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We undertook Zr isotope measurements on zircon, titanite, biotite, amphibole, and whole rocks from the La Posta pluton (Peninsular Ranges, southern California) together with trace element analyses and U-Pb age measurements to understand the controls on Zr isotope fractionation in igneous rocks, including temperature, crystallization sequence, and kinetic effects. We find large (>0.6‰) Zr isotope fractionations (expressed as δ94/90Zr) between titanite and zircon forming at approximately the same temperature. Using equilibrium fractionation factors calculated from ionic and ab initio models, we infer the controls on Zr isotope evolution to include the relative order in which phases appear on the liquidus, with titanite fractionation resulting in isotopically lighter melt and zircon fractionation resulting in isotopically heavier melt. While these models of Zr fractionation can explain δ94/90Zr variations in zircon of up to ∼1.5‰, crystallization order, temperature and presence of co-crystallizing phases do not explain all aspects of the intracrystalline Zr isotopic distribution in zircons in the La Posta pluton or the large range of Zr isotopic values among zircons (>2‰). Without additional constraints, such as knowledge of co-crystallizing phases and a better understand of the true causes of Zr isotope fractionation, Zr isotopes in zircon remains an ambiguous proxy of magmatic evolution. [ABSTRACT FROM AUTHOR]

Geochemical data on widely distributed igneous rocks of southern Tibet are used to reconstruct paleo-crustal thickness during the 50+ million years that have elapsed since the onset of the India-Asia collision. We use two approaches, one based on Nd isotopes and an assimilation-recharge model for granitic magma genesis and another empirical method based on trace element geochemistry (La/Yb). The focus is on granitic rocks of two age ranges in a segment of the southern Lhasa Block between approximately 89.5° and 92.5°E longitude. One age range, 45 to 62 Ma, spans the time of the onset of collision and for which we infer the geochemistry of granitic rocks reflects mainly pre-collision structure. The other age range is 21 to 9 Ma for the Nd isotopic approach, and 32 to 9 Ma for La/Yb, where the geochemistry must reflect post-collision structure. Our results suggest that the pre- and syn -collision southern margin of the Lhasa block, that portion now located within 50–60 km of the Indus-Yarlung suture (IYS) and south of 29.8°N latitude was relatively thin, about 25–35 km thick until 45 Ma. At approximately 29.8°-29.9°N latitude there was a pronounced crustal discontinuity, and north of that latitude (for a distance that we cannot constrain), the inferred crustal thickness was greater, at least 50–55 km, as indicated by latest Cretaceous and Early Tertiary granitoids and ignimbrites that have large fractions of assimilated continental crust and high La/Yb ratios. Post-collision Nd isotopic and La/Yb data from granitoids younger than 32 Ma suggest that the southern margin south of 29.8°N was thickened substantially to at least 55–60 km (based on Nd isotopes) and possibly as much as 70–75 km (based on La/Yb) by Early to mid-Miocene time. These observations require that thickening of the southern Lhasa Block margin in the period 45–32 Ma was non-uniform; the crust now within 60 km of the suture was thickened by approximately 40 km whereas the crust north of 29.9°N latitude was thickened much less, or not at all. The region currently between 29.8°N and the YTS may have been the highest elevation mountain terrane in the period from roughly 30 to 20 Ma. The amount of Miocene denudation reflects this difference, as there is evidence of substantially more denudation near the IYS than in the region north of 29.9°N. Some of the difference in thickening could be due to magmatic additions from the mantle in the region south of 29.8°N, but there is need for at least 30 km of tectonic thickening between 45 and 32 Ma. The non-uniform thickening suggests that the high elevations at the southern margin of the Himalaya-Tibet orogen propagated southward by about 200 km, from north of Lhasa to their present position, during the period from 50 to 20 Ma. Present crustal thickness requires an additional 10–15 km of more uniform post-Miocene thickening. Unlabelled Image • Nd isotopes and La/Yb ratios in granitic rocks indicate radical changes in crustal thickness between 45 and 32 Ma • Syn-collision granites indicate that the southern margin (south of 29.9°N) of the Tibet crust was thin until 45 Ma • Between 45 and 32 Ma, the crust south of 29.9°N thickened dramatically to ≥ 70km • The crust north of 30°N was not thickened in this interval, an inference compatible with thermochronology • The 30 to 45 km thickening of the southern margin crust was due to both magmatic and tectonic processes • Nd isotope and La/Yb approaches for estimating crustal thickness are in substantial agreement • Modern crustal thickness requires 10-15 km of additional, post-Miocene crustal thickening across the entire area [ABSTRACT FROM AUTHOR]

