Zircon Dissolution Research Articles

Zircon Zr[sbnd]Hf isotope disequilibrium during crustal anatexis: A record from the Mesozoic migmatite in the eastern Gangdese arc, South Tibet

Stable zirconium (Zr) isotopes have become the focus of attention as potential tracers of magma crystallization and differentiation processes. However, the effects of partial melting, especially disequilibrium melting, on Zr isotope fractionation remain unclear. Here we report in-situ zircon UPb ages, major and trace elements, and ZrHf isotope data for the migmatites in the eastern Gangdese arc, South Tibet. The leucosomes from the metatexite and diatexite migmatites are generally concordant with the foliation of the paleosome and display diffuse and gradational contacts with the melanosomes, indicating that they are in-source leucosomes. Zircon in the paleosome, the protolith of the migmatites, have UPb age of 361 ± 2 Ma and low 176Hf/177Hf ratios of 0.282399–0.282482. They exhibit intragrain variation in δ94/90Zr values, increasing from the core (−0.10 ‰ to +0.36 ‰) to the rim (+0.14 ‰ to +0.55 ‰) domains. The negative correlation between δ94/90Zr values and Zr/Hf ratios suggests preferential incorporation of light Zr isotopes during the crystallization of the zircon in the protolith. Most zircon in the leucosomes from the metatexite and diatexite migmatites exhibit distinct overgrowth rims around the inherited cores. The inherited cores have consistent UPb ages (ca. 360 Ma), trace elements, and Hf isotopic compositions (0.282362–0.282449) with zircon in the paleosome, indicating in situ partial melting. The inherited cores display δ94/90Zr values ranging from −0.12 ‰ to +0.31 ‰, consistent with the zircon core domains in the paleosome. The overgrowth rims have UPb ages of ca. 102 Ma and display higher 176Hf/177Hf ratios (0.282592–0.282900) and δ94/90Zr values (+0.01 ‰ to +0.63 ‰) than their inherited cores. The elevated 176Hf/177Hf for the overgrowth rims resulted from the retention of unradiogenic 177Hf in the residue due to limited zircon dissolution and more release of radiogenic 176Hf to the melts because of the garnet breakdown, indicating disequilibrium melting. The elevation of δ94/90Zr for the overgrowth rims relative to the inherited cores was not caused by the disequilibrium melting of other Zr-bearing minerals but was mainly controlled by the Zr isotopic composition and dissolution degree of the zircon in the protolith. The preferential melting of isotopically heavy zircon rims would elevate the δ94/90Zr values in the melts relative to their zircon-bearing protoliths. Crustal anatexis may be another important mechanism contributing to the Zr isotope fractionation during crustal differentiation.

Hf-Nd isotope decoupling in S-type granitoids caused by disequilibrium melting of metasedimentary rocks: Evidence from the North Qinling Terrane, Central China

The intrinsic assumption of radiogenic isotope tracing is that isotope systems maintain equilibrium between melt and source. However, this assumption is not naturally correct, particularly for S-type granites, which are formed by partial melting of metasedimentary rocks. The S-type metagranites from the North Qinling Terrane show various degrees of HfNd isotope decoupling and discrepancies between whole-rock and zircon Hf isotopes. Zircon UPb dating yields emplacement ages from 946 to 890 Ma. Their high δ18O values, (87Sr/86Sr)i ratios, and negative εNd(t) values overlapped with metasedimentary rocks in the Qinling Group imply that they are derived from partial melting of the Qinling Group. The low Rb/Ba, Rb/Sr, Al2O3/TiO2, and high CaO/Na2O ratios indicate that their sources are metagraywackes. The negative correlation between decoupling degrees of HfNd isotopes and Zr concentrations suggests that the incomplete dissolution of zircon during partial melting results in the decoupling. The discrepancy between whole-rock and zircon Hf isotopes can be attributed to the combined effect of incomplete dissolution of zircons and the release of various Hf isotope compositions from partly dissolved inherited zircons to local melts. The compiled literature data suggest that whole-rock HfNd isotope decoupling of most metagraywacke-derived S-type granites is caused by incomplete melting of zircons, whereas those of metapelite-derived S-type granites may be controlled by both incomplete melting of zircons and source inheritance.

