Trace Element Systematics Research Articles

Clues to the origin of metal in Almahata Sitta EL and EH chondrites and implications for primitive E chondrite thermal histories

Enstatite (E) chondrites are a group of texturally highly variable meteorites formed under strongly reducing conditions giving rise to unique mineral and chemical characteristics (e.g., high abundances of various sulfides and Si-bearing metal). In particular the abundant metal comprises a range of textures in E chondrites of different petrologic type, but available in situ siderophile trace element data on metal are limited. Nine samples of E chondrites from the recent Almahata Sitta fall [one EH3, two EL3/4, two EL6, two EL impact melt rocks (IMR), two EH IMR] were investigated in this study in addition to St. Mark’s (EH5) and Grein 002 (EL4/5), with a focus on the nature of their metal constituents. Special attention was given to metal–silicate intergrowths (MSSI) that occur in many primitive E chondrites, which have been interpreted as post-accretionary asteroidal impact melts or primitive nebular condensates. This study shows that siderophile trace element systematics in E chondrite metal are independent of petrologic type of the host rock and distinct from condensation signatures. Three basic types of siderophile trace element signatures can be distinguished, indicating crystallization from a melt, thermal equilibration upon metamorphism/complete melting, and exsolution of schreibersite–perryite–sulfide. Textural and mineral–chemical constraints from EL3/4s are used to evaluate previously proposed formation processes of MSSI (impact melting vs. nebular condensation) and elucidate which other formation scenarios are feasible. It is shown that post-accretionary (in situ) impact melting or metallic melt injection forming MSSI on the thin section scale, and nebular condensation, are unlikely formation processes. This leads to the conclusion that MSSIs are pre-accretionary melt objects that were formed during melting processes prior to the accretion of the primitive E chondrites. The same can be concluded for metal nodules in the EH3 chondrite examined. The pre-accretionary origin of MSSIs in E chondrites is consistent with a growing body of evidence for early differentiation followed by impact disruption of early formed planetesimals in all major chondrite types.

Continental rift and oceanic protoliths of mafic–ultramafic rocks from the Kechros Complex, NE Rhodope (Greece): implications from petrography, major and trace-element systematics, and MELTS modeling

The whole-rock chemistry of eclogites, partially amphibolitized eclogites, and dyke amphibolites from the metamorphic Kechros complex in the eastern Rhodope Mountains preserves evidence of the geodynamic framework for the origin of their protoliths. Major and trace-element concentrations define two distinct protolith groups for the eclogites. The low-Fe–Ti (LFT) eclogites have low-TiO_2 content (<0.67 wt%), negative high field strength element anomalies, and variable enrichments in large ion lithophile elements (LILE). The rare earth element (REE) patterns are characterized by strong light-REE (LREE) enrichment and heavy-REE (HREE) depletion. The high-Fe–Ti (HFT) eclogites have small to moderate LILE enrichment and lack Nb anomalies. The REE patterns of the HFT eclogites are characterized by LREE depletion and relatively flat MREE–HREE patterns. The rock compositions and petrographic features of the LFT eclogites resemble gabbros formed in a continental rift environment with minor to moderate contamination of a mantle-derived mafic magma by continental crust, whereas the HFT eclogites resemble mafic rocks formed in extensional oceanic environments. We interpret the HFT suite to represent a later stage in an evolution from continental rift to open ocean, following the origin of the LFT suite. Dyke amphibolite compositions, except for probable SiO_2 loss associated with metamorphic dehydration reactions, appear to represent liquid compositions quenched in conduits through the lower crust. MELTS modeling shows that dyke amphibolite compositions can be related to each other by fractional crystallization under strongly oxidizing conditions at ~0.5 GPa pressure, and all can be derived from a low-degree melt of modified fertile peridotite from around 1.7 GPa. Cumulates crystallized from the parental liquids of the amphibolites under oxidizing conditions may have yielded the protoliths of the HFT suite.

Mesozoic–Cenozoic mantle evolution beneath the North China Craton: A new perspective from Hf–Nd isotopes of basalts

We report here the whole-rock Hf–Nd isotopic data on Mesozoic–Cenozoic basaltic lavas from Liaoning, Shandong, Hebei and Jiangsu–Anhui regions in the North China Craton (NCC). These lavas can be readily subdivided into two groups. The older (>110Ma) lavas are relatively depleted in high field strength elements (HFSEs), and have enriched Sr–Nd–Hf isotopic compositions. The younger (<110Ma) lavas, by contrast, have OIB-like trace element systematics and more depleted Sr–Nd–Hf isotopic compositions. Among several possibilities, the most straightforward explanation is that the >110Ma lavas may have largely derived from the “ancient metasomatized lithosphere” or contain contributions from ancient continental crust in the process of lithosphere thinning, whereas the <110Ma lavas with more depleted Sr–Nd–Hf isotopic compositions were most likely derived from the asthenosphere when the lithosphere had already been thinned with the ancient lithospheric mantle being removed. We thus propose that the thinning of the ancient cratonic mantle lithosphere beneath the NCC must have been largely completed by ~110Ma. In this context, we infer that ancient sub-continental lithospheric material, once entering the asthenosphere, may be important as an enriched component for intra-plate basaltic magmatism, including ocean island basalts (OIBs). In addition, all these basaltic lavas plot along the terrestrial array in the Hf–Nd isotopic space, suggesting that the mantle source isotopic variation is largely controlled by simple magmatic processes, i.e., low-degree melt metasomatic enrichment (e.g., elevated Hf/Lu and Nd/Sm ratios) and melt extraction-related depletion (e.g., lowered Hf/Lu and Nd/Sm ratios).

