Low Rare Earth Element Abundances Research Articles

Detection of Bastnäsite-Rich Veins in Rare Earth Element Ores Through Hyperspectral Imaging

Spectral indices are a well-established remote sensing tool showcased in many studies pertaining to fields such as agriculture, forest ecology, geology, soil sciences, vegetation, water resources, and urban development. In the case of geology, several indices already exist for the purpose of highlighting regions with interesting lithologies, such as iron oxides, aluminous clays, carbonates, and mafic to ultramafic minerals, based on signature absorption features in their reflectance spectra. However, in the literature, most indices often depend on the few, wide spectral bands used by spaceborne multispectral imagers such as Landsat 8 OLI, Terra ASTER, and Sentinel-2 MSI. Hyperspectral imagers are able to glean more detailed compositional information of a rock surface by operating with a myriad of narrow spectral bands, though the usage of these bands in spectral indices for mining and exploration purposes remains largely unexplored. We propose a new bastnäsite index capable of detecting the presence of rare earth elements and mapping their relative abundances across rock surfaces. Using four reference bands and four absorption bands in the visible to near infrared wavelength range, we apply this novel technique in a laboratory setting to ore samples extracted from the carbonatite body localized in the Sulfide Queen mine in Mountain Pass, California. Visual inspection is used to compare an index map to corresponding reflectance spectra for areas of known high and low rare earth element abundances, demonstrating its potential as a fast and straightforward means to assess rare earth element ore grade remotely.

Carbonatitic pockets in intra-ocean arc volcanics (Qilian orogen): Petrogenesis and implications for carbon recycling in subduction zones

Carbonatites in island arcs are crucial for understanding the processes of magmatism and carbon recycling in subduction zones. Here we report carbonatitic pockets in Ordovician intra-oceanic island-arc volcanic rocks and in Ocean island basalts (OIB) of Cambrian ophiolites in the South Qilian Accretionary Belt, NW China. All these carbonatitic pockets are composed of relatively pure calcites, some of which have accessory minerals of apatite, sphene, hematite, magnetite and allanite. The 87Sr/86Sr(i) ratios of calcite range from 0.703833 to 0.706732, concentrated at about 0.705, which is close to the composition of Enriched Mantle 1 (EMI) affected by fluids from the subduction zone. The C-O isotopic compositions show trends that evolve away from primary carbonatite (δ13CPDB = −9.0 ~ −1.0 ‰, 18OSMOW = 10.1– 16.5 ‰), probably caused by calcite fractional crystallization. High Sr, Th, U, Pb and low Nb, Zr, Hf, Ti contents and 87Sr/86Sr(i) ratios indicate that these carbonatitic pockets derived from carbonatite magma, and may have been altered by subduction-zone fluids. Carbonatitic pockets in ankaramites represent primary mantle-derived carbonate melts/liquids with low REE (rare earth elements) abundances (<10 ppm) and δ13CPDB. Elevated REE contents and LREE/HREE (light / heavy rare earth elements) ratios suggest that carbonatitic pockets in high-Al andesite may have experienced fractionation crystallization. The EMI-type carbonatitic pockets occurrence in the two different rock assemblages may reflect an interaction between an oceanic arc and an oceanic plateau, deriving from the recycling of Proterozoic marine carbonates through the lower mantle.

Early Cretaceous bimodal volcanic rocks in the Yinshan belt, North China Craton: age, petrogenesis, and geological significance

Determining the age and petrogenesis of the Mesozoic magmatic rocks may provide crucial evidence regarding the tectonic setting and evolution of the North China Craton (NCC). In this paper, we present new zircon U–Pb ages and geochemical data for bimodal volcanic rocks from Zhuozi area in the eastern Yinshan belt, NCC, to discuss their petrogenesis and tectonic implications. The volcanic rock suite consists of basalts, rhyolites, and trachytes. According to the results of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), the rhyolitic and trachytic rocks have zircon U–Pb ages of 136.9 ± 0.8 Ma and 134.0 ± 1.7 Ma, respectively, representing their eruption ages. The basalts are enriched in large-ion lithophile elements (LILE; Ba, Rb, Sr, and Th) and light rare-earth elements (LREE), and are relatively depleted in high field strength elements (HFSE; Nb, Ta, and Ti). In addition, the basalts are characterized by high fractionations of rare-earth elements (REE), with (La/Yb)N = 24.87–42.85. These features suggest that they were derived from low-degree partial melting of enriched lithospheric mantle metasomatized by subduction-related components. The rhyolitic rocks have low mean Al2O3 (12.26 wt%) and TiO2 (0.14 wt%) contents and Mg# (14.92) values, with relatively low total REE abundances. They have high alkali and Zr, Ce, Y, and Nb contents and high Ga/Al ratios, with notably positive Pb and negative Sr, Ba, and Eu anomalies observed in the primitive mantle-normalized trace element diagram. These features indicate that the rocks exhibit characteristics of A1-type granite and likely derived from the partial melting of a crustal source. The trachytic rocks are show moderate trace elements and SiO2, MgO, Fe2O3T, and TiO2 contents. They were possibly generated by fractional crystallization of the underplating basaltic magmas and assimilated minor crustal materials. The geochemical characteristics, in combination with the regional geology, suggest that the bimodal volcanic rocks formed in an intraplate extensional setting likely associated with the retreat of the subducted Paleo-Pacific plate.