Minimizing the effects of Pb loss in detrital and igneous U–Pb zircon geochronology by CA-LA-ICP-MS

Abstract. Detrital zircon geochronology by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) is a widely used tool for determining maximum depositional ages and sediment provenance, as well as reconstructing sediment routing pathways. Although the accuracy and precision of U–Pb geochronology measurements have improved over the past 2 decades, Pb loss continues to impact the ability to resolve zircon age populations by biasing affected zircon toward younger apparent ages. Chemical abrasion (CA) has been shown to reduce or eliminate the effects of Pb loss in zircon U–Pb geochronology but has yet to be widely applied to large-n detrital zircon analyses. Here, we assess the efficacy of the chemical abrasion treatment on zircon prior to analysis by LA-ICP-MS and discuss the advantages and limitations of this technique in relation to detrital zircon geochronology. We show that (i) CA does not systematically bias LA-ICP-MS U–Pb dates for 13 reference materials that span a wide variety of crystallization dates and U concentrations, (ii) CA-LA-ICP-MS U–Pb zircon geochronology can reduce or eliminate Pb loss in samples that have experienced significant radiation damage, and (iii) bulk CA prior to detrital zircon U–Pb geochronology by LA-ICP-MS improves the resolution of age populations defined by 206Pb/238U dates (Neoproterozoic and younger) and increases the percentage of concordant analyses in age populations defined by 207Pb/206Pb dates (Mesoproterozoic and older). The selective dissolution of zircon that has experienced high degrees of radiation damage suggests that some detrital zircon age populations could be destroyed or have their abundance significantly modified during this process. However, we did not identify this effect in either of the detrital zircon samples that were analyzed as part of this study. We conclude that pre-treatment of detrital zircon by bulk CA may be useful for applications that require increased resolution of detrital zircon populations and increased confidence that 206Pb/238U dates are unaffected by Pb loss.

Himalayan-like Crustal Melting and Differentiation in the Southern North American Cordilleran Anatectic Belt during the Laramide Orogeny: Coyote Mountains, Arizona

Abstract The southern US and northern Mexican Cordillera experienced crustal melting during the Laramide orogeny (c. 80–40 Ma). The metamorphic sources of melt are not exposed at the surface; however, anatectic granites are present throughout the region, providing an opportunity to investigate the metamorphic processes associated with this orogeny. A detailed geochemical and petrochronological analysis of the Pan Tak Granite from the Coyote Mountains core complex in southern Arizona suggests that prograde metamorphism, melting, and melt crystallization occurred here from 62 to 42 Ma. Ti-in-zircon temperatures (TTi-zr) correlate with changes in zircon rare earth elements (REE) concentrations, and indicate prograde heating, mineral breakdown, and melt generation took place from 62 to 53 Ma. TTi-zr increases from ~650 to 850 °C during this interval. A prominent gap in zircon ages is observed from 53 to 51 Ma and is interpreted to reflect the timing of peak metamorphism and melting, which caused zircon dissolution. The age gap is an inflection point in several geochemical-temporal trends that suggest crystallization and cooling dominated afterward, from 51 to 42 Ma. Supporting this interpretation is an increase in zircon U/Th and Hf, a decrease in TTi-zr, increasing zircon (Dy/Yb)n, and textural evidence for coupled dissolution–reprecipitation processes that resulted in zircon (re)crystallization. In addition, whole rock REE, large ion lithophile elements, and major elements suggest that the Pan Tak Granite experienced advanced fractional crystallization during this time. High-silica, muscovite± garnet leucogranite dikes that crosscut two-mica granite represent more evolved residual melt compositions. The Pan Tak Granite was formed by fluid-deficient melting and biotite dehydration melting of meta-igneous protoliths, including Jurassic arc rocks and the Proterozoic Oracle Granite. The most likely causes of melting are interpreted to be a combination of (1) radiogenic heating and relaxation of isotherms associated with crustal thickening under a plateau environment, (2) heat and fluid transfer related to the Laramide continental arc, and (3) shear and viscous heating related to the deformation of the deep lithosphere. The characteristics and petrologic processes that created the Pan Tak Granite are strikingly similar to intrusive suites in the Himalayan leucogranite belt and further support the association between the North American Cordilleran anatectic belt and a major orogenic and thermal event during the Laramide orogeny.

Fluid effect on zircon O and U-Pb isotopes during ultrahigh-pressure metamorphism: Insights from the Dora-Maira Massif of the Western Alps

Zircon geochemistry such as U-Pb and O isotopes have been widely used in dating and tracing complex geological processes. However, it still remains unclear how fluid action affects zircon geochemistry during metamorphic and metasomatic processes in subduction zones. Here a systematic study on zircon U-Pb dating, O isotopes and trace elements as well as whole-rock O isotopes was carried out for the coesite-bearing whiteschists, jadeite quartzites and granitic gneisses from the Dora-Maira Massif, Western Alps. Whole-rock and zircon geochemistry supports a common protolith, i.e., Permian S-type granites, for the above three types of rocks and an intense fluid metasomatism during the Alpine orogeny to form whiteschists and jadeite quartzites. Zircon cores in all samples have nearly identical δ18O values (9‰–11‰), whereas their apparent 206Pb/238U ages show a greater variability due to Pb loss during metamorphism. Zircon rims formed in the late Eocene to early Oligocene can be categorized into two types. Type-I rims occur in granitic gneisses and jadeite quartzites. They have high δ18O values consistent with zircon cores, but much lower contents of P and Y as well as lower Th/U ratios than the cores. Their growth can be attributed to internal metamorphic fluid action at the UHP metamorphic stage. Type-II rims occur in whiteschists and jadeite quartzites. They have remarkably lower δ18O values (5‰–8‰) and Th/U ratios (<0.01), compared with zircon cores and Type-I rims. Their growth can be ascribed to external fluids during the metasomatic process. Some zircon domains in whiteschists and jadeite quartzites show a positive correlation between δ18O values and apparent 206Pb/238U ages, which suggest the simultaneous impacts on U-Pb-O isotopes during external fluid metasomatism. This process can be attributed to the fluid-assisted dissolution and recrystallization of protolith zircons. Especially, coesite inclusions that would have been expected to occur only in young zircon rims formed during UHP metamorphism are also observed in the relict magmatic zircon cores, indicating that the fluid-related metasomatism at the UHP metamorphic conditions also affected these pre-existing protolith-related cores. Therefore, fluid action in subduction zones reveals significant impacts on both the U-Pb and O isotope systems of zircon, especially when external metasomatic fluids are involved. Therefore, a detailed study on zircon, including microstructure, mineral inclusion and geochemical data of different growth and recrystallization domains, is needed in order to unravel continental crustal evolution based on zircon U-Pb ages and O isotope compositions.