A Record of Paleoproterozoic Subduction Preserved in the Northern Slave Cratonic Mantle: Sr–Pb–O Isotope and Trace-element Investigations of Eclogite Xenoliths from the Jericho and Muskox Kimberlites

Paleoproterozoic eclogite xenoliths from the Jericho and Muskox kimberlites located in the northern Slave craton, Canada, display geochemical and isotopic evidence of a recycled oceanic lithosphere origin. The eclogites comprise a diverse suite of kyanite- and diamond-bearing and bimineralic xenoliths that have a wide range of geochemical compositions. Calculated whole-rock eclogite compositions are broadly basaltic to picritic, ranging from 5 to 13 wt % MgO. Calculated whole-rock trace-element compositions for the eclogites show three overall REE patterns: the first pattern resembles that of oceanic gabbros with prominent positive Eu anomalies and flat heavy REE; the second pattern is similar to the first but displays marked light REE depletion that is interpreted to record partial melt loss; the third pattern lacks Eu anomalies and instead displays both gradually increasing REE contents from La to Lu similar to boninitic melts and ‘humped’ middle REE patterns similar to those observed in eclogites formed by reaction with subduction fluids. Several eclogites have garnet and/or clinopyroxene δ 18 O values >6‰, which coupled with the trace-element systematics of all the eclogites is interpreted to reflect derivation from recycled oceanic lithosphere protoliths. Lead isotope data for eclogite clinopyroxenes form a broad array that extends from highly radiogenic isotope compositions similar to the host Jericho kimberlite to an intersection with the Stacey–Kramers terrestrial two-stage Pb evolution curve at c. 2·2 Ga. The slope of this data array defines a secondary Pb isochron with an associated age of 1·7 ± 0·3 Ga. Although it is possible that the Pb isotope array is actually a mixing line between the eclogite protolith and the host Jericho kimberlite, both the secondary isochron age and the c. 2·2 Ga Stacey–Kramers model age are consistent with Paleoproterozoic protolith oceanic crust formation. Eclogite xenoliths and eclogitic diamond inclusions from the Slave craton almost exclusively yield c. 2 Ga ages, which are broadly compatible with formation during subduction associated with the c. 1·9–1·8 Ga Wopmay orogeny at the western margin of the Slave craton. Trends in the Sr–Pb isotopic compositions of the eclogite clinopyroxene show that seawater alteration of the oceanic crust protoliths was an important process during protolith formation, and cryptic metasomatism further increased the Pb and Sr isotope values. This metasomatism is probably associated with the Jurassic kimberlite magmatism; however, contributions from the Mesoproterozoic Mackenzie event cannot be completely excluded.

The oldest zircons of Africa—Their U–Pb–Hf–O isotope and trace element systematics, and implications for Hadean to Archean crust–mantle evolution

More than 450 detrital zircon grains from two Limpopo Belt quartzite samples were investigated by a combination of scanning electron imaging, U–Pb dating, δ18O, Lu–Hf isotope and trace element analyses in order to get robust information about the early Earth's crust–mantle evolution. The detrital zircon grains have crystallization ages between 3.95Ga and 3.18Ga, show ɛHft between +1 to −15 (±1 ɛ-unit), and δ18OVSMOW mostly between +5.5 and +8.1‰ (±0.2‰). Pristine zircon domains reveal Ti-in-zircon temperatures between 700 and 865°C, and Th/U of 0.3–2.3. Trace elements point to zircon formation in predominately granitoid rocks. Metamorphic zircon rims have ages ≤2.65Ga, ɛHf2.65Ga∼−15, δ18O=7.0–8.1‰, and Th/U mostly <0.1. Nine zircon grains define an ɛHft-age array (I), which starts from a chondritic uniform reservoir (CHUR) at about 4.5Ga, and requires 176Lu/177Hf=0.020, indicative for mafic crust. Most zircon grains, however, plot on or above an ɛHft-age array (II), which runs parallel to array I, 176Lu/177Hf=0.021, and starts from CHUR at 4.01Ga. Oxygen isotope compositions of δ18O>5.5 indicate that the magmatic host rocks of the zircons have been formed either by melting of altered mafic crust, which interacted with cold water prior to granitoid formation, and/or that ancient sedimentary and/or magmatic rocks were involved in the melting process. The new U–Pb–Hf–δ18O datasets together with compiled data from worldwide sources indicate a significant gap of about 5 epsilon units between arrays I and II. Furthermore, they illustrate that many Hadean zircon analyses plot well below array I, and some above CHUR. These findings support an interpretation that the Hadean Earth was covered by a long-lived, mafic protocrust, perhaps forming a partially open “stagnant lid”. This protocrust was affected by internal reworking, but also injected and overlain by (ultra)mafic rocks derived from chondritic and (highly) depleted mantle sources. At <4.3Ga, the mafic protocrust was locally transformed into a TTG crust, perhaps caused by enhanced lower crust foundering, related to enhanced volcanic resurfacing and secular cooling of the Hadean lithosphere. Eventually, this heterogeneous Hadean protocrust became completely substituted by a new crust, which started to evolve from the mantle at <4.01Ga

