Abstract Carbonatites, usually occurring within intra-continental rift-related settings, have strong light rare earth element (LREE) enrichment; they rarely contain economic heavy REE (HREE). Here, we report the identification of Late Triassic HREE-Mo-rich carbonatites in the northernmost Qinling orogen. The rocks contain abundant primary HREE minerals and molybdenite. Calcite-hosted fluid inclusions, inferred to represent a magmatic-derived aqueous fluid phase, contain significant concentrations of Mo (~17 ppm), reinforcing the inference that these carbonatitic magmas had high Mo concentrations. By contrast, Late Triassic carbonatites in southernmost Qinling have economic LREE concentrations, but are depleted in HREE and Mo. Both of these carbonatite types have low δ 26 Mg values (−1.89 to −1.07‰), similar to sedimentary carbonates, suggesting a recycled sediment contribution for REE enrichment in their mantle sources. We propose that the carbonatites in the Qinling orogen were formed, at least in part, by the melting of a subducted carbonate-bearing slab, and that 10 Ma younger carbonatite magmas in the northernmost Qinling metasomatized the thickened eclogitic lower crust to produce high levels of HREE and Mo.
Abstract The genesis and primary compositions of carbonatite melts are enigmatic owing to their reactivity and resulting contamination in the mantle and crust. This overprints earlier uncontaminated compositions and obscures their origins. The subvolcanic Badou carbonatite in the North China Craton is characterized by brecciated textures and comprises pristine clinopyroxene, mica, and apatite phenocrysts in a mixed, carbonate–silicate matrix. Here, we provide mineralogical, textural, and major and trace elemental results to reveal the mantle derivation of the Badou carbonatite melt, and its compositional modification during interaction with crustal materials. Clinopyroxene and apatite phenocrysts have initial Sr isotopes (0.70920–0.71037) similar to brecciated calcite (0.71028–0.71159) and the whole rock (0.70953–0.71061), indicating a common source. Rare Mg-rich clinopyroxene and mica contain high Cr and Ni contents, recording direct mantle derivation without immiscibility from silicate melts. Abundant feldspars and aegirine, occurring as both phenocrysts and fine-grained matrix, formed an antiskarn at relatively low temperatures indicating extensive silica contamination upon crustal emplacement. Silica contamination is further indicated by britholite-rich apatite rims and relict quartz cores in clinopyroxene. Zircon xenocrysts were partly assimilated, indicated by Zr and Hf enrichment in late-stage silicates, and partly underwent decomposition to baddeleyite in a decreasing silica activity environment. The assimilation process consumed the carbonate melt, forming refractory silicate minerals and CO2 vapor, leading to near-surface gas overpressure with a decrease in magma viscosity. This elevates the potential for explosive activity, which, in turn, provides a positive feedback mechanism for silica assimilation. The complex evolution of the carbonatite melt in the crust might be responsible for the compositional gap between the natural rocks and experimental outcomes, and results in different eruption styles.
The Bayan Obo REE–Fe–Nb deposit is the largest light rare earth element (REE) deposit in the world, and it has undergone serious hydrothermal reworking processes. However, the origin and nature of metasomatic fluids in this deposit remain controversial. To determine the origin of metasomatic fluids, we conducted a detailed mineralogical and geochemical investigation on the apatite from the ∼ 1.8-km drill core in the Bayan Obo deposit. The textural and geochemical variations of the apatite could trace the hydrothermal metasomatism during REE mineralization. Four types of apatites were identified according to their cathodoluminescence characteristics, including turbid euhedral apatite (Ap-0), rounded pinkish–violet apatite veinlets (Ap-1), yellowish–green apatite aggregates (Ap-2), and apatite with a pinkish–violet core and a yellowish–green rim (Ap-3). According to the in-situ Sr isotopic compositions, these apatites were classified into two groups. Ap-0, Ap-1, and Ap-2 display constant and low 87Sr/86Sr ratios (0.7030–0.7035), whereas Ap-3 displays various Sr isotopic compositions (0.7047–0.7105). The in-situ Th–Pb age of the bastnäsite associated with the Ap-0 ranges from 390 to 425 Ma. The Sr isotopic compositions and geochronological study indicate that metasomatic fluids in the Bayan Obo deposit were obtained from different sources, including the Silurian Bainaimiao arc plutons and sedimentary materials. The four apatite types display various REE distribution patterns, which could account for hydrothermal fluid properties. F-rich and Na-poor fluids metasomatized the precursor Ap-0, releasing REEs from apatites and resulting in low REE contents of Ap-0. Ap-1 and Ap-2 were derived from the same fluid but were characterized by different trace element compositions, which could be attributed to the evolution of hydrothermal fluids with changes in the Na content and F/Cl ratio. The primary Ap-3 was derived from a REE-rich source and was altered by Na-poor and Cl-rich fluids. Metasomatic fluids with different origins and variable chemical compositions result in complex mineral assemblages and geochemical features in the Bayan Obo deposit.