A new chemical separation procedure for the determination of rare earth elements and yttrium abundances in carbonates by ICP-MS

The determination of rare earth elements (REEs) and Y in carbonates can be complicated by low REE abundances and the presence of significant amounts of Ba resulting in problematic interferences when analysed by ICP-MS. We describe here a novel ion-exchange method using the DGA resin (TODGA), combined with addition of a Tm spike, which allows the separation of the REEs+Y as a whole prior to analysis using an Element XR ICP-MS. This method was validated with results obtained on three different reference carbonate materials (CAL-S, JLs-1 and BEAN, an in-house standard), yielding reproducibility levels better than 3% (RSD) in most cases. This new separation scheme is particularly well suited for carbonate samples having very low REE contents, but could be equally applied to various rock types and organic-rich sample matrices whenever quantitative Ba removal is required.

Provenance and geochemical variations across the Ediacaran–Cambrian transition in the Soltanieh Formation, Alborz Mountains, Iran

AbstractThe Soltanieh Formation in the Alborz Mountains of northern Iran is not only a key lithostratigraphic unit for reconstruction of the Iranian geological history, but also a globally outstanding succession to reveal variations in seawater composition across the Precambrian–Cambrian (PC–C) transition. Mineralogical and geochemical data from a continuous stratigraphic record of Lower and Upper Shale members of the Soltanieh Formation are used to define their provenance, tectonic setting as well as geochemical variations during the PC–C transition. The Soltanieh mudrocks are composed of quartz and plagioclase, with minor constituents of illite, chlorite and montmorillonite. The chemical index of alteration, A-CN-K (Al2O3– CaO + Na2O – K2O) relations, index of compositional variability, and Th/Sc versus Zr/Sc ratios indicate low chemical weathering in source areas, compositionally immature and first-cycle sediments. Immobile trace-element ratios and discrimination diagrams, chondrite-normalized rare Earth element (REE) patterns and negative Eu anomaly, along with low total REE abundances and negligible Ce anomalies, demonstrate that the Soltanieh Formation was mainly derived from proximal felsic-intermediate Cadomian magmatic arc sources and deposited in a continental-arc-related basin on the proto-Tethyan active margin of Gondwana. The palaeoredox indicators exhibit a remarkable change in environmental condition from a suboxic to an oxic state across the PC–C transition from the Kahar Formation to the Upper Shale Member of the Soltanieh Formation. Moreover, a significant upwards increase of P, Ba, and Ca is likely associated with enhanced fluxes of nutrient elements during the PC–C transition, coeval with the building of collisional mountain belts during the amalgamation of Gondwana.

Improved Precision and Accuracy of Quantification of Rare Earth Element Abundances via Medium-Resolution LA-ICP-MS.

Laser ablation ICP-MS enables streamlined, high-sensitivity measurements of rare earth element (REE) abundances in geological materials. However, many REE isotope mass stations are plagued by isobaric interferences, particularly from diatomic oxides and argides. In this study, we compare REE abundances quantitated from mass spectra collected with low-resolution (m/Δm = 300 at 5% peak height) and medium-resolution (m/Δm = 2500) mass discrimination. A wide array of geological samples was analyzed, including USGS and NIST glasses ranging from mafic to felsic in composition, with NIST 610 employed as the bracketing calibrating reference material. The medium-resolution REE analyses are shown to be significantly more accurate and precise (at the 95% confidence level) than low-resolution analyses, particularly in samples characterized by low (<μg/g levels) REE abundances. A list of preferred mass stations that are least susceptible to isobaric interferences is reported. These findings impact the reliability of REE abundances derived from LA-ICP-MS methods, particularly those relying on mass analyzers that do not offer tuneable mass-resolution and/or collision cell technologies that can reduce oxide and/or argide formation. Graphical Abstract ᅟ.