Lithological controls of dissolved trace elements: carbonate versus silicate

Trace elements in river waters can give information about chemical weathering and aqueous environments, but the systematics of trace elements in river waters have been poorly understood. Here, concentrations of dissolved trace elements were determined to investigate factors controlling their seasonal and spatial variations in the Han River Basin (HR), South Korea. Two major branches, the North Han River (NHR) and South Han River (SHR), differed largely in concentrations of their trace elements (e.g., As, B, Co, Li, Ni, and U), with the NHR having low concentrations and the SHR having high concentrations. This difference can be mainly attributed to the underlying lithology of the watersheds, namely, silicates in the NHR and carbonates plus clastic sediments containing black slate rich in trace elements in the SHR. Seasonally, concentrations of trace elements such as Al, Co, Fe, and Rb in the NHR were higher in summer, indicating that these trace elements exist as a colloid-borne form because of intensive physical weathering in the NHR. In contrast, the SHR showed little or no seasonal variation in concentrations of those trace elements.

IMMOBILE TRACE ELEMENT SYSTEMATICS OF OCEANIC ISLAND BASALTS: THE ROLE OF OCEANIC LITHOSPHERE IN CREATING THE GEOCHEMICAL DIVERSITY

The Earth’s mantle is known to be heterogeneous at different scales, which has been generally linked to the presence of diverse mantle reservoirs, some of which are believed to have remained isolated for long periods of time. When oceanic island basalts (OIBs) are subdivided into five distinct end-member groups on the basis of Sr-Nd-Pb isotope systematics, which include DM, EM1, EM2, HIMU and C, trace element systematics do not appear to be effective discriminators as isotopes, though an end-member signature may become dominant relative to others within a specific ratio range. Melting of a lithologically heterogeneous source or melting of distinct sources, which is followed by melt mixing, appears to be an important mechanism in creating variable geochemi- cal signatures in OIB genesis. Ratio-based trace element modeling suggests that sole involvement of eclogitic components cannot explain the entire elemental variation observed in OIBs; a peridotitic component must have been involved in the genesis of all types of end-member signatures. Combined trace element and isotope systematics are consistent with the involvement of metasomatized oceanic lithosphere (crust + lithospheric mantle) with/without sediments. This mechanism, which involves slab components uprising within the plumes, may also have been the main reason causing the geochemical diversity in the Tethyan mantle.

A petrogenetic model for the comagmatic origin of chassignites and nakhlites: Inferences from chlorine‐rich minerals, petrology, and geochemistry

AbstractTwelve samples belonging to the chassignite and nakhlite subgroups of Martian meteorites were investigated using a variety of micro‐beam analytical techniques to gain insight into the petrogenesis of these two meteorite classes. There are a striking number of geochemical similarities between the chassignites and nakhlites, including mineralogy and petrology, crystallization age, cosmic‐ray exposure age, and radiogenic isotopic compositions. However, there are also geochemical differences, namely in trace element systematics of pyroxenes, that have led some authors to conclude that the nakhlites are comagmatic with each other, but not comagmatic with the chassignites. On the basis of data presented here, we propose a model in which these differences can be reconciled by the addition of an exogenous Cl‐rich fluid to the chassignite‐nakhlite magma body shortly after the formation of the cumulate horizon that was sampled by the Chassigny meteorite. This model is supported by the textural and chemical associations of the volatile‐bearing minerals apatite, amphibole, and biotite, which record a history starting with the addition of a Cl‐ andLREE‐enriched fluid to the magma body. As the magma continued to crystallize, it eventually reached chloride saturation and degassed a Cl‐rich fluid phase. Depending on the provenance of the Cl‐rich fluid, this model could explain how the chassignites and nakhlites originated from anLREE‐depleted source, yet all exhibitLREE‐enriched bulk‐rock patterns. Additionally, the model explains the range in oxygen fugacity that is recorded by the chassignites and nakhlites because eventual exsolution and loss of Cl‐rich fluid phases near the end of crystallization of the nakhlite sequence leads to auto‐oxidation of the magma body due to the preferential partitioning of Fe2+into the fluid phase.

