Light Rare Earth Element Abundances Research Articles

On the overabundance of light rare earth elements in terrestrial zircons and its implication for Earth’s earliest magmatic differentiation

We present whole-rock and zircon rare earth element (REE) data from two early Archaean gneisses (3.81 Ga and 3.64 Ga) from the Itsaq gneiss complex, south-west Greenland. Both gneisses represent extremely rare examples of unaltered, fresh and relatively undeformed igneous rocks of such antiquity. Cathodoluminescence imaging of their zircons indicates a single crystallisation episode with no evidence for either later metamorphic and/or anatectic reworking or inheritance of earlier grains. Uniform, single-population U/Pb age data confirm the structural simplicity of these zircons. One sample, a 3.64 Ga granodioritic gneiss from the Gothåbsfjord, yields a chondrite-normalised REE pattern with a positive slope from La to Lu as well as substantial positive Ce and slight negative Eu anomalies, features generally considered to be typical of igneous zircon. In contrast, the second sample, a 3.81 Ga tonalite from south of the Isua Greenstone Belt, has variable but generally much higher light REE abundances, with similar middle to heavy REE. Calculation of zircon/melt distribution coefficients ( D REE zircon/melt) from each sample yields markedly different values for the trivalent REE (i.e. Ce and Eu omitted) and simple application of one set of D REE zircon/melt to model the melt composition for the other sample yields concentrations that are in error by up to two orders of magnitude for the light REE (La–Nd). The observed light REE overabundance in the 3.81 Ga tonalite is a commonly observed feature in terrestrial zircons for which a number of explanations ranging from lattice strain to disequilibrium crystallisation have been proposed and are further investigated herein. Regardless of the cause of light REE overabundance, our study shows that simple application of zircon/melt distribution coefficients is not an unambiguous method for ascertaining original melt composition. In this context, recent studies that use REE data to claim that >4.3 Ga Hadean detrital zircons originally crystallised from an evolved magma, in turn suggesting the operation of geological processes in the early Earth analogous to those of the present day (e.g. subduction and melting of hydrated oceanic crust), must be regarded with caution. Indeed, comparison of terrestrial Hadean and >3.9 Ga lunar highland zircons shows remarkable similarities in the light REE, even though subduction processes that have been used to explain the terrestrial zircons have never operated on the Moon.

A hibonite‐corundum inclusion from Murchison: A first‐generation condensate from the solar nebula

Abstract— Through freeze‐thaw disaggregation of the Murchison (CM) carbonaceous chondrite, we have recovered a ˜90 times 75 μm refractory inclusion that consists of corundum and hibonite with minor perovskite. Corundum occurs as small (˜10 μm), rounded grains enclosed in hibonite laths (˜10 μm wide and 30–40 μm long) throughout the inclusion. Perovskite predominantly occurs near the edge of the inclusion. The crystallization sequence inferred petrographically‐corundum followed by hibonite followed by perovskite‐is that predicted for the first phases to form by equilibrium condensation from a solar gas for Ptot ≤5 times 10−3 atm. In addition, the texture of the inclusion, with angular voids between subhedral hibonite laths and plates, is also consistent with formation of the inclusion by condensation. Hibonite has heavy rare earth element (REE) abundances of ˜40 × CI chondrites, light REE abundances ˜20 × CI chondrites, and negative Eu anomalies. The chondrite‐normalized abundance patterns, especially one for a hibonite‐perovskite spot, are quite similar to the patterns of calculated solid/gas partition coefficients for hibonite and perovskite at 10−3 atm and are not consistent with formation of the inclusion by closed‐system fractional crystallization. In contrast with the features that are consistent with a condensation origin, there are problems with any model for the formation of this inclusion that includes a molten stage, relic grains, or volatilization. If thermodynamic models of equilibrium condensation are correct, then this inclusion formed at pressures <5 times 10−3 atm, possibly with enrichments (<1000x) in CI dust relative to gas at low pressures (below 10−4 atm). Both hibonite and corundum have δ17O ≅ δ18O ≅ −50%, indicating formation from an 16O‐rich source. The inclusion does not contain radiogenic 26Mg and apparently did not contain live 26Al when it formed. If the short‐lived radionuclides were formed in a supernova and injected into the early solar nebula, models of this process suggest that 26Al‐free refractory inclusions such as this one formed within the first ˜6 times 105 years of nebular collapse.