Chemical abrasion: the mechanics of zircon dissolution

Abstract. Chemical abrasion is a technique that combines thermal annealing and partial dissolution in hydrofluoric acid (HF) to selectively remove radiation-damaged portions of zircon crystals prior to U–Pb isotopic analysis, and it is applied ubiquitously to zircon prior to U–Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS). The mechanics of zircon dissolution in HF and the impact of different leaching conditions on the zircon structure, however, are poorly resolved. We present a microstructural investigation that integrates microscale X-ray computed tomography (µCT), scanning electron microscopy, and Raman spectroscopy to evaluate zircon dissolution in HF. We show that µCT is an effective tool for imaging metamictization and complex dissolution networks in three dimensions. Acid frequently reaches crystal interiors via fractures spatially associated with radiation damage zoning and inclusions to dissolve soluble high-U zones, some inclusions, and material around fractures, leaving behind a more crystalline zircon residue. Other acid paths to crystal cores include the dissolution of surface-reaching inclusions and the percolation of acid across zones with high defect densities. In highly crystalline samples dissolution is crystallographically controlled with dissolution proceeding almost exclusively along the c axis. Increasing the leaching temperature from 180 to 210 ∘C results in deeper etching textures, wider acid paths, more complex internal dissolution networks, and greater volume losses. How a grain dissolves strongly depends on its initial radiation damage content and defect distribution as well as the size and position of inclusions. As such, the effectiveness of any chemical abrasion protocol for ID-TIMS U–Pb geochronology is likely sample-dependent. We also briefly discuss the implications of our findings for deep-time (U-Th)/He thermochronology.

Using Zircons to Disentangle Back-Veining and Hybridization of Diorite Dykes: an Example From the Gangdese Arc, Tibet

AbstractThermo-chemical modeling and chemical data suggest that the genesis of arc melts commonly involves re-melting of older intrusions, triggered by the injection of mantle-derived melts, followed by magma mixing. Remelting and mixing may lead to complex zircon populations, which can be used to gain insights into the conditions of mixing. This paper investigates a case where such processes can be studied through the compositional and thermal record provided by zircons preserved in a diorite dyke swarm that remelted host rock tonalites in the Gangdese Batholith in Tibet. Autocrystic zircons from the diorite yield consistent ages of 46 to 47 Ma even though they can be altered, having highly enriched trace elements, reaching ~1 wt % in Fe, Ca, Y, U, Th, and anomalously high values of LREE and Ti. Granitic magmas derived from the tonalite, back-veined the dykes and mixed with the dioritic mush, transferring small quantities of 77–79 Ma zircon xenocrysts. The xenocrysts are euhedral with little evidence for resorption, indicating that they were apparently stable during the process of tonalite anatexis and transfer to the diorite magma. This requires that: (i) tonalite melting occurred at low temperatures with minimal zircon dissolution, and (ii) the diorite either cooled rapidly before significant resorption of the grains or was already saturated in zircon when mixing occurred. Zircon saturation temperatures of the diorite are relatively low, indicating that xenocrystic zircons were transferred to a highly crystalline dioritic mush. This requires either liquefaction by injection of the back-veining felsic magma to allow for mixing, or pervasive throughflow of the diorite mush by the felsic magma leaving behind zircon xenocrysts. The findings suggest that the dykes triggered low-T, water-fluxed melting of the host tonalite and that these anatectic melts invaded the diorite mush initially through the interstices leading to mixing. This may have caused the breakdown of the crystalline framework leading to liquefaction and renewed magma flow. Dyking and assimilation of wall-rock through back-veining as recorded in these rocks could be common in transcrustal arcs. However, this process could be hidden due to a combination of the similarity in the isotopic and chemical nature of arc rocks and resorption of zircon xenocrysts during mixing. This process may explain some complex chemistry of arc magmatic rocks and their minerals that are not easily explained by endmember models, such as pure re-melting of older arc rocks or fractional crystallization of mantle-derived melts.

