Light Rare Earth Element Depletion Research Articles

An Integrated Apatite Geochronology and Geochemistry Tool for Sedimentary Provenance Analysis

AbstractSediment provenance studies utilizing detrital geochronology are often limited by source‐rock non‐uniqueness with respect to mineral age. Thus, the development of integrated techniques which permit identification of source rock age and lithology are highly desirable. In this case study from the southern Massif Central, France, we target modern‐river sediment from the river Tarn to assess the utility of combined U‐Pb and multiple trace‐element geochemical analysis of detrital apatite as a provenance tool. The study area was chosen because the sediment source areas chiefly comprise a relatively simple mix of medium‐ to low‐grade Variscan metasediments and late Variscan granitoids, which should yield detrital apatite readily distinguishable by age, trace‐element chemistry, or both. Based on comparison with previously published apatite trace‐element data from metasedimentary rocks and granitoids, pelitic apatite in the river Tarn detritus is primarily distinguished by high Sr/Mn, light rare‐earth element depletion (LREE) and low actinide contents, whereas granitic apatite is characterized by much lower Sr/Mn, and high LREE and actinide abundances. These source rock determinations are highly consistent with apatite trace‐element data from Tarn tributaries that drain either predominantly metapelitic or granitoid catchments. U‐Pb analysis of detrital rutile was also undertaken on those catchments for comparative purposes. As pelitic and granitic apatite can be readily distinguished, samples that have experienced downstream sediment mixing can then be comprehensively characterized. Using this method we identify a source lithology for nearly every analyzed apatite grain in the river sediment, even though 59% of the analyzed grains do not yield reliable U‐Pb ages.

Geology and geochemical constrains on the origin and depositional setting of the Kpwa–Atog Boga banded iron formations (BIFs), northwestern Congo craton, southern Cameroon

The Kpwa–Atog Boga (KAB) area lies within the Paleoproterozoic Nyong unit which defines the northwestern edge of the Archean Congo craton in southern Cameroon. Rock types in the area are mainly comprises of garnet gneiss associated with banded iron formations (BIFs). The BIFs and its host rocks were subjected to amphibolite facies metamorphism. The Rare Earth Elements (REE) patterns and the discrimination ratios (e.g., Al2O3/TiO2 Cr/Th Th/Sc La/Sc) of the garnet gneiss suggest that they are metasedimentary rocks derived from a felsic parent sources that have undergone intensive weathering. The KAB BIFs exhibit significant concentration of TFe2O3 + SiO2 (>93 wt%), and lower Al2O3 and TiO2 contents and high field strength elements (e.g., Hf, Nb, Ta, and Zr). This involves little contamination with continental detritus at their depositional environment. The Post Archean Australian Shale (PAAS)-normalized REE patterns of the KAB BIFs show enrichment in heavy REE and the depletion in light REE with positive La anomalies, which imply that these BIFs precipitated in ambient seawater. Moreover, the contribution of low-temperature hydrothermal fluids during the deposition of the studied BIFs were highlighted through their high superchondritic Y/Ho ratio (mean: 31.48), weak positive Eu/Eu∗(SN) ratio (mean: 1.83) and the low contents of transition metals. This suggests that the geochemical characteristics of the KAB BIFs and those of the Paleoproterozoic superior-type BIFs are almost similar. In summary, low-temperature hydrothermal fluids mixed with seawater contributed to the deposition of the KAB BIFs on the continental margin. This depositional setting similar to its host rocks is consistent with the Paleoproterozoic geological history recorded within the Nyong unit. This preliminary study reveals that the KAB BIFs are associated with low grade gold (<5 ppb), which could constitute further prospective mineralization besides the iron ore in the KAB area.

Geochemistry of the Eocene limestones of the Jaisalmer basin, Rajasthan, India: Implications on depositional conditions and sources of rare earth elements

Major, trace and rare earth elements (REE) concentration of the Eocene limestones, Jaisalmer Basin, Rajasthan, India are analysed to reconstruct the depositional conditions and to identify sources of REEs. Among the major oxides, CaO is the dominant oxide followed by SiO2 in the studied limestones. Trace element Ba dominates over the other trace elements and it shows negative correlation with CaO. The Sr, occurring in small concentration, shows positive correlation with CaO. Other trace elements such as V, Zr, Sc, Y, Rb, Ni, Pb Co, Cu, U occur in small concentrations. The studied limestones show a positive correlation of ΣREE with Fe2O3, Ni, Th, Sc, and Y. These limestones possess sea-water like shale-normalized REE + Y pattern with light REE depletion, slight Gd enrichment, slightly positive La anomaly, positive Y anomaly, positive Eu anomaly, negative Ce anomaly and superchondritic Y/Ho ratio from 23.12 to 28.57. The dominance of CaO and low percentage of MgO suggest that mineral phase is calcite and there is absence of dolomitization. The occurrence of SiO2 and Al2O3 in appreciable percentages may be because of the siliciclastic input during the limestone precipiatetion. The low concentration of Uranium (0.4-3.7) and authigenic Uranium (Average Total U-Th/3 value = 0.74) indicate that the studied limestones were precipitated in oxic condition from seawater. The depletion of LREE suggests that the limestones were precipitated from the seawater. The positive correlation of ΣREE with Al2O3 Fe2O3, Ni, Th, Sc, and Y and negative correlation with CaO suggest an input of siliciclastic sediments from the land during limestone precipitation. The negative Ce anomaly, slightly positive La anomaly, slight Gd enrichment, positive Y anomaly, and positive Eu anomaly also suggest that the limestone was precipitated from the seawater with some siliciclastic input from continent. The low values of the Y/Ho ratio (23.12 to 28.57) in the studied limestones suggest some modification of the seawater by the input of freshwater in a coastal environment. The REEs of the studied limestones are correlable with the shallow sea water REEs with exception of a few elements. We envisage a coastal/shallow marine depositional environment where mixing of the continental material in sea water appears feasible.

