The Effects of High-Grade Metamorphism on Cr-Spinel from the Archean Sittampundi Complex, South India

We investigated the crystal and structural behavior of Cr-bearing spinels from the Archean chromitites of Sittampundi (India), which had been subjected to very high-grade metamorphism. The structural data show that their oxygen positional parameters are among the highest ever recorded for Cr-bearing spinels with similar Cr# and Mg# and very similar to those found for other Archean occurrences. The general agreement between electron microprobe and Mössbauer data indicates that the analyzed spinels are stoichiometric. It is therefore most likely that the PH2O and Ptotal values as well as both the oxygen fugacity and the temperature reached during high-grade metamorphism inhibited the possibility of the non-stoichiometry of chromites, contrary to what can happen in ophiolites, where non-stoichiometry has recently been documented.

Acidic fluids in the Earth’s lower crust

Fluid flux through Earth’s surface and its interior causes geochemical cycling of elements in the Earth. Quantification of such process needs accurate knowledge about the composition and properties of the fluids. Knowledge about the fluids in Earth’s interior is scarce due to limitations in both experimental methods and thermodynamic modeling in high/ultrahigh pressure–temperature conditions. In this study, we present halogen (Cl, F) measurements in apatite grains from the mafic (metagabbro), and felsic (two-pyroxene granulite, charnockite, hornblende-biotite gneiss) rocks preserved in the Nilgiri Block, southern India. Previous experiments show that it is difficult to incorporate Cl in apatite compared to F at high pressure and temperature conditions. Based on regional trends in Cl and F content in apatite (with highest Cl content 2.95 wt%), we suggest the presence of acidic C–O–H fluids in the lower crust (~20–40 km deep) during the high-grade metamorphism of these rocks. These fluids are capable of causing extreme chemical alterations of minerals, especially refractory ones. They also have significant potential for mass transfer, causing extensive geochemical variations on a regional scale and altering the chemical and isotope records of rocks formed in the early Earth. Our findings have important relevance in understanding speciation triggered by acidic fluids in the lower crust, as well as the role of fluids in deep Earth processes.

Syn-metamorphic sulfidation of the Gamsberg zinc deposit, South Africa

The Mesoproterozoic Aggeneys-Gamsberg ore district, South Africa, is one of the world´s largest sulfidic base metal concentrations and well-known as a prime example of Broken Hill-type base metal deposits, traditionally interpreted as metamorphosed SEDEX deposits. Within this district, the Gamsberg deposit stands out for its huge size and strongly Zn-dominated ore ( >14 Mt contained Zn). New electron microprobe analyses and element abundance maps of sulfides and silicates point to fluid-driven sulfidation during retrograde metamorphism. Differences in the chemistry of sulfide inclusions within zoned garnet grains reflect different degrees of interaction of sulfides with high metal/sulfur-ratio with a sulfur-rich metamorphic fluid. Independent evidence of sulfidation during retrograde metamorphism comes from graphic-textured sulfide aggregates that previously have been interpreted as quenched sulfidic melts, replacement of pyrrhotite by pyrite along micro-fractures, and sulfides in phyllic alteration zones. Limited availability of fluid under retrograde conditions caused locally different degrees of segregation of Fe-rich sphalerite into Zn-rich sphalerite and pyrite, and thus considerable heterogeneity in sphalerite chemistry. The invoked sulfur-rich metamorphic fluids would have been able to sulfidize base metal-rich zones in the whole deposit and thus camouflage a potential pre-metamorphic oxidation. These findings support the recently established hypothesis of a pre-Klondikean weathering-induced oxidation event and challenge the traditional explanation of Broken Hill-type deposits as merely metamorphosed SEDEX deposits. Instead, we suggest that the massive sulfide deposits experienced a complex history, starting with initial SEDEX-type mineralization, followed by near-surface oxidation with spatial metal separation, and then sulfidation of this oxidized ore during medium- to high-grade metamorphism.

Retrograded Eclogite From Chicheng, North China Craton: New Insights Into the Fidelity of U-Pb-Hf-O Isotopes in Zircon During High-grade Metamorphism

Zircon is the most abundantly used mineral for dating igneous and metamorphic events and for tracing source characteristics. Understanding the geochemical behavior of the U-Pb-Hf-O isotope systems during high-grade metamorphism is therefore important for accurate interpretation of the isotopic information. We report zircon U-Pb-Hf-O isotopes and trace elements of retrograded eclogites and host gneisses from Chicheng, North China Craton, with the aim to obtain new insights into the fidelity of U-Pb-Hf-O isotopes in zircon as recorders of high-grade metamorphism. U-Pb dating suggested that the Chicheng mélange experienced eclogite facies metamorphism at ~1.84 Ga, and then exhumed to amphibolite facies at 320–300 Ma. Zircons with Paleoproterozoic ages formed in metamorphic melts-derived from the gneiss during the eclogite facies metamorphism. Zircons with ages of 300–320 Ma formed by recrystallization of peak metamorphic zircons during fluid-assisted amphibolite-facies retrograde metamorphism. This process led to the near-complete resetting not only of U-Pb ages but also of Hf-O isotopic compositions of the peak metamorphic zircons, while preserve REE patterns. These results contrast with the sluggish Hf diffusion rate predicted from experimental studies, and support findings that isotopic data from metamorphic zircons in retrograded high-grade metamorphic rocks need not be faithful recorders of their sources.