Trace element systematics of tourmaline in pegmatitic and hydrothermal systems from the Variscan Schwarzwald (Germany): The importance of major element composition, sector zoning, and fluid or melt composition

An extensive data set on the compositional variation of tourmaline from granitic pegmatites, from migmatitic gneisses and from various types of hydrothermal veins from the Schwarzwald, Germany, is provided. The investigated tourmalines are members of the alkali and X-vacant groups representing mostly dravite–schorl solid solutions with some analyses belonging to the foitite–Mg–foitite series. Oxygen isotope data on quartz–tourmaline pairs indicate formation temperatures between 550 and 350°C for most of the quartz–tourmaline veins.Most of the tourmalines show strong sector zonation, fractionating certain major (e.g., Na, Mg), minor (e.g., Ti, Ca) and trace elements (e.g., Sr, Pb, REE) among the different crystallographic sectors of the crystals. We conclude that comparison of tourmaline compositions for petrogenetic interpretations must be based on the concentrations in the neutral a sector in order to eliminate the effects of intra-crystalline fractionation.Most trace element contents vary over several orders of magnitude with median concentrations generally between 0.1 and 10μg/g. The highest concentrations are found for Zn, V, Sr, Sc, Sn and Li (median concentrations of several tens to hundreds of μg/g) whereas very low median concentrations (<0.1μg/g) were found for U, Th, Bi, Tl and Cs. The concentration of several trace elements (e.g., Sr, REE, Co) correlates with the major element composition of the tourmaline, which implies potential crystal chemical effects on their incorporation. Consequently, the general assumption that tourmaline is a passive monitor for the chemical compositions of the melts or fluids from which it crystallizes has to be questioned. Nevertheless, our study demonstrates that the trace element composition of tourmaline allows to distinguish between geochemically distinct fluid sources, and the trace element composition of tourmaline may therefore be used to unravel the chemical complexity of granite-related hydrothermal systems.

Insights into early Earth from Barberton komatiites: Evidence from lithophile isotope and trace element systematics

Major, minor, and lithophile trace element abundances and Nd and Hf isotope systematics are reported for two sets of remarkably fresh, by Archean standards, samples of komatiitic lavas from the 3.48Ga Komati and the 3.27Ga Weltevreden Formations of the Barberton Greenstone Belt (BGB) in South Africa. These data are used to place new constraints on the thermal history of the early Archean mantle, on the timing of its differentiation, and on the origin and chemical nature of early mantle reservoirs and their evolution through time.Projected moderate to strong depletions of highly incompatible lithophile trace elements and water in the mantle sources of both komatiite systems, combined with the partitioning behavior of V during lava differentiation, are consistent with anhydrous conditions during generation of the komatiite magmas. Komati and Weltevreden lavas are inferred to have erupted with temperatures of ∼1600°C, and, thus, represent the hottest known lavas on Earth. The calculated mantle potential temperatures of ∼1800°C for both komatiite systems are 150–200°C higher than those of contemporary ambient mantle. Combined, these observations are consistent with the origin of these BGB komatiite magmas in mantle plumes in the lower mantle.New Sm–Nd and Lu–Hf isotopic data allow precise determination of initial ε143Nd=+0.46±0.10 and +0.50±0.11 and initial ε176Hf=+1.9±0.3 and +4.7±0.8 for the Komati and the Weltevreden system komatiites, respectively. These positive initial values reflect prior fractionation of Sm/Nd and Lu/Hf in the mantle early in Earth history. Conversely, μ142Nd values are 0.0±2.4 and +2.2±4.1 for the Komati and the Weltevreden systems, respectively. These values overlap, within uncertainties, those of modern terrestrial rocks, thus, limiting the magnitudes of possible Sm/Nd fractionations generated by early Earth processes in the sources of these rocks. Combined 142,143Nd and Hf isotope and lithophile trace element systematics are consistent with formation and long-term isolation of deep-seated mantle domains with fractionated Sm/Nd and Lu/Hf at ca. 4400Ma. These domains were likely generated as a result of crystallization of a primordial magma ocean, with Mg-perovskite and minor Ca-perovskite acting as fractionating phases. The inferred mantle domains were evidently mixed away by 2.7Ga on the scale of mantle reservoirs sampled by late Archean komatiite lavas emplaced worldwide.

Geochemistry of komatiites from the Tipasjärvi, Kuhmo, Suomussalmi, Ilomantsi and Tulppio greenstone belts, Finland: Implications for tectonic setting and Ni sulphide prospectivity