Petrology and geochemical characteristics of dolomite in the Middle Permian Maoukou Formation, central Sichuan

Dolomites of the Maokou Formation in the central Sichuan were mainly developed in the middle-upper part of Member 2 and the lower part of Member 3 of Maokou Formation, it could be divided into fine-to medium-grained calcareous dolomite, fine-to medium-grained dolomite and breccia dolomite according to petrology characteristic, and the latter two were dominant. Through contrast of geochemical characteristic between micritic limestone and different types of dolomite, the dolomite was characterized by low Fe content, high Mn content and low Sr content relative to the micritc limestone. The micritic limestone and different types of dolomite had similar rare earth element (REE) distribution patterns which were characterized by depletion of light REE, weak positive anomaly of La and negative anomaly of Ce, and the dolomite also had weak positive anomaly of Eu. Compared with the micritic limestone, the fine-to medium-grained dolomite and breccia dolomite had low values of δ18O and obvious high ratios of 87Sr/86Sr. REE distribution patterns of different types of dolomite indicated that some geochemical characteristics of primitive limestone were preserved during the dolomitization process in the Maokou Formation, while weak Eu positive anomaly of dolomite and isotopic difference in dolomite and limestone might be caused by high-temperature thermal fluids during the dolomitization process. The dolomitization of the Maokou Formation was controlled by some factors: (1) strata residual seawater and hydrothermal fluids derived from magmatism were major sources of Mg during the dolomitization process; (2) heating effect of Emeishan large igneous province provided abnormal high paleo-geotemperature for dolomitization; (3) fault systems offered rapid migration channels for abnormal geothermal convection and dolomitization fluid; (4) limestone of grain-shoal facies with good porosity and permeability was more prone to dolomitization than micritic limestone. Abnormal geotemperature and abundant Mg supply derived from Emeishan Large Igneous Province were key factors for dolomitization in the Maokou Formation, and different types of dolomites were formed in different periods of the dolomitization process.

Geochemistry, U-Pb zircon geochronology and Sm-Nd isotopes of the Xincai banded iron formation in the southern margin of the North China Craton: Implications on Neoarchean seawater compositions and solute sources

Banded iron formations (BIFs) that are distal from submarine volcanic activity and generally lack direct relationships with volcanic rocks have been extensively used as proxies to infer the chemical composition of ancient seawater and trace the effects of continental weathering at the time of their deposition. Here, we discuss the major and trace elements and Sm-Nd isotopic geochemistry of the poorly-studied Xincai BIF, located west of the Superior-type Huoqiu BIF along a NW-SE trending supracrustal belt in the southern margin of the North China Craton, to understand the compositions and solute sources of seawater during the Xincai BIF precipitation. U-Pb zircon dating of granitic intrusive rock and biotite schist interlayer of the BIF constrains the depositional age of the Xincai BIF to 2.5–2.7 Ga for the first time, indicating that the Xincai BIF could be part of the Huoqiu BIF or consist of contemporaneous sediments formed in a similar tectonic-depositional environment. Shale-normalized rare earth element and yttrium (REY) patterns of the bulk and individual band samples display features that are characteristic of other Archean BIFs with light REE depletion relative to heavy REE, positive LaSN, GdSN and YSN anomalies, indicative of pure chemical sedimentation. The marine origin of these samples is further supported by their super-chondritic Y/Ho ratios (average Y/Ho = 47). The very low Al, Ti and high field strength element contents of the Xincai BIF indicate an essentially detritus-free component. The bulk BIF samples show a narrow range of εNd(t) values from −1.5 to +1.0, and most of the samples have negative εNd(t) values, −0.3 on average, which is different from Archean BIFs with mantle-derived depleted Nd isotopes. Deviations toward negative εNd(t) values of the iron bands (−0.5 to −0.1) and the silica bands (−4.7 to −0.5) further indicate a continental Nd contribution. In addition, positive EuSN anomalies that were found in all of the bulk and individual band samples indicate that a high-T hydrothermal input also contributed to their REY signatures. It is considered that both high-T hydrothermal fluids and ambient seawater with solutes from erosion and weathering of a nearby landmass can provide their REY signatures.