Abstract The genesis of the Bayan Obo giant rare earth element (REE) deposit has been debated for several decades. Here, we report the isotopic effects of dynamic recrystallization in the H8 carbonatite, which is the principal ore carrier in the deposit. We studied fresh drill core to a depth of 1.78 km and documented the elemental and C-O-Sr isotope evolution of rock-forming dolomite during its deformation and reaction with fluids. The precursor dolomite and the products of its recrystallization differ in δ13CVienna-PeeDee Belemnite (V-PDB) (–1.09 to 2.37 vs. –3.59 to 0.79‰, respectively) and 87Sr/86Sr (0.70241–0.70394 vs. 0.70288–0.71409, respectively), and show a similar δ18Ovienna-standard mean ocean water (V-SMOW) range (10.3–16.9‰). The strong negative shift in δ13CV-PDB indicates that, locally, there was as much as 40% loss of CO2 from the precursor dolomite, although most of the recrystallized dolomite experienced decarbonation on a smaller scale. Clumped monazite grains associated with apatite in paragenetically similar samples yielded variable in situ Th-Pb dates (980–340 Ma), whereas those in monomineralic veinlets give a consistent age of ~400 Ma and consistent initial Nd isotope ratios. This indicates that the wide range of dates may not represent real REE depositional events and that the primary REE minerals deposited in the Mesoproterozoic underwent isotopic reequilibration and REE remobilization in the mid-Paleozoic. Recrystallization and decarbonation of dolomite in the H8 unit were facilitated by its reaction with subduction-derived silica- and halogen-rich fluid, genetically linked to plate-convergence processes along the northern margin of the North China craton, and did not require an influx of REEs from an external source.
Barite is an abundant sulfate mineral in nature. Especially, the variety of morphologies of barite is often driven by the mixing of Ba-bearing hydrothermal fluid and sulfate-bearing seawater around hydrothermal chimneys. In order to better understand the factors affecting the morphology and precipitation mechanism(s) of barite in seafloor hydrothermal systems, we synthesized barite by a new method of in-situ mixing of BaCl2 and Na2SO4 solutions at 200 °C while varying Ba concentrations and ratios of Ba2+/SO42−, and at room temperature for comparison. The results show that barite synthesized by in-situ mixing of BaCl2 and Na2SO4 solutions at 200 °C forms a variety of morphologies, including rod-shaped, granular, plate-shaped, dendritic, X-shaped, and T-shaped morphologies, while room temperature barites display relatively simple, granular, or leaf-like morphologies. Thus, temperature affects barite morphology. Moreover, dendritic barite crystals only occurred at conditions where Ba2+ is in excess of SO42− at the experimental concentrations. The dendritic morphology of barite may be an important typomorphic feature of barite formed in high-temperature fluids directly mixing with excess Ba2+ relative to SO42−.
Rare earth elements (REEs + Y) play an important role in modern industry. Heavy REEs (HREEs) are particularly critical because of their relative scarcity in nature. Global HREE resources are predominantly present in granitoid weathering crusts in southern China. Although it is well known that REEs are generally enriched in alkaline rocks, in contrast, the parental granitoids in most Chinese HREE deposits are peraluminous. Here, we examined different REE mineral compositions, distribution patterns, and Nd isotope ratios of the granitoid complex in Zudong, South China, whose weathering crusts form the largest HREE deposits globally. The complex is composed of granodiorite, biotite-muscovite, and muscovite alkali-feldspar granites, whose REE patterns change from HREE depletion to enrichment relative to light REEs (LREEs) with increasing negative Eu anomalies. They are not fractional crystallization products from the same parental melts owing to the different zircon U-Pb ages (∼191, ∼169, and ∼154 Ma for granodiorite, biotite-muscovite, and muscovite granites, respectively). However, magmatic zircons from the three types of granitoids show consistent initial Hf isotope compositions [εHf(t) = 3.4 to 8.6], indicating they evolved from the same sources. The granodiorite contains primary LREE minerals with low εNd(t) values (−13.8 to −10.1), decoupled from high zircon Hf isotopes. This indicates a garnet-containing basement source resulted in Lu (HREE) remaining preferentially in the garnet phase, with Hf entering the melt. This contradicts the high HREE concentration recorded in the biotite-muscovite and muscovite granites, which contain abundant HREE minerals associated with muscovite, fluorite, and recrystallized quartz. The whole-rocks and their secondary REE minerals show variable and higher initial Nd isotope ratios [εNd(t) = −3.7 to 2.9 and −11.4 to −7.1 for biotite-muscovite and muscovite granites, respectively] than the early granodiorites and their primary REE minerals, indicating minimal 143Nd was derived from radioactive decay of 147Sm in the basement and wall rocks. This constrained the contribution of external REE-, volatile-rich liquids, which drove the segregation of highly evolved silicate melts from the magma chamber. This increased volatile saturation and fluid exsolution, which may have mobilized the HREEs and metasomatized the granites during their emplacement. Therefore, we infer that external fluid metasomatism appears to be crucial for the HREE enrichment in highly fractionated peraluminous granites.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)