Lu Hf System Research Articles

Thermal evolution of the North China Craton revealed by microsampling garnet geochronology on single xenoliths

Granulite xenoliths, sourced from the lower cratonic crust and brought to the Earth's surface within volcanic rocks, offer unique insights into the formation and destruction of cratons. Age determinations are essential for constructing the thermal history of cratons. However, accurate dating is complicated by the xenoliths' extended exposure to high temperatures and their complex thermal histories. Specifically, the effects of extended lower crust residency and entrainment heating on coupled Sm–Nd and Lu–Hf systematics remain to be explored. Six xenoliths hosted in the Mesozoic dioritic porphyry from the eastern North China Craton were sampled from center to edge using micro-drilling/sawing techniques. From each xenolith, two to four concentric zones were successfully extracted for coupled Lu–Hf and Sm–Nd geochronological analysis. These xenoliths predominantly exhibit a granoblastic texture, with minerals that are mostly unzoned. Textural and chemical analyses of minerals indicate a decompression process following the granulite-facies metamorphism, with subsequent multiple reheating events. Thermobarometric calculations suggest granulite-facies equilibration at temperatures of ∼750 °C and pressures of 1.2–2.0 GPa. Consistent Lu–Hf and U–Pb ages of ca. 1.7 Ga, obtained from garnet, zircon, and titanite in chemical and textural equilibrium, are attributed to granulite-facies metamorphism. The Paleoproterozoic Lu–Hf ages and the Neoproterozoic Sm–Nd dates are indistinguishable across all concentric zones within each xenolith, whereas Sm–Nd ages are consistently ∼1.0 Ga younger than the corresponding Lu–Hf ages. This discrepancy suggests a complex history for this section of the lower cratonic crust, marked by a granulite-facies metamorphic event around 1.7 Ga, followed by prolonged cooling. A significant heating event during the Tonian, likely due to regional mantle plume activity, is inferred to have partially or completely reset the Sm–Nd isochrons. This thermal event and subsequent thermal relaxation on the samples at different residence depths thereby decoupled the Sm–Nd system from the Lu–Hf system, signifying a thermal pulse at the southern margin of North China Craton. The Mesozoic entrainment heating event appears to have had minimal impact on the Lu–Hf/Sm–Nd systematics of garnet and U–Pb of zircon/titanite. Our data highlight the potential of coupled microsampling Lu–Hf and Sm–Nd geochronology on individual xenoliths to reveal tectonic episodes within the lower crust that might be undetected by accessory mineral U–Pb geochronology.

Laser ablation (in situ) Lu-Hf geochronology of epidote group minerals

Epidote group minerals, including allanite, clinozoisite and epidote are common in a range of metamorphic, igneous and hydrothermal systems, and are stable across a wide range of pressure–temperature (P–T) conditions. These minerals can incorporate substantial amounts of rare earth elements (REEs) during their crystallisation, making them potential candidates for Lu–Hf geochronology to provide age constraints on various geological processes. Here we report on a first exploration into the feasibility of in situ Lu–Hf geochronology for epidote group minerals from various geological settings and compare the results with age constraints from other geochronometers. Magmatic allanite samples from pegmatites and monzogranites in the Greenland anorthosite complex, Coompana Province and Qingling Orogen provided dates consistent with magmatic events spanning from c. 2660 to 1171 Ma. In the Qingling pegmatites, a younger phase of hydrothermal allanite was dated at c. 215 Ma, consistent with the timing of regional REE mineralisation. Allanite from the Yambah Shear Zone, Strangways Metamorphic Complex, yielded Lu–Hf age of c. 430 Ma. It predates the garnet and apatite growth at c. 380 Ma, suggesting the Lu–Hf system can be preserved in allanite during prograde amphibolite-facies metamorphism. Additionally, Lu–Hf dates for hydrothermal clinozoisite and epidote are consistent with the timing of hydrothermal alteration and mineralisation in a range of settings, demonstrating the utility of the technique for mineral exploration. Despite the current lack of matrix-matched reference materials, the successful application of laser ablation Lu–Hf geochronology to epidote group minerals offers valuable geochronological insights into various geological processes that can be difficult to access through other geochronometers.

In situ apatite and carbonate Lu-Hf and molybdenite Re-Os geochronology for ore deposit research: Method validation and example application to Cu-Au mineralisation