Komatiites and komatiitic basalts within the Neo-Mesoarchaean, ca. 2.8Ga greenstone belts in the western part of the Karelian craton in Finland formed in diverse palaeoenvironments. The lavas in the Tipasjärvi and Kuhmo greenstone belts formed in an oceanic plateau environment, whereas the komatiites of the Suomussalmi belt formed in a continental rift setting, consistent with Nd and Pb isotope data of Suomussalmi felsic volcanics indicating the presence of older (>3Ga) crust in the belt. The komatiites and komatiitic basalts of the Ilomantsi greenstone belt show trace element systematics similar to those of modern continental arc volcanics, implying interaction of the lavas with felsic arc volcanics. PGE contents are low (2–4ppb Pt and Pd each) relative to most other Al-undepleted komatiites from elsewhere, interpreted to result from equilibration with sulphides during emplacement. Xenocrystic zircon grains, up to 3.2Ga in age, in some felsic volcanics and granitoids that are coeval with the komatiites indicate the presence of older continental basement in the Ilomantsi area. The Ilomantsi komatiites are interpreted to have formed at an active continental margin where basement rocks were extensively intruded and assimilated by felsic plutonic massifs. The komatiites of the Tulppio belt in northern Lapland resemble those of the Tipasjärvi, Kuhmo and Suomussalmi belts, but due to limited data, their emplacement environment remains uncertain.The Ni sulphide prospectivity of the Finnish komatiite belts appears to be relatively low, despite the identification of geochemical signatures of PGE depletion and crustal contamination in the Ilomantsi belt and, to a lesser degree, Suomussalmi and Tulppio belts, the presence of locally sulphide-bearing felsic volcanics and sedimentary rocks in the footwall of some of the komatiites, and the discovery of several small magmatic sulphide deposits in the Kuhmo, Suomussalmi, Ilomantsi and Tulppio belts. The main reasons are that the Finnish komatiites are relatively differentiated compared to those in mineralised belts elsewhere, and the Finnish belts contain a relatively large proportion of compound sheet flows relative to adcumulates, suggesting that dynamic lava channel environments where significant amounts of sulphide-bearing substrate could be assimilated and deposited are uncommon. Furthermore, the samples analysed to date lack the signature of scattered, but widespread PGE enrichment that is characteristic of mineralized komatiite belts and that has been interpreted to reflect wide-scale saturation of the magmas in Ni-sulphide melt.

Integrated field, satellite and petrological observations of the November 2010 eruption of Erta Ale

Erta Ale volcano, Ethiopia, erupted in November 2010, emplacing new lava flows on the main crater floor, the first such eruption from the southern pit into the main crater since 1973, and the first eruption at this remote volcano in the modern satellite age. For many decades, Erta Ale has contained a persistently active lava lake which is ordinarily confined, several tens of metres below the level of the main crater, within the southern pit. We combine on-the-ground field observations with multispectral imaging from the SEVIRI satellite to reconstruct the entire eruptive episode beginning on 11 November and ending prior to 14 December 2010. A period of quiescence occurred between 14 and 19 November. The main eruptive activity developed between 19 and 22 November, finally subsiding to pre-eruptive levels between 8 and 15 December. The estimated total volume of lava erupted is ∼0.006 km3. The mineralogy of the 2010 lava is plagioclase + clinopyroxene + olivine. Geochemically, the lava is slightly more mafic than previously erupted lava lining the caldera floor, but lies within the range of historical lavas from Erta Ale. SIMS analysis of olivine-hosted melt inclusions shows the Erta Ale lavas to be relatively volatile-poor, with H2O contents ≤1,300 ppm and CO2 contents of ≤200 ppm. Incompatible trace and volatile element systematics of melt inclusions show, however, that the November 2010 lavas were volatile-saturated, and that degassing and crystallisation occurred concomitantly. Volatile saturation pressures are in the range 7–42 MPa, indicating shallow crystallisation. Calculated pre-eruption and melt inclusion entrapment temperatures from mineral/liquid thermometers are ∼1,150 °C, consistent with previously published field measurements.

ACCUMULATION OF TRACE ELEMENTS IN VESUVIANITE DURING FLUID-ROCK INTERACTION: AN EXAMPLE FROM A ZONED SKARN DEVELOPED BETWEEN A METAMORPHOSED BASIC DIKE AND MARBLE IN THE HIRAO LIMESTONE, FUKUOKA, JAPAN

Garnet–pyroxene–wollastonite zoned skarns in the Hirao limestone, Fukuoka, Japan, were formed during metamorphic activity associated with the emplacement of the Hirao granodiorite. In this area, rocks along the contact between a marble unit and a metamorphosed basic dike (MB) were replaced by clinopyroxene, garnet, wollastonite, and vesuvianite during infiltration metasomatism involving H 2 O-rich fluids. Whole-rock analysis of skarn-, marble-, and MB-hosted mineral zone samples indicates that the garnet zone skarn rocks are enriched in rare earth elements (REE). Marble trace element systematics indicate that Ce, alkali elements, Th, Pr, Zr, and Nb were all added during metamorphic activity. The majority of these elements were derived from metamorphic fluids, although some originated from the MB. Micro-scale mineral analysis was performed using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), revealing that vesuvianite preferentially hosts the REE within the garnet zone rocks, even though it is only present in small amounts (3% modal abundance). The vesuvianite commonly co-exists with a Ca-rich garnet and both minerals can contain significant concentrations of REE. The results suggest that the distribution of the REE in garnet and vesuvianite may be a function of fluid composition, with REE-enriched vesuvianite forming during interaction with light rare earth element (LREE)-enriched but heavy rare earth element (HREE)-unenriched fluids. The correlation of REE concentrations and Eu anomalies in vesuvianite indicates that the fluid was REE-enriched but Eu-depleted..

