Light Rare Earth Element Depletion Research Articles

Magma‐Mush Interactions in the Lower Oceanic Crust: Insights From Atlantis Bank Layered Series (Southwest Indian Ridge)

AbstractMagma migration and differentiation processes are key to understanding the development and evolution of oceanic magma reservoirs. To provide new quantitative geochemical constraints on these processes, we applied a high‐resolution approach to study an interlayered section of the lower oceanic crust sampled at Atlantis Bank, on the (ultra)slow‐spreading Southwest Indian Ridge. The section is characterized by sharp grain‐size layering between fine‐ and coarse‐grained olivine gabbros that is representative of other layered structures described at the Atlantis Bank oceanic core complex. The textures and fabrics of the layers and the nature of their contacts indicate formation by intrusion of a magma (i.e., crystal‐bearing) into an almost solidified coarse‐grained mush. Petrographic observations and in situ incompatible trace element signatures indicate that the fine‐ and coarse‐grained layers record reactive porous migration of melts. Widespread reactive porous flow occurred prior to intrusion within the coarse‐grained gabbro, producing mineral compositions enriched in incompatible elements. The intrusive fine‐grained lithology records a late stage event of localized reactive melt percolation in cm‐scale structures, which lead to strong light rare earth elements depletion relative to heavy rare earth elements. In addition, we highlight the occurrence of interactions at the contacts between layers and partial modification in compositions of the intruded lithology. This layered section likely represents a contact between two larger magma bodies emplaced within the lower crust during accretion, where the type of melt migration (intrusion or porous flow) and the modalities of melt percolation (widespread or localized) strongly govern the composition of the crustal lithologies.

Middle Jurassic–Lower Cretaceous glendonites from the eastern Barents Shelf as a tool for paleoenvironmental and paleoclimatic reconstructions

Glendonites are carbonate pseudomorphs after ikaite which use as indicators of near-freezing temperatures. We describe the first glendonites of Middle–Late Jurassic to Early Cretaceous age from the Barents Shelf, including the first glendonites of Late Barremian–Early Aptian age ever discovered on Earth and provide the detailed mineralogical-geochemical and isotopic studies. Mesozoic glendonites are well-known across the Arctic realm, however the ikaite-calcite transformation and implication of glendonite findings are still debatable. Pseudomorphs are mainly composed of calcite. Cathodoluminescence and scanning electron microscopy studies reveal three types (I-III) of calcite: type I - elongated, rounded and irregular calcite; type II - blocky, fibrous and needle-like calcite/siderite; type III - blocky calcite in pores. Elemental analyses distinguish geochemical differences between the calcite types: type I comprises pure CaCO3; type II calcite contains Mg and P (in all studied samples), Fe (Bajocian–Callovian), S (Middle Volgian) or Sr (Upper Barremian–Lower Aptian); type III calcite is depleted in Mg, P and Fe. Concentrations of Mn, Fe, Sr and calculated Mn/Ca, Fe/Ca and Sr/Ca ratios are suggestive of diagenetic alteration. Bulk δ18O values range from −5.39 to −1.71‰ VPDB, supporting overprinting of primary values during burial diagenesis, while bulk δ13C values range from −33.3 to −22.6‰ VPDB, providing the influence of organic matter on ikaite-glendonite formation. However, 87Sr/86Sr values range from 0.7070 to 0.7110, mainly corresponding to Jurassic–Cretaceous seawater. Rare-earth elements characterize porewater REE-patterns, with slight middle REE-enrichment and depletion in light and heavy REE. Weak positive and negative Ce anomalies indicate fluctiating from anoxic to oxic conditions during ikaite-glendonite formation. Our new biostratigraphic data refine the ages of glendonite-bearing horizons in the Barents Shelf region, allowing a more definitive correlation with coeval glendonite occurrences across the Arctic realm and with global climatic changes during the Jurassic–Cretaceous.