Age and nature of the Jurassic–Early Cretaceous mafic and ultramafic rocks from the Yilashan area, Bangong–Nujiang suture zone, central Tibet: implications for petrogenesis and tectonic Evolution

ABSTRACTThe Jurassic–Early Cretaceous Yilashan mafic–ultramafic complex is located in the middle part of the Bangong–Nujiang suture zone, central Tibet. It features a mantle sequence composed of peridotites and a crustal sequence composed of cumulate peridotites and gabbros that are intruded by diabases with some basalts. This article presents new whole-rock geochemical and geochronological data for peridotites, gabbros, diabases and basalts to revisit the petrogenesis and tectonic setting of the Yilashan mafic–ultramafic complex. Zircon laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) U–Pb ages of three diabase samples are 169.6 ± 3.3 Ma, 132.5 ± 2.5 Ma, and 133.6 ± 4.9 Ma, respectively. These ages together with previous studies indicate that the Yilashan mafic–ultramafic complex probably formed during the Jurassic–Early Cretaceous. The peridotites exhibit nearly U-shaped REE patterns and are distinct from abyssal peridotites. The diabase and basalt samples show arc features with selective enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILEs; e.g. Rb, U, and Sr) and depletion in high field strength elements (HFSEs; e.g. Nb, Ta, and Ti). The gabbro samples display cumulate features with selective enrichment in LILEs (e.g. Rb, Ba, and Sr) but depletion in LREEs and HFSEs (e.g. Nb, Zr, and Ti). Combing the positive εNd(t) values (+6.1 to +10.0) and negative zircon εHf(t) values (–16.5 to –11.7 and –13.6 to –0.4) with older Hf model ages for the mafic rocks, these signatures suggest that the Yilashan mafic and ultramafic rocks likely originated from an ancient lithospheric mantle source with the addition of asthenospheric mantle materials and subducted fluids coupled with limited crustal contamination in a continental arc setting as a result of the southward subduction of the Bangong–Nujiang Tethys Ocean beneath the Lhasa terrane during the Jurassic–Early Cretaceous.

Geochemistry of rare earth elements in a neutral mine drainage environment, Anjir Tangeh, northern Iran

This study investigates the geochemical characteristics of rare earth elements and yttrium (REE+Y) in the neutral mine drainage (NMD) and river water in the vicinity of the Anjir Tangeh coal washing plant, Mazandaran province, northern Iran. The NMD water had a neutral pH along with high sulphate and bicarbonate concentrations. The total concentrations of REE in the NMD water were lower than in the river water. According to the North American Shale Composite (NASC) normalised concentrations, the patterns of NMD showed heavy REE (HREE) enrichment while the river water represented middle REE (MREE) enhancement. Both negative europium (Eu) and positive gadolinium (Gd) anomalies were observed in the drainage and river water within the study area. The sum of REE concentration increased gradually downstream of the discharging point of the plant reject water, in contrast to decreasing of Al, Fe, Mn and Cu+Pb+Zn concentrations. The river sediment with high Fe concentrations was enriched with lanthanum (La) and cerium (Ce). The overall REE pattern of the NMD approximates those of coal except for the depletion in light REE (LREE) and the enrichment of Eu. Dominant inorganic REE species in the river water included Ln (CO3)2− and LnCO3+, while LnCO3+, Ln (CO3)2− and Ln (SO4) + were the main complexes in the NMD. The saturation index indicated that Al and Fe were both oversaturated in the drainage and river water, which can control the fractionation of REE by the sorption process. According to both hierarchical cluster analysis (HCA) and principal component analysis (PCA), pH, Al, and P were the significant parameters in the REE distribution.

Light rare earth element depletion during Deepwater Horizon blowout methanotrophy

Rare earth elements have generally not been thought to have a biological role. However, recent work has demonstrated that the light REEs (LREEs: La, Ce, Pr, and Nd) are essential for at least some methanotrophs, being co-factors in the XoxF type of methanol dehydrogenase (MDH). We show here that dissolved LREEs were significantly removed in a submerged plume of methane-rich water during the Deepwater Horizon (DWH) well blowout. Furthermore, incubation experiments conducted with naturally methane-enriched waters from hydrocarbon seeps in the vicinity of the DWH wellhead also showed LREE removal concurrent with methane consumption. Metagenomic sequencing of incubation samples revealed that LREE-containing MDHs were present. Our field and laboratory observations provide further insight into the biochemical pathways of methanotrophy during the DWH blowout. Additionally, our results are the first observations of direct biological alteration of REE distributions in oceanic systems. In view of the ubiquity of LREE-containing MDHs in oceanic systems, our results suggest that biological uptake of LREEs is an overlooked aspect of the oceanic geochemistry of this group of elements previously thought to be biologically inactive and an unresolved factor in the flux of methane, a potent greenhouse gas, from the ocean.