Late Silurian bimodal volcanism of southwestern New Brunswick, Canada: Products of continental extension

Silurian–Devonian bimodal volcanism—a common feature of the northern Appalachians of the northeastern United States and eastern Canada—forms major belts of igneous rocks including the Coastal volcanic belt of New England and New Brunswick. The Coastal volcanic belt is one of the largest bimodal volcanic provinces in the world. Controversy remains regarding the tectonic setting of this volcanism; suggestions range from volcanic arc, to extensional continental rift, transpressional basin, backarc, or plume-modified subduction zone. The mapped Passamaquoddy Bay volcanic sequence of the Coastal volcanic belt of southwestern New Brunswick has a minimum thickness of 4 km and comprises 63 lithologic units that are divided into four distinct cycles of mafic-felsic volcanism. A preliminary U-Pb zircon date indicates an age of 423 Ma, similar to the age of the Cranberry Island bimodal volcanic series of the Coastal volcanic belt in Maine. Our analysis of 115 volcanic rocks for major and trace element concentrations indicates that the sequence is subalkalic and bimodal (basalt-rhyolite) with within-plate tectonic affinities. Most mafic rocks range in composition from basalt to basaltic andesite. The Mg# [defined as 100 × MgO/(MgO + FeO), where FeO = FeO T × 0.9] of the basalts ranges from 30 to 70, and there is a trend toward more primitive compositions upward in the section. The light rare earth element abundances of the basalts are enriched to levels of 30–130 times chondritic values. The flows are enriched in incompatible elements with respect to primitive mantle and show distinct Nb, Hf, and Sr anomalies. We interpret the basalts to be mantle melts modified by crustal contamination and mantle metasomatism from a previous subduction event. Most of the chemical variation can be explained by replenishment and fractionation of small magma chambers. The rhyolitic rocks are crustal melts, modified by crystal fractionation. Within each of the rhyolitic cycles there is an upward trend toward more evolved compositions as a result of an evolving magma chamber and magma-chamber zonation. We suggest that the rocks were erupted in an extensional environment and that there was a significant zone of continental extension during the Late Silurian in the northern Appalachians. However, controversy remains regarding the precise nature of the tectonic setting and the duration and extent of this volcanism.

High mass resolution ion microprobe analysis of rare earth elements in silicate glass, apatite and zircon: lack of matrix dependency

The concentrations of all rare earth elements (REE: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) in silicate glass, apatite and zircon were measured by an ion microprobe method with a high mass resolving power of 9300 at 1% peak height. Observed contents of REE in apatite extracted from a dacite agree well with those measured by the isotope dilution-thermal ionization mass spectrometry (ID-TIMS) method within 30% error at 2 σ. The contents of heavy REE in several zircons are consistent with those measured by ID-TIMS and instrumental neutron activation analysis (INAA), while the light REE abundances are significantly lower by ion microprobe than by the other methods. The discrepancy is partly due to the presence of 1–5-μm inclusions of apatites within zircon crystals. Secondary ion yields of REE observed in a silicate glass show a very similar trend to those in apatite and zircon. Since the chemical composition of the silicate glass is appreciably different from the apatite and zircon, relative secondary ion yields of REE are probably independent from matrix effect, which is the most important finding of this work. Calculated partition coefficients of REE between melt and apatite agree well with reported values while the coefficients between melt and zircon are discrepant from those in literature. The REE coefficients in zircon increase with increase of the ionic radius logarithmically, suggesting power law dependence of the coefficients.

Constraints on the Trace Element Composition of the Archean Mantle Root beneath Somerset Island, Arctic Canada