Differences in the behaviour of trace and rare-earth elements in oxidizing and reducing soil environments: Case study of Terra Rossa soils and Cretaceous palaeosols from the Istrian peninsula, Croatia

This study compares the differences between the distribution of trace elements and rare-earth elements (REEs) formed under reducing and oxidizing soil conditions during pedogenesis on carbonate bedrock. Terra rossa (TR) soils, representing pedogenesis under oxic conditions, and Cretaceous palaeosols (CP), representing pedogenesis under reducing conditions, were sampled on the Istrian peninsula. They were studied by ICP-MS, ICP-OES, XRF, XRD, sequential extraction and statistical analyses. The differences in trace-element behaviour between the TR and CP stem from different redox conditions, but the most remarkable difference was observed in the behaviour of the REEs. Statistical analyses revealed that in TR soils all the REEs showed a very positive correlation, while in CPs the light REEs and heavy REEs showed an internal, very positive correlation. TR soils have almost twice as much REEs as CPs. This difference is pedogenetic, as both materials have a very similar amount of REEs in the residual fraction. While TR soils have the same amount of REEs in fractions other than the residual fraction, CPs have almost no REEs in these fractions. Different REE patterns obtained from sequential extraction, such as a middle-REE enrichment and a positive Ce anomaly in TR soils and light-REE depletion, heavy-REE enrichment, positive Ce and Eu anomalies in CPs, contributed to an understanding of the redox and pedogenetic processes. This study successfully emphasized the influence of different redox conditions on the behaviour of trace and rare-earth elements during pedogenesis on a carbonate bedrock and the ability of the REEs to track pedogenetic processes.

Origin of Graphite–Diamond-Bearing Eclogites from Udachnaya Kimberlite Pipe

Abstract Kimberlite-borne mantle eclogites represent an important diamond source rock. Although the origin and stability of diamond, as opposed to its low-pressure polymorph graphite, have been studied for decades, their relationship in rare natural samples where both polymorphs coexist remains poorly constrained. To shed new light on this issue, seven graphite–diamond-bearing eclogites from the kimberlite pipe Udachnaya, Siberian craton were comprehensively investigated with respect to their petrography, mineral chemical composition and omphacite 87Sr/86Sr, acquired in situ by laser ablation multicollector inductively coupled plasma mass spectrometry. The calculated P–T conditions for basaltic group eclogites (Eu/Eu* < 1) correspond to a pressure range of 4·8–6·5 GPa and temperatures of 1060–1130 °C, whereas gabbroic eclogites with positive Eu- and Sr-anomalies have a smaller pressure variation (4·8–5·8 GPa), but a larger range in temperature (990–1260 °C). Reconstructed bulk compositions for gabbroic eclogites indicate an oceanic crustal origin for their protoliths, with accumulation of plagioclase and olivine ± clinopyroxene (gabbronorite or olivine gabbro). The protoliths of basaltic eclogites probably formed from the complementary residual melt. The presence of coesite and low Mg# in basaltic eclogites suggest that their light rare earth element depletion was the result of <10 % partial melting during subsequent subduction and emplacement into the cratonic lithosphere. Extremely unradiogenic 87Sr/86Sr (0·70091–0·70186 for six of seven samples) not only provides new evidence for the Archean age (2·5–2·9 Gyr) of Yakutian graphite–diamond-bearing eclogites and for formation of their protoliths in a depleted mantle source, but also suggests that they were not significantly metasomatically overprinted after their formation, despite their extended residence in the cratonic mantle lithosphere. The mineralogical and petrographic features indicate that the primary mineral association includes garnet, omphacite, ± coesite, ± kyanite, ± rutile, graphite, and diamond. Graphite occurs in the samples in the form of idiomorphic crystals (the longest dimensions being 0·4–1 mm) in garnet and kyanite and extends beyond their grain boundaries. Diamonds occur as octahedral cubic transparent, slightly colored or bright yellow crystals as large as 0·1–2 mm. Furthermore, idiomorphic and highly ordered graphite occurs as inclusions in diamond in four samples. The carbon isotope composition for diamond and graphite has a narrow range (−4 to −6·6 ‰) for both groups (gabbroic and basaltic), indicating a mantle source and limiting the role of subducted isotopically light biogenic carbon or reduction of isotopically heavy carbonate in diamond crystallization. Importantly, the presence of graphite and diamond inclusions in garnet, omphacite, and kyanite in three samples indicates a co-formation close in time to eclogitization. Combined, the petrographic and geochemical evidence suggests that both polymorphic carbon modifications can form in the diamond stability field, as also suggested by experiments and some natural examples, although the exact mechanism remains unresolved. Furthermore, this study provides natural evidence that graphite can be preserved (metastably) deep within the diamond stability field, without recrystallizing into diamond, for a long time, ≥2·5 Gyr.