Oceanic island basalts in ophiolitic mélanges of theCentralChinaOrogen: An overview

We present an overview of the internal structure of the 10 ophiolitic mélanges in the Central China Orogen (CCO) with a focus on the geochemical character and tectonic evolution of the ophiolitic mélange‐related ocean island basalt (OIB) and mafic rock assemblages. The ophiolitic mélanges in CCO are generally complicated and usually consist of metamorphic peridotites (serpentinite), cumulates, gabbros, basaltic lavas (pillows), and abyssal radiolarian cherts. The ages of ophiolitic mélanges range from Mesoproterozoic to Carboniferous. The OIB‐type basalts and mafic rocks in CCO occur as tectonic blocks within the mélanges that are composed of limestones, radiolarian cherts, and turbidites, possessing formation characteristics of seamounts (oceanic islands/plateau). The mafic rocks in ophiolitic mélanges of CCO display uniform chondrite‐normalized rare earth element (REE) patterns with light REE enrichment and heavy REE depletion, no obvious Eu anomalies or negative Nb, Ta, and Ti anomalies, and primitive mantle‐normalized trace element patterns with significant large‐ion lithophile element enrichment, similar to those of modern OIB and the Hawaiian alkaline basalts. The OIB‐type basalts and mafic rocks are considered as accreted seamount fragments in an accretionary complex of CCO and may represent plume‐related magmatism within the Proto‐Tethys Ocean and Paleo‐Tethys Ocean. Our study provides further insights into the processes of multiple subduction and long‐lasting accretionary histories with seamounts in the CCO.

A Transgressive Depositional Setting for the Paleogene Shahejie Formation in the Qikou Depression, Eastern China: Inferences from the REE Geochemistry of Carbonates

Rare earth element (REE) and Y concentrations, and 87Sr/86Sr ratios were analyzed in 33 carbonate samples from the Paleogene Shahejie Formation in the Qikou depression, eastern China, with the goal of determining depositional environments and post-depositional conditions of carbonates in the region. The REE and Y concentrations were normalized to the post-Archean Australian shale (PAAS) standard. The La*PAAS/Yb*PAAS ratios of 0.35–1.52, where *PAAS indicates values for the PAAS standard, show light REE enrichment and heavy REE depletion in most samples. Values of La*PAAS (0.775–1.284) and Ce*PAAS (0.822–1.224), coupled with a relatively flat REE distribution, indicate that the Shahejie carbonates were deposited in lacustrine environments. Values of Y anomalies (1.009–1.527) and Y/Ho ratios (28.43–45.00) in the Shahejie Formation are greater than those of lacustrine carbonates and closer to those of marine carbonates, indicating that diagenetic fluids were probably influenced by seawater. In the carbonates from Well Kou-42, Eu anomalies (1.171–1.604), 87Sr/86Sr ratios (0.708 001–0.710 893), and high homogenization temperatures (104–151 oC) suggest that the carbonates were affected by hydrothermal fluids. The REEs and Sr isotope ratios show that the carbonates from the Shahejie Formation in the Qikou depression were deposited in lacustrine environments, and were influenced by seawater and hydrothermal fluids.

Deciphering the diagenetic alteration degree in thrombolites across the Permian-Triassic boundary and the evaluation of REY as a proxy of palaeoseawater

The thrombolites across the Permian-Triassic boundary (PTB) are widely distributed in South China. In order to examine the utility of rare earth element and yttrium (REY) in the thrombolites as a proxy of palaeoseawater, the petrographic and geochemical (Sr, Mn, Fe, REY, δ13C, δ18O) features of thrombolites from Cili and Taiping sections are comparatively studied to determine the diagenetic alteration degrees of the thrombolites and the impact of diagenesis on REY concentrations and distribution patterns. The thrombolites from Cili section, digitate in mesoscopic morphology, are latest Permian in age. In contrast, most of the thrombolites in Taiping section are mottled and formed in the earliest Triassic. The variation in thrombolite morphology across the PTB is probably related to increasing amount of metazoan and increasing intensity of bioturbation after the end-Permian mass extinction. Compared to the thrombolites from Taiping section, those from Cili section underwent more extensive diagenetic alteration, which are characterized by more dolomitic content, lower Sr concentrations, more negative δ18O values, and higher Mn/Sr ratios.The REY concentrations are higher in the thrombolites from Taiping section (5–303ppm, average 48ppm) than in the thrombolites from Cili section (17–34ppm, average 25ppm). Shale-normalized REY distribution patterns of the thrombolites from both sites are similar to those of oxygenated seawater, which are characterized by positive La anomalies (Pr/Pr∗=1.1–1.5), negative Ce anomalies (Ce/Ce∗=0.2–0.9), and light rare earth element (LREE) depletion. The preserved seawater like REY distribution pattern indicates that diagenesis did not alter the REY distribution patterns. The thrombolite samples from Cili section, compared to the counterparts from Taiping, have less positive La anomaly and less negative Ce anomaly. For the thrombolites from Cili section, a positive correlation exists between Ce anomaly and siliciclastic content, suggesting contamination of Ce anomaly by terrigenous input. The results suggest that ancient thrombolites that are isolated from terrigenous inputs are better proxy candidates of palaeoseawater redox condition than thrombolites located closer to land sources.