The development of laser ablation inductively coupled plasma quadrupole tandem mass spectrometry (LA-ICP-Q-MS/MS) opens new opportunities to rapidly date a variety of hydrothermal minerals. Here we present in situ Lu-Hf and Re-Os dates for hydrothermal apatite and molybdenite, respectively. We further report the first in situ Lu-Hf dates for bastnäsite, dolomite, and siderite, and assess their potential for constraining ore deposit geochronology. For method validation, we report isotope-dilution Lu-Hf dates for apatite reference material Bamble-1 (1102 ± 5 Ma) and calcite reference material ME-1 (1531 ± 7 Ma), enabling improved accuracy on matrix-matched calibration for LA-ICP-MS/MS Lu-Hf dating. The new methods are applied to the Vulcan Iron-Oxide Copper-Gold (IOCG) prospect in the Olympic Cu-Au Province of South Australia. Such deposits have been difficult to accurately date, given the general lack of reliable mineral geochronometers that are cogenetic with IOCG mineralisation. Hydrothermal apatite Lu-Hf dates and molybdenite Re-Os dates demonstrate that mineralisation at Vulcan largely occurred at ca. 1.6 Ga, contemporaneous with the world class Olympic Dam deposit. Our data also indicates that the Lu-Hf system in apatite is more robust than the U-Pb system for determining the timing of primary apatite formation in an IOCG system. We further demonstrate that dolomite can retain Lu-Hf growth ages over an extended time period (>1.5 billion years), providing constraints on the timing of primary ore mineral crystallisation during brecciation and IOCG mineralisation. Finally, late Neoproterozoic (ca. 589–544 Ma) and Carboniferous (ca. 334 ± 7 Ma) Lu-Hf dates were obtained for texturally late Cu-bearing carbonate veins, illustrating that the carbonate Lu-Hf method allows direct dating of Cu remobilisation events. This has important implications for mineral exploration as the remobilised Cu may have been transferred to younger deposits hosted in Neoproterozoic sedimentary basins overlaying the Olympic IOCG province.

Testing in-situ apatite Lu–Hf dating in polymetamorphic mafic rocks: a case study from Palaeoproterozoic southern Australia

In mafic systems where primary mineral assemblages have witnessed moderate- to high-temperature hydrous overprinting and deformation, little is known about the retentivity of the Lu–Hf isotopic system in apatite. This study presents apatite laser-ablation Lu–Hf and U–Pb geochronology, zircon geochronology, and detailed petrological information from polymetamorphic mafic intrusions located in the central-western Gawler Craton in southern Australia, which records an extensive tectonometamorphic history spanning the Neoarchaean to the Mesoproterozoic. Zircon records magmatic crystallisation ages of c. 2479–2467 Ma, coinciding with the onset of the c. 2475–2410 Ma granulite-facies Sleafordian Orogeny. The amphibole-dominant hydrous assemblages which extensively overprint the primary magmatic assemblages are hypothesised to post-date the Sleafordian Orogeny. The Lu–Hf and U–Pb isotopic systems in apatite are used to test this hypothesis, with both isotopic systems recording significantly younger ages correlating with the c. 1730–1690 Ma Kimban Orogeny and the c. 1590–1575 Ma Hiltaba magmatic event, respectively. While the early Mesoproterozoic apatite U–Pb ages are attributed to thermal re-equilibration, the older Lu–Hf ages are interpreted to reflect re-equilibration facilitated primarily by dissolution-reprecipitation, but also thermally activated volume diffusion. The mechanisms of Lu–Hf isotopic resetting are distinguished based on microscale textures and trace element abundances in apatite and the integration of apatite-amphibole textural relationships and temperatures determined from the Ti content in amphibole. More broadly, the results indicate that at low to moderate temperatures, apatite hosted in mafic rocks is susceptible to complete recrystallisation in rocks that have weak to moderate foliations. In contrast, at higher temperatures in the absence of strain, the Lu–Hf system in apatite is comparatively robust. Ultimately, the findings from this study advance our understanding of the complex role that both metamorphism and deformation play on the ability of mafic-hosted apatite to retain primary Lu–Hf isotopic signatures.

Neoarchean synkinematic metamorphic peak in the Isua supracrustal belt (West Greenland)

Abstract We present petrological data and seven Lu-Hf garnet–amphibole–whole rock ages obtained from a single garnet-hornblende-mica schist sample from the Isua supracrustal belt (West Greenland). Garnets grew during prograde metamorphism toward regional amphibolite-facies peak conditions, and a mylonitic foliation formed during and after garnet growth. Garnet crystals show typical prograde zoning with no visible traces of a relict garnet generation. They do show various degrees of retrogression. While some crystals are perfectly euhedral with only minor chemical alteration along cracks, others are elongated in the foliation and either grew in this shape or were deformed. Six garnet splits were separated from crushed single crystals and one from a crushed bulk sample. Individual three-point garnet–hornblende–whole rock ages scatter between 2.603 ± 0.018 Ga and 2.432 ± 0.059 Ga for single garnets. The garnet split from the bulk sample defines an age of 2.463 ± 0.031 Ga, the data point farthest from the regression line for all data points (2.551 ± 0.074 Ga, mean square of weighted deviates = 25). We interpret these data to indicate partial retrogression of a Neoarchean garnet population not significantly older than the oldest obtained three-point age. Well-preserved garnet zoning, regional peak temperatures well below the closing temperature of the Lu-Hf system, and the small scatter of Lu-Hf ages preclude an interpretation of the observed metamorphism and deformation as being Eoarchean in age.

Hafnium isotope systematics of zircon in high-grade metamorphic rocks of the Anabar shield, Siberia: Radiogenic Hf without mantle input?