Peridotites that sample Archean mantle roots are frequently in- INTRODUCTION compatible trace element enriched despite their refractory major The mantle underlying many Archean cratons has anelement compositions. To constrain the trace element budget of the omalously high seismic velocities to depths of 350–400 km lithosphere beneath the Canadian craton, trace element and rare ( Jordan, 1988; Grand, 1994), indicating the presence of earth element (REE) abundances were determined for a suite of cold refractory roots, depleted in the fusible major elegarnet peridotites and garnet pyroxenites from the Nikos kimberlite ments compared with fertile mantle (Boyd & Mertzman, pipe on Somerset Island, Canadian Arctic, their constituent garnet 1987; McDonough, 1990). These deep residual peridotite and clinopyroxene, and the host kimberlite. These refractory mantle roots probably contribute to the stability of Archean xenoliths are depleted in fusible major elements, but enriched in continental lithosphere because of their lower density and incompatible trace elements, such as large ion lithophile elements higher viscosity compared with that of the surrounding (LILE), Th, U and light rare earth elements (LREE). Mass asthenospheric mantle (Boyd & McCallister, 1976; Jorbalance calculations based on modal abundances of clinopyroxene dan, 1979; Pollack, 1986). Mantle xenoliths that are and garnet and their respective REE contents yield discrepancies hosted by kimberlites and alkaline basalts are our only between calculated and analyzed REE contents for the Nikos bulk window into the subcontinental lithosphere. They provide rocks that amount to LREE deficiencies of 70‐99%, suggesting essential evidence on the chemical composition and evoluthe presence of small amounts of interstitial kimberlite liquid tion of the upper mantle to depths of >200 km. Studies (0·4‐2 wt %) to account for the excess LREE abundances. These of these mantle xenoliths enable us to characterize the results indicate that the peridotites had in fact depleted or flat abundance and distribution of major, minor and trace LREE patterns before contamination by their host kimberlite. LREE elements in peridotites and between their constituent and Sr enrichment in clinopyroxene and low Zr and Sr abundances minerals. Most subcratonic peridotite samples have in garnet in low-temperature peridotites (800‐1100°C) compared undergone a complex history of melt extraction that has with high-temperature peridotites (1200‐1400°C) suggest that the changed their chemical composition and resulted in shallow lithosphere is geochemically distinct from the deep lithosphere depletion of the residual mantle in fusible major elements beneath the northern margin of the Canadian craton. The Somerset such as Fe, Al and Ca (e.g. Nixon, 1987; Herzberg, 1993; mantle root appears to be characterized by a depth zonation that Boyd et al., 1997). In contrast to their depletion in may date from the time of its stabilization in the Archean. incompatible major elements, however, many peridotite

Measurement of isotopes of light rare earth elements in the form of oxide ions: a new development in thermal ionization mass spectrometry

An isobaric interference free thermal ionization mass spectrometry (TIMS) method is presented here for the precise isotope measurements of light rare earth elements (LREEs). LREE isotopes can be measured in the form of monoxide ion (MO+), but the previous determinations of LREE isotopes in the form of oxide were hindered, notably by the 17O and 18O related isobaric interferences. In the present study, 16O-enriched oxygen was introduced into the ion source chamber via a specially designed gas inlet system. It was observed that the isobaric interferences caused by 17O and 18O were remarkably reduced by up to 6 and 70 times, respectively, compared to those caused by natural oxygen bleeding. Isotopic ratios of La, Ce, and Nd obtained without isobaric interference corrections show the excellent precision and accuracy in the present study. The results also shed some light on the formation mechanism of the measured oxide species in TIMS. Furthermore it promises more precise determination of LREE abundances and La-Ce and Sm-Nd ages in geochemistry and cosmochemistry.

Geomicrobiological controls on light rare earth element, Y and Ba distributions during granite weathering and soil formation

In this study we examine the redistribution of trivalent and tetravalent light rare earth elements (LREEs), Ba and Y during chemical weathering of granites from southern New South Wales, Australia. In essentially abiotic zones in the lower weathering profile, primary allanite is dissolved, and apatite is extensively replaced by secondary LREE lanthanide phosphates such as rhabdophane and florencite. This association is attributed to the relatively high concentrations of phosphorus at dissolving apatite surfaces and low solubility products for lanthanide phosphates. Bulk chemical data from the lower profile indicate considerable enrichment of Y, La and Nd. In contrast, in granites weathered in proximity to the soil zone, secondary lanthanide phosphates are rare and phosphate surfaces are often colonized by bacteria and fungal hyphae. Bulk chemical data show that Y, La and Nd decrease in abundance with increasing weathering. Low dissolved phosphate concentration due to microbial uptake of phosphorus suppresses secondary phosphate precipitation and also leads to dissolution of secondary lanthanide phosphates formed prior to colonization. In the most highly weathered rocks and soils, secondary phosphates are extremely rare, and only Ce oxides remain. Bulk chemical data show Ce concentrations in excess of 1200 ppm in some soils. Retention of Ce as Ce-oxides reflects the low mobility of (tetravalent) Ce under oxidizing conditions. These observations explain heterogeneities in LREE abundances in weathering profiles, development of extreme Ce anomalies, and greatly elevated concentrations of trivalent LREEs in some regions.