>1.8 Ga cold subduction of lithospheric mantle: Evidences from the Fengzhen olivine-bearing garnet pyroxenite xenoliths in Trans-North China Orogen

Whether modern-style subduction exists in Paleoproterozoic has been hotly debated because of the scarcity of robust petrological evidence. Here, we present a comprehensive study of olivine-bearing garnet pyroxenite xenoliths hosted in the Paleoproterozoic Fengzhen carbonatite. The carbonatite is located in the conjugate area between the Paleoproterozoic Trans-North China Orogen (TNCO) and the Khondalite Belt in North China Craton (NCC) with a dated age of 1810 ± 3 Ma. Petrographically, four-stages of evolution have been identified in the studied olivine-bearing garnet pyroxenite: 1) the formation of the protolith spinel websterite (S1), 2) the prograde metamorphism of spinel-lherzolite facies to garnet-lherzolite facies (M1), 3) retrograde metamorphism to Ariegite subfacies (M2) with formation of symplectite-I, and 4) Seiland subfacies (M3) forming symplectite-II. The clinopyroxene in the xenoliths display high Mg# (Mg2+/(Mg2++Fe2+)*100 = 89–94), strongly depleted HREE (heavy REE) and HFSE (high field strength elements; e.g., Nb, Zr, Ti) and enriched LREE (light REE) and LILE (large ion lithophile elements; e.g., Th and U). Similarly, the whole-rock chemistry exhibits high Mg# (85–88) and high Al2O3 + CaO (20.2–21.9 wt%) with enrichment in LREE and LILE (Th, U) and depletion in HREE and HFSE (e.g., Nb, Ta, Zr, Hf, Ti). The geochemical signatures imply that they might originate from the refractory mantle wedge in subduction zone and have been metasomatized by crust-derived melts. Major mineral thermobarometry yields the peak PT conditions of 26–33 kbar and 890–962 °C corresponding to a cold subduction geothermal gradient (307 ± 35 °C/GPa) with a depth up to 110 km for olivine-bearing garnet pyroxenites, which is also consistent with the results by zirconium-in-rutile and REE thermobarometers. This studies suggest that that the modern-style subduction of the lithospheric mantle was initiated at least since 1.8 Ga.

Middle–late Permian mantle plume/hotspot–ridge interaction in the Sumdo Paleo-Tethys Ocean region, Tibet: Evidence from mafic rocks