The timing for the onset of plate tectonics, along with the secular changes in the tectonic settings of continental crust formation, continue to be debated. Recent interpretations based on the increase in zircon 176Hf/177Hf ratios at the time of crystallisation (expressed as εHf(t) with respect to chondritic evolution) have been used to ascertain changes in geodynamic settings in the early Earth, specifically in the 3.8–3.6 Ga interval. This increase is widely interpreted as a change in magma generation, from source(s) dominated by ancient crust to source(s) dominated by juvenile inputs from the mantle. At issue, Hf isotope variations remain limited in the early Earth due to the long decay of the LuHf system. This feature, along with the scarcity of rocks and minerals of Eo/Mesoarchaean and Hadean ages, generate large uncertainties over the nature and the timing of the interactions between mantle and crustal reservoirs. The distinction between mantle and crustal sources becomes much clearer in the Palaeoproterozoic, and the study of ancient terranes with several billion years of protracted crustal evolution may hold the key to unlock complex mantle–crust interactions. Here we investigate high-grade metamorphic rocks from the Anabar shield, which contain zircons that crystallised between the Eoarchaean (oldest core at 3814 ± 16 Ma) and the Palaeoproterozoic (youngest core at 2251 ± 15 Ma and youngest rim at 1910 ± 21 Ma). The combination of in situ UPb and Hf isotope analyses in zircon indicates the formation of the continental crust in the Siberian Craton in the Eoarchaean, and a conspicuous metamorphic event at 2.0–1.9 Ga. We demonstrate that 2.0–1.9 Ga zircon ages reflect recrystallisation processes under subsolidus conditions, involving the breakdown of high-Lu/Hf minerals (i.e. garnet and pyroxene). The εHf(t) shift at 2.0–1.9 Ga towards more radiogenic values may not be related to a change in magmatic style and sources, but rather to resetting of the LuHf system in response to heating and metamorphic reactions on a mineral scale. Our findings challenge the widely-evoked mechanism of changes in tectonic style and magma sources to account for vertical arrays in the εHf(t) versus crystallisation age space. This calls for considering alternative options, including those based on petrographic data, when interpreting Hf isotope variations in the Hadean/Archaean detrital zircon record.

Apatite laser ablation Lu[sbnd]Hf geochronology: A new tool to date mafic rocks

Mafic rocks are the most common type of igneous rocks on Earth, however, constraining the crystallization age of mafic rocks can be challenging. Apatite is a common accessory phase in mafic rocks and is amenable to dating using the U–Pb system. However, the U–Pb system in apatite has a relatively low closure temperature (∼350°-550 °C) and is therefore prone to resetting by later thermal and metasomatic events. Here, a recently developed LuHf dating method using laser ablation reaction-cell mass spectrometry is applied to apatite from mafic rocks. The LuHf system in apatite has a higher closure temperature (∼650°-750 °C) compared to U–Pb, increasing the chances of obtaining primary crystallization ages. Furthermore, the laser-ablation method allows rapid data collection compared to traditional solution-based LuHf dating techniques. Four study areas were selected to compare the LuHf vs U–Pb systematics of apatite in mafic igneous rocks: the Paleoproterozoic Sudbury Igneous Complex (Canada), the Neoproterozoic Borborema Province (NE Brazil), the Paleoproterozoic Fennoscandian Shield (Finland), the Archean Yilgarn Craton and adjacent Mesoproterozoic Albany Fraser Orogen (Western Australia). For all analyzed samples that have apatite trace element compositions typical of an undisturbed primary mafic igneous lithology, the LuHf system retains primary igneous apatite crystallization ages, whereas the U–Pb system in the same grains often records isotopic disturbance or a cooling age. In few cases, the LuHf system has also been disturbed in response to recrystallization, however, such disturbance is readily detected with trace element data. Hence, this study demonstrates the potential of laser ablation apatite LuHf dating to obtain primary crystallization ages for otherwise difficult to date mafic rocks.

Robust laser ablation Lu–Hf dating of apatite: an empirical evaluation

Abstract Recent developments in laser-ablation Lu–Hf dating have opened a new opportunity to rapidly obtain apatite ages that are potentially more robust to isotopic resetting compared to traditional U–Pb dating. However, the robustness of the apatite Lu–Hf system has not been systematically examined. To address this knowledge gap, we conducted four case studies to determine the resistivity of the apatite Lu–Hf system compared to the zircon and apatite U–Pb system. In all cases, the apatite U–Pb system records a secondary (metamorphic or metasomatic) overprint. The apatite Lu–Hf system, however, preserves primary crystallization ages in unfoliated granitoids at temperatures of at least c. 660°C. Above c. 730°C, the Lu–Hf system records isotopic resetting by volume diffusion. Hence, in our observations for apatites of ‘typical’ grain sizes in granitoids ( c. 0.01–0.03 mm 2 ), the closure temperature of the Lu–Hf system is between c. 660 and c. 730°C, consistent with theoretical calculations. In foliated granites, the Lu–Hf system records the timing of recrystallization, while the apatite U–Pb system tends to record younger cooling ages. We also present apatite Lu–Hf dates for lower crustal xenoliths erupted with young alkali basalts, demonstrating that the Lu–Hf system can retain a memory of primary ages when exposed to magmatic temperatures for a relatively short duration. Hence, the apatite Lu–Hf system is a new insightful addition to traditional zircon (or monazite) U–Pb dating, particularly when zircons/monazites are absent or difficult to interpret due to inheritance or when U and Pb isotopes display open system behaviour. The laser-ablation-based Lu–Hf method allows campaign-style studies to be conducted at a similar rate to U–Pb studies, opening new opportunities for magmatic and metamorphic studies.