Accreted Eclogites with Oceanic Plateau Basalt Affinities in Ecuador

High-pressure, regionally-metamorphosed rocks (the Raspas Formation) outcrop extensively (more than 10 km long and 3 km wide) within a terrane including extensively serpentinized harzburgite in E 1 0 r o Province, Ecuador. The high-pressure rocks consist of pelitic schists, eclogite, eclogite amphibolite and mafic rocks retrograded into amphibolites and greenschists. Exhumation of these high-pressure rocks would have occurred at about 132 Ma, based on K/Ar ages obtained from phengite (Feininger, 1980). Eclogites are composed chiefly of euhedral garnet, omphacite, black stubby amphibole and quartz. The garnet is peppered with abundant rutile inclusions, pyroxene and tiny quartz crystals. This metamorphic assemblage equilibrated under conditions of about P(total) = 1.3 GPa 1.5-2 GPa ( 45-60 km depth) and a T of 600 ~ On the basis of the major element compositions, the protoliths of the eclogites were basaltic (e.g. 97CE1:SiO2 = 49%; MgO = 7.58%; TiO2 = 1.48%; Fe203 =12.44%) and/or cumulate gabbroic (e.g. 97CE4A: SiO2 = 47%, MgO = 10.36%; TiO 2 = 0.46%; Fe203 = 8.12%). Trace element abundances have been determined both by bulk solution ICP-MS as well as laser ablation-ICP-MS analysis of individual ( 50 gm spots) mineral grains. The metabasalt has slightly enriched light rare earth element (LREE) abundances relative to heavy (H)REE [chondrite-normalised (cN) (La/Yb) CN = 1.3] with a slight negative Eu anomaly. In contrast, the metagabbro is characterised by depleted LREE abundances relative to HREE [(La/Yb)cN = 0.54] and a marked positive Eu anomaly. Relative to N-MORB, the metabasalt is Nb-, Ta-, Th-, U-, La-, and Ceenriched, and Srand Rbdepleted. In contrast, the gabbro is depleted in most of these the elements, additionally in Nb, Zr, and Hf, but has significant enrichments in Ba and Sr. Omphacite and amphibole separates have complementary REE abundance patterns, i.e. amphibole is depleted in LREE relative to the HREE [(La/Yb) cN = 0.41] while omphacite is enriched [(La/Yb)cN = 5.95]. Absolute REE concentrations of amphibole and omphacite are low (about 10 times chondritic abundances). In terms of isotopic characteristics, omphacite has an eNd(T=lSOMa) = +7.96, which is slightly higher than that of the amphibole (+6.82), but both ratios fall within the range of OIB. The metabasalt has a relatively high (87Sr/86Sr)i [0.70649; eSr(T-lS0Ma) = +30] which is likely related to oceanic hydrothermal alteration. Remarkably, the trace element abundances (absolute and relative) and isotopic characteristics of both eclogitised basalt and gabbro do not differ significantly from those of unmetamorphosed equivalents of an oceanic plateau origin, elsewhere in Ecuador (e.g. basalts and diabases of the Pifidn Formation, San Juan cumulate gabbro; refer to figure). We note further that the subcreted portion of the accreted

Mixed Caledonian appinite magmas: implications for lamprophyre fractionation and high Ba-Sr granite genesis

Ach'Uaine Hybrid appinites represent a rare example of lamprophyric magmas that were demonstrably exactly contemporaneous with felsic differentiates, preserved within a suite of minor, hypabyssal intrusions emplaced at the end of the Caledonian orogeny in northern Scotland. Numerous small stocks, bosses and dykes show outcrop-scale relationships characteristic of mingling between lamprophyric and syenitic magmas, and are commonly cut by sharp-sided granite veins. The mafic rocks are characterised by Ni and Cr abundances and MgO sufficiently high to signal derivation from a mantle source within which radiogenic 87Sr/86Sr and nonradiogenic 143Nd/144Nd ratios require significant time-integrated incompatible element enrichment. This is manifest in high Ba, Sr and light REE abundances and incompatible element ratios in the derived magmas directly comparable with those of high Ba-Sr granitoids and related rocks. Quantitative major element, trace element, radiogenic and stable isotope modelling is consistent with early fractionation of clinopyroxene and biotite, accompanied by minor crustal assimilation, having driven the evolving lamprophyric magma to cogenetic syenite. Subsequent derivation of granite required a major change to feldspar-dominated crystal fractionation with continued, still minor contamination. The elemental and isotopic characteristics of the granitic terminus are so similar to high Ba-Sr granitoids both locally and worldwide, that these too may have had large mantle components and represent significant juvenile additions to the crust.