The Sumdo Paleo-Tethys Ocean was the southernmost part of the Paleo-Tethys Ocean and was located on the Qinghai–Tibet Plateau. Doubts exist as to the internal dynamics and evolution of the Sumdo Paleo-Tethys Ocean region during the Permian, owing to insufficient knowledge regarding the remnants of oceanic crust. Here, we report results for petrology, laser ablation–inductively coupled plasma–mass spectrometry zircon UPb dating, whole-rock geochemistry, and zircon Hf and whole-rock SrNd isotopes of mafic rocks in the Sumdo area. Geochemical characteristics of these mafic rocks allow them to be divided into two groups, A and B. Group A rocks are characterized by high SiO2 and TiO2 contents, low Al2O3 contents, and Na2O > K2O. Their chondrite-normalized rare-earth-element (REE) and primitive-mantle-normalized trace-element patterns are similar to those of N-MORB. The samples exhibit enrichment in U and depletion in Th, depletion in light REEs, and high 143Nd/144Nd values. Group B rocks have high SiO2, TiO2, and Al2O3 contents. Their chondrite-normalized REE patterns and primitive-mantle-normalized trace-element patterns are similar to those of E-MORB. Group B rocks are enriched in Nb and Ta, depleted in Th and U, and show high 87Sr/86Sr and low 143Nd/144Nd values. Zircon UPb dating of five meta-gabbro rock samples yielded ages of 268.9 ± 1.6 Ma (STN66), 273.8 ± 2.2 Ma (STN79), 265.1 ± 4.1 Ma (STE26), 258.7 ± 4.4 Ma (STE89), and 261.0 ± 3.1 and 263.7 ± 5.4 Ma (STE41). STN79 shows N-MORB geochemical characteristics and has εHf(t) values of +12.4 to +14.6, whereas STE41 displays E-MORB geochemical characteristics with εHf(t) values of −2.6 to +0.4. The Sumdo mafic rocks were not contaminated by subducted sediments and/or slab-derived fluids, and formed in a mid-ocean ridge setting. Combining results of previous studies with our new geochemical data for mafic rocks in the Sumdo area, we infer the occurrence of plume/hotspot–ridge interaction in the Sumdo Paleo-Tethys Ocean during the middle–late Permian. Mixing of mantle plume/hotspot melt and depleted asthenospheric mantle explains the origin of basaltic magma that formed rocks with the observed geochemical and isotopic characteristics in the Sumdo area.

Geochemistry of apatite in Late Permian coals, Bowen Basin, Australia

Apatite may be a common accessory mineral in coal seams, but interpretation of its origins can vary from syndepositional mineral detritus, early precipitation during diagenesis, and post coalification precipitation and replacement from hydrothermal or other fluids. It is hypothesised that paragenesis is reflected in the modes of occurrence in different stratigraphic units of the Late Permian in the Bowen Basin. From samples analysed in this study, apatite in the youngest unit, the Rangal Coal Measures, most commonly occurs as pore-apatites (i.e. apatites infilling preserved plant cell cavities of fusinite and semifusinite macerals and commonly associated with kaolinite) and less commonly as fracture-apatites (i.e. apatites infilling micro-fractures) and detrital-apatites intermixed with the organic coal layers (Detrital-B). In the tuffaceous Fort Cooper Coal Measures and equivalents, apatite occurs more commonly as detrital-apatites intermixed with the lithic layers (Detrital-A) (Detrital. Within the basal unit, the Moranbah Coal Measures and its equivalents, the mode is commonly encrusting-apatites (i.e. small apatite crystallites encrusting silicate minerals). Almost all apatites in the coal samples tested by electron microprobe microanalysis (EPMA), regardless of stratigraphic location and mode, are fluorapatite. Although there are some locality-driven enrichments, the minor and trace-element chemistry vary more so by mode of occurrence. For comparison, apatites from dykes intersecting the coal measures and from tuffs were also added to the study. These had lower fluorine (F) but measurable chlorine (Cl) contents and are enriched in light rare earth elements (LREE), similar to the Durango apatite that is of magmatic origins. Most of the detrital-A apatites are also enriched in LREE similar to the tuff-apatites, but with variable enrichment in the rare earth and yttrium elements. Detrital-B apatites have minimal Cl but also show the magmatic LREE trend. The pore- and fracture-apatites exhibited two trends. In the samples from tuffaceous coals (e.g., Fort Cooper Coal Measures), the pore- and fracture apatites tended to follow the magmatic LREE trend. In the Rangal Coal Measures, the pore- and fracture-apatites have elevated F contents, minimal Cl contents and depleted LREE contents (or Middle REE enrichment). Depletion of LREE could occur through leaching in an acidic environment, which is suggested by the abundance of kaolinite. Apatite precipitates under more neutral conditions although pore-apatites are commonly considered “early” and fracture-apatites “late” or post coalification. Their geochemical similarity suggests they have similar fluid origins and/or timing, with geothermal fluids moving through the porosity afforded by the structured inertinite group macerals and fractures. Verification of that geothermal source still requires further work and a technique that can analyse the isotopic composition—e.g., carbon (13C/12C), oxygen (18O/16O) and strontium (87Sr/86Sr)—of micron-sized crystals in situ.