In situ Lu–Hf geochronology of calcite

Abstract. The ability to constrain the age of calcite formation is of great utility to the Earth science community, due to the ubiquity of calcite across a wide spectrum of geological systems. Here, we present the first in situ laser ablation inductively coupled tandem quadrupole mass spectrometry (LA-ICP-MS/MS) Lu–Hf ages for calcite, demonstrating geologically meaningful ages for iron oxide copper gold (IOCG) and skarn mineralisation, carbonatite intrusion, and low-grade metamorphism. The analysed samples range in age between ca. 0.9 and ca. 2 Ga with uncertainties between 1.7 % and 0.6 % obtained from calcite with Lu concentrations as low as ca. 0.5 ppm. The Lu–Hf system in calcite appears to be able to preserve primary precipitation ages over a significant amount of geological time, although further research is required to constrain the closure temperature. The in situ approach allows calcite to be rapidly dated while maintaining its petrogenetic context with mineralisation and other associated mineral processes. Therefore, LA-ICP-MS/MS Lu–Hf dating of calcite can be used to resolve the timing of complex mineral paragenetic sequences that are a feature of many ancient rock systems.

Detrital apatite Lu–Hf and U–Pb geochronology applied to the southwestern Siberian margin

AbstractApatite is increasingly used in sedimentary provenance studies. However, detrital apatite U–Pb geochronology can be challenging due to the presence of non‐radiogenic Pb, its intermediate closure temperature (~350–550°C) and/or age‐resetting by metamorphic/metasomatic processes. The Lu–Hf system in apatite has a higher closure temperature (~675–750°C) and is, therefore, more robust to thermal resetting. Here we present the first detrital apatite Lu–Hf age spectra. We have developed a laser‐ablation Lu–Hf dating technique, using reaction‐cell mass spectrometry, that allows rapid cost‐effective analysis, required for detrital apatite studies. The method is best suited to Precambrian detritus, permitting greater radiogenic Hf ingrowth. Using samples from Siberia, we demonstrate: (1) excellent correlations between U–Pb and Lu–Hf dates for apatites from igneous protoliths; and (2) that Lu–Hf dating can detect primary age information in metamorphic grains. Hence, when used in tandem with U–Pb zircon and apatite geochronology, Lu–Hf apatite dating provides a powerful new tool for provenance studies.

Zircons underestimate mantle depletion of early Earth

The mechanism and timing of crustal growth and differentiation on early Earth are debated. Evidence of crustal differentiation is detectable as deviations from Earth’s assumed chondritic uniform reservoir (CHUR) as crust is extracted from the mantle leading to a melt-depleted reservoir. For the long-lived zircon Lu-Hf system, no incontrovertible evidence of significant mantle depletion >3.8 Ga exists. We conduct combined U-Pb and Lu-Hf isotopic analyses for the detrital zircon from the Caozhuang supracrustal sequence in North China. The zircon Hf isotopic compositions are broadly scattered along the CHUR evolution line. However, given the possibility of potential systematic biases in zircon petrogenesis and the unique tectonic setting of early Earth, we posit that magmatism controlled by the nascent forms of plate tectonics during the Eoarchean could have likely hidden the degree of ancient crust-mantle differentiation. The non-depleted zircon Hf isotopes observed in North China and globally during early Earth may in verity imply the existence of ubiquitous depleted mantle domains at that time.

In-situ Lu[sbnd]Hf geochronology of garnet, apatite and xenotime by LA ICP MS/MS

LuHf geochronology is a powerful method to constrain the temporal evolution of geological systems. Traditional application of this dating method requires time-consuming chemical separation of the parent (176Lu) and daughter (176Hf) isotopes that is commonly accompanied by loss of textural context of the analysed minerals. In contrast, In-situ (laser-ablation based) LuHf geochronology offers a number of advantages including rapid analysis with high spatial resolution, as well as control on textural relationships of the analysed mineral. However, laser-ablation based LuHf geochronology has been hindered by isobaric interferences of 176Yb and 176Lu on 176Hf that have effectively masked reliable determination of 176Lu and 176Hf. We present a methodology that resolves these interferences using LA-ICP-MS/MS (laser ablation tandem inductively coupled mass spectrometry) and NH3 gas to separate Hf from Lu. Both Lu, Yb, and Hf react with NH3 to form a variety of product ions. By measuring high order reaction products (e.g. Hf(NH)(NH2)(NH3)3+), we demonstrate that 176Hf can be measured interference-free from 176Lu and 176Yb with sufficient sensitivity to yield useful geochronological age data.The novel in-situ LuHf technique has been successfully applied to a variety of Palaeozoic and Precambrian-aged garnet, apatite and xenotime samples, including published reference materials. The resulting age uncertainties are as low as ~0.5% (95% conf. interval). The technique has the potential to obtain spatially-resolved LuHf ages in garnet-bearing samples that would be difficult to obtain by conventional techniques. The method also offers the opportunity for rapid “campaign style” geochronology in complex terrains that record poly-metamorphic histories. In apatite, the expected higher closure temperature of the LuHf system compared to the commonly used UPb system allows high-temperature thermal history reconstructions. In addition, LuHf dating of apatite allows dating of samples with low U and high common Pb (e.g. mafic and low-grade metamorphic rocks and ore deposits). Furthermore, apatite tends to incorporate little to no common Hf, allowing single grain ages to be calculated, which opens new doors for detrital provenance studies. In situ LuHf dating of xenotime offers an additional avenue to UPb dating, and may be particularly beneficial to dating of rare earth element ore deposits that often have complex temporal records of development.