A unique ultrarefractory inclusion from the Murchison meteorite

Abstract— Through freeze‐thaw disaggregation of the Murchison meteorite, we have recovered a refractory inclusion, HIB‐11, that is unique in terms of its texture, mineral compositions, and bulk composition. It consists of anhedral, Y‐rich (1.6 wt% Y2O3) perovskite and lathlike spinel grains enclosed in a matrix of fine‐grained, Sc‐rich (10.5 wt% SC2O3 avg.), Ti‐rich (12.6 wt% TiO2 avg., reporting all Ti as TiO2) clinopyroxene. The chondrite‐normalized rare earth element (REE) pattern is complex, with light REE (LREE) at ∼10× C1, abundances increasing from Gd through Ho (the latter at ∼104× C1), decreasing through Yb at 200× C1, and Lu at ∼400× C1. The pattern reflects several stages of high‐temperature volatility fractionation. Removal of Lu and Er from the source gas in the first condensation event was followed by partial to complete removal of the somewhat less refractory heavy REE, Gd through Ho, in the HIB‐11 precursors by condensation from the fractionated residual gas in a second event. Both of these events probably reflect condensation of REE into ZrO2 or a mixed Zr‐, Sc‐, Ti‐, Y‐oxide at temperatures too high for hibonite stability. A second, lower‐temperature component, which was subsequently added, had fractionated (Nd‐poor, Ce‐rich) LREE abundances that resulted from condensation from a gas that had undergone prior removal of the more refractory LREE, resulting in enrichment in Ce and the most volatile REE, Eu and Yb. The aggregate was then melted and quickly cooled, forming a fine‐grained spherule. This is the first reported inclusion in which the two most refractory REE, Lu and Er, are strongly fractionated from the other REE. An absence of mass fractionation among the Ti isotopes indicates that HIB‐11 is not an evaporative residue, implying that volatility fractionation of trace elements took place during condensation. The fact that the two most refractory heavy REE could be separated from the other, only slightly less refractory heavy REE suggests that a wide variety of REE patterns is possible, and that ultrarefractory inclusions with other unusual REE patterns, important recorders of nebular condensation, may yet be discovered.

Origin of blueschist-facies clasts in the Mariana forearc, Western Pacific.

In the Mariana forearc region, blueschist-facies metabasite clasts have been recovered from a serpentinite seamount during Ocean Drilling Program Leg 125. To clarify the origin of these metamorphic clasts, major and rare earth elements (REE), Ba, and Sr were measured in seven blueschist-facies metabasite clasts. Two out of seven clasts have zig-zag REE patterns reflecting the lanthanide tetrad effect, suggesting intense interaction with sea water. The other five clasts, however, show REE patterns with light REE depletion similar to N-type mid-ocean ridge basalts. Barium and Sr enrichments, relative to light REE abundances, are not conspicuous and most clasts show Sr and/or Ba depletions, indicating that they have chemical affinity with mid-ocean ridge basalt. This result is supported by the similarity of the TiO2-Al2O3 systematics of the clasts to those of mid-ocean ridge basalts. These MORB-like clasts metamorphosed under blueschist-facies conditions must have been taken into uprising serpentinite diapir at a depth where materials from oceanic crust extensively accreted. Two possible sources for the MORB-like protolith of blueschist-facies clasts are considered: (1) the oceanic crust of the subducted Pacific plate, (2) oceanic crust trapped within the area between trench axis and volcanic front when the subduction of the Pacific plate started. The second possibility is likely because the subducting Pacific plate crust is overlain by 600-800 m of oceanic sediment which is not represented in the clast population. It is proposed that the clasts provide evidence of tectonic erosion of oceanic crust which was trapped in the Mariana forearc at the initiation of subduction.

Clastic metasediments of the Early Proterozoic Broken Hill Group, New South Wales, Australia: Geochemistry, provenance, and metallogenic significance