Tracing fault‐related fluids using in situ trace elements: A case study in Upper Cambrian dolostone in Gucheng area, Tarim Basin, NW China

The Gucheng area, located in the east of the Tarim Basin, is one of the important gas exploration targets, and the Upper Cambrian dolostone is the main pay layer. The dolostone is considered to be altered by hydrothermal fluids, but the hydrothermal altering degrees are still unclear. In this study we report the petrography and in situ trace elemental data, and try to discuss the hydrothermal fluids and their impact on the Upper Cambrian dolostones. Based on the petrography and CL properties, five types of dolomites are identified. MD1 and MD2 are the matrix dolomites. The crystal size of MD1 range from approximately 30 to 100 μm, while the MD2 is larger than 200 μm. Cement dolomite (CD) occurring only in vugs and fractures is subdivided into three types depending on the CL properties. CD‐zone1 is the core of CD with dull CL colours. CD‐zone2 comprises a bright red CL colour zone and a dull CL colour zone, and CD‐zone3 is the rim of CD dolomite with bright red CL colours. MD2 and CD‐zone1 show light rare earth elements (LREE) depletion, middle rare earth elements (MREE) enrichment and negative Eu anomalies. The REEY patterns of CD‐zone3 dolomite are marked by LREE enrichment and positive Eu anomalies. CD‐zone2 is the transitional mineral phase that shows slightly LREE depletion and negative Eu anomalies. The REEY patterns of CD‐zone1 suggest that CD‐zone1 is likely formed by local redistribution of older dolomites in a closed system. The REEY patterns of CD‐zone3 may indicate a magmatic associated hydrothermal fluid. Based on the petrography and in situ trace elemental data, the Permian magmatic fluid, which is recorded in CD‐zone2 and CD‐zone3 dolomites, may be only limited within fractures, while unlikely significantly altered the matrix dolomite in the Upper Cambrian.

Petrogenesis and tectonic evolution of the Palaeozoic to Mesozoic Niuxinshan granitoids in the North Qilian orogen,NWChina

Numerous granitoid suites exposed in the Qilian orogen, NE Tibetan Plateau preserve important records of the tectonic evolution of this orogen. Here we present zircon U–Pb, zircon rare earth elements (REEs), and whole‐rock geochemical and Sr–Nd–Hf isotopic data on Niuxinshan granitoids in the North Qilian orogen, with a view to constrain their petrogenesis and timing of emplacement, as well as to gain new insights into the Early Palaeozoic tectonic evolution of the Qilian orogen. Zircon U–Pb data show multiple concordant ages of ~517, 455–425, 397–389, and 351–319 Ma for syenogranite, and ages of ~758 and 724 Ma together with some scattered Early Mesozoic discordant ages for porphyritic granite, suggesting episodic magmatism from Neoproterozoic to Early Mesozoic. Zircon ∑REEs range from 276.43 to 4,582.32 ppm, and are characterized by depletion of light REEs (LREEs) and enrichment heavy REEs (HREEs) with negative Eu anomalies, indicating a mixture of magmatic and hydrothermal zircons. Whole‐rock geochemical data show similar variation of LREEs, large‐ion lithophile elements (LILEs), HREEs, and high‐field‐strength elements (HFSEs) with zircon REEs, with Eu, Ba, P, and Ti negative anomalies and Rb, U, and Pb positive anomalies. Based on their formation ages, two groups of granitoids are identified. Group I rocks (Early Palaeozoic) are represented by I‐type granites whereas Group II rocks (Early Mesozoic) are of S‐type. Whole‐rock Sr–Nd–Hf isotopes display (87Sr/86Sr)iratios of 0.707085–0.709591, εNd(t) values of −5.3 to −2.3, εHf(t) values of −2.2 to 0, and two‐stage model (Nd and Hf) ages of 1.6–1.4 Ga for Group I rocks, and of 0.728156–0.734113, −9.4 to −9.3, −10.6 to −6.9, and 2.1–1.9 Ga for Group II rocks. Accordingly, we infer that the magma for Group I was sourced from the partial melting of basaltic protoliths in the mafic lower crust and underwent fractional crystallization. Group II was derived from metasedimentary protoliths through reworking of ancient Palaeoproterozoic crustal components. Integrating the results from this study with those from previous studies on Niuxinshan granitoids, we argue that the southward subduction model of the North Qilian Ocean beneath the Qilian Block during the Early Palaeozoic might be a more reasonable scenario. However, the Early Mesozoic magmatic event reported here remains equivocal in the North Qilian orogen.