통가열도 해저화산 표층 퇴적물 내 열수변질의 화학적 특성

통가열도 내 우리나라 해저열수광상 독점 탐사지역에 위치하는 5개 해저화산(TA12, TA19, TA22, TA25, TA26) 주변 표층 퇴적물의 열수변질 여부를 규명하고, 열수변질 특성을 확인하기 위하여 각 해저화산에서 채취된 29개의 표층 퇴적물에 대해 미량 원소와 희토류 원소 분석을 실시하였다. 분석 결과 TA12, TA19, TA22 해저화산에서는 열수에 의한 퇴적물의 변질작용이 없거나 매우 적은 것으로 나타났으며, TA25와 TA26 해저화산의 일부 지역은 현재에도 열수의 영향을 받고 있는 것으로 나타났다. 열수의 영향이 있는 해저화산 사이에도 부화된 미량 원소의 종류에 차이가 있는데, TA25 해저화산의 경우 주로 Ni, Cu, Sn, Zn, Pb, Cr, Cd, Sb, W, Ba, Ta, Rb, Sr, As가, TA26 해저화산에서는 주로 Cu, Sn, Zn, Pb, Cd, Sb, Ba, Rb, Sr이 부화되어 있다. 미량 원소의 부화가 확인된 지역은 희토류 원소의 부화도 관찰된다. 상부지각 희토류 원소 농도로 표준화된 희토류 원소의 분포형태는 가벼운 희토류 원소(LREE)가 매우 낮고 중간 희토류 원소(MREE)에서 무거운 희토류 원소(HREE)로 갈수록 증가하는 형태를 보이며, TA25와 TA26 해저화산의 일부 정점에서 Eu의 부화가 확인된다. 또한 TA26 해저화산의 일부 정점에서 Ce이 부화가 관찰되었는데, 이는 주변지역에서 채취된 열수 내 희토류 원소의 높은 Ce분포형태와 매우 유사하다. 또한 TA25와 TA26 해저화산은 부화된 원소의 특징이 다르게 나타나는데, TA25 해저화산에서는 Cu를 포함한 미량 원소가, TA26 해저화산에서는 Ce와 Eu를 포함한 희토류 원소가 우세하게 부화되어 있는 것으로 나타났다. 조사결과 나타난 각 해저화산 표층 퇴적물 내 부화된 미량 원소 및 희토류 원소의 종류와 농도는 차후 통가열도 지역의 열수광상 탐사시에 지역적인 열수변질의 지시자로써 유용하게 활용될 것으로 판단된다. We analyzed 29 surface sediment samples in five submarine volcanoes (TA12, TA19, TA22, TA25, and TA26) located in the southern part of the Tonga arc for trace elements and rare earth elements to investigate characteristics of the hydrothermal alteration of surface sediments. Based on analytical results of trace element and rare earth element (REE), surface sediments of TA12, TA19, and TA22 submarine volcanoes, which are located in the northern part of the study area, were very little or not influenced by hydrothermal fluids. In contrast, some stations of TA25 and TA26 submarine volcanoes were strongly affected by hydrothermal fluids. However, these two submarine volcanoes showed different features in element concentration in the sediments. Some stations of TA25 submarine volcano showed enrichment of Ni, Cu, Sn, Zn, Pb, Cr, Cd, Sb, W, Ba, Ta, Rb, Sr, and As, however, those of TA26 submarine volcano showed enrichment of Sn, Zn, Pb, Cd, Sb, Ba, Rb, and Sr. Stations which enriched trace elements were observed, enriched REEs were also observed. Average upper continental crust (UCC)-normalized REE patterns of the surface sediments generally showed low light REE (LREE) abundances and increased heavy REE (HREE) abundances. Eu enrichment was identified at several stations of TA25 and TA26 submarine volcanoes. In addition, enrichment of Ce was found at some stations of TA26 submarine volcano and these enrichment patterns were similar with hydrothermal fluid of near stations. Furthermore, TA25 and TA26 submarine volcanoes showed different enrichment characteristics of trace elements and REE. Trace elements were concentrated at TA25 submarine volcano. TA26 submarine volcano, on the other hand, observed highly enrichment of REE especially, Eu and Ce. As a result of the investigation, the characteristics and concentrations of REEs and trace elements in the surface sediments of each submarine volcano can be applied to identify hydrothermal alteration of sediments during exploration for hydrothermal deposits.

Mafic forearc cumulates and associated rocks in the central high-pressure belt of the Acatlán Complex of southern México: geochemical constraints

The high-pressure (HP) Piaxtla Suite at Tehuitzingo contains peridotites, gabbros, and serpentinized peridotites, as well as granitoids and metasedimentary rocks. The HP mafic rocks are characterized by low SiO2 (38–52 wt.%) and high Mg# (∼48–70), Ni (100–470 ppm), and Cr (180–1750 ppm), typical of cumulate compositions. Trace elements and rare earth element (REE) primitive mantle-normalized patterns display generally flat profiles, indicative of derivation from a primitive mantle with two distinct patterns: (1) gabbroic patterns are characterized by a positive Eu anomaly, low REE abundances, and slightly depleted high REE (HREE) relative to low REE (LREE), typical of cumulus olivine, pyroxene, and plagioclase; and (2) mafic-intermediate gabbroic patterns exhibit very flat profiles characteristic of olivine and clinopyroxene as cumulus minerals. Their Nb/Y and Zr/TiO2 ratios suggest a subalkaline character, whereas low Ti/V ratios indicate that the Tehuitzingo cumulates are island arc tholeiitic basalts that resemble modern, immature oceanic, forearc magmas. These cumulates have high values of ϵ Nd(t) = 5.3–8.5 and 147Sm/144Nd = 0.18–0.23, which renders calculations of model ages meaningless. Our data are consistent with the Tehuitzingo arc rocks being part of a tectonically extruded Devonian–early Carboniferous arc developed along the west margin of Gondwana.