Whole-rock analyses of samples of pelite, psammite, and psammopelite from the Early Proterozoic Broken Hill Group (Willyama Supergroup) in the Broken Hill Block, New South Wales, Australia, reveal distinctive geochemical signatures. Major-element data show high Al2O3 and K2O, low MgO and Na2O, and relatively high Fe2O3TMgO ratios, compared to average Early Proterozoic clastic metasediments. High field strength elements (HFSE) are especially abundant, including Nb (most 15–27 ppm), Ta (most 1.0–2.2 ppm), Th (17–36 ppm), Hf (4–15 ppm), and Zr (most 170–400 ppm); Y (33–74 ppm) is also high. Concentrations of ferromagnesian elements are generally low (Sc = < 20 ppm, Ni = ≤ 62 ppm, Co = <26 ppm; Cr = most < 100 ppm). Data for rare earth elements (REEs) show high abundances of light REEs (LaCN = 116–250 × chondrite; LaCN = 437 in one sample), high LaCNYbCN ratios (5.6–13.9), and large negative Eu anomalies (EuEu∗ = 0.32–0.57).The geochemical data indicate derivation of the metasedimentary rocks of the Broken Hill Group by the erosion mainly of felsic igneous (or meta-igneous) rocks. High concentrations of HFSE, Y, and REEs in the metasediments suggest a provenance dominanted by anorogenic granites and(or) rhyolites, including those with A-type chemistry. Likely sources of the metasediments were the rhyolitic to rhyodacitic protoliths of local quartz + feldspar ± biotite ± garnet gneisses (e.g., Potosi-type gneiss) that occur within the lower part of the Willyama Supergroup, or chemically similar basement rocks in the region; alternative sources may have included Early Proterozoic anorogenic granites and(or) rhyolites in the Mount Isa and(or) Pine Creek Blocks of northern Australia, or in the Gawler craton of South Australia.Metallogenic considerations suggest that the metasediments of the Broken Hill Block formed enriched source rocks during the generation of pegmatite-hosted deposits and concentrations of La, Ce, Nb, Ta, Th, and Sn in the region. Li, Be, B, W, and U in pegmatite minerals of the district may have been acquired during granulite-facies metamorphism of the local metasediments.

Chemical properties of Central African carbonado and its genetic implications

Carbonado diamonds from Central African Republic exhibit very intense laser-induced photoluminescence due to the presence of the radiation damages. From the spectral profiles of photoluminescence obtained for these samples, the carbonado stones were grouped into three groups depending on a variation in the structure of radiation-damage products; Group-A carbonado with 504 nm band (3H band), Group-B carbonado with 575 nm band in addition to weak background peaks centered at 512 and 580 nm, and the Group-AB carbonado with features intermediate between Group-A and Group-B. When heating Group-A carbonado, Group-A was found to transform to Group-B at temperatures higher than 400°C. Thermogravimetry and Differential Thermal Analysis (TG-DTA) clarified the difference in thermal properties between Group-A and Group-B. While Group-A carbonado yields one intense and narrow exothermic peak at 800°C, Group-B carbonado exhibits a broad exothermic peak at 785°C. The distinct difference in the oxidation rate could be attributed to the difference in the density of micropores in carbonado. Chondrite-normalized rare earth element (REE) patterns for the bulk carbonado samples show negative Eu anomalies, and the inclinations of the patterns are similar to those of kimberlites and much steeper than that of continental crust as represented by continental shales. In the chondrite-normalized value, La/Lu ratios were of the order of 10 2 and the light REE abundances were as high as 10 3 relative to chondrite. Further, 1384 cm −1 absorption band was observed in IR (infrared) spectra of both Group-A and Group-B, which suggests the presence of nitrogen platelet in carbonado. These REE and IR results imply that the seed crystal of carbonado was formed at high pressure and high temperature and was subjected to the annealing process to generate nitrogen platelets. The genesis of Central African carbonado is discussed as a reconciliation of these experimental results.

Noble metal abundances and characteristics of six granitic magma series, Archean Abitibi Belt, Pontiac Subprovince; relationships to metallogeny and overprinting of mesothermal gold deposits

Noble metal abundances and characteristics of six granitic magma series, Archean Abitibi Belt, Pontiac Subprovince; relationships to metallogeny and overprinting of mesothermal gold deposits

Origin of Archean lamprophyre dykes, Superior Province, Canada: rare earth element and Nd?Sr isotopic evidence