A New Partially Phosphorylated Polyvinyl Phosphate-PPVP Composite: Synthesis and Its Potentiality for Zr (IV) Extraction from an Acidic Medium

A newly synthesized partially phosphorylated polyvinyl phosphate derivative (PPVP) was functionalized to extract Zirconium (IV) from Egyptian zircon sand. The specifications for the PPVP composite were approved effectively via different techniques, namely, FT-IR, XPS, BET, EDX, TGA, 1H-NMR, 13C-NMR, GC-MS, XRD and ICP-OES analyses, which demonstrated a satisfactory synthesis of PPVP and zircon dissolution from Egyptian zircon sand. Factors controlling parameters, such as pH values, shaking time, initial zirconium concentration, PPVP dose, nitrate ions concentration, co-ions, temperature and eluting agents, have been optimized. At 25 °C, pH 0, 20 min shaking, 0.05 mol/L zirconium ions and 0.5 mol/L nitrate ions, PPVP has an exciting preservation potential of 195 mg/g, equivalent to 390 mg/L zirconium ions. From the extraction–distribution isotherm, the practical outcomes of Langmuir’s modeling are better than the Freundlich model. With a theoretical value of 196.07 mg/g, which is more in line with the experimental results of 195 mg/g. The zirconium ions adsorption onto the PPVP composite follows the pseudo-second-order kinetics with a theoretical capacity value of 204.08 mg/g. According to thermodynamic potential, the extraction process was expected to be an exothermic, spontaneous and beneficial extraction at low temperatures. The thermodynamic parameters ΔS (−0.03 kJ/mol), ΔH (−12.22 kJ/mol) and ΔG were also considered. As the temperature grows, ∆G values increase from −2.948 kJ/mol at 298 K to −1.941 kJ/mol at 338 K. Zirconium ions may be eluted from the working loaded PPVP by 0.025M HNO3, with a 99% efficiency rate. It was found that zirconium ions revealed good separation factors towards some co-ions such as Hf4+ (28.82), Fe3+ (10.64), Ti4+ (28.82), V5+ (86.46) and U6+ (68.17). A successful alkali fusion technique with NaOH flux followed by the extraction with PPVP is used to obtain a high-purity zirconia concentrate with a zircon content of 72.77 % and a purity of 98.29%. As a result of this, the improved factors could finally be used.

Long-lived dacitic magmatic systems and recharge dynamics in the Jemez Mountains volcanic field, western USA

We examine the connected history of dacite-dominant volcanic rocks of the Tschicoma Formation, erupted between 5.5 and 2 Ma from the Jemez Mountains volcanic field, western USA. Zircon samples from two separate eruptions have continuous SHRIMP U–Pb age spectra spanning 0.84–1.08 Myr duration (3.12–3.96 Ma and 3.50–4.58 Ma, respectively), following an episode of zircon crystallization 0.28–0.50 Myr earlier (at 4.46 Ma and 4.86 Ma, respectively). Zircon chemical variations, as well as ubiquitous resorption textures that commonly show large core-rim age differences (up to 720–740 kyr), suggest that they grew in separate melt lenses. Zircons were likely stored at near-solidus or even sub-solidus conditions after crystallization, but may have been reactivated in response to at least four major magma recharge events every 300–400 kyr and smaller events in between. A cycle of zircon dissolution (from heating), recrystallization (during cooling), and storage repeated in different locations in the Tschicoma mush system throughout its lifespan; each recharge-induced heating stage may last for several hundred to more than a thousand years based on calculations of zircon dissolution. We envisage the melt lenses to be distributed in a crystal mush zone, coalescing into a single magma batch as magma recharge occurs shortly before eruption. Once active, increasing magma supply rates may trigger large-scale partial melting of the pre-existing mush and caldera-forming eruptions.

Sr-Nd-Hf Isotopic Disequilibrium During the Partial Melting of Metasediments: Insight From Himalayan Leucosome