Trace and rare earth elements as indicators of provenance and depositional environments of Lias cherts in Gumushane, NE Turkey

Trace elements and rare earth elements (REEs) of Lias-aged cherts in the Gumushane area were studied in order to understand their origin and depositional environment. Twenty three chert samples from five stratigraphic sections were analysed by inductively coupled plasma-mass spectrometry, X-ray diffraction, and mineralogical investigation. Lias cherts in the study area are microcrystalline, cryptocrystalline quartz, and megaquartz depending on mineralogical content. Trace elements of the cherts were compared with PAAS, Co, Y, and Th had stronger depletions in the five sections, whereas V, Ni, Zr, Nb, and Hf had smaller depletions. The distribution of Zr, Hf, and Ta yields Zr/Hf, Zr/Ta and Hf/Ta ratios (25/645, 37/665, and 0.18/3, respectively) that differ from those of chondrites and average upper continental crust, suggesting that these elements are likely non-detrital but are sourced from seawater. Th/U ratios range from 0.04 to 0.45 and are lower than those of the upper continental crust (average: 3.9). Lias-aged cherts have low total REE abundances and stronger depletions in five sections of the PAAS and chondrite-normalised plots. The cherts are characterised by a positive Eu anomaly (average: 4.9) and LREE-enrichment (LaN/YbN=average: 3.5). In addition, about one-half of the cherts exhibit positive Ce anomaly (range: 0.25–2.58), chondritic Y/Ho values (range: 3.3–60), and low (La/Ce)N values (average: 1.8). REE and trace element abundance in Lias cherts indicate that these elements were likely derived from hydrothermal solutions, terrigenous sources, and seawater. The REE patterns of the cherts show that they were probably deposited close to a continental margin.

Shallow slab fluid release across and along the Mariana arc‐basin system: Insights from geochemistry of serpentinized peridotites from the Mariana fore arc

Shallow slab devolatilization is not only witnessed through fluid expulsion at accretionary prisms, but is also evidenced by active serpentinite seamounts in the shallow fore‐arc region of the Mariana convergent margin. Ocean Drilling Program (ODP) Leg 195 recovered serpentinized peridotites that present a unique opportunity to study the products of shallow level exchanges between the upper mantle and slab‐derived fluids. Similar to samples recovered during ODP Leg 125, the protoliths of these fore‐arc serpentinized peridotites are mantle harzburgites that have suffered large volume melt extraction (up to 25%) prior to interactions with fluids released from the downgoing Pacific Plate. Samples recovered from both ODP legs 125 and 195 show U‐shaped rare earth element (REE) patterns and very low REE abundances (0.001–0.1 × chondrites). Relative to global depleted mantle values these rocks typically have 1–2 orders of magnitude lower high field strength elements, REE, Th, and U contents. Interestingly, all fore‐arc rocks thus far examined show extreme enrichments of fluid mobile elements (FME: B, As, Cs, Sb, Li). Because the elemental and B, Li, and Sr isotope systematics in these fore‐arc serpentinites point to nonseawater‐related processes, studies of elemental excesses and anomalous isotopic signatures allow assessment of how much of the subducted inventory is lost during the initial subduction process between 10 and 40 km. On the basis of similar but substantial enrichments of FME in the Mariana fore‐arc samples recovered at ODP legs 125 and 195, we report large slab inventory depletions of B (∼75%), Cs (∼25%), As (∼15%), Li (∼15%), and Sb (∼8%); surprisingly low (generally less than 2%) depletions of Rb, Ba, Pb, U, Sr; and no depletions in REE and the high field strength elements (HFSE). Such slab‐metasomatized mantle wedge materials may be dragged to depths of arc magma generation, as proposed by Tatsumi (1986) and Straub and Layne (2002) and thus represent an unexplored class of mantle material, different in its origins, physical properties and geochemical fingerprint from mantle rocks traditionally used in modeling a wide range of subduction zone processes.