Given the global dearth of Hadean (>4 Ga) rocks, 4.4–4.0 Ga detrital zircons from Jack Hills, Narryer Gneiss Complex (Yilgarn Craton, Western Australia) constitute our best archive of early terrestrial materials. Previous Lu–Hf investigations of these zircons suggested that felsic (low Lu/Hf) crust formation began by ∼4.4 to 4.5 Ga and continued for several hundred million years with evidence of the least radiogenic Hf component persisting until at least ∼4 Ga. However, evidence for the involvement of Hadean materials in later crustal evolution is sparse, and even in the detrital Jack Hills zircon population, the most unradiogenic, ancient isotopic signals have not been definitively identified in the younger (<3.9 Ga) rock and zircon record. Here we show Lu–Hf data from <4 Ga Jack Hills detrital zircons that document a significant and previously unknown transition in Yilgarn Craton crustal evolution between 3.9 and 3.7 Ga. The zircon source region evolved largely by internal reworking through the period 4.0–3.8 Ga, and the most ancient and unradiogenic components of the crust are mostly missing from the record after ∼4 Ga. New juvenile additions to the crust at ca. 3.9–3.8 Ga are accompanied by the disappearance of unradiogenic crust ca. 3.9–3.7 Ga. Additionally, this period is also characterized by a restricted range of δ 18 O after 3.8 Ga and a shift in several zircon trace element characteristics ca. 3.9–3.6 Ga. The simultaneous loss of ancient crust accompanied by juvenile crust addition can be explained by a mechanism similar to subduction, which effects both processes on modern Earth. The oxygen isotope and trace element information, although less sensitive to tectonic setting, also supports a transition in zircon formation environment in this period. [ABSTRACT FROM AUTHOR]

Abstract: Cretaceous magmatism in southern Lhasa subterrane, Tibetan plateau has been investigated for many years and a series of models have been proposed to illustrate their petrogenesis and geodynamic implications. But rare work has been done on the Cretaceous magmatism in central Lhasa subterrane. Here we report the petrology, zircon in situ U–Pb geochronology, Hf isotopes, trace element, and whole-rock elements and Sr–Nd isotopic geochemical data of the host granodiorites, and gabbroic and dioritic enclaves in Mamba area, central Lhasa subterrane. Zircon U–Pb dating for a Mamba host granodiorite yields a crystallization age of ~84Ma, with in situ Hf isotopic analyses for 18 spots of the same zircons of εHf (t) ranging from −7.5 to −0.3. A dioritic enclave (85.2Ma) is coeval with the host granodiorite and shows similar zircon Hf isotopic compositions (εHf (t)=−4.0 to +0.2). Mamba granodiorites (SiO2 =66.6–67.5wt.%) and dioritic enclaves (SiO2 =53.9–57.6wt.%) are high-K calc-alkaline, and a gabbroic enclave is shoshonitic (K2O=2.81%). All these samples are metaluminous, and enriched in large ion lithophile elements (LILEs, such as Rb, Ba, K, U, Th) and depleted in high field strength elements (HFSEs, e.g., Nb, Ta, Ti, and Zr). The host granodiorites are enriched in light rare earth elements (REEs), depleted in heavy REEs with weakly negative Eu anomalies (δEu=0.86–0.88), with high Al2O3 (15.0–15.7wt.%), high Sr/Y ratio (58.1–68.3) and Sr (680–755ppm), and low Y (10.8–13.0ppm) abundance, suggesting adakitic affinities. Mamba adakitic granodiorites, gabbroic and dioritic enclaves exhibit homogeneous Sr isotopes ((87Sr/86Sr)i =0.7066–0.7067, 0.7073, and 0.7067, respectively) and Nd isotopes (εNd(t)=−5.7 to −4.4, −4.0, and −3.6, respectively). These geochemical features allowed us to conclude that the adakitic host granodiorites and mafic (gabbroic–dioritic) enclaves were derived from magma mixing between ancient thickened lower crust and enriched fluid-metasomatized mantle. The distance between Mamba and the suture zone was more than 200km when the intrusives emplaced at ~85Ma, which implies that these rocks cannot be resulted from the mid-ocean ridge subduction. Combining of the intra-plate environment indicated by the gabbroic enclave of this study, the presence of the coeval bimodal igneous rocks in the similar latitude in central Lhasa subterrane, and other records in late Cretaceous sedimentary basin, the Mamba ~85Ma magmatism were attributed to the back-arc extension of Neo-Tethyan Ocean. [Copyright &y& Elsevier]

