Characterization of magma from inclusions in zircon: Apatite and biotite work well, feldspar less so

Detrital zircon grains are employed to decipher sediment provenances and crustal evolution, and they provide unique evidence of Hadean crust-mantle differentiation processes. We demonstrate that mineral inclusions in zircon provide valuable information on the conditions under which zircon crystallized. Zircon grains from selected plutonic rocks from Dronning Maud Land, Antarctica, contain inclusions of apatite, biotite, amphibole, and pyroxenes that accurately reflect the chemical compositions of the equivalent phases in the host-rock matrix, and the compositions of the whole rocks. High concentrations of Y and low concentrations of Sr in apatite inclusions in zircon are diagnostic of evolved, felsic granitoid host rocks. In contrast, the relative abundances and compositions of plagioclase and alkali feldspar inclusions in zircon are decoupled from the composition of the whole rock, and they are generally indicative of granitic melts regardless of the bulk rock. This is best explained by the late crystallization of zircon relative to the bulk of the feldspars. We conclude that inclusions of apatite and mafic phases in zircon constrain the potential source rocks of detrital zircon, whereas feldspar inclusions do not

The lithium isotopic composition of orogenic eclogites and deep subducted slabs

We have modelled the Li budget of subducting oceanic crust during dehydration, using recently established input parameters. The results show that the entire prograde metamorphic process, up to anhydrous eclogite, can account for a decrease in δ7Li of only ≤ 3‰. Given that altered, oceanic crust entering the subduction zone should have Li isotopic compositions between − 3 and + 14‰, dehydration cannot account for markedly light Li isotopic compositions (δ7Li + 6‰) and very light (δ7Li < 0‰) compositions, as low as − 21.9‰. Notably some of the samples with highest Li concentrations are also isotopically light, which would not be predicted by isotopic fractionation as a consequence of Li loss during dehydration. Li abundances in excess of 30 μg/g in orogenic HPM rocks of basaltic composition (eclogites) are higher than any value of altered MORB and presumably result from addition of Li after the onset of subduction, most probably during eclogitisation or exhumation. Hence we propose that light-δ7Li values are generated by kinetic fractionation of the Li isotopes during diffusive influx of Li from the country rocks into the exhuming eclogite bodies. Our conclusions are in stark contrast to the previously accepted model, as we predict the deeply subducted eclogites to have a Li isotopic signature heavier than the mantle

Metasomatic tourmaline in hybrid contact-bands between gneiss and peridotite in the Ulten Zone of the Eastern Italian Alps: chemistry and boron isotopic composition.

Ultramafic rocks in the Ulten Zone (UZ) high-pressure m\ue9lange, in the Eastern Italian Alps, embedded in crustal gneisses, preserve a series of metasomatic hybrid bands generated by infiltration of aqueous fluids. These hybrid bands at the peridotite\u2013gneiss contact comprise phlogopite and anthophyllite with accessory zircon, apatite and allanite. Here, we report the rare occurrence of centimeter-sized crystals of metasomatic tourmaline in two different UZ outcrops, at Mt. Hochwart and Malga Preghena. Tourmaline has a dravite\u2013uvite-rich composition and shows optical and chemical zoning, with slightly variable Ti, Ca and Fe contents. Secondary-ion mass spectrometry (SIMS) analyses provide information on the intragrain B-isotopic zonation of the grains, which is correlated with optical and chemical zoning. The range in d11B is from \u20137.4 to +0.3\u2030 for the Mt. Hochwart tourmaline, generally with an isotopically lighter core and heavier rim. The d11B range for Malga Preghena tourmaline is from +1.2\u2030 in the core to \u20133.9\u2030 in the rim. Variable proportions of fluids derived from both the local crustal gneisses and a subducting slab control the B-isotope composition in the metasomatic tourmaline from the UZ. Fluid mixing in hybrid crustal\u2013ultramafic bands can be monitored by metasomatic tourmaline, which may also have a significant impact on the B isotope budget of the subducted slab and the slab\u2013mantle interface

Silicate melt inclusions in the new millennium: A review of recommended practices for preparation, analysis, and data presentation

International audienceMineral-hosted melt inclusions have become an important source of information on magmatic processes. As the number of melt inclusion studies increases, so does the need to establish recommended practice guidelines for collecting and reporting melt inclusion data. These guidelines are intended to ensure certain quality criteria are met and to achieve consistency among published melt inclusion data in order to maximize their utility in the future. Indeed, with the improvement of analytical techniques, new processes affecting melt inclusions are identified. It is thus critical to be able to reprocess any previously published data, such that reporting the raw data is one of the first “recommended practices” for authors and a publication-criteria that reviewers should be sensitive to. Our guidelines start with melt inclusion selection, which is a critical first step, and then continue on to melt inclusion preparation and analysis, covering the entire field of methods applicable to melt inclusions