Supra-subduction Zone Pyroxenites from San Jorge and Santa Isabel (Solomon Islands)

Peridotites associated with pyroxenites (with rare olivine and spinel) are exposed on the islands of San Jorge and Santa Isabel in the Solomon Islands. Orthopyroxenite occurs in large outcrops (≤100 m2) whereas websterite and clinopyroxenite occur as layers and veins/dykes in peridotites. The bulk compositions of the pyroxenites are characterized by high Mg2+/(Mg2+ + Fe2+) (0·78–0·91) and low Al2O3 (<2·7 wt %). Low rare earth element abundances are coupled with large ion lithophile element enrichments and positive Sr and Pb anomalies (primitive mantle-normalized) relative to adjacent rare earths. Temperatures of equilibration for the pyroxenites are between 950 and 1050°C. These relatively low temperatures, combined with the occurrence of primary fluid inclusions, suggest that the pyroxenites formed by interaction of peridotite protoliths with an aqueous fluid. Bulk-rock and mineral compositions of the orthopyroxenites are similar to those of mantle-derived pyroxenites, whereas the websterites have closer chemical affinity with crustal arc cumulates. Nevertheless, field relationships plus petrological, textural and geochemical evidence are consistent with formation of all pyroxenite types in supra-subduction zone mantle, resulting from metasomatism of peridotite by subducted Pacific Plate-derived fluid. Such a setting for pyroxenite has not previously been reported in detail. We propose that these processes produce mantle pyroxenite with compositions similar to crustal pyroxenite.

Trace element fractionation during high-grade metamorphism and crustal melting—constraints from ion microprobe data of metapelitic, migmatitic and igneous garnets and implications for Sm–Nd garnet chronology

Rare earth element (REE) and other trace element (Y, Sr, Ti, Cr, V, Na) abundances in garnet from a garnet-bearing metapelite, a pelitic migmatite, a syn-tectonic granite and a post-tectonic leucogranite were measured by secondary ion mass spectrometry (SIMS) in order to identify the effective variables on the trace element distribution between garnet and the host rock. Garnet from the garnet-bearing metapelite, the pelitic migmatite and the syn-tectonic granite is zoned with respect to REE. The cores are enriched by a factor of 2–3 relative to the rims. For the garnets from the garnet-bearing metapelite equilibrium distribution following a simple Rayleigh fractionation is responsible for the decreasing concentrations in REE from core to rim. Garnet from the pelitic migmatite shows a more complex trace element pattern following distinct enrichment and depletion patterns for Ti, V, Cr and REE from core to rim. These features suggest disequilibrium between garnet and the associated melt in which the enrichment of trace elements probably correspond to a period of open-system behaviour in these rocks at a time when the garnet, originally nucleated in the metamorphic environment was incorporated into the melt. The garnet from the syn-tectonic granite shows stepwise decreasing concentrations in REE from core to rim: a REE-rich core can be distinguished from a broad REE-depleted rim. Notably, from core to rim an inflection of the Yb / Er and Yb / Dy ratios is visible. Whereas the decrease of HREE abundance in the core region of the garnet from the syn-tectonic granite may arise from equilibrium partitioning during garnet growth, the inflection can be interpreted as a result of partial melting. Garnet cores with high Yb / Er and Yb / Dy > 1 nucleated in the metamorphic environment without the presence of a melt whereas the rims with lower Yb / Er and Yb / Dy < 1 crystallized in the presence of a melt. Garnet from the leucogranite has lower REE abundances and is considered to be of igneous origin. In contrast to garnet from the other samples, its core has low trace element abundances, whereas its rim is significantly enriched in REE but depleted in Ti. These features suggest that only the outermost rim was in equilibrium with the melt. For this garnet, liquid diffusion controlled partitioning is more likely to explain the extreme trace element variation. An evaluation of Sm and Nd concentrations in garnet and a comparison of Sm–Nd and U–Pb garnet ages and U–Pb monazite ages form the terrane indicate that the observed LREE systematics in the different garnet species are a primary feature and are not homogenized by volume diffusion during high grade amphibolite facies conditions.