High Ba–Sr adakitic charnockite suite from the Nagercoil Block, southern India: Vestiges of Paleoproterozoic arc and implications for Columbia to Gondwana

The Nagercoil block is the southernmost crustal segment of the Southern Granulite Terrane (SGT) in India and is mainly composed of charnockitic rocks and felsic gneisses (charnockite suite). In this study, we present petrologic, geochemical, zircon U–Pb, REE, and Hf isotopic studies on the charnockites and leucogneiss from the Nagercoil block. Based on field investigations and petrologic studies, the charnockites can be divided into garnet-bearing and garnet-absent anhydrous granulite facies rocks with orthopyroxene. The charnockites and leucogneiss show transition from adakites to non-adakitic magmatic rocks, with enrichment in LREEs (light rare earth elements) and LILEs (large ion lithophile elements), and depletion in HREEs (heavy rare earth elements) and HFSEs (high field strength elements). Some of the charnockites and the leucogneiss show typical HSA (high silica adakite) characters, (high SiO2, Al2O3, Ba–Sr, La/Yb, and Sr/Y). The HSA is considered to have formed from the interaction of slab derived melts and peridotitic mantle wedge. The high Ba–Sr features were possibly inherited from subducted oceanic crust melting under high thermal gradient during Precambrian. The magmas were underplated and subjected to fractional crystallization. Zircon grains from the charnockite and leucogneiss show zoned magmatic cores surrounded by structureless metamorphic rims. Magmatic zircon grains from the charnockites show ages ranging from 1983 ± 8.8 Ma to 2046 ± 14 Ma, and the metamorphic domains show an age range of 502 ± 14 Ma to 547 ± 8.7 Ma. Zircon from the leucogneiss yielded magmatic and metamorphic ages of 1860 ± 20 Ma and 575.6 ± 8.8 Ma. Both charnockites and leucogneiss show two prominent age peaks at 1987 Ma and 568 Ma. The REE data of the zircon grains show LREE depletion and HREE enrichment, with the metamorphic grains showing more depletion in HREE. Zircon Hf isotopic data of the magmatic cores of zircon grains from the charnockite yielded εHf(t) values from −1.17 to 0.46 with TDM and TDMC and age peaks at 2392 Ma and 2638 Ma, suggesting Neoarchean to Paleoproterozoic juvenile sources. We suggest that the high Ba–Sr adakitic charnockite suite from the Nagercoil block formed in a Paleoproterozoic magmatic arc setting during the assembly of the Columbia supercontinent, and underwent high-grade metamorphism associated with the amalgamation of the Gondwana supercontinent during the late Neoproterozoic–Cambrian. Our study provides new insights into the vestiges of Columbia fragments within the Gondwana assembly with two distinct cycles of crustal evolution.

Carboniferous ridge subduction in the Xingmeng Orogenic Belt: Constraints from geochronological, geochemical, and Sr-Nd-Hf isotopic analysis of strongly peraluminous granites and gabbro-diorites in the Xilinhot micro-continent