Abstract: Two distinctive modes of post-collisional magmatism are evident along a transect through the eastern Himalaya at ca. 92°E. The first comprises Eocene igneous complexes emplaced into the central Tethyan Himalaya Series (THS) (Dala granitoids, 44.1±1.2Ma) and the core of the Yala-Xiangbo dome (Yala-Xiangbo granitoids, 42±5Ma). Trace element characteristics and Rb–Sr and Sm–Nd isotopes indicate that the Dala granitoids formed from sub-equal mixtures of Gangdese-type magmas and rocks of the Greater Himalayan Crystallines and/or THS. The Yala-Xiangbo granitoids are geochemically complex but interpreted to be evolved equivalents of the Dala granitoid suite. The undeformed Dala plutons constrain the timing of deformation of THS metasediments to be >44Ma and their compositions indicates that components of the Indian foreland sequences had been accreted to the hanging wall of the main Himalayan decollement by that time. The second magmatic mode comprises Miocene intrusions (Arunachal- and Tsona-types) similar in most respects to the High Himalayan Leucogranites documented throughout the range. Although fluid-absent melting of metapelites appears to be the dominant origin of Miocene Himalaya leucogranites, thermometry and trace element data for the Arunachal-type intrusions suggest that they formed under fluid-present conditions. Sr and Nd-isotopes indicate that Lesser Himalayan metasediments were involved during the partial melting that produced the Arunachal-type leucogranites. [Copyright &y& Elsevier]

Abstract: Four distinct granitoid suites outcrop across a N–S transect in the eastern Himalaya between the Main Central Thrust (MCT) and the Indus Tsangpo suture. The Arunachal and Tsona leucogranites outcrop, respectively, in the MCT hanging-wall and adjacent the South Tibetan Detachment. Both crystallized at about 19Ma, and appear to be lateral equivalents of the High Himalayan Leucogranites. In the central Tethyan Himalaya, the Dala granitoids form a suite of undeformed plutons emplaced into deformed Tethyan Himalayan Sequence metasediments. In the northern Tethyan Himalaya, the Yala-Xiangbo granitoids are part of an igneous complex in the core of the Yala-Xiangbo dome. Both the Dala and Yala-Xiangbo granitoids have Eocene magmatic ages (44.1±1.2Ma and 42±5Ma, respectively) that are notably older than the vast majority of post-collisional granitoids along the main Himalayan Arc. Pelitic units from the core of the Yala-Xiangbo dome record two episodes of amphibolite-grade metamorphism and monazite growth coinciding with well-documented episodes of Eohimalayan and Neohimalayan metamorphism. The thermochronological histories of the Dala and Yala-Xiangbo units suggest that the eastern Tethyan Himalaya was tectonically quiescent from ca. 40Ma until ca. 15Ma. Miocene exhumation of the Dala and Yala-Xiangbo units, and all North Himalayan Domes studied to date, appears best explained by a structural control, such as concurrent north-directed motion on the Great Counter Thrust. We hypothesize that an extension of the Ninety East Ridge may have been responsible for Eocene high heat-flow beneath the eastern Himalaya and explain the localization of magmatism there. [Copyright &y& Elsevier]