MELT RE-FERTILIZATION IN HESS DEEP PERIDOTITES

Abyssal peridotites drilled from Hess Deep, at the intersection of the East-Pacific Rise and the Galapagos spreading center, are mostly strongly depleted, with high Cr# in spinel, low modal abundances of CPX and very depleted pyroxene mineral compositions. They have very low Ti, Na and Al in both OPX and CPX relative to most abyssal peridotites and have very low rare earth element abundances. Modal data for this suite show that they are harzburgites, with olivine 80 ±3, OPX 18 ±2, CPX 2±1, and spinel ~1 volume per cent. (Dick and Natland, 1996). A subset of peridotites from the Hess Deep have been re-fertilized in some melt components, resulting in large enrichments in Na in CPX, and these samples, as well as some very depleted ‘typical’ Hess Deep peridotites, have been analyzed for trace element abundances in CPX by ion probe. Enrichment factors in the re-fertilized samples range up to 35x for Na, 5x for Ti, greater than 140x for Ce, 80x for Sm, 2.5x for Yb, >10 for Sr and 40x for Zr. All of our samples are spinel harzburgite, with no plagioclase or alteration after plagioclase recognized, though parts of these cores do contain plagioclasebearing dunite and harzburgite. Enrichments are extremely localized, with multiple thin sections taken from one rock sample showing different degrees of enrichment. The incompatible trace element enrichments are accompanied by only minor shifts in Mg# in pyroxenes and olivines, suggesting that these rocks interacted with melts much more enriched than the last melts that had been extracted from them during fractional melting, with relatively minor outright trapping of melt into these rocks. Rare earth element ratios, such as (Ce/Sm)n and (Sm/Yb)n, vary by a factor of ~30. Strong enrichments in light and medium rare earth elements (REEs) are accompanied by minor shifts in heavy REEs, producing the steep trend seen in Figure 1. Cr-spinels in most Hess Deep peridotites have Cr#’s at 51 to 55, but one of our samples with metasomatic enrichments has been shifted to a Cr # of 46.6. Ti in spinel in these samples shows only minor effects from the metasomatic interactions. Modal abundances of CPX remain quite low in the metasomatized samples from this suite, ranging from ~1 to ~3.5 %, with the highest value in the most enriched sample. Melt-rock reactions and melt-trapping clearly have led to substantial enrichments in various elements in some samples from Hess Deep, but it remains unclear to what extent these processes have effected abyssal peridotites worldwide. Figure 2 shows Na2O versus TiO2 in abyssal peridotites, along with a fractional melting curve, and it is clear that abyssal peridotite CPX compositions in the whole suite are not consistent with melting as the sole process that has determined their compositions. Incorporating dynamic melting models into the calculation does not change this conclusion. Nor does incorporating more complex functions for changing distribution coefficients with pressure. Models that attempt to explain the overall geochemistry of abyssal peridotites include porous reactive flow (Asimow, 1999) or melt trapping into more extensively melted peridotite (Elthon, 1992), but no models have yet fully explained the geochemistry of these rocks.

Trace element distribution in calcite–dolomite carbonatites from Spitskop: inferences for differentiation of carbonatite magmas and the origin of carbonates in mantle xenoliths

We have studied trace element distribution in intrusive calcite–dolomite carbonatites from South Africa, in which primary igneous textures are remarkably well preserved. These rocks contain few non-carbonate minerals and show no effects of crustal contamination. The trace element compositions of carbonates and fine-grained interstitial aggregates were determined in situ by laser ablation inductively coupled plasma mass spectrometry while whole rock samples were analyzed by solution inductively coupled plasma mass spectrometry. A range of rare earth element (REE) contents and various distribution patterns have been found for minerals in each rock. Calcite phenocrysts typically have low REE abundances (1–20×primitive mantle) and nearly flat primitive mantle-normalized distribution patterns with no or minor enrichments in light REE. In comparison, interstitial calcite in the same rocks is strongly enriched in light over heavy REE (La/Yb N≈200). Fine-grained interstitial material (apatite-bearing) has high contents of REE and of highly incompatible trace elements. These in situ analyses have documented significant chemical differences between distinct textural groups of carbonate minerals (phenocrysts and interstitial grains) which must be related to their crystallization sequence. These results indicate incompatible behavior of REE during fractional crystallization of carbonatite liquids and suggest strong enrichments of residual liquids in highly incompatible elements. Sr abundances show relatively little variation, consistent with carbonate/melt partition coefficients higher than for REE but close to or less than unity. The REE abundances in calcite phenocrysts are nearly as low as those reported for individual carbonate grains from mantle peridotite xenoliths and are lower than in composite carbonate-rich pockets found in some xenoliths. These results strongly indicate that the carbonates in mantle xenoliths are crystal cumulates from primitive carbonate-rich melts rather than quenched carbonatite liquids. Furthermore, relatively low REE abundances (and high Sr) may be common in primary mantle-derived carbonate liquids. Strong enrichments in highly incompatible elements found in many carbonatites may require complex fractionation of initial liquids during ascent as well as derivation from highly enriched mantle domains.