Archean continental crust formed by magma hybridization and voluminous partial melting

Archean (4.0–2.5 Ga) tonalite–trondhjemite–granodiorite (TTG) terranes represent fragments of Earth’s first continents that formed via high-grade metamorphism and partial melting of hydrated basaltic crust. While a range of geodynamic regimes can explain the production of TTG magmas, the processes by which they separated from their source and acquired distinctive geochemical signatures remain uncertain. This limits our understanding of how the continental crust internally differentiates, which in turn controls its potential for long-term stabilization as cratonic nuclei. Here, we show via petrological modeling that hydrous Archean mafic crust metamorphosed in a non-plate tectonic regime produces individual pulses of magma with major-, minor-, and trace-element signatures resembling—but not always matching—natural Archean TTGs. Critically, magma hybridization due to co-mingling and accumulation of multiple melt fractions during ascent through the overlying crust eliminates geochemical discrepancies identified when assuming that TTGs formed via crystallization of discrete melt pulses. We posit that much Archean continental crust is made of hybrid magmas that represent up to ~ 40 vol% of partial melts produced along thermal gradients of 50–100 °C/kbar, characteristic of overthickened mafic Archean crust at the head of a mantle plume, crustal overturns, or lithospheric peels.

Zircon recrystallisation microstructures andthe implications for U-Pb dating

<p>Uranium-lead dating of zircon has been used extensively in geochronological studies based on the widespread occurrence of zircon and its resistance to chemical and physical weathering. Previous research has shown that despite their apparent robustness, many zircons contain evidence for recrystallisation, such as the replacement of the primary oscillatory zoning by unzoned zircon. This replacement is characterised by rims, patches and embayments of unzoned zircon which can either completely replace the primary zoning or preserve faint remnants within the unzoned zircon.  In some samples, the unzoned zircon contains lower U and Pb concentrations, implying that the zircon U-Pb age may be reset during the replacement (Pidgeon, 1992). Interestingly, zircons have also been found in which there is no apparent difference in U-Pb age between the zoned and unzoned zircon (Schaltegger et al., 1999). To better understand the replacement of zoned by unzoned zircon, it is important to study the microstructures present within recrystallised zircon to understand possible mechanisms causing recrystallisation. Multiple mechanisms may explain the trace element distribution within (partially) recrystallised zircon: annealing of radiation damaged (metamict) zircon, annealing of lattice strain imposed by alternating U concentrations in oscillatory zoning, enhanced diffusion along fast-diffusivity pathways (such as low-angle subgrain boundaries or fractures) and coupled dissolution-reprecipitation.  The mechanism(s) by which zircons recrystallise remain poorly understood, as well as the effect of the formation of different microstructures on corresponding zircon U-Pb dates. Understanding these phenomena is therefore of vital importance for correctly interpreting U-Pb ages in zircon.</p><p>This work focusses on investigating the microstructures that are present within recrystallised zircons from both metamorphic and igneous environments from the Jack Hills, Australia (Pidgeon, 1992) and the island of Lewis and Harris, Scotland (Van  Breemen  et  al.   1971). Suites of zircons from these areas have been imaged with cathodoluminescence, which is a powerful tool for obtaining high resolution images of the internal structures of zircons. Within these suites, zircons are present which show complex zoning patterns and (partial) recrystallisation; these will be studied in greater detail using EDS, EBSD and SHRIMP. Preliminary results of EDS on the inclusions show that inclusions are composed of feldspars, thorite, quartz and apatite, which were most likely included during the primary crystallisation of the zircon. EBSD measurements will provide additional data on the crystallographic orientation of recrystallized zones and the state of metamictization of the zircons, and may show if zircon has deformed crystal-plastically forming subgrain boundaries.</p><p><strong> </strong></p><p><strong>References</strong></p><p>Pidgeon, R. T. (1992). Recrystallisation of oscillatory zoned zircon: some geochronological and petrological implications. Contributions to Mineralogy and Petrology, 110(4), 463-472.</p><p>Schaltegger, U., Fanning, C. M., Günther, D., Maurin, J. C., Schulmann, K., & Gebauer, D. (1999). Growth, annealing and recrystallization of zircon and preservation of monazite in high-grade metamorphism: conventional and in-situ U-Pb isotope, cathodoluminescence and microchemical evidence. Contributions to Mineralogy and Petrology, 134(2-3), 186-201.</p><p>Van Breemen, O., Aftalion, M., & Pidgeon, R. (1971). The age of the granitic injection complex of harris,outer hebrides.Scottish Journal of Geology,7(2), 139–152.</p>