Radiogenic isotopes of granitoids are widely applied to fingerprint the source of granitoids and study the magma mingling and assimilation processes, aiming to decipher the planetary differentiation. This weapon is based on the assumption that crustal melts inherit the radiogenic isotopes of protoliths. However, complicated melting processes in the crust would drive the radiogenic isotopes of melt away from the source, thus calling for a clear understanding of the behaviors of radiogenic isotopes during crustal melting. This study investigated the joint behavior of Rb-Sr, Sm-Nd, and Lu-Hf isotope systems during the melting of metasediments. Leucosome, schist, and leucogranite samples were collected from the Nyalam, South Tibet, where the leucosome was produced by muscovite dehydration melting of metapelites. Results show that the leucosome has a broad range of Sr-Nd-Hf isotopes (87Sr/86Sri: 0.763,48–0.875321, εNd(t): 14.6–11.6, εHf(t): 15.0–4.4) that deviate from the metasediments. We attributed it to the non-modal or disequilibrium melting of metapelites. The variation of Sr isotopes of leucosome is formed due to preferential entry into the melt of muscovite relative to plagioclase during melting. The changing 87Sr/86Sri and 87Rb/86Sr of leucosome define an errorchron at ∼ 500 Ma indicating an early Paleozoic tectono-thermal event supported by the U-Pb age of zircon cores (∼460 Ma) in the leucosome. The Nd isotopes of leucosome are mainly controlled by the preferential dissolution of apatite relative to monazite. The inadequate dissolution of zircon caused more depleted initial Hf isotopes compositions in the leucosome than the source. However, the leucosome with a higher crystallization temperature has radiogenic isotopic composition closer to the source. It indicates that the melting temperature greatly affects the isotope equilibrium between source and melt.

Dissolution and precipitation behaviors of zircon under the atmospheric environment

The reactive behavior of zircon in three different solutions: 0.1 M HCl (aq), ultrapure water, and 0.1 M NaOH (aq) was examined under normal temperature and pressure conditions. Dissolved Si and Zr concentrations show that considerable Zr precipitated on the surface in all immersion cases. Bulk changes in the zircon were insignificant. However, the obvious surface change of the zircon immersed in 0.1 M NaOH (aq) was confirmed. Porous layer comprising an amorphous SiOx phase and needle crystals of ZrSiO4 around pores were detected on the near-surface area of the zircon immersed in 0.1 M NaOH (aq). Consequently, the reaction was limited when zircon was immersed in ultrapure water. Zr precipitated on the surface after the dissolution of ZrSiO4 in 0.1 M HCl (aq). Dissolved Zr and Si in 0.1 M NaOH (aq) precipitated on the surface, then the dissolution of ZrSiO4 formed a porous layer, and most dissolved Zr precipitated as zircon crystals.

Experimental Annealing of Zircon: Influence of Inclusions on Stability, Intracrystalline Melt Migration, Common Lead Leaching, and Permeability to Fluids.

Zircon derived from crustal rocks can survive dissolution into hot basalts during magma hybridization and rock assimilation if it is shielded as an inclusion phase in early-formed phenocrysts or in minerals from non-disaggregated xenoliths. Under these conditions, zircon can be thermally shocked, triggering recrystallization of metamict domains and reaction with its hosted mineral inclusions. This work simulates this process by performing thermal annealing experiments on zircon grains with variable degrees of metamictization. These were embedded in cristobalite powder under a N2 atmosphere at 1 bar and 1300 °C. The thermal annealing produces recrystallization of metamict domains, melting of multi-phase mineral inclusions, nanopore formation, and microcrack propagation by thermo-elastic stress. The porosity enhances intracrystalline melt mobility, leaching out trace-element and mineral impurities. Baddeleyite was formed at temperatures below the thermal decomposition of pure zircon by two mechanisms: (i) recrystallization of metamict domains assisted by silica migration from the reaction site and (ii) incongruent zircon dissolution into molten mineral inclusions with a high CaO/SiO2 ratio. Highly metamict zircons with elevated common Pb and radiogenic Pb loss, which were impossible to date with SHRIMP, lost all their common Pb and some radiogenic Pb upon annealing, producing well-fitted discordias with a significant upper intercept age.

Multiple crustal melting pulses and Hf systematics in zircons

This paper focuses on the U–Pb ages and Lu–Hf isotopes in zircons in a long-lived anatectic terrane, to understand both the nature of melting events and controls on Hf-isotope ratios in the zircon record. We focus on rocks from the Wuyi-Yunkai orogeny, South China, that mainly comprises Proterozoic basement rocks that generated numerous granitoids during Early Paleozoic granulite facies metamorphism. We investigate zircons from three anatectic granites and one migmatite. Zircon U–Pb dates for individual samples vary between ~465 and ~410 Ma, covering the duration of the orogeny. U–Pb dates for two samples define three normal distributions with populations spaced by ~15–20 Myr, indicating protracted and pulsed melting events. The inherited pre-orogenic zircon cores yield a wide spread of εHf(435) values (−39.0 to −2.8), whilst all Early Paleozoic, magmatic syn-orogenic cores and rims have a narrower range shifted to higher values ranging between −14.3 and + 5.5 and no systematic variation with the date of the analytical spot. The data suggest that the magmatic syn-orogenic zircons grew from isotopically heterogeneous melts with ~9 to 16 ε units variation for individual samples. The narrower ranges compared to the inherited cores suggest that some magma homogenization must have occurred, and the upward shift of the εHf(435) values is likely due to the influence of non-zircon Hf rather than due to mixing with an external less-evolved magma. Thus, the samples record a ~55 Myr, multi-pulsed melting event, during which heterogeneous crustal magmas had their Hf isotopic composition controlled by inherited zircon dissolution modified by contributions from non-zircon phase.