Abstract: Several lines of isotopic evidence – the most direct of which is from Hadean Jack Hills zircons – suggest a very early history of crust formation on Earth that began by about 4.5Ga. To constrain both the fate of the reservoir for this crust and the nature of crustal evolution in the sediment source region of the Jack Hills, Western Australia, during the early Archean, we report here initial 176Hf/177Hf ratios and δ18O systematics for <4Ga Jack Hills zircons. In contrast to the significant number of Hadean zircons which contain highly unradiogenic 176Hf/177Hf requiring a near-zero Lu/Hf reservoir to have separated from the Earth’s mantle by 4.5Ga, Jack Hills zircons younger than ca. 3.6Ga are more radiogenic than –13ε (CHUR) at 3.4Ga in contrast to projected values at 3.4Ga of –20ε for the unradiogenic Hadean reservoir indicating that some later juvenile addition to the crust is required to explain the more radiogenic younger zircons. The shift in the Lu–Hf systematics together with a narrow range of mostly mantle-like δ18O values among the <3.6Ga zircons (in contrast to the spread towards sedimentary δ18O among Hadean samples) suggests a period of transition between 3.6 and 4Ga in which the magmatic setting of zircon formation changed and the highly unradiogenic low Lu/Hf Hadean crust ceased to be available for intracrustal reworking. Constraining the nature of this transition provides important insights into the processes of crustal reworking and recycling of the Earth’s Hadean crust as well as early Archean crustal evolution. [Copyright &y& Elsevier]

Abstract: The 40K–40Ca decay system has not been widely utilized as a geochronometer because quantification of radiogenic daughter is difficult except in old, extremely high K/Ca domains. Even these environments have not heretofore been exploited by ion microprobe analysis due to the very high mass resolving power (MRP) of 25,000 required to separate 40K+ from 40Ca+. We introduce a method that utilizes doubly-charged K and Ca species which permits isotopic measurements to be made at relatively low MRP (~5000). We used this K–Ca ‘double-plus’ approach to address an enduring controversy in 40Ar/39Ar thermochronology revolving around exsolved alkali feldspars from the 1166Ma Klokken syenite (southern Greenland). Ion microprobe 40K–40Ca analysis of Klokken samples reveal both isochron and pseudoisochron behaviors that reflect episodic isotopic and chemical exchange of coarsely exsolved perthites and a near end-member K-feldspar until ≤719Ma, and perhaps as late at ~400Ma. Feldspar microtextures in the Klokken syenite evolved over a protracted interval by non-thermal processes (fluid-assisted recrystallization) and thus this sample makes a poor model from which to address the general validity of 40Ar/39Ar thermochronological methodologies. [ABSTRACT FROM AUTHOR]

Abstract: Hydrothermal treatment of closely sized muscovite aggregates in a piston-cylinder apparatus induced 40Ar∗ loss that is revealed in 40Ar/39Ar step heating spectra. Age spectra and Arrhenius data, however, differ from that expected from a single diffusion length scale. A numerical model of episodic loss assuming the presence of multiple diffusion domains yields excellent fits between synthetic and actual degassing spectra. We used this model to isolate 40Ar∗ loss from the grains that remained intact during hydrothermal treatment at 10kbar permitting calculation of diffusion coefficients in the temperature range 730-600°C. Diffusion data generated in this manner yield an activation energy (E) of 63±7kcal/mol and frequency factor (D o) of 2.3 cm2/s. Experiments at 20kbar yield diffusivities lower by about an order of magnitude and correspond to an activation volume of ∼14cm3/mol. Together, these parameters predict substantially greater retentivity of Ar in muscovite than previously assumed and correspond to a closure temperature (T c) of 425°C for a 100μm radius grain cooling at 10°C/Ma at 10kbar (T c =405°C at 5kbar. Age and log (r/r o) spectra for the run products show strong correlations indicating that muscovites can retain Ar diffusion boundaries and mechanisms that define their natural retentivity during vacuum step heating. This may permit the application of high resolution, continuous 40Ar/39Ar thermochronology to low grade, regionally metamorphosed terranes. [Copyright &y& Elsevier]