Platinum-group elemental and Sr-Nd-Os isotopic geochemistry of the ∼635 Ma mafic intrusions in the northern margin of the Yangtze Block: A link of metasomatized subcontinental lithospheric mantle and Ni-Cu-(PGE) sulfide mineralization

A number of ∼635Ma mafic-ultramafic intrusions occur in the Nanyang region and Suizhou-Zaoyang (Suizao) region along the northern margin of the Yangtze Block. The intrusions in the Suizao region are mainly composed of troctolite, olivine gabbro and gabbronorite. The parental magmas of the intrusions are estimated based on the non-modal melting using two fresh samples with the lowest REE concentrations and the results show that the parental magmas may have La/Sm of ∼2.5, Sm/Yb of ∼2.5 and La/Yb of ∼6.2 and could be produced by 5–10% of partial melting of the mantle composed of spinel-garnet (50:50) lherzolite. The estimated compositions of the parental magmas also display the enrichment of light REE and weak depletion of HFSE (Th, U, Nb and Ta) on the primitive mantle-normalized trace element patterns and have Ba/Nb, Ba/La and Ba/Th distinctly higher than MORB and OIB, consistent with the derivation from metasomatized subcontinental lithospheric mantle (SCLM) rather than convecting mantle. The rocks of the intrusions have radiogenic 187Os/188Os(i) of 0.137–0.247, which are likely attributed to low degrees of crustal contamination and no involvement of convecting mantle-derived magmas. They consistently have εNd(t) values of −5.4 to −10.4, much lower than the value of +2.5 for the mantle inferred from Neoproterozoic evolutionary trend and could be interpreted as the mixing of the parental magmas with 7–10% of Archean basement of the Yangtze Block. The rocks of the intrusions commonly have <0.1wt% S and 0.08–17.5ppb PGE (Os, Ir, Ru, Rh, Pt and Pd). They have Cu/Pd (7900–6,500,000) much higher than the value of the primitive mantle (∼7000), which is likely resulted from the retention of sulfide in the mantle source due to the low degrees of partial melting of the mantle. We conclude that the low PGE concentrations of the rocks can be attributed to the sulfide retention in metasomatized SCLM source and no substantial interaction of metasomatized SCLM with the melts from convecting mantle. The ∼635Ma mafic intrusions in the Suizao region may thus have little potential for economically important Ni-Cu-(PGE) sulfide mineralization.

Geochemical characteristics of mafic and ultramafic rocks from the Naga Hills Ophiolite, India: Implications for petrogenesis

The Naga Hills Ophiolite (NHO) represents one of the fragments of Tethyan oceanic crust in the Himalayan Orogenic system which is exposed in the Phek and Kiphire districts of Nagaland, India. The NHO is composed of partially serpentinized dunite, peridotite, gabbro, basalt, minor plagiogranite, diorite dyke and marine sediments. The basalts are mainly composed of fine grained plagioclase feldspar, clinopyroxene and orthopyroxene and show quenching and variolitic textures. The gabbros are characterized by medium to coarse grained plagioclase, orthopyroxene and clinopyroxene with ophitic to sub-ophitic textures. The ultramafic cumulates are represented by olivine, Cpx and Opx. Geochemically, the basalts and gabbros are sub-alkaline to alkaline and show tholeiitic features. The basalts are characterized by 44.1–45.6 wt.% of SiO2 with 28–38 of Mg#, and the gabbros by 38.7–43.7 wt.% of SiO2, and 26–79 of Mg#. The ultramafic rocks are characterized by 37.4–52.2 wt.% of SiO2, and 80–88 of Mg#. In multi-element diagrams (spidergrams) both basalts and gabbros show fractionated trends with strong negative anomalies of Zr, Nb, Sr and a gentle negative anomaly of P. However, the rare earth element (REE) plots of the basalts and gabbros show two distinct patterns. The first pattern, represented by light REE (LREE) depletion, suggests N-MORB features and can be interpreted as a signature of Paleo-Tethyan oceanic crust. The second pattern, represented by LREE enrichment with negligible negative Eu anomaly, conforms to E-MORB, and may be related to an arc tectonic setting. In V vs. Ti/1000, Cr vs. Y and AFM diagrams, the basalts and gabbros plot within Island Arc Tholeiite (IAT) and MORB fields suggesting both ridge and arc related settings. The ultramafic rocks exhibit two distinct patterns both in spidergrams and in REE plots. In the spidergram, one group displays highly enriched pattern, whereas the other group shows near flat pattern compared to primordial mantle. In the REE plot, one group displays steeper slopes [(La/Yb)N = 4.340–4.341], whereas the other displays moderate to flat slopes [(La/Yb)N = 0.97–1.67] and negative Eu-anomalies. Our study suggests that the ultramafic rocks represent two possible mantle sources (fertile and refractory).