Preferential dissolution of uranium-rich zircon can bias the hafnium isotope compositions of granites

Abstract Hafnium (Hf) isotopes in zircon are important tracers of granite petrogenesis and continental crust evolution. However, zircon in granites generally shows large Hf isotope variations, and the reasons for this are debated. We applied U-Pb geochronology, trace-element, and Hf isotope analyses of zircon from the Miocene Himalayan granites to address this issue. Autocrystic zircon had εHf values (at 20 Ma) of–12.0 to–4.3 (median =–9). Inherited zircon yielded εHf values (at 20 Ma) of–34.8 to +0.3 (median =–13); the majority of εHf values were lower than those of autocrystic zircon. The εHf values of inherited zircon with high U concentrations resembled those of autocrystic zircon. Geochemical data indicates that the granites were generated during relatively low-temperature (&amp;lt;800 °C) partial melting of metasedimentary rocks, which, coupled with kinetic hindrance, may have led to the preferential dissolution of high-U zircon that could dissolve more efficiently into anatectic melt due to higher amounts of radiation damage. Consequently, Hf values of autocrystic zircon can be biased toward the values of U-rich zircon in the source. By contrast, literature data indicate that granites generated at high temperatures (&amp;gt;820–850 °C) generally contain autocrystic and inherited zircons with comparable Hf isotope values. During higher-temperature melting, indiscriminate dissolution of source zircon until saturation is reached will result in near-complete inheritance of Hf isotope ratios from the source. Our results impose an extra layer of complexity to interpretation of the zircon Hf isotope archive that is not currently considered.

Metamictization and fluid-driven alteration triggering massive HFSE and REE mobilization from zircon and titanite: Direct evidence from EMPA imaging and LA-ICP-MS analyses

The major and trace element composition of the main HFSE-bearing phases from highly evolved miaskitic syenites has been investigated to assess their stability, an important control on HFSE retention and mobility during post-magmatic hydrothermal activity. The major HFSE carrier, abundant zircon (up to 5 vol%), has undergone intensive metamictization, caused by high initial U and Th along with the presence of early magmatic U-thorite inclusions. Subsequent to metamictization, zircon experienced pervasive fluid-driven alteration, as evidenced by strong depletion in Zr and Si, enrichment in non-formula elements (Ca, Fe, Al, P), deficient EMPA analytical totals, high porosity, and low BSE-intensities of strongly altered crystal interiors. Element maps suggest that extensive HFSE remobilization from altered zircon was controlled by interaction with oxidized aqueous solutions circulating through abundant microfractures that formed by volume expansion due to zircon metamictization. Enhanced HFSE mobility in hydrothermal fluids is reflected by the presence of a hydrated amorphous Zr-Th-U-Si-phase, filling microveinlets intimately associated with metamict zircons. In addition to HFSE remobilization from altered zircon, titanite released a significant amount of REE during interaction with the hydrothermal fluids that concurrently caused a fractionation of Eu in titanite, producing increased EuN/EuN* (4.16–22.2) in altered crystals. Extensively altered titanite grains with elevated EuN/EuN* are considerably depleted in ∑REE + Y compared to unaffected crystals, showing that degree of positive Eu anomalies, relating to post-magmatic metasomatic overprint, appears to be a sensitive indicator for an intensity of hydrothermal alteration and even the total amount of released incompatible elements. Pronounced LREE enrichment accompanied by HREE depletion in the strongly altered crystals indicates greater LREE retention in titanite and higher HREE mobility in the hydrothermal system probably caused by complexing with fluoride ligands. Fluorine was most likely supplied through amphibole and biotite chloritization, as suggested by notably low F in the mafic silicates along with the formation of secondary F-rich titanite at the expense of altered biotite. Moreover, the dissolution of zircon and titanite and resistance of apatite to pervasive alteration indicates that remobilization of HFSE and REE from these phases occurred under alkaline conditions (pH 8–12), presumably controlled by potassium-dominated complexes transporting HFSE along with REE over long distances, as indicated by a close spatial linkage of syenite outcrops with U-mineralization. Our results clearly demonstrate: (1) extensive chemical dissolution of zircon controlled by its self-induced radiation damage; (2) high solubility of titanite and metamict zircon under alkaline conditions; (3) enhanced mobility and long-distance transport of HFSE and REE in alkali- and F-rich hydrothermal systems.