The Xingmeng Orogenic Belt evolved through a long-lived orogeny involving multiple episodes of subduction and accretion. However, there is a debate on its tectonic evolution during the Late Paleozoic. Here, we report geochemical, geochronological, and isotopic data from strongly peraluminous granites and gabbro-diorites from the Sunidzuoqi–Xilinhot region. Zircon U–Pb ages suggest that the intrusive rocks were emplaced during the Early Carboniferous (333–322 Ma). The granites exhibit geochemical characteristics similar to S-type granites, with high SiO2 (72.34–76.53 wt.%), Al2O3 (12.45–14.65 wt.%), and A/CNK (1.07–1.16), but depleted Sr, Nb, and Ta contents. They exhibit positive εNd(t) and εHf(t) values (−0.3 to 2.8 and 2.7–5.7, respectively) and young Nd and Hf model ages (TDM2(Nd)=853–1110 Ma and TDM2(Hf)=975–1184 Ma), suggesting that they may be the partial melting products of heterogeneous sources with variable proportions of pelite, psammite, and metabasaltic rocks. The meta-gabbro-diorites from the Maihantaolegai pluton have low SiO2 (47.06–53.49 wt.%) and K2O (0.04–0.99 wt.%) contents, and demonstrate slight light rare earth element (REE) depletion in the chondrite-normalized REE diagrams. They have high zircon εHf(t) values (14.41–17.34) and young Hf model ages (TDM2(Hf)= 230–418 Ma), indicating a more depleted mantle source. The variations of the Sm/Yb and La/Sm ratios can thus be used to assess the melting degree of the mantle source from 5% to 20%, suggesting a quite shallow mantle melting zone. We propose that the petrogenesis and distribution of the strongly peraluminous granites and gabbro-diorites, as well as the tectonic architecture of the region, can be explained by a ridge subduction model. Based on these results, and previous studies, we suggest a southward ridge subduction model for the Sunidzuoqi–Xilinhot region.

Petrogenesis of heterogeneous mantle peridotites with Ni-rich olivine from the Pujada Ophiolite, Philippines

Nickel (Ni) enrichment (>0.40 wt%) in olivine was previously recognized in mantle xenoliths and volcanic rocks. Though this enrichment is often attributed to subarc metasomatic processes, the occurrence of Ni rich olivines in mantle peridotites remains to be established. We found peridotites that randomly contain unusually high-Ni olivines (up to Ni = 0.53 wt%) from the mantle section of the Pujada Ophiolite, Philippines. The Pujada mantle peridotites can be classified into three groups based on their occurrence and spinel Cr#: Group I - massive lherzolites and harzburgites with Cr# = 0.10–0.40, Group II - lherzolite-dunite and harzburgite-dunite interlayers with Cr# = 0.10–0.40, and Group III - interlayered harzburgites and dunites with Cr# >0.40. Rare earth element composition of clinopyroxenes shows general light rare earth element (LREE) depletion and relatively flat heavy rare earth element (HREE) with minor LREE enrichment. The high-Ni olivines are associated to the occurrence of secondary orthopyroxenes, high modal abundance of orthopyroxenes (>30%) and LREE enrichment. The Ni enrichment in olivine is the result of the diffusion of Ni from a pyroxenitic melt. The crystallization of a hybridized melt formed by the combination of pyroxenite melt, primitive MORB melt and siliceous slab-derived melt, shows the same case of formation for the high Ni-olivine phenocrysts in alkali basalts from Hawaii, Kamchatka and the Mexican Volcanic Belt.

Petrography and Geochemistry of the Carboniferous Ortokarnash Manganese Deposit in the Western Kunlun Mountains, Xinjiang Province, China: Implications for the Depositional Environment and the Origin of Mineralization

AbstractThe Upper Carboniferous Ortokarnash manganese ore deposit in the West Kunlun orogenic belt of the Xinjiang province in China is hosted in the Kalaatehe Formation. The latter is composed of three members: (1) the 1st Member is a volcanic breccia limestone, (2) the 2nd Member is a sandy limestone, and (3) the 3rd Member is a dark gray to black marlstone containing the manganese carbonate mineralization, which, in turn, is overlain by sandy and micritic limestone. This sequence represents a single transgression-regression cycle, with the manganese deposition occurring during the highstand systems tract. Geochemical features of the rare earth elements (REE+Y) in the Kalaatehe Formation suggest that both the manganese ore and associated rocks were generally deposited under an oxic water column with Post-Archean Australian Shale (PAAS)-normalized REE+Y patterns displaying characteristics of modern seawater (e.g., light REE depletion and negative Ce anomalies). The manganese ore is dominated by fine-grained rhodochrosite (MnCO3), dispersed in Mn-rich silicates (e.g., friedelite and chlorite), and trace quantities of alabandite (MnS) and pyrolusite (MnO2). The replacement of pyrolusite by rhodochrosite suggests that the initial manganese precipitates were Mn(IV)-oxides. Precipitation within an oxic water column is supported by shale-normalized REE+Y patterns from the carbonate ores that are characterized by large positive Ce (>3.0) anomalies, negative Y (~0.7) anomalies, low Y/Ho ratios (~20), and a lack of fractionation between the light and heavy rare earth elements ((Nd/Yb)PAAS ~0.9). The manganese carbonate ores are also 13C-depleted, further suggesting that the Mn(II) carbonates formed as a result of Mn(III/IV)-oxide reduction during burial diagenesis.