Trace element systematics in granulite facies rutile: implications for Zr geothermometry and provenance studies

AbstractMetamorphic rutile from granulite facies metapelitic rocks of the Archean Pikwitonei Granulite Domain (PGD; Manitoba, Canada) provides constraints on the systematics of trace elements in rutile during high‐temperature conditions and subsequent slow cooling. Compositional profiles and maps of the Zr concentrations in rutile grains (120–600 μm) from three metapelitic gneisses were acquired by electron probe micro‐analysis, using a spatial resolution of down to 2 μm. Simultaneously, profiles were analysed for Nb, Cr and V, which have significantly different diffusion characteristics in rutile. The profiles of all elements show relatively homogeneous concentrations within most grains, but significant inter‐grain differences even within a single thin section. Some rutile grains display a slight concentration decrease from a neighbouring garnet towards the matrix for all measured elements. The lack of diffusion profiles for all analysed elements shows that these are highly immobile in rutile and that distributions of these elements are primary and preserve prograde information. The Nb and Cr concentrations overlap with ranges that are ascribed to different provenances indicating that source discrimination based on these elements is not possible in all cases. High retentiveness for Zr implies that the Zr‐in‐rutile geothermometer is highly robust to diffusive re‐equilibration, even during very slow cooling (&lt;2 °C Ma−1) from granulite facies conditions. Most grains have high Zr contents (3000–4600 ppm). Differences between high Zr contents suggest that during growth under vapour‐absent conditions there may not be saturation of Zr in rutile, even if zircon is present. Therefore, several rutile grains need to be analysed in a sample to obtain a useful minimum peak temperature. The highest Zr concentrations correspond to ∼900 °C. This is significantly higher than previous peak temperature estimates of 820 °C based on two‐feldspar thermometry. On a regional scale this implies that part of the PGD was affected by ultra‐high temperature (UHT) metamorphism. It also implies that rutile is able to preserve primary compositions even to UHT conditions. This study shows that, if combined with textural information, Zr‐in‐rutile has the potential to be a very useful tool for estimating rutile crystallization temperatures and peak metamorphic conditions. For granulite facies rocks, Zr‐in‐rutile yields more reliable peak metamorphic temperatures than most other exchange geothermometers, which tend to partially re‐equilibrate by diffusion during cooling.

Craton formation in Late Archean subduction zones revealed by first Greenland eclogites

It is now well established that the early continental crust was formed by melting of basaltic lithologies such as amphibolite and eclogite. However, considerable uncertainty surrounds the geologic environment in which melting took place. Commonly invoked options range between melting at the underside of oceanic plateaus above mantle plumes or melting of oceanic lithosphere during shallow subduction. Distinguishing between these scenarios has important implications for the early evolution of continents. We use the first eclogites discovered from the North Atlantic craton (NAC) to constrain the formation of the deep root to this continent. Late Archean eclogite xenoliths (2.7 ± 0.3 Ga) from a kimberlite in West Greenland are broadly coeval with a major regional episode of tonalite-trondhjemite-granodiorite (TTG) magmatism. Major and trace element systematics of the eclogites reveal a highly refractory character that is mirrored by NAC peridotites. Moreover, the refractory eclogites define a complementary relationship to the Late Archean TTG granitoids from the NAC, and their elevated garnet δ 18 O values along with negative Eu anomalies suggest seafloor-altered oceanic crust as the most viable eclogite protolith. These results from Greenland provide strong support for a model in which early continental crust grew by melting of basaltic slabs in subduction zones, where tectonic stacking of down-going oceanic lithosphere provided the mechanism that coupled formation of cratonic crust and mantle.

U-series isotope systematics of mafic magmas from central Oregon: Implications for fluid involvement and melting processes in the Cascade arc

The Cascade arc is the warm-slab subduction zone global end member, where a broad variety of primitive magmas with highly variable slab fluid signatures have erupted in close spatial and temporal proximity. A number of petrogenetic models have been proposed to explain the occurrence of such diverse magmas, but the source(s) of these magmas and the timing of fluid addition to the sub-arc mantle remain controversial. We present uranium-series isotope data ( 238U– 230Th– 226Ra) for eighteen mafic lavas from the Three Sisters region of the central Oregon Cascades, and for a further six lavas from the rear-arc Newberry Volcano. The majority of these samples have geochemical characteristics (e.g. Nb/Zr, Ba/Zr, 87Sr/ 86Sr) consistent with previously described calc-alkaline basalts from this region of the arc, and indicative of limited fluid involvement at some stage in their genesis. Trace element and long-lived radiogenic isotope modeling suggests that this fluid was derived from dehydration of subducting sediments, and was added to an enriched, garnet-bearing mantle wedge source. The trace element systematics of the lavas are consistent with small degree (< 10%) melts of this fluid-modified source. All samples display ( 230Th/ 238U) and ( 226Ra/ 230Th) ≥ 1, similar to values measured in fresh MORB and other parts of the arc. Results of a dynamic melting model support the interpretation that these lavas are small degree melts of an asthenospheric source, and do not allow for a lithospheric mantle source. However, the U-series data do not permit us to determine whether fluid addition was the trigger for melting, or whether the lavas were generated from a secular equilibrium source that had experienced fluid addition > 350 ka prior to melting. Regardless, modern fluid input is limited and melting is dominantly occurring in response to upwelling and decompression.