Evaluating crustal contributions to enriched shergottites from the petrology, trace elements, and Rb-Sr and Sm-Nd isotope systematics of Northwest Africa 856

The origin of the incompatible trace element (ITE) characteristics of enriched shergottites has been critical for examining two contradicting scenarios to explain how these Martian meteorites form. The first scenario is that it reflects ITE enrichment in an early-formed mantle reservoir whereas the second scenario attributes it to assimilation of ancient Martian crust (∼4–4.5Ga) by ITE-depleted magmas. Strongly differentiated shergottite magmas may yield added constraints for determining which scenario can best explain this signature in enriched shergottites. The meteorite Northwest Africa (NWA) 856 is a basaltic shergottite that, unlike many enriched shergottites, lacks olivine and has undergone extensive differentiation from more primitive parent magma. In similarity to other basaltic shergottites, NWA 856 is comprised primarily of compositionally zoned clinopyroxenes (45% pigeonite and 23% augite), maskelynite (23%) and accessory minerals such as ulvöspinel, merrillite, Cl-apatite, ilmenite, pyrrhotite, baddeleyite and silica polymorph. The CI-chondrite normalized rare earth element (REE) abundance patterns for its maskelynite, phosphates, and its whole rock are flat with corresponding light-REE depletions in clinopyroxenes. The 87Rb-87Sr and 147Sm-143Nd internal isochron ages are 162±14 (all errors are ±2σ) Ma and 162.7±5.5Ma, respectively, with an initial εNdI=−6.6±0.2. The Rb-Sr isotope systematics are affected by terrestrial alteration resulting in larger scatter and a less precise internal isochron age. The whole rock composition is used in MELTS simulations to model equilibrium and fractional crystallization sequences to compare with the crystallization sequence from textural observations and to the mineral compositions. These models constrain the depth of initial crystallization to a pressure range of 0.4–0.5GPa (equivalent to 34–42km) in anhydrous conditions at the Fayalite-Magnetite-Quartz buffer, and consistently reproduce the observed mineralogy throughout the sequence with progressive crystallization. The Ti/Al ratios in the clinopyroxenes are consistent with initial crystallization occurring at these depths followed by polybaric crystallization as the parent magma ascended to the surface. The REE abundances in the clinopyroxenes and maskelynite are consistent with progressive crystallization in a closed system.The new results for NWA 856 are combined with other shergottite data and are compared to mixing and assimilation and fractional crystallization (AFC) models using depleted shergottite magmas and ancient Martian crust as end-members. The models indicate that the range of REE abundances and ratios, when taken in isolation, can be successfully explained for all shergottites by crustal contamination. However, no successful crustal contamination model can explain the restricted εNdI of −6.8±0.2 over the wide range of Mg# (0.65–0.25), and corresponding trace element variations from enriched shergottites to depleted shergottites. The findings indicate that the origin of the long-term ITE-enriched signature in enriched shergottites and the geochemical variability seen in shergottites is not a result of crustal contamination but instead reflects ancient mantle heterogeneity.

Ancient-depleted and enriched mantle lithosphere domains in northern Madagascar: geochemical and isotopic evidence from spinel-to-plagioclase-bearing ultramafic xenoliths

Mantle xenoliths hosted in Cenozoic alkaline rocks of northern Madagascar (Massif d'Ambre and Bobaomby volcanic fields) are spinel lherzolites, harzburgites and rare websterites. Petrography, electron microprobe, LA-ICP-MS and thermal ionization mass spectrometry techniques allowed to recognize domains characterized by variable degree of partial melting and extent of re-enrichment processes: 1) refractory spinel-to-spinel+plagioclase-lherzolites, with clinopyroxenes having marked LREE (Light Rare Earth Elements) depletion ((La/Yb)N~0.2) and very high 143Nd/144Nd (0.513594), which represent a limited and shallow portion of old mantle that suffered low degree partial melting (2–3%) and was later accreted to the lithosphere. These lherzolites acted as a low-porosity region, being, in places, percolated by small volumes of melts shortly before eruption; 2) lherzolites and harzburgites that suffered variable degrees of partial melt extraction (up to 15%), assisted and/or followed by pervasive, porous flow infiltration of alkaline melts in a relatively large porosity region, leading to the creation of a wide area rich in secondary mineral phases (i.e. olivine, clinopyroxene and pargasitic amphibole), enriched in incompatible elements (e.g., LaN/YbN in clinopyroxene up to 15) and having radiogenic Sr and unradiogenic Nd; 3) websterites and wehrlite-bearing samples that record differentiation processes of alkaline melts highly enriched in Th, U and LREE, not yet documented in the erupted volcanics of northern Madagascar. The mantle xenoliths of northern Madagascar show a regional decrease of the equilibration temperature from to SW (up to 1180°C, Nosy Be Archipelago) to the NE (up to 900°C, Bobaomby district). A significant lithologic and geochemical variation of the shallow lithospheric mantle beneath northern Madagascar is noted, in contrast with the relatively uniform geochemical and isotopic composition of the host alkali basalt and basanite lavas.

