lavas-and-upper-mantle-1q3m4oqiid.pdf

Oxygen Isotope Variations of Basaltic Lavas and Upper Mantle Rocks

Granitoid gneisses and supracrustal rocks that are 3,800–4,000 Myr old are the oldest recognized exposures of continental crust1. To obtain insight into conditions at the Earth's surface more than 4 Gyr ago requires the analysis of yet older rocks or their mineral remnants. Such an opportunity is presented by detrital zircons more than 4 Gyr old found within 3-Gyr-old quartzitic rocks in the Murchison District of Western Australia2,3. Here we report in situ U–Pb and oxygen isotope results for such zircons that place constraints on the age and composition of their sources and may therefore provide information about the nature of the Earth's early surface. We find that 3,910–4,280 Myr old zircons have oxygen isotope (δ18O) values ranging from 5.4 ± 0.6‰ to 15.0 ± 0.4‰. On the basis of these results, we postulate that the ∼4,300-Myr-old zircons formed from magmas containing a significant component of re-worked continental crust that formed in the presence of water near the Earth's surface. These data are therefore consistent with the presence of a hydrosphere interacting with the crust by 4,300 Myr ago.

Oxygen-isotope Evidence From Ancient Zircons for Liquid Water at the Earth's Surface 4,300 Myr Ago

Calculation of oxygen isotope fractionation in metal oxides

The effect of Rayleigh distillation by outgassing of SO2 and H2S on the isotopic composition of sulfur remaining in silicate melts is quantitatively modelled. A threshold mole fraction of sulfur in sulfide component of the melts is reckoned to be of critical importance in shifting the δ34S of the melts mith respect to the original magmas. The partial equilibrium fractionation in a magmatic system is evaluated by assuming that a non-equilibrium flux of sulfur occurs between magmatic volatiles and the melts, while an equilibrium fractionation is approached between sulfate and sulfide within the melts.

Sulfur isotope fractionation in magmatic systems: Models of Rayleigh distillation and selective flux

Zircon U-Pb isotopic data on the early Cretaceous Dalongshan granitoids in the Yangtze Foldbelt of Southeast China provides evidence for the incorporation of old Archean crustal material. The age of inherited radiogenic lead is determined by the three-dimensional U-Pb discordia plane method to be about 3330±180 Ma, indicating that the early Archean basement probably exists underneath the northwest part of the Yangtze Foldbelt. The nature of the source region of initial lead in the zircon demands that the Archean basement is responsible for a lower crust or a depleted mantle.

Zheng Yong-Fei

Papers

Calculation of oxygen isotope fractionation in anhydrous silicate minerals

Calculation of oxygen isotope fractionation in metal oxides

Sulfur isotope fractionation in magmatic systems: Models of Rayleigh distillation and selective flux

Early Archean inheritance in zircon from Mesozoic Dalongshan granitoids in the Yangtze Foldbelt of Southeast China