Abstract: Large deviations in ɛ Hf(T) from bulk silicate Earth seen in >4 Ga detrital zircons from Jack Hills, Western Australia, have been interpreted as reflecting a major differentiation of the silicate Earth at 4.4 to 4.5 Ga. We have expanded the characterization of 176Hf/177Hf initial ratios (Hf) in Hadean zircons by acquiring a further 116 laser ablation Lu–Hf measurements on 87 grains with ion microprobe 207Pb/206Pb ages up to 4.36 Ga. Most measurements employed concurrent Lu–Hf and 207Pb/206Pb analyses, permitting assessment of the age of the volumetrically larger domain sampled by laser drilling against the spatially more restricted ion microprobe ages. Our new results confirm and extend the earlier observation of significant negative deviations in ɛ Hf(T) throughout the Hadean, although no positive ɛ Hf(T) values were documented in this study. Monte Carlo modelling of these data yields an essentially uniform spectrum of model ages between 4.56 and 4.20 Ga for extraction of the zircons'' protoliths from a chondritic reservoir. To assess whether the five data plotting close to solar system initial Hf (Hf o) are statistically robust, we derived the error propagation equation for a parameter, ɛ o, which measures the difference of a sample from Hfo. Our analysis suggests that this limited data is indicative of source sequestration in a crustal-type Lu/Hf environment prior to 4.5 Ga. Oxygen isotope data and Ti thermometry from Hadean zircons show little obvious correlation with Hf, consistent with their derivation through fusion of a broad suite of crustal rock types under water-saturated conditions. Together with other isotopic and trace element data obtained from these ancient zircons, our results indicate essentially continuous derivation of crust from the mantle from 4.5 to 4.2 Ga with concurrent recycling into the mantle and internal crustal re-working. These results represent further evidence that by 4.35 Ga, portions of the crust had taken on continental characteristics. [Copyright &y& Elsevier]

A recurring challenge to ion microprobe U–Pb zircon geochronology has been to discriminate between preservation of original igneous zircon populations and inherited grains. This has proved particularly problematic in studying the polyphase metamorphic rocks that dominate Early Archean gneissic terranes. For example, differing interpretations exist for the origin of complex zircon populations investigated by U–Pb ion microprobe zircon geochronology from a granitoid (orthogneiss) body, previously used to establish a minimum age for a supracrustal enclave on Akilia (island) in southern West Greenland. We describe a method whereby the geochemistry of the Akilia orthogneiss, coupled with a U–Th–Pb vs. age depth profile in a zircon from the same rock, permits direct assessment of zircon inheritance. Results reveal evidence for three phases of concentric zircon growth at 3.83 Ga, ∼3.6 Ga and 2.7–2.5 Ga; zircon growth at both ∼3.6 Ga and 2.7–2.5 Ga is consistent with precipitation from a metamorphic fluid. Depth profile U–Th–Pb data demonstrate that only the >3.8-Ga zircon core could have crystallized from the host rock. We conclude that the magmatic age of this rock is 3.83±0.01 Ga, not ∼3.65 Ga as has been previously proposed. The U–Th–Pb zircon depth profile technique has wider applications to resolving other geochronological debates in high-grade metamorphic terranes where zircons populations are diverse and individual grains record complex overgrowth and dissolution/re-precipitation features. [Copyright &y& Elsevier]

We apply a rapid and accurate in situ technique to make U–Pb isotopic measurements of complexly intergrown uranium minerals and oxygen isotopic analyes of uraninite from the unconformity-type Cigar Lake uranium deposit. Secondary uranium minerals intergrown with uraninite, such as coffinite, USiO4·nH2O and calciouranoite, CaU2O7·5H2O, were identified by high-resolution transmission electron microscopy (HRTEM).In situ U–Pb results from three stages of uraninite and coffinite define well-correlated arrays on concordia with upper intercepts of 1461±47, 1176±9, and 876±14 Ma (±1σ). These ages are interpreted as the minimum ages of mineralization correlate with the timing of clay mineral alteration (∼1477 Ma) associated with these unconformity-type uranium deposits, the ages of magnetization events at 1600–1450 and 900 Ma from the Athabasca Basin, and the Grenvillian Orogeny at ∼1100 Ma. In situ U–Pb isotopic analyses of uraninite and coffinite can document the Pb*/U heterogeneities that occur on the scale of 15–30 μm, thus providing relatively accurate information regarding the timing of fluid interactions associated with the evolution of these deposits.The high spatial resolution and precision of the ion microprobe allow us to measure δ18O values of 20–100 μm unaltered portions of uraninites from Cigar Lake. The range of δ18O values (−33.9 to −20.4‰) are among the lowest reported for unconformity-type deposits, confirming that conventional fluorination analyses of material sampled at the mm-scale are insufficient to avoid contamination from isotopically heavier coffinite and calciouranoite. [Copyright &y& Elsevier]