Geochemical and textural investigations of the Eoarchean Ukaliq supracrustals, Northern Québec (Canada)

Structural, geochronological and geochemical studies of pre-3.75 Ga rocks of volcano-sedimentary protoliths in the Inukjuak domain in the Superior Province in Québec have been mostly focused on the Nuvvuagittuq Supracrustal Belt (NSB). The Porpoise Cove outcrops – at the southwestern limit of the NSB – are the de-facto “type locality” for the supracrustals of the Inukjuak Complex. Yet, it remains unclear whether the NSB rocks are geochemically distinct from, or are in fact common to, a host of other supracrustal enclaves locked in the dominantly gneissic Inukjuak domain. Here, we report detailed textural and geochemical studies for a suite of rocks from the Ukaliq Supracrustal Belt (USB), located approximately 3 km northeast of the NSB. We find that the USB and NSB have a similar protracted metamorphic history; both experienced amphibolite grade metamorphism and contain granitoid gneiss sheets, quartz-magnetite rocks (banded iron-formation s.l.) and quartz-biotite schists within amphibolitized rocks of basaltic affinity with local retrogressions. If the various Inukjuak supracrustal belts were once a part of a larger coherent (now dismembered) terrane, they should show similar emplacement ages and source chemistry. New zircon UPb geochronology from five gneissic units and two quartz-biotite (metasedimentary) schists reveal the oldest emplacement ages across all units of each individual rock type to be 3.68 ± 0.07 Ga (granitoid gneisses) and 3.65 ± 0.06 Ga (quartz-biotite schists). These new ages are similar to those documented as likely minimum emplacement ages of the NSB determined by UPb geochronology. Zircons from the quartz-biotite schist were also analyzed by ion microprobe for their UPb geochronology and were found to yield statistically identical, albeit more precise, ages than those obtained by LA-ICP-MS. Possible detrital zircons from the USB quartz-biotite schists were analyzed by ion microprobe for their coupled δ30SiNBS28 and δ18OVSMOW values with respective values between −0.75 and − 0.07‰ (δ30SiNBS28), and + 5.61 and + 6.59‰ (δ18OVSMOW). The δ18OVSMOW values, which are on average above mantle-derived zircon, indicate contribution of altered, non-mantle, derived material into the source of the rocks that weathered to form the quartz-biotite schists. Zircon mineral inclusions (quartz, feldspar, apatite, biotite, muscovite and other unrecognized Fe/Al/Si rich phases), along with the major- and trace element contents for the rocks were analyzed to substantiate this interpretation. Together with δ30SiNBS28, δ18OVSMOW, our results suggest that lithologies like authigenic silica and serpentinized rocks re-melted to form the parent melts that gave rise to zircons found in the USB quartz-biotite schists. Additional LuHf studies reported here on the same zircons also show similarities with NSB zircons. The εHf values showed a positive correlation with the measured UPb age from −22.7 ± 0.8 to +1.9 ± 1.1. The LuHf system also reveals that the USB, extracted at ca. 3.8 Ga, carries isotopic signatures of an older Hadean reservoir, having been formed from an Eoarchean mafic melt that incorporated them. Taken together, this supports a co-genetic origin for the NSB and the USB.

Formation of early Archean Granite-Greenstone Terranes from a globally chondritic mantle: Insights from igneous rocks of the Pilbara Craton, Western Australia