Formation of orange hibonite, as inferred from some Allende inclusions

Abstract— We studied three fluffy Type A refractory inclusions from Allende that contain orange hibonite. The melilite in the present samples is very Al‐rich, averaging Åk6, Åk14, and Åk12 in the three samples studied. Hibonite in two inclusions, unlike that in Murchison, has low rare earth element abundances of &lt;10 × CI; in the other inclusion, the hibonite, melilite and perovskite have Group II‐like patterns. The hibonite and melilite in all three inclusions studied have excess 26Mg consistent with (26Al/27Al)I = 5 × 10−5. Much of the hibonite and some of the spinel in these inclusions is corroded. These phases are found enclosed in melilite, but based on bulk compositions and phase equilibria, hibonite should not be an early‐crystallizing phase in these inclusions. We conclude that the hibonite and probably some of the spinel is relic. Reversely zoned melilite, rounded spinel and isotopically heavy Mg in the inclusions probably reflect reheating events that involved melting and evaporation. Alteration of the gehlenitic melilite gave rise to some rare phases, including corundum and nearly pure CaTs pyroxene.Studies have shown that blue hibonite contains Ti3+ while orange hibonite does not (Ihinger and Stolper, 1986; Beckett et al., 1988). Orange hibonite formed either under oxidizing conditions (such as at oxygen fugacities at least seven orders of magnitude greater than that of a solar gas at 1700 K), or under conditions reducing enough (e.g., solar) that it contained Ti3+, which was later oxidized in situ. Although V and Ce oxides are volatile at the temperature and range of oxygen fugacities at which orange hibonite is known to be stable, we find that (a) the hibonite is V‐rich (∼1 wt% V2O3) and (b) there are no negative Ce anomalies in Allende hibonite. This indicates that the hibonite did not form by condensation under oxidizing conditions. In addition, there are slight excesses of Ti + Si cations relative to Mg + Fe cations (up to 0.1 of 0.8 cations per 19 oxygen anions), probably reflecting the original presence of Ti3+. The results of this study strongly support the suggestion (Ihinger and Stolper, 1986) that Allende hibonite originally formed under reducing conditions and was later oxidized. Oxygen fugacities within ∼2–3 orders of magnitude of that of a solar gas are implied; otherwise, strong Ce and V depletions would be observed.

Characteristics of rare-earth elements, Ba, Sr and Rb abundances in natural zeolites.

Abundances of rare earth elements (REE), Ba, Sr and Rb were determined for several kinds of zeolites occurred in cavities of volcanic rocks from Japan with relevance to peculiar crystal chemical properties of zeolites. Because of the scarcity of REE data of zeolites, the following points would be noteworthy: (1) REE abundances for those zeolites are extremely low, ranging from about 0.01 to 0.1 in chondrite normalized unit; (2) Mutual fractionation among REE is rather small aside from small Eu and Ce anomalies; (3) Three different zeolite species from Maze area, Niigata Prefecture, show considerable resemblance with respect to REE, Ba and Sr concentrations. It is suggested that the REE incorporation in zeolites seems to be affected by their genetic process or formation environment rather than by difference in zeolite species. Moreover, relatively low REE abundances in zeolites may reflect extremely low REE abundances in the solutions from which those zeolites were precipitated. The Ce anomalies for the Maze samples show that those zeolites have not been deposited from magmatic solutions but from meteoric water.

Classification of mafic clasts from mesosiderites: Implications for endogenous igneous processes

We have analyzed thirteen igneous pebbles from the Vaca Muerta, EET87500 and Bondoc mesosiderites by electron microprobe and instrumental neutron activation and combined these data with literature data for forty-three analyzed mesosiderite clasts. We classify these well-characterized clasts into the following five principal groups: 1. 1) Polygenic and monogenic cumulates (39%) are coarse-grained gabbros that are highly depleted in incompatible elements (relative to H chondrites); they formed at moderate depth either as residues of low-degree partial melting of pre-existing cumulate eucrites or as cumulates from parent melts similar to cumulate eucrites. 2. 2) Polygenic basalts (30%) are finer-grained rocks with positive europium anomalies, La/Lu ratios < 1, and lower rare earth element abundances than basaltic eucrites. It seems likely that these rocks were formed near their parent body surface by remelting mixtures of major amounts of basaltic eucrites and lesser amounts of cumulate eucrites. 3. 3) Quench-textured rocks comprise two compositional groups, (a) those which resemble basaltic eucrites (5%) and (b) those which resemble cumulate eucrites (2%). The quench-textured rocks are probably monogenic; they formed most likely when small-scale impacts at their parent body surface totally melted small amounts of basaltic or cumulate eucrite material. 4. 4) Monogenic basalts (11%) resemble basaltic eucrites and formed by endogenous igneous processes on the mesosiderite parent body (MPB). The monogenic basalts and those quench-textured rocks that resemble basaltic eucrites in composition plot subparallel to the Nuevo Laredo trend eucrites on diagrams of incompatible element concentration vs. mg ∗. Previous work has shown that the Nuevo-Laredo trend eucrites were probably produced by fractional crystallization; it seems likely that the monogenic basalts from the MPB had a similar origin. 5. 5) Ultramafic rocks are cumulates consisting mainly of large crystals of orthopyroxene (9%) or olivine (4%). Orthopyroxenite clasts closely resemble diogenites and were formed most likely by endogenous igneous processes. Olivine crystals vary significantly in major element composition; some may be endogenous, but others may have been derived from the disrupted mantle of a different parent body.