A biotic origin of isotopically light graphite in Hadean zircon remains contested, in part because it is unclear how biogenic carbon in sediments behaves during diagenesis, metamorphism and anatexis, and how it can be preserved as inclusions in zircon. Here, we report the discovery of graphitic inclusions in zircon from Rumburk granite in the Lusatian Block of eastern central Europe as the first example of such inclusion-bearing zircon in a well-constrained geological and petrological context. Most graphite inclusions are trapped at the interface between inherited zircon interiors of early Cambrian–Proterozoic age and late Cambrian (496.2 ± 2.3 Ma; 95% confidence) zircon overgrowths that crystallized at ∼700 °C under highly reducing conditions (typically −4.6 log units relative to the fayalite-magnetite-quartz buffer). Zircon overgrowths are also enriched in xenotime component (up to 93 µmol/g P) and δ18O (average δ18O = 7.7‰), both typical for S-type granitic melts. Raman microspectroscopy reveals crystalline and disordered graphitic carbon, but because overgrowth temperatures reached >700 °C, sufficient for complete graphitization, disordering is presumably secondary and caused by in-situ irradiation from the zircon host. Because organic materials were avoided during sample preparation, and inclusions were excavated by ion beam sputtering, exposure to potential C-bearing contaminants can be dismissed. Inclusions range in δ13C from −44.6 to −7.5‰ with a dominant mode at δ13C = −34‰ that is positively skewed. Rayleigh-type graphite precipitation from a CO 2 -CH 4 fluid with a starting composition equivalent to regional black shale (δ13C = −32‰) explains inclusion carbon isotopic range and distribution. Collectively, these observations suggest that graphite was initially trapped in voids formed via dissolution-reprecipitation of trace element enriched, and possibly metamict, detrital zircon in metasedimentary protoliths when exposed to C-O-H fluids during prograde metamorphism. Subsequently, graphite-filled voids in inherited zircon became enclosed when zircon rims crystallized from highly reduced S-type granitic melts. Besides partial disordering of graphitic carbon due to irradiation, there is no indication for post-entrapment alteration, demonstrating that graphitic inclusions in zircon can preserve isotopic biosignatures over hundreds of millions of years. [ABSTRACT FROM AUTHOR]

Despite the robust nature of zircon in most crustal and surface environments, chemical alteration, especially associated with radiation damaged regions, can affect its geochemistry. This consideration is especially important when drawing inferences from the detrital record where the original rock context is missing. Typically, alteration is qualitatively diagnosed through inspection of zircon REE patterns and the style of zoning shown by cathodoluminescence imaging, since fluid-mediated alteration often causes a flat, high LREE pattern. Due to the much lower abundance of LREE in zircon relative both to other crustal materials and to the other REE, disturbance to the LREE pattern is the most likely first sign of disruption to zircon trace element contents. Using a database of 378 (148 new) trace element and 801 (201 new) oxygen isotope measurements on zircons from Jack Hills, Western Australia, we propose a quantitative framework for assessing chemical contamination and exchange with fluids in this population. The Light Rare Earth Element Index is scaled on the relative abundance of light to middle REE, or LREE-I = (Dy/Nd) + (Dy/Sm). LREE-I values vary systematically with other known contaminants (e.g., Fe, P) more faithfully than other suggested proxies for zircon alteration (Sm/La, various absolute concentrations of LREEs) and can be used to distinguish primary compositions when textural evidence for alteration is ambiguous. We find that zircon oxygen isotopes do not vary systematically with placement on or off cracks or with degree of LREE-related chemical alteration, suggesting an essentially primary signature. By omitting zircons affected by LREE-related alteration or contamination by mineral inclusions, we present the best estimate for the primary igneous geochemistry of the Jack Hills zircons. This approach increases the available dataset by allowing for discrimination of on-crack analyses (and analyses with ambiguous or no information on spot placement or zircon internal structures) that do not show evidence for chemical alteration. It distinguishes between altered and unaltered samples in ambiguous cases (e.g., relatively high Ti), identifying small groups with potentially differing provenance from the main Jack Hills population. Finally, filtering of the population using the LREE-I helps to more certainly define primary correlations among trace element variables, potentially relatable to magmatic compositional evolution. [ABSTRACT FROM AUTHOR]