The continental crust grows via juvenile additions from the mantle. However, the timing of initial continent stabilisation and the rate of subsequent continental growth during the first billion years of Earth history is widely debated, in part due to uncertainty over the composition of the mantle source of new crust. Well-preserved Archean granite-greenstone terranes, as present within the Pilbara Craton (Western Australia), provide insights into the sources of felsic magmas and the processes of continental growth and evolution in the distant geological past at the regional scale. Here, we present zircon U-Pb, O and Hf isotope data from ancient gneissic and granitic rocks of the Pilbara Craton, to decipher magma sources and the timing and processes of craton growth. There is no evidence for depleted mantle compositions, and the simplest interpretation is that the crust of the Pilbara Craton was generated from mantle with a chondritic Hf isotope composition. Our results indicate crustal addition at ~3.59 Ga, represented by emplacement of gabbroic to anorthositic rocks. We suggest that the formation of these igneous rocks, and the foundering of the complementary residues, triggered extensive melting of hot, upwelling mantle, leading to the subsequent accumulation of the >12 km thick greenstone belt eruptive sequences from 3.53 Ga, with emplacement of coeval felsic magmas at depth. This process shaped the initial crustal configuration of the proto-craton, which subsequently underwent gravitationally driven overturn and reworking to generate stable, cratonic continental crust with the distinctive dome and keel architecture. The zircon Hf and O isotope signatures of the Pilbara igneous rocks from ~3.59–3.4 Ga do not support remelting of an ancient (> 3.8 Ga) basement, and reinforce the overwhelmingly chondritic to near-chondritic zircon Hf isotope composition of Eoarchean meta-igneous rocks from a number of different Archean cratons. A corollary of this remarkable global consistency is that a significant volume of the mantle maintained a chondritic composition for the Lu-Hf system from the formation of the Earth into the Paleoarchean (up to 3.6–3.5 Ga), as would be the case if stabilised volumes of felsic continental crust prior to 3.5 Ga were relatively small. One implication is that the common assumption of a linear evolution of depleted mantle from 4.5 Ga to the present day is inappropriate for determining the timing and volume of continental crust extraction in the Archean. The nearly identical early evolution of the Pilbara and Kaapvaal cratons suggests a common process to generate Archean granite-greenstone terranes that does not require extensive reworking of ancient crust, but rather involves juvenile crustal addition above persistent zones of upwelling, chondritic mantle.

Multispecies Diffusion of Yttrium, Rare Earth Elements and Hafnium in Garnet

AbstractWe report experimental data for Y, La, Lu and Hf diffusion in garnet, in which diffusant concentrations and silica activity have been systematically varied. Experiments were conducted at 950 and 1050 °C, at 1 atm pressure and oxygen fugacity corresponding to the quartz–fayalite–magnetite buffer. At Y and REE concentrations below several hundred ppm we observe both slow and fast diffusion mechanisms, which operate simultaneously and correspond to relatively high and low concentrations, respectively. Diffusivity of Y and REE is independent of silica activity over the studied range. General formulae for REE diffusion in garnet, incorporating data from this and previous studies, are logDREE(f)(m2 s−1)=−10·24(±0·21)−221057(±4284)2·303RT(K) for the ‘fast’ REE diffusion mechanism at 1 atm pressure, and logDREE(s)(m2 s−1)=−9·28(±0·65)−265200(±38540)+10800(±2600)×P(GPa)2·303RT(K) for the ‘slow’ REE diffusion mechanism. These slow and fast diffusion mechanisms are in agreement with previous, apparently conflicting, datasets for REE diffusion in garnet. Comparison with high-pressure experiments suggests that at high pressures (>∼1 GPa minimum) the fast diffusion mechanism no longer operates to a significant degree. When Y and/or REE surface concentrations are greater than several hundred ppm, complex concentration profiles develop. These profiles are consistent with a multi-site diffusion–reaction model, whereby Y and REE cations diffuse through, and exchange between, different crystallographic sites. Diffusion profiles of Hf do not exhibit any of the complexities observed for Y and REE profiles, and can be modeled using a standard (i.e. single mechanism) solution to the diffusion equation. Hafnium diffusion in garnet shows a negative dependence on silica activity, and is described by logDHf(m2 s−1)=−8·85(±0·38)−299344(±15136)+12500(±900)×P(GPa)2·303RT(K)−0·52(±0·09)×log⁡10aSiO2. In many natural garnets, diffusion of both Lu and Hf would be sufficiently slow that the Lu–Hf system can be reliably used to date garnet growth. In cases in which significant Lu diffusion does occur, preferential retention of 176Hf/177Hf relative to 176Lu/177Hf will skew isochron relationships such that their apparent ages may not correspond to anything meaningful (e.g. garnet growth, peak temperature or the closure temperature of Lu or Hf). Late-stage reheating events are capable of causing larger degrees of preferential retention of 176Hf/177Hf relative to 176Lu/177Hf and partial to full resetting of the Sm–Nd system within garnet, thus increasing the separation between garnet Lu–Hf and Sm–Nd isochron dates, owing to the fact that these systems are more significantly disturbed through diffusion as more radiogenic 176Hf and 143Nd have accumulated.

The Belomorian eclogite province (eastern Fennoscandian Shield, Russia): Meso-Neoarchean or Late Paleoproterozoic?