Pre-Keweenawan anorthosite inclusions in the Keweenawan Beaver Bay and Duluth Complexes, northeastern Minnesota

Large numbers of calcic anorthosite inclusions varying in size from centimetres to several hundred metres occur in the Beaver River gabbro unit of the Beaver Bay Complex exposed along the North Shore of Lake Superior. Lesser numbers of similar inclusions occur in the Duluth Complex. Both host rocks crystallized at about 1.13 b.y. B.P. and are of Keweenawan age. The inclusions have been divided into four groups, based on outcrop and petrographic data, as follows: (1) Tectonite inclusions that were sheared and recrystallized. The tectonites consist of plagioclase (An 76–78 ), minor to accessory olivine (Fo 78 ), and accessory magnetite-ilmenite. (2) Igneous inclusions that have igneous structures and textures. Plagioclase ranges in composition from An 54 to An 80 . Principal mafics are augite (Wo 40 En 45 Fs 15 ) and orthopyroxene (Wo 2 En 80 Fs 18 ), but more iron-rich pyroxenes (Wo 38 En 40 Fs 22 ), including pigeonite (Wo 10 En 55 Fs 35 ), occur as much smaller grains in most igneous inclusions and are comparable to pyroxenes in host rocks. Textural evidence and mineral chemistry suggest that the Fe-rich pyroxenes, as well as the more albitic plagioclase, represent contamination. (3) Intermediate inclusions whose textures are intermediate between the tectonites and igneous rocks. Some of these appear to have been incompletely recrystallized, and some were modified to more igneous-appearing rocks by reaction with host rocks. (4) Cataclastic and brecciated inclusions having brittle deformation that is not apparent in the host rocks. Representatives of each group were analyzed for rare-earth elements (REE) and Sm-Nd, Rb-Sr isotopes. The chemical data correlate with the textural groups. For example, tectonites have low REE abundances with Nd about one times chondrites, whereas those in the igneous-textured group have higher REE abundances with Nd up to as much as seven times chondrites. Isotopic data show that the isotopic systems of the inclusions were disturbed and that contamination was introduced. Tectonites have 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios that are less than those of the Beaver River gabbro when recalculated to 1.13 b.y. Igneous-textured inclusions have 143 Nd/ 144 Nd ratios that are lower than tectonite values at 1.13 b.y., but 87 Sr/ 86 Sr ratios vary widely, with some being greater than ratios in the host Beaver River gabbro. None of the inclusions has an isotope ratio that would allow a co-magmatic relationship with either the Duluth Complex or the Beaver River gabbro. Sm-Nd data from an incompletely equilibrated inclusion suggest recrystallization between 1.14 b.y. B.P. and 1.9 b.y. B.P., perhaps as a result of the 1.8 b.y. B.P. Penokean event. Calculations of a crystallization age for the inclusions are speculative but suggest crystallization at or before 1.9 b.y. B.P.

Proterozoic age and cumulate origin for granulite xenoliths, Lesotho

Rare earth element (REE), 143Nd/144Nd and 87Sr/86Sr analyses on a range of granulite xenoliths from kimberlite pipes in Lesotho demonstrate that useful whole rock ages can be obtained from such lower crustal material. The characteristic positive Eu-anomalies, relatively low REE abundances and large variation in Sr contents suggest that many of the xenoliths are crystal cumulates rather than residua after partial melting. They yield an Sm–Nd whole rock age of 1,400 ± 100 Myr and are unrelated to the younger magmatic rocks of the Karoo.

Lewisian gneiss geochemistry and Archaean crustal development models

The geochemistry of Lewisian amphibolite-facies gneisses from northwest Scotland is described with particular reference to the rare earth elements (REE) and compared with the geochemistry of Lewisian granulite-facies gneisses. The results show that there are no significant differences between “Laxfordian” amphibolite-facies and “Scourian” granulite-facies gneisses in terms of REE and other immobile trace elements (at equivalent silica levels), although the mobile radioactive heat-producing elements, K, Rb, Th, U, are significantly lower in the granulites. In both types the basic gneisses have moderately fractionated REE patterns while the intermediate and acid gneisses have strongly fractionated REE patterns with low heavy REE abundances and decreasing levels of total REE with increasing SiO 2. The most silicic gneisses develop large positive europium anomalies. These gross chemical similarities between gneisses from intermediate (amphibolite-facies) and lower (granulite-facies) crustal levels constrain models for the evolution of the Archaean crust. The depletion of K, Rb, Cs, Th and U in granulites, but not other incompatible trace elements cannot be explained by magmatic processes. The positive Eu anomaly in the more siliceous gneisses of both facies is a function of the primary processes of crustal generation and not secondary processes such as intracrustal melting or fractional crystallisation. Fractionation of radioactive heat-producing elements from other trace elements is a result of granulite-facies metamorphism with these elements being removed by an active fluid phase. The apparent lack of partial melting in lower crustal granulites suggests a model for Archaean crustal growth largely through underplating by primary tonalitic magmas.