Peritectic minerals record partial melting of the deeply subducted continental crust in the Sulu orogen

AbstractThe deeply subducted continental crust commonly undergoes partial melting during its exhumation from mantle to crustal depths. Peritectic minerals are commonly produced during decompressional melting, providing an important record of the timing, P–T conditions, and mechanisms of crustal anatexis in continental subduction zones. A combined study of petrography, mineral inclusions, mineral major and trace element compositions, phase equilibrium modelling, zircon and titanite U–Pb dating, and zircon Hf–O isotopes was carried out to characterize peritectic minerals in ultrahigh‐pressure (UHP) metamorphic gneisses from the Sulu orogen, China. Field and petrographic observations reveal that the crustal anatexis was extensive and is recorded by folded leucosomes, corroded phengite relicts, cuspate K‐feldspar and quartz, pseudomorphs of melt films along grain boundaries, and multiphase crystal inclusions in zircon, garnet, and phengite. The multiphase crystal inclusions represent the crystallized product of anatectic melt droplets trapped in the peritectic zircon and garnet as well as the residual UHP phengite. The peritectic origin of both zircon and garnet is identified by their compositional characteristics, and the peritectic origin of garnet is further supported by phase equilibrium modelling which shows an increase in garnet contents as partial melting proceeds. The modelled phase diagrams also indicate that the two peritectic minerals were produced through two episodes of phengite dehydration melting. The first occurred at 237 ± 3 Ma under P–T conditions of ≥2.6 GPa and 810–880°C, corresponding to thermobaric ratios of ~312–338℃/GPa. The second took place at 222 ± 2 Ma under P–T conditions of 1.5–2.1 GPa and 730–760℃, corresponding to thermobaric ratios of 362–487℃/GPa. In comparison to peak UHP metamorphic P–T conditions of 2.8–4.0 GPa and 720–870℃ for thermobaric ratios of 218–257℃/GPa, it appears that the decompressional melting is associated with an increase in thermal gradients during the exhumation in the different stages. Whereas the first is caused by decompression during initial exhumation of the deeply subducted continental crust, the second is caused by decompression during relamination of the UHP slices into the collision‐thickened orogenic lower crust. Peritectic amphibole and titanite were also recognized by comparing their petrographic textures with natural samples and experimental products. They were produced through water‐fluxed melting at 217 ± 3 Ma under lower P–T conditions, suggesting the liberation of aqueous fluids from the underlying exhuming UHP rocks. All of the peritectic minerals generally show compositional inheritance from their precursor minerals. Nevertheless, they commonly underwent dissolution when anatectic P–T conditions deviate significantly from those of peritectic reactions. This is indicated by the dissolution of peritectic zircon and garnet and by the occurrence of anhedral and embayed peritectic amphibole. As a consequence, the behaviour of peritectic minerals provides a snapshot of crustal anatexis for geochemical differentiation in collisional orogens.

Zircon stability grids in crustal partial melts: implications for zircon inheritance

Zircon inheritance is a common phenomenon in igneous rocks, although more frequent in granitoids. Zircons inherited from granite magmas mostly come from the source, not from wall rocks or xenoliths. Consequently, they can provide invaluable information about the source materials, melting temperature, and melt segregation conditions. Miller et al. (Geology 31:529–532, 2003) divided granite rocks according to their zircon saturation temperature (TZr) into “hot” (TZr > 800 °C, with little or no inherited zircon) and “cold” (TZr < 800 °C; with abundant inherited zircon). Nevertheless, we have found that coeval and neighboring two-mica granites with TZr < 750 °C, presumably derived from similar sources, may have a radically different inheritance, from about 95 to near 0%. This paper aims to understand the reasons for these differences, in particular, and the survival of source zircons in granitoids, in general. To this end, we modeled the relationships between source composition, temperature, pressure, water content, zircon solubility, and melt fraction, on one hand; and melt production and zircon solution rates, on the other hand. Our results foresee that zircon survival during crustal melting is more probable if the source is a fertile peraluminous metasedimentary rock than if it is a metaluminous source with similar SiO2. Elevated zircon inheritance is characteristic of mid-crustal S-type, water-rich granite magmas generated within 4.5 and 6 kbar. Moderate or no inheritance is characteristic of water-poor granite magmas, because their sources require higher temperatures to produce the same melt fraction. Fast melt extraction does not cause perceptible effects on our models, because melt generation is slower than zircon dissolution, except in the case of crustal underplating by hot mafic magmas. We propose to refine the “hot” and “cold” classification by splitting the “cold” granites (TZr < 800 °C) into two categories, “dry” with little inheritance and “wet” with a very high zircon inheritance. Wet granites require a source water-fluxed from outside. They are characteristic of mid-crustal anatectic complexes with highly fertile gneisses alternating with unfertile mica-rich metapelites. We suggest that the extra water should come in most cases from dehydration reactions in the unfertile metasedimentary rocks beneath the crustal section undergoing anatexis.