Abstract: We report the first U–Th–Pb ion microprobe depth profiles of four Hadean zircons from the Jack Hills and Mount Narryer supracrustal belts of the Narryer Gneiss Complex (NGC), Western Australia. This ultra-high spatial resolution technique probes the age and origin of sub-micron features in individual crystals that can record episodes of zircon growth. Near-surface grain dates of 2700Ma or older are coincident with post-depositional growth/modification. Some ages may coincide with documented pre-deposition metamorphic events for the NGC and igneous emplacement at ca. 3700Ma. Separate events that do not correlate in time with known geologic episodes prior to the preserved rock record are also present on pre-4000Ma zircons. We find evidence for a ∼3.9Ga event, which is coterminous within age uncertainty with one or several large basin-forming impacts (e.g. Nectaris) on the Moon attributed to the late heavy bombardment of the inner solar system. [Copyright &y& Elsevier]

Monazite grains from Nepal and Vietnam were compositionally analyzed with an electron microprobe and dated (Th–Pb) with an ion microprobe. Five sources of uncertainty explain age distributions from single samples that appear inconsistent with a single population: (1) Pb loss due to diffusion, (2) dissolution/reprecipitation reactions along a retrograde path, (3) analytical uncertainties, (4) analyses of overlapping age domains, and (5) episodic monazite growth. The influence of these factors is sample-dependent, but can be evaluated: (1) using peak metamorphic conditions and X-ray element maps to assess potential polymetamorphism or retrogression, (2) obtaining other geochronologic data including previous work or dating other minerals in the sample, (3) evaluating any method-related uncertainty including counting statistics for electron microprobe analyses or calibration reproducibility for ion microprobe analyses, and (4) ascertaining the potential growth mechanism of the monazite grain including dissolution of detrital grains or production from rare earth element (REE) oxide or allanite. Chemical contents of monazite grains analyzed in this study fail to reflect timing information or mineral growth mechanisms. Instead of relying on monazite chemical composition, major (Mn, Fe, Mg, Ca) and minor (Y) element garnet-zoning patterns and peak P–T conditions should be used to facilitate age interpretation. This thermobarometric data records the sample's thermal history, changes in garnet growth rate and mechanisms, and accessory mineral breakdown. ... [Copyright &y& Elsevier]

We examine a database containing the results of 40Ar/39Ar step-heating experiments performed on 194 basement K-feldspars to recover thermal history information. Qualitative examination of 40Ar/39Ar systematics reveals that about half of the K-feldspars examined are sufficiently well behaved to be suitable for thermal history analysis. Correlation algorithms are developed to quantitatively assess the degree to which age and 39Ar release spectra are compatible with the same volume diffusion process. Upon applying these methods, we find that 65% of all samples yield correlation coefficients in excess of 0.8, whereas roughly 40% give values above 0.9. We further compare the observed correlation behavior with that predicted from the multidiffusion domain model and find good agreement for samples with correlation coefficients above 0.9. In contrast, hydrous phases unstable under in vacuo heating and K-feldspars with highly disturbed age spectra yield poorly correlated age and diffusion properties. The high degree of correlation exhibited by the majority of K-feldspars we have analyzed validates extrapolation of experimentally determined diffusion properties to conditions attending natural Ar loss within the crust. Despite this, a significant number of basement K-feldspars analyzed by the step-heating method yield 40Ar/39Ar systematics that are clearly problematic for thermal history analysis. We numerically explore the effects of low-temperature alteration of K-feldspar on thermochronological analysis and identify a range of conditions under which information is progressively lost. Finally, we demonstrate the insensitivity of thermal history calculations to detailed knowledge of the diffusion mechanism by introducing the heterogeneous diffusion model. We find that the multidiffusion domain approach can successfully recover imposed thermal histories from heterogeneous diffusion-type crystals and conclude that most details of the interpretive model employed are of secondary importance. The only requirement for recovering thermal histories from K-feldspar 40Ar/39Ar step-heating results is that argon loss proceeds by volume diffusion and that laboratory argon release adequately mimics the natural diffusion boundaries and mechanisms—a requirement implicitly met by those samples exhibiting high degrees of correlation. [Copyright &y& Elsevier]