Hornblende- and clinopyroxene-phyric lamprophyre dykes exposed in the Roaring River Complex, Superior Province are alkaline, nepheline-normative, basaltic compositions (>50 wt% SiO2), that range from primitive to fractionated [Mg/(Mg + total Fe)=0.66–0.40; Ni=200–35 ppm], and which have high abundances of light rare earth elements (REE) [(Ce/Yb)n=16–26, Cen=60–300; n = chondrite normalized], Sr (870–1,800 ppm), P2O5 (0.4–1.3 wt%), and Ba (150–900 ppm). Crystal fractionation of the lamprophyres produced coeval gabbro and clinopyroxenite cumulate bodies. A whole-rock Sm−Nd isochron for lamprophyres and gabbro-pyroxenite yields a crystallization age of 2,667±51 Ma Ma (I=0.50929±0.0004; ɛNd = + 2.3 α 0.7). Whole-rock Sr isotope data are scattered, but suggest an initial 87Sr/86Sr ratio of ≈ 0.7012, similar to bulk Earth. The elevated levels of light REEs and Sr in the lamprophyres were not due to crustal contamination or mixing with contemporaneous monzodioritic magmas, but a result of partial melting of a mantle source which was enriched in these and other large-ion-lithophile elements (LILEs) shortly before melting. The lamprophyres were contemporaneous with mantle-derived, high-Mg, LILE-enriched monzodiorite to granodiorite of the Archean “sanukitoid” suite. Both suites have concave-downward light REE profiles, suggesting that depleted mantle was common to their source regions, but the higher light REE abundances, higher Ba/La ratios, and lower ɛNd values (+1.3±0.3) of the parental monzodiorites suggest a more enriched source. The lamprophyres and high-Mg monzodiorites were derived from a mineralogically and compositionally heterogeneous, LILE-enriched mantle lithosphere that may have been part of a mantle wedge above a subducting plate in an arc environment.

Actinide abundances in ordinary chondrites

Measurements of 244Pu fission Xe, U, Th, and light REE (LREE) abundances, along with modal petrographic determinations of phosphate abundances, were carried out on equilibrated ordinary chondrites in order to define better the solar system Pu abundance and to determine the degree of variation of actinide and LREE abundances. Our data permit comparison of the directly measured Pu/ U ratio with that determined indirectly as ( Pu/ Nd) × ( Nd/ U) assuming that Pu behaves chemically as a LREE. Except for Guareña, and perhaps H chondrites in general, Pu concentrations are similar to that determined previously for St. Séverin, although less precise because of higher trapped Xe contents. Trapped 130Xe 136Xe ratios appear to vary from meteorite to meteorite, but, relative to AVCC, all are similar in the sense of having less of the interstellar heavy Xe found in carbonaceous chondrite acid residues. The Pu/U and Pu/Nd ratios are consistent with previous data for St. Séverin, but both tend to be slightly higher than those inferred from previous data on Angra dos Reis. Although significant variations exist, the distribution of our Th/U ratios, along with other precise isotope dilution data for ordinary chondrites, is rather symmetric about the CI chondrite value; however, actinide/(LREE) ratios are systematically lower than the CI value. Variations in actinide or LREE absolute and relative abundances are interpreted as reflecting differences in the proportions and/or compositions of more primitive components (chondrules and CAI materials?) incorporated into different regions of the ordinary chondrite parent bodies. The observed variations of Th/U, Nd/U, or Ce/U suggest that measurements of Pu/U on any single equilibrated ordinary chondrite specimen, such as St. Séverin, should statistically be within ±20–30% of the average solar system value, although it is also clear that anomalous samples exist.

Composition and isotopic constraints on the petrogenesis of alkaline arc lavas: Lihir Island, Papua New Guinea

The SiO2‐undersaturated lavas from Lihir island, Papua New Guinea, like most arc lavas are highly enriched in Sr, Ba, K, Rb, and Cs and depleted in Hf, Ta, Nb, and Ti relative to ocean floor basalts and oceanic island basalts. These alkali‐rich lavas have arc trace element signatures and Nd, Sr, and Pb isotopic systematics. However, they are not a product of present‐day subduction, as this volcanism has tapped mantle which was enriched by prior subduction episodes. The narrow range of Pb (206pb/204pb, 18.74–18.76) and Nd (143Nd/144Nd, 0.51297–0.51304) isotopic compositions suggest a cogenetic origin for these lavas. During the fractionation of the primitive Lihir lavas, elements normally considered incompatible (i.e., the light rare earth elements (LREE), Rb, Th, and P) have high bulk solid/melt partition coefficients (0.15–1.5). Relatively higher partition coefficients during forma‐tion of the evolved lavas produced crossing rare earth element (REE) patterns, and primitive lavas have higher incompatible elements abundances than evolved lavas. This results from (1) changes in the amount of apatite in the fractionating assemblage and (2) the abnormal partitioning of trace elements into apatite microphenocrysts which nucleate at the crystal‐liquid interface of rapidly growing clinopyroxene phenocrysts. The Lihir lavas have lower alkali, Sr, Ba, K, Rb, Cs, and LREE abundances than other Tabar‐Feni lavas. They are derived from a less enriched mantle source rather than by a higher degree of melting of a source similar to that of the other islands. The similarity of Sm/Nd ratios of these undersaturated arc lavas to those of tholeiitic and calc‐alkaline arc lavas and the moderate chondrite‐normalized La/Yb (La/Ybcn = 3–7) indicates that there has been limited enrichment of the LREE relative to the heavy REE during generation of the arc‐modified source mantle. The alkaline nature of these lavas reflects their generation, in a tensional tectonic environment, from a “fossil” arc mantle region that has undergone extreme arc enrichment of alkali and alkaline earth elements during two earlier subduction episodes.