Geochronology, geochemistry and tectonic significance of the ore-associated granites at the Kaladawan Fe–Mo ore field (Altyn), NW China

The Kaladawan Fe–Mo ore field (Altyn, Xinjiang) in Northwest (NW) China contains six deposits, with a total reserve of 60Mt Fe and 10Kt Mo metal. The orebodies are hosted in lower Paleozoic andesite, dacite, phyllite and marble with well-developed skarn alteration. The Kaladawan granites are newly U–Pb dated to be Early Ordovician (476.1±3.3Ma), largely coeval with the Fe–Mo mineralization (molybdenite Re–Os: 480.3±3.2Ma). The granites contain high SiO2, K2O and Al2O3, low TiO2, MgO and CaO, with high K2O/Na2O ratios (1.26–1.58) and A/CNK values (1.00–1.08), showing peraluminous high-K calc-alkaline affinity. The rocks are characterized by large ion lithophile element (LILE) and light rare earth element (LREE) enrichments and depletions of Sr, Ba, Nb, Ta, Ti and P, and with negative Eu anomalies. The rocks have initial 87Sr/86Sr ratios of 0.7066 to 0.7112 and εNd(t) values of −1.4 to −1.1, with TDM2(Nd) ages of 1.32–1.30Ga. Zircon εHf(t) values range from 2.9 to 6.4, with TDM2(Hf) ages of 1.26–1.04Ga. The new geochemical and isotopic data suggest that the Kaladawan granites are highly fractionated I-type, and likely formed by fractional crystallization of a magma that was derived from partial melting of a mixture of crustal and mantle materials. Deposits in the Kaladawan Fe–Mo field are skarn-type and may have occurred in an active continental margin, via the contact metamorphism and metasomatic reaction between granite-derived fluids and the wall rocks.

Paleo-seawater REE compositions and microbial signatures preserved in laminae of Lower Triassic ooids

Signals indicative of paleo-seawater rare earth element (REE) composition and microbial activity were identified in marine ooids from the Lower Triassic of South China using in situ laser ablation–inductively coupled plasma-mass spectroscopy (LA-ICP-MS) together with petrographic and isotope geochemical techniques. Quantitative trace element profiles were generated across successive growth laminae in the cortices of selected ooids. The shale-normalized REE and yttrium (Y) data of these layers show 1) light rare earth element depletion (mean LaSN/YbSN=0.36±0.14), 2) positive La anomalies (mean (La/La*)SN=2.02±1.25), and 3) high ratios of Y/Ho (mean 53±12), which are similar to modern seawater compositions and Bahamian ooids. These findings imply that ooids in the study samples faithfully record Early Triassic paleo-seawater chemical signals such as REE+Y distributions. Petrographic observations revealed dark-colored layers with intense fluorescence containing structures that resemble microbial filaments or extracellular polymeric substances. These layers also possess more enriched REE compositions and higher concentrations of nutrient-like elements (e.g., Zn and Ba) than adjacent recrystallized light-colored laminae. Most ooid laminae contain negative Ce anomalies indicative of a well-oxygenated environment of formation, but some laminae have minimal negative anomalies suggesting the influence of suboxic porewater conditions. The present study provides a new perspective on the role of microbes in ooid genesis and on the utility of ooids as proxies for paleo-seawater chemistry.

Sedimentary environments and mechanisms of organic matter enrichment in the Mesoproterozoic Hongshuizhuang Formation of northern China

The Mesoproterozoic Hongshuizhuang Formation in Jixian, northern China, is a hydrocarbon source unit containing abundant organic matter. We investigated the mechanisms controlling this organic matter accumulation by analyzing the sedimentology and geochemistry of the Qinghe section, in the Kuancheng region of northern China. Trace element geochemistry suggests that organic matter accumulation was controlled primarily by marine productivity, which was the main determinant of redox conditions. Elevated marine productivity and consequent anoxic-sulfidic bottom water conditions were coincident with accumulation of the organic-rich (TOC>1.0%) lower part of the Hongshuizhuang Formation. Deposition of the organic-lean upper Hongshuizhuang Formation appears to have been coeval with diminished productivity and a transition to oxic-suboxic redox conditions. Enhanced organic matter accumulation in the lower Hongshuizhuang Formation was promoted by a warm, humid paleoclimate and submarine hydrothermal activity. Average Rb/Sr ratios (3.42) and negative carbon isotope excursion reflect a warm climate, likely caused by the release of volcanogenic CO2. Warm conditions would have promoted continental weathering, and increased the flux of nutrients into the oceans, enhancing marine productivity. An increased supply of nutrients and enhanced marine productivity would have depleted oxygen in the bottom water environment and favored the proliferation of sulfate-reducing bacteria, leading to anoxic-sulfidic conditions. The relative stability of δ13Ccarb values suggests that organic matter enrichment in the Hongshuizhuang Formation reflects changes in surface water productivity and redox conditions, caused by climate-induced sea-level fluctuations. Shale-normalized rare earth element (REE) distributions show a depletion in light REEs and enrichment in heavy REEs, positive Eu anomalies, and mean Y/Ho and Ba/Sr ratios of 33.2 and 9.91, respectively, suggesting that deposition of the Hangshuizhuang Formation was accompanied by submarine hydrothermal activity.