LA-ICP-MS trace element analysis of pyrite from the Xiaoqinling gold district, China: Implications for ore genesis

The Xiaoqinling district, the second largest gold producing district in China, is located on the southern margin of the North China Craton. It consists of three ore belts, namely, the northern ore belt, the middle ore belt and the southern ore belt. Pyrite from the Dahu gold deposit in the northern ore belt and Wenyu and Yinxin gold deposits in the southern ore belt were investigated using a combination of ore microscopy and in-situ laser-ablation inductively-coupled plasma-mass spectrometry (LA-ICP-MS). A range of trace elements was analyzed, including Au, Te, Ag, Pb, Bi, Cu, Co, Ni, Zn, Mo, Hg, As and Si. The results show that there are no systematic differences between the trace element compositions of pyrite in the three deposits from different ore belts. In general, Au concentrations in pyrite are low (from < 0.01 ppm to 2.2 ppm) but Ni concentrations are rather high (up to 8425 ppm). A four-stage mineralization process is indicated by microscopic and field observations and this can be related to the systematic trace element differences between distinct generations of pyrite. Stage I precedes the main gold mineralization stage; pyrite of this stage has the lowest Au concentrations. Stages II and III contributed most of the gold to the ore-forming system. The corresponding pyrite yielded the highest concentrations of Au and Ni. Our microscopic observations suggest that pyrite in the main gold mineralization stage precipitated simultaneously with molybdenite that has been previously dated as Indosinian (~ 218 Ma by Re–Os molybdenite dating), indicating the Indosinian as the main gold mineralization stage. The Indosinian mineralization age and the geological and geochemical features of these gold deposits (e.g., low salinity, CO 2-rich ore fluids; spatial association with large-scale compressional structures of the Qinling orogen; δ 18O and δD data suggestive of mixing between metamorphic and meteoric waters; δ 34S and Pb-isotopic data that point to a mixed crustal-mantle source) all point to typical orogenic-type gold deposits. High Ni concentrations (up to 8425 ppm) of pyrite possibly linked to deep-seated mafic/ultramafic metamorphic rocks provide further evidence on the orogenic gold deposit affinity, but against the model of a granitic derivation of the mineralizing fluid as previously suggested by some workers. Generally low Au concentration in pyrite is also consistent with those from worldwide orogenic gold deposits. Therefore, the gold mineralization in the Xiaoqinling district is described as orogenic type, and is probably related to Indosinian collision between the North China Craton and the Yangtze Craton.

A radiogenic Os component in the oceanic lithosphere? Constraints from Hawaiian pyroxenite xenoliths

Platinum Group Element (PGE) concentrations in garnet pyroxenite xenoliths from Oahu, Hawaii, are significantly lower than those in mantle peridotites and show fractionated patterns (e.g. Pd N/Os N = 2–10, Pd N/Ir N = 4–24; N = chondrite normalized) and very high Re N/Os N ratios (∼9–248). Mass balance calculations show that the bulk rock pyroxenite PGE inventory is controlled by the presence of sulfide phases. The 187Os/ 188Os ratios of these pyroxenites vary from subchondritic to suprachondritic (0.123–0.164); and the 187Os/ 188Os ratios show good correlations with bulk rock and clinopyroxene major and trace element compositions, and bulk rock PGE and sulfur abundances. These observations suggest that the Os isotope compositions in these pyroxenites largely reflect primary processes in the oceanic mantle and Pacific lithosphere. In contrast, bulk rock 187Os/ 188Os ratios do not correlate with other lithophile isotopic tracers (e.g. Rb–Sr, Sm–Nd, Lu–Hf) which show limited isotopic variability ( Bizimis et al., 2005). This and the lack of 187Os/ 188Os vs. Re/Os correlations suggest that the range in Os isotope ratios is not likely the result of mixing between long-lived depleted and enriched components or aging of these pyroxenites within the Pacific lithosphere after its formation at a mid-oceanic ridge setting some 80–100 million years ago. We interpret the Os isotopes, PGE and lithophile element systematics as the result of melt–lithosphere interaction at the base of the Pacific lithosphere. The major and trace element systematics of the clinopyroxenes and bulk rock pyroxenites and the relatively constant lithophile element isotope systematics are best explained by fractional crystallization of a rather homogenous parental magma. We suggest that during melt crystallization and percolation within the lithosphere, the parental pyroxenite melt assimilated radiogenic Os from the grain boundaries of the peridotitic lithosphere. This radiogenic Os component may reside in the grain boundary sulfides or other trace phases, and may be due to fluids or melts that had previously percolated through the basal part of the lithosphere during its transit from a mid-oceanic ridge to its present position above the Hawaiian plume. As the solidus of the parental pyroxenite melt is lower than the solidus of the lithospheric peridotite, we envision that the pyroxenite–parent melt selectively assimilated the grain boundary sulfide phases with lower melting temperature as it percolated through the lithosphere, without significantly reacting with the silicate minerals. Thus while the parental melt of these pyroxenites originate within the Hawaiian plume, melt–lithosphere interaction during progressive crystallization may have selectively enriched the resulting melts with radiogenic Os, thereby decoupling Os from the lithophile element isotopes, but retaining a link between Os, PGE and fractional crystallization systematics. In this model, Oahu pyroxenites essentially represent melts from different stages of this melt–mantle reaction process at the base of the lithosphere, and we suggest that this process may also explain the similar Os vs. lithophile element decoupling seen in the rejuvenated volcanism in Oahu and Kauai. We further show that the pyroxenites do not posses the requisite Pt/Re ratios, where upon, recycling and aging would generate the coupled enrichments of 186Os– 187Os isotope ratios observed in Hawaiian and other lavas.