Postcollisional potassium-rich magmatism in southern Tibet provides an important insight into the deep processes inducing accelerated plateau uplift and associated geologic episodes during early Miocene. As a major outcropping of postcollisional magmatic rocks in the southwestern Tibet, ultrapotassic and potassic volcanism in Xungba basin occurred coevally at ∼23 Ma, consisting of bimodal-like volcanic sequence. The mantle-derived ultrapotassic rocks (Group 1) are latites and marked by both mantle and crustal geochemical signatures, similar to other younger (19–10 Ma) ultrapotassic rocks elsewhere in the southern Tibet. The high Th/Yb ratios, low Ba/La and Hf/Sm ratios observed in ultrapotassic rocks strongly imply derivation from a metasomatized lithospheric mantle regions enriched by inputs of pelagic sediment and carbonate during previous Tethyan oceanic subduction, while their relatively high SiO 2 and Dy/Yb, low Ni/MgO and CaO/Al 2 O 3 , and convex upward 87 Sr/ 86 Sr–δ 18 O V-SMOW relationship suggest additional contributions of overthickened lower crust and ancient basement except for enriched mantle sources. The potassic rocks (Group 2), which underlie Group 1 rocks, are intermediate to silicic and exhibit adakitic geochemical affinities with high Sr/Y and La/Yb ratios, and low Y and Yb concentrations. The fingerprint of overthickened lower crust identified both from ultrapotassic and potassic rocks may be an important feature for magmatism occurred under the background of continental collision. And the bimodal volcanic sequence of Xungba postcollisional magmatism may further corroborate that the removal of lower part of over-thickened lithospheric mantle at depth have triggered an extension setting associated with Miocene plateau uplift. [ABSTRACT FROM AUTHOR]

Abstract: We present a record of seasonal and inter-annual oxygen (δ 18O) and carbon (δ 13C) isotope ratios from an 81-year-old stalagmite from Moondyne Cave, southwest Australia. The growth history of stalagmite MND-S1 is known since it grew on a cave boardwalk that was installed in 1911 and removed in 1992. This stalagmite provides an excellent test of speleothem climate proxies because the regional climate is strongly seasonal (wet winter/dry summer) and has experienced a 200 mm (20%) reduction of mean rainfall since the mid-1960s, and a 0.8 °C temperature rise since ∼1953. Seasonal variations in calcite δ 18O were measured in situ by high spatial resolution ion microprobe, whilst inter-annual variations of δ 18O and δ 13C were measured by conventional gas-source mass spectrometry. Comparison of the speleothem stable isotopes and instrumental temperature records reveals that δ 18O variations are too large to be driven by temperature alone, and are in the opposite sense. However, daily rainfall δ 18O measurements show that the mean seasonal range in δ 18O of rainfall in southwest Australia is large (2‰) and inversely correlated with rainfall amount. A rainfall driver for the speleothem δ 18O is confirmed by the detection of seasonal shifts of 0.7–1.5‰ in speleothem δ 18O that track rainfall δ 18O, smoothed by storage in the overlying limestone. The seasonal range in speleothem δ 18O is larger than any inter-annual and decadal variation observed in the record. The prominent annual cycles in speleothem δ 18O revealed by ion microprobe analysis indicate that subtle changes in the frequency of intense winter rainfall events, or possibly also moisture sources, could produce significant changes in mean speleothem δ 18O. The ion microprobe results also raise the possibility that the masses of speleothem calcite deposited in winter and summer could vary as a function of the seasonal drip rate and carbonate saturation state of these waters. If this is the case, then small changes in the relative masses of calcite deposited in winter and summer could produce significant shifts in mean δ 18O and δ 13C that have a complex relation to climate. This finding should be generally applicable to the interpretation of long-term trends in speleothem geochemical records for shallow cave sites where seasonal variations in geochemical tracers are relatively large, including most of the sub-tropical monsoon belts and mid to high latitudes with distinct rainfall seasons. [Copyright &y& Elsevier]