Inglés

Metabasites belonging to the Arquia Complex and outcropping along the Pijao-Genova strip (western edge of the Central Cordillera of the Colombian Andes) consist of chlorite schists, actinolite schists with and without garnet, amphibolites, garnet-amphibolites, eclogites and metabasalts. Some bodies of serpentinized peridotites are observed along with metabasites. Both rock types are confined to the tectonic block bordered by the Cauca-Almaguer and Silvia-Pijao fault systems and their satellite faults. Metabasites were affected by a regional metamorphism, reaching their peak of metamorphism at the amphibolite facies. The Rare Earth Elements (REEs) (normalized to the C1 chondrite) from the studied rocks show a Light REEs depletion and a flat or no-fractionated Heavy REEs patterns. The metabasites´ REEs patterns, along with their La/Sm ratio < 0.6 and positive Nb, Ta and Ti anomalies, may suggest their protoliths are MORBtype related. The lithologic association between metabasites, few metapelites and deformed ultramafic rocks suggest that the rocks of the Complex Arquia can represent remnants of an ophiolite sequence of N-MORB type series. The whole rock-garnet Lu-Hf geochronology from metabasites yield an age of 128.7±3.5 Ma. This age has been interpreted as the age at which these rocks reached the eclogite facies, which in turn coincides with the roll-back process of the subducted ocean lithosphere and the Quebradagrande magmatic arc formation. Keywords: Petrology; metabasites; Arquia Complex; Cordillera Central; Colombia.

Mineralogical evidence for crystallization conditions and petrogenesis of ilmenite-series I-type granitoids at the Baogutu reduced porphyry Cu deposit (Western Junggar, NW China): Mössbauer spectroscopy, EPM and LA-(MC)-ICPMS analyses

Primary ore-forming minerals retain geochemical signatures of magmatic crystallization information and can reveal the petrochemical conditions prevalent at the time of their formation. The Baogutu deposit is a typical reduced porphyry Cu deposit. Amphibole and biotite Fe3+/ΣFe ratios, minerals (feldspar, biotite, amphibole, zircon and apatite), in situ elemental and apatite Nd isotopic compositions were determined by Mössbauer spectroscopy, electron probe microanalysis, and laser ablation multiple-collection inductively coupled plasma mass spectrometry, respectively, to investigate the magma oxidation state, petrogenesis, source features, and to constrain the carbon species at magmatic stages for the intrusive phases. The results show that the primary plagioclase and amphibole in the mineralized diorite to granodiorite porphyry and post ore hornblende diorite porphyry are distinct (An26-55 versus An60-69; Mg-hornblende versus tschermakite). In particular, the amphibole shows distinct major and trace element compositions with light rare earth element enrichments and negative Eu anomalies in Mg-hornblende and light rare earth element depletions and no Eu anomalies in tschermakite. All the analyzed biotites are primary igneous phases with a biotite phenocryst profile showing significant variations of Zn, Cr, Sc and Sr from core to rim. These results may indicate the occurrence of mixing between two distinct magmas during mineral formation. Titanium in zircon and Si∗ in amphibole thermometries indicate that magma crystallized at >900°C and continued to ∼650°C. In situ apatite Nd isotope (εNd(t)=5.6–7.6, TDM2=620–460Ma), indicate absence of significant reduced sedimentary contamination and the source of juvenile lower crust. Slightly decreasing Fe3+/ΣFe ratios from biotite and amphibole to whole rock indicate decreasing oxygen fugacity during magma crystallization. Recalculated biotite compositions according to Fe3+/ΣFe ratios indicate fO2 values of less than Ni-NiO buffer (NNO) which show slightly lower values than that estimated according to zircon/melt distribution coefficients Ce anomalies (∼ΔNNO+0.6). These values are consistent with the features of reduced porphyry Cu deposits. Crystallization of other mineral phases significantly affects the reliability of oxybarometer of zircon/melt distribution coefficients Eu anomalies and Mn contents in apatite. This oxidation state suggests that only CO2 was present at the magmatic stage, and implies that CH4 formed during CO2 reduction occurring later hydrothermal alteration. The alteration of primary amphibole to actinolite released Ti, Al, Fe, Mn, Na and K to the fluid with later precipitation of titanite, albite and minor ilmenite and magnetite during actinolite alteration.