Geochemistry of Late Proterozoic basaltic rocks from southeastern Cape Breton Island, Nova Scotia

Late Proterozoic volcanic rocks of the Fourchu Group from the Avalon Zone in southeastern Cape Breton Island, Nova Scotia, are composed predominantly of mafic and felsic types with subordinate intermediate units that were all affected by subgreenschist- to greenschist-facies metamorphism. The rocks crop out in four fault blocks (Coastal, Stirling, East Bay Hills, and Coxheath) and have geochemical characteristics of ensialic orogenic volcanic suites. The basaltic rocks range from tholeiitic to calc-alkaline and show a distinct compositional zonation that resembles the across-arc variation observed in recent volcanic-arc systems. The variations include a progressive increase in abundances of light rare-earth elements, Th, Zr, Hf, Nb, and Ta and in the ratios of Zr/SiO2, Th/SiO2, Zr/Y, La/Yb, and Th/Hf from the Coastal block in the southeast to the Coxheath block in the northwest. The zonation may be explained in terms of a northwesterly-dipping subduction zone, with the trench lying to the southeast of Nova Scotia.

Fe-Rich Archean Tholeiites Derived from Melt-Enriched Mantle Sources: Evidence from the Kolar Schist Belt, South India

Tholeiitic amphibolites in the circa 2700 Ma Kolar Schist Belt, south India, are intimately interlayered with minor komatiitic amphibolites. The tholeiitic amphibolites, which are similar to other Archean tholeiitic rocks, are Fe-enriched, with varying from about 9 to 13 wt %. Olivine and plagioclase have similar calculated saturation surface temperatures ranging from about 1125 to 1225°C. On many trace and major element plots there is a distinct compositional gap between the komatiitic and tholeiitic amphibolites that cannot be explained by density filtering because the calculated melt densities for the komatiitic (2.73-2.79) and tholeiitic (2.61-2.72) amphibolites nearly overlap. The rare earth element (REE) patterns for the tholeiites are flatter and have lower light REE abundances than those for many of the associated komatiitic rocks. Modeling of major and trace element data suggests that the tholeiitic rocks are not related to the komatiitic rocks by: (1) differentiation of magmas of the komatiitic rocks, (2) melting of the komatiitic mantle source at the same depths at which the komatiites were derived, (3) differentiation of liquids produced by melting the komatiite source at shallow levels, or (4) shallow-level fractional crystallization (open or closed-system) of a pyrolite-derived parental melt common to both komatiite and tholeiite. The tholeiitic rocks appear to have resulted from sources melting at about 35 km depth with variable Fe/Mg ratios much greater than that of pyrolite. The most likely candidate for their source is an Archean litho-sphere previously intruded by variable amounts of mantle-derived melts.

Trace-element variations in acidic rocks from the inner Zone of southwest Japan

Rare-earth elements (REE), Rb, Sr and Ba abundances were measured for Cretaceous to early Paleogene acidic and related intermediate rocks (49 volcanic and 7 plutonic rocks) collected from a wide range of the Inner Zone of southwest Japan. Regional variations in both north-south and west-east directions and discrepancies with respect to age are not found in most of the trace-element concentrations, but Ba and light REE (LREE) abundances are roughly correlated with the longitude of the sampling site, increasing eastwards. Despite such regional variations, most of the acidic rocks have some common features showing LREE-enriched REE patterns with large negative Eu anomalies, enrichment of Ba relative to LREE, large and variable Rb/Sr ratios, and a good correlation between Eu/Eu★ and Sr concentration. Furthermore, most of the acidic rocks (69–74% SiO 2) tend to show significant trends: Eu/Eu★ values decrease with increasing Sm/Nd ratios, which are correlated with Lu concentrations. On the basis of model calculations, these trends as well as the REE patterns can be explained by a partial melting model in which source material having moderately LREE-enriched pattern melts to leave a gabbroic residue. It is also suggested that regional variations of La (LREE and Ba) may reflect regional heterogeneity of source materials.