A critical discussion of competing models of the geodynamic nature (oceanic or continental subduction) and age (Meso-Neoarchean or Late Paleoproterozoic) of the eclogite facies metamorphism in the Belomorian eclogite province (BEP) is based on the systematic analysis of the sum of previously known and newly obtained data characterizing the geological structure of the Salma eclogite association and features of zircons from eclogites, including the isotopegeochronological and geochemical characteristics, composition and distribution of mineral inclusions. Regular changes in the REE trends and crystallization-recrystallization temperature of porous zircons in eclogite-metagabbro illustrate the sequence of magmatic and metamorphic events in the Meso-Neoarchean and Paleoproterozoic. The susceptibility to recrystallization of zircons is due to partial metamictness and porous structure. The earliest (~2.9 Ga) zircon zones retain mag-matic-type REE trends. The microinclusions of the prenite-pumpelliite and greenschist facies minerals and the increase in the LREE and MREE concentrations indicate hydrothermal metamorphism in the spreading ridge and on the ocean floor at 2.9–2.82 Ga. Prenite, pumpelliite, albite, actinolite, chlorite, diaspore and saponite also form inclusions in the eclogitic garnet. An increase of LREE and MREE, the disappearance of the Ce positive anomaly, a change from negative to positive Eu anomaly at 2.82–2.78 Ga indicate that plagioclase was removed during the formation of the ‘garnet + omphacite’ eclogite association and the replacement of sphene with rutile. The eclogite facies metamorphism linked with subduction of the oceanic crust is also indicated by the microinclusions of garnet and rutile in zircon. The crystallization temperature in 700–900 °C range of the round-oval zircons from eclogites-metagabbronorites records the Neoarchean granulite facies metamorphism at 2.77–2.70 Ga, the negative Eu anomalies in the cores and rims of zircons indicate the participation of plagioclase in the metamorphic crystallization. Late (2.1–1.7 Ga) rims of porous zircons that occurred at 600–680 °C are distinguished by minimal REE concentrations, a change from a positive Eu anomaly to a negative one, and the appearance of a negative Ce anomaly, which indicates the presence of plagioclase, reducing type of fluids and, accordingly, low water activity that is characteristic of high-temperature metamorphism under stretching condition and mantle-plume activity. The deep Sm-Nd system reworking in the Belomorian tectonic province, including BEP, at ~1.9 Ga was caused by the crustal heating that spread from the Lapland granulite belt border in the west-south-westward direction. The Lu-Hf system in zircon reworking with a significant increase in radiogenic Hf indicates the recrystallization of a long-existing garnet, in which a significant amount of radiogenic 176Hf accumulated by 1.9 Ga as a result of the 176Lu decay. This contradicts the earlier suggestion of the eclogite garnet primary crystallization in the late Paleoproterozoic (1.94–1.89 Ga).

Reconciliation of discrepant U–Pb, Lu–Hf, Sm–Nd, Ar–Ar and U–Th/He dates in an amphibolite from the Cathaysia Block in Southern China

We present a combined garnet Lu–Hf and Sm–Nd, zircon and monazite U–Pb, hornblende Ar–Ar, and zircon U–Th/He geochronology study on a garnet-bearing amphibolite from the Cathaysia Block in Southern China. Pseudosection modeling and garnet–clinopyroxene thermometry indicate that the amphibolite attained peak metamorphic conditions of approximately 0.9 GPa and 780 °C. The Lu–Hf garnet date of 402.7 ± 1.0 Ma and Sm–Nd date of 331.6 ± 3.6 Ma define a ~ 70 Myr difference for the same garnet fractions. Reconciling the large discrepancies between these three geochronological systems with the observed major element diffusion profiles in garnet requires that REEs and Hf were fully re-equilibrated at peak temperatures and that subsequent rapid cooling prevented any disruption of the Lu–Hf or Sm–Nd systems throughout cooling from high temperatures. The Sm–Nd garnet age was partially reset during a thermal pulse associated with intrusion of leucocratic veins, which is inferred to have occurred at ca. 226 Ma based on the coinciding hornblende Ar–Ar (222.3 ± 0.9 Ma) and monazite U–Pb (226.2 ± 2.3 Ma) dates. The T–t path of a reheating event at 226 Ma necessary to produce both the major divalent cation diffusion profiles in garnet and partial resetting of the Sm–Nd garnet age, such that it records an apparent age of ~ 332 Ma, requires an effective diffusion radius of 45–60 μm in garnet during reheating, which is in agreement with the observation of pervasive chlorite-filled fractures within single garnet grains. Diffusion kinetics of Lu and Hf in garnet are sufficiently slow that this reheating episode would not impact the Lu–Hf system, and the 402.7 ± 1.0 Ma garnet age likely reflects the onset of rapid cooling from peak P–T conditions. The zircon U–Pb date of 434.8 ± 2.7 Ma predates the Lu–Hf garnet date by approximately 30 Myr and is interpreted to reflect zircon crystallization during prograde metamorphism. The ~ 226 Ma reheating event is contemporaneous with the intrusion and crystallization of the widespread Triassic A-type granites and alkaline syenites across the Cathaysia Block, indicating an extensional environment. We interpret the zircon U–Th/He date of 143.1 ± 4.2 Ma as reflecting a third thermal pulse based on the concurrence with the early Cretaceous volcanic rocks and granitoids, which overlies the amphibolite.