When zircon drowns: Elusive geochronological record of water-fluxed orthogneiss melting in the Velay dome (Massif Central, France)

Zircon U-Pb geochronology is routinely performed to unravel the timing and duration of melting events within the continental crust. A comprehensive understanding of the zircon behavior during anatexis is therefore paramount to accurately assess the impact of crustal melting on orogenic processes. We report on an anatectic system from the southern part of the Velay dome (Variscan French Massif Central) encompassing late Carboniferous migmatites developed at the expense of peraluminous orthogneisses and associated with (leuco)granite bodies intrusive at higher structural levels. Our zircon U-Pb geochronological survey (494 analyzed grains) evidences a very scant record of the Variscan anatectic event. Indeed, oscillatory-zoned melt-precipitated zircon grains extracted from metatexites, leucosomes and (leuco)granites almost systematically yielded the late Neoproterozoic crystallization age of the felsic igneous protolith. Field and petrographic observations, whole-rock geochemical signatures, phase equilibrium and thermodynamically-constrained trace element modelling collectively indicate that partial melting of the orthogneisses took place at T ~ 700 °C and was fluxed by ingress of external water. The lack of new zircon growth is ascribed to a range of processes ultimately related to the low melting temperature including limited zircon solubility in the melt phase, occlusion of zircon in non-reacting source biotite and sluggish zircon dissolution kinetics. Peraluminous orthogneisses, yet highly fusible due to their “eutectic” subsolidus mineral assemblage, exhibit a very limited zircon record of low temperature water-fluxed melting. Thus, zircon has little potential to provide reliable chronological constraints on water-fluxed melting episodes affecting such lithologies.

The evolution of the Sesia Zone (Western Alps) from Carboniferous to Cretaceous: insights from zircon and allanite geochronology

Microscale dating of distinct domains in minerals that contain relics of multiple metamorphic events is a key tool to characterize the polyphase evolution of complex metamorphic terranes. Zircon and allanite from five metasediments and five metaintrusive high-pressure (HP) rocks from the Eclogite Micaschist Complex of the Sesia Zone were dated by SIMS and LA-ICP-MS. In the metasediments, zircon systematically preserves detrital cores and one or two metamorphic overgrowths. An early Permian age is obtained for the first zircon rim in metasediments from the localities of Malone Valley, Chiusella Valley and Monte Mucrone (292 ± 11, 278.8 ± 3.6 and 285.9 ± 2.9 Ma, respectively). In the Malone Valley and Monte Mucrone samples, the early Permian ages are attributed to high-temperature metamorphism and coincide with the crystallization ages of associated mafic and felsic intrusions. This implies that magmatism and metamorphism were coeval and associated to the same tectono-metamorphic extensional event. In the Malone Valley, allanite from a metasediment is dated at 241.1 ± 6.1 Ma and this age is tentatively attributed to a metasomatic/metamorphic event during Permo-Triassic extension. Outer zircon rims with a late Cretaceous age (67.4 ± 1.9 Ma) are found only in the micaschist from Monte Mucrone. In metagabbro of the Ivozio Complex, zircon cores yield an intrusive age for the protolith of 340.7 ± 6.8 Ma, whereas Alpine allanite are dated at 62.9 ± 4.2 and 55.3 ± 7.3 Ma. The Cretaceous ages constrain the timing of the HP metamorphic stage. The presence of zircon overgrowth only in the central area of the Eclogite Micaschist Complex is attributed to local factors such as (1) multiple fluid pulses at HP that locally enhanced zircon dissolution and recrystallization, and (2) slightly higher temperatures reached in this area during HP metamorphism.

Zircon survival in shallow asthenosphere and deep lithosphere

Abstract Zircon (ZrSiO4) is the most frequently used geochronometer of terrestrial and extraterrestrial processes. To shed light on question of zircon survival in the Earth's shallow asthenosphere, high-temperature experiments of zircon dissolution in natural mid-ocean ridge basaltic (MORB) and synthetic haplobasaltic melts have been performed at temperatures of 1250–1300 °C and pressures from 0.1 MPa to 0.7 GPa. Zirconium measurements were made in situ by electron probe microanalyses (EPMA) at high current. Taking into account secondary fluorescence effects in zircon-glass pairs during EPMA, a zirconium diffusion coefficient of 2.87E-08 cm2/s was determined at 1300 °C and 0.5 GPa. When applied to the question of zircon survival in asthenospheric melts of tholeiitic basalt composition, the data are used to infer that typical 100 mm zircon crystals dissolve rapidly (~10 h) and congruently upon reaction with basaltic melt at pressures of 0.2–0.7 GPa. We observed incongruent (to crystal ZrO2 and SiO2 in melt) dissolution of zircon in natural mid-ocean ridge the basaltic melt at low pressures &amp;lt;0.2 GPa and in the haplobasaltic melt at 0.7 GPa pressure. Our experimental data raise questions about the origin of zircon crystals in mafic and ultramafic rocks, in particular, in shallow oceanic asthenosphere and deep lithosphere, as well as the meaning of the zircon-based ages estimated from these minerals. The origin of zircon in shallow (ultra-) mafic chambers is likely related to the crystallization of intercumulus liquid. Large zircon megacrysts in kimberlites, peridotites, alkali basalts, and carbonatite magmas suggest fast transport and short interaction durations between zircon and melt. The origin of zircon megacrysts is likely related to metasomatic addition of Zr into the mantle as an episode of mantle melting should eliminate them on geologically short timescales.