Collisional accretion of a Late Ordovician oceanic island arc, northern Tasman Orogenic Zone, Australia

A distinctive Late Ordovician volcano-sedimentary terrane, embracing the Carriers Well Formation and Everetts Creek Volcanics and dismembered slivers now structurally intercalated in the adjoining Wairuna Formation, is located within the Broken River Province of the northern Tasmanides. It abuts a basement of mafic–ultramafic rocks (Gray Creek Complex) and overlying Early Ordovician deep marine sedimentary and volcanic strata (Judea Formation) which host tonalitic plutons. The terrane lies at the western, inboard-margin of the Camel Creek Subprovince, a broad tract of multiply deformed mid-Paleozoic turbidites with minor basalt and chert variously interpreted as the infill of a backarc basin or an accretionary wedge. U–Pb dates from detrital zircon indicate a maximum Late Silurian age for siliciclastic rocks from the previously undated Wairuna Formation. Geochemistry of volcanic rocks from the volcano-sedimentary terrane show them to be largely of mafic to intermediate compositions of calc-alkaline affinity, comparable with broadly coeval Macquarie Arc volcanic suites of the southern Tasmanides. Trace-element systematics identify a subduction relationship for the volcanic suite and V/Ti employed as a discrimination tool identifies the terrane as representing an oceanic island arc, consistent with its sedimentary facies which include volcaniclastic mass flow deposits, limestone, and radiolarian chert. Continent-derived sandstone in the sedimentary assemblage, confirmed by the ages of detrital zircon from a sandstone sample from the Carriers Well Formation, indicates that the oceanic island arc developed proximal to the Late Ordovician continental margin of East Gondwana. Its nature and location bear on the tectonic setting of the entire Camel Creek Subprovince, for which interpretation as an Early Silurian–Early Devonian accretionary wedge is favoured. Collision of the island arc with the continental margin, and associated deformation of part the intervening oceanic crustal tract, now represented by the Gray Creek Complex and its sedimentary cover (Judea Formation) registers the initiation of subduction accretion in late Early Silurian (Llandoverian) time. It marks early-stage orogenesis in the Broken River Province, accurately timed by stratigraphic relationships in the basinal succession developed in the Graveyard Creek Subprovince immediately to the west of the arc assemblage. Tectonism was regionally developed in north Queensland at this time, coeval with the Benambran Orogeny of the Lachlan Orogen in which the Macquarie Arc was likewise accreted to the East Gondwana margin. Benambran orogenesis marks a general phase of shortening, and removal by subduction, of oceanic crust and inversion of continent-derived overlying sedimentary cover along the East Gondwana margin.

Deep melting of old subducted oceanic crust recorded by superchondritic Nb/Ta in modern island arc lavas

Niobium–tantalum systematics of slab-derived melts are powerful tracers that discriminate residual high-pressure rutile-bearing eclogite from low-pressure garnet-bearing amphibolite in subducting plates. Previously reported low Nb–Ta ratios in modern slab melts suggested a predominance of shallow melting in the presence of residual amphibole and that deep melting of rutile-bearing eclogitic slabs, devoid of residual amphibole, is volumetrically insignificant. This study evaluates Nb/Ta in combination with other trace element systematics of modern intra-oceanic and slab melt-related arc lavas from the south-western volcanic chain of the Solomon Islands that cover over 1000km of the SW Pacific plate border. After a change of subduction polarity, an old subducted Pacific slab and a recently subducting Indian–Australian slab are both present beneath the arc. Solomon arc lavas show sub- to superchondritic Nb–Ta ratios (ca. 10 to 27) which is the largest range ever reported in modern island arc lavas. The large range of Nb/Ta likely results from enrichment of the depleted sub-arc mantle by two distinct slab-derived melts in addition to fluids. One minor slab melt component is derived from the shallow and recent subducting Indian–Australian plate where amphibole is still a significant residual phase. The second slab melt component is predominant in Solomon arc lavas and can be attributed to deep rutile–eclogite-controlled melting of old subducted Jurassic Pacific oceanic crust where residual amphibole is entirely absent or insignificant. The deep Pacific slab melt component is the most likely origin of the extremely high and superchondritic Nb/Ta signatures that produce the upper half of the observed range of Nb/Ta in Solomon arc lavas. The slab melt component that enriched the sub-arc mantle with an unusually high Nb/Ta signature is derived from an initially intact Pacific plate that was probably subject to a slab break-off event and subsequent melting at depths exceeding 100km. The geochemical evidence presented here shows that old and cold subducted oceanic crust, which is initially not torn, may resist shallow melting but can melt at greater depths instead. The resulting slab melts are generated in the presence of residual rutile-bearing eclogite and significantly fractionate Nb–Ta ratios which may be of relevance at a global scale.