Post-peak metamorphic evolution of the Sumdo eclogite from the Lhasa terrane of southeast Tibet

A reconstruction of the pressure–temperature–time (P–T–t) path of high-pressure eclogite-facies rocks in subduction zones may reveal important information about the tectono-metamorphic processes that occur at great depths along the plate interface. The majority of studies have focused on prograde to peak metamorphism of these rocks, whereas after-peak metamorphism has received less attention. Herein, we present a detailed petrological, pseudosection modeling and radiometric dating study of a retrograded eclogite sample from the Sumdo ultrahigh pressure belt of the Lhasa terrane, Tibet. Mineral chemical variations, textural discontinuities and thermodynamic modeling suggest that the eclogite underwent an exhumation–heating period. Petrographic observations and phase equilibria modeling suggest that the garnet cores formed at the pressure peak (∼2.5GPa and ∼520°C) within the lawsonite eclogite-facies and garnet rims (∼1.5GPa and <650°C) grew during post-peak amphibole eclogite-facies metamorphism. The metamorphic evolution of the Sumdo eclogite is characterized by a clockwise P–T path with a heating stage during early exhumation, a finding that conflicts with previously reported heating-compression P–T paths for the Sumdo eclogite. A garnet–whole rock Lu–Hf age of 266.6±0.7Ma, which is consistent with the loosely constrained zircon U–Pb age of 261±15Ma within uncertainty, was obtained for the sample. The peak metamorphic temperature of the sample is lower than the Lu–Hf closure temperature of garnet, which combined with the general core-to-rim decrease in the Mn and Lu concentrations and the occurrence of a second maximum Lu peak in the inner rim, is consistent with the Lu–Hf system skewing to the age of the garnet inner rim. Thus the Lu–Hf age likely reflects late eclogite-facies metamorphism. The new U–Pb and Lu–Hf ages, together with previously published radiometric dating results, suggest that the overall growth of garnet spans an interval of ∼7millionyears, which is a minimum estimate of the duration of the eclogite-facies metamorphism of the Sumdo eclogite.

Meteorite zircon constraints on the bulk Lu−Hf isotope composition and early differentiation of the Earth

Knowledge of planetary differentiation is crucial for understanding the chemical and thermal evolution of terrestrial planets. The (176)Lu-(176)Hf radioactive decay system has been widely used to constrain the timescales and mechanisms of silicate differentiation on Earth, but the data interpretation requires accurate estimation of Hf isotope evolution of the bulk Earth. Because both Lu and Hf are refractory lithophile elements, the isotope evolution can be potentially extrapolated from the present-day (176)Hf/(177)Hf and (176)Lu/(177)Hf in undifferentiated chondrite meteorites. However, these ratios in chondrites are highly variable due to the metamorphic redistribution of Lu and Hf, making it difficult to ascertain the correct reference values for the bulk Earth. In addition, it has been proposed that chondrites contain excess (176)Hf due to the accelerated decay of (176)Lu resulting from photoexcitation to a short-lived isomer. If so, the paradigm of a chondritic Earth would be invalid for the Lu-Hf system. Herein we report the first, to our knowledge, high-precision Lu-Hf isotope analysis of meteorite crystalline zircon, a mineral that is resistant to metamorphism and has low Lu/Hf. We use the meteorite zircon data to define the Solar System initial (176)Hf/(177)Hf (0.279781 ± 0.000018) and further to identify pristine chondrites that contain no excess (176)Hf and accurately represent the Lu-Hf system of the bulk Earth ((176)Hf/(177)Hf = 0.282793 ± 0.000011; (176)Lu/(177)Hf = 0.0338 ± 0.0001). Our results provide firm evidence that the most primitive Hf in terrestrial zircon reflects the development of a chemically enriched silicate reservoir on Earth as far back as 4.5 billion years ago.

Cathodoluminescence guided zircon Hf isotope depth profiling: Mobilization of the Lu–Hf system during (U)HP rock exhumation in the Woodlark Rift, Papua New Guinea

Cathodoluminescence image guided Hf isotope depth profiling by laser ablation of zircons from two quartzofeldspathic host gneisses constrains the Lu–Hf system's behavior during rapid exhumation of (U)HP rocks in the Woodlark Rift, Papua New Guinea. Investigation of the depth profiling technique in individual and composite zircon standards demonstrates that it is possible to resolve ~8μm thick domains in which εHf(present) differs by as little as 4units. In a metasedimentary gneiss, 2.89±0.29Ma zircon overgrowths on Cretaceous aged inherited cores have radiogenic εHf(present) indicating growth in a medium that was originally in equilibrium with garnet undergoing recrystallization (the ‘garnet effect’ of Zheng et al., 2005). In a separate gneiss sample that originated as an exhumation related anatectic melt, 3.66±0.13Ma zircons lacking inheritance contain sub-domains that differ from each other by >15 εHf(present). Some of these sub-domains are radiogenic and can be explained by the ‘garnet effect’, whereas others also contain highly elevated Lu and Yb in addition to their radiogenic Hf compositions, thus necessitating a medium derived from the complete breakdown of garnet. Zircons in this sample also contain non-radiogenic sub-domains that grew in the presence of Hf mobilized from the surrounding rocks of the subducted and metamorphosed remnants of the Australian continental margin. The results confirm that rapid exhumation of (U)HP rocks can result in the following: 1) transmission of radiogenic Hf (and sometimes Lu and the other HREE) from garnet bearing mafic lithologies into the quartzofeldspathic gneisses, and 2) mobilization and transport of unradiogenic Hf present within the quartzofeldspathic remnants of subducted continental crust.