WITHDRAWN: Petrogenesis of heterogeneous mantle peridotites with Ni-rich olivine from the Pujada Ophiolite, Philippines

Abstract Nickel (Ni) enrichment (>0.40 wt%) in olivine was previously recognized in mantle xenoliths and volcanic rocks. Though this enrichment is often attributed to subarc metasomatic processes, the occurrence of Ni rich olivines in mantle peridotites remains to be established. We found peridotites that randomly contain unusually high-Ni olivines (up to Ni = 0.53 wt%) from the mantle section of the Pujada Ophiolite, Philippines. The Pujada mantle peridotites can be classified into three groups based on their occurrence and spinel Cr#: Group I - massive lherzolites and harzburgites with Cr# = 0.10–0.40, Group II - lherzolite-dunite and harzburgite-dunite interlayers with Cr# = 0.10–0.40, and Group III - interlayered harzburgites and dunites with Cr# >0.40. Rare earth element composition of clinopyroxenes shows general light rare earth element (LREE) depletion and relatively flat heavy rare earth element (HREE) with minor LREE enrichment. The high-Ni olivines are associated to the occurrence of secondary orthopyroxenes, high modal abundance of orthopyroxenes (>30%) and LREE enrichment. The Ni enrichment in olivine is the result of the diffusion of Ni from a pyroxenitic melt. The crystallization of a hybridized melt formed by the combination of pyroxenite melt, primitive MORB melt and siliceous slab-derived melt, shows the same case of formation for the high Ni-olivine phenocrysts in alkali basalts from Hawaii, Kamchatka and the Mexican Volcanic Belt.

Geochronology and Geochemistry of Gabbros from Moa‐Baracoa Ophiolitic Massif, Eastern Cuba: Implication for Early Cretaceous SSZ Magmatism

AbstractA series ophiolitic massifs in the northeastern part of Cuba are generally considered to be relics of proto‐Caribbean or Caribbean lithosphere, emplaced during Paleogene collision of the NW Caribbean and North American plates (Iturralde‐Vinent et al., 2016). However, neither their age nor their geodynamic settings are well constrained. They are generally considered to be Creataceous in age but they have been interpreted as having formed in a back‐arc basin, forearc basin or plume setting (e.g. Giunta et al., 2009; Marchesi et al., 2016; Kerr et al., 1999). The Moa‐Baracoa ophiolitic massif consists of mantle harzburgite, a Moho transition zone, layered and isotropic gabbro, and pillow basalts. The isotropic gabbros, collected west of Moa (20°35′ 17.99″N, 75°02′56.58″W), are composed mainly of subhedral plagioclase and subhedral to euhedral clinopyroxene with minor primary brown amphibole. The plagioclase grains are fresh, whereas the clinopyroxene very weakly altered and partly replaced by secondary amphibole. Cathodoluminescence (CL) images of zircon grains separated from isotropic gabbro display oscillatory zoning and broad zoning, analogous to typical igneous zircon in gabbro. Ten zircon grains with oscillatory zoning in CL images yielded a mean U‐Pb SHRIMP age of 136.7±1.8 Ma. Isotropic gabbro samples have low SiO2 (47.22 wt%–49.25 wt%), TiO2 (0.13wt%–0.26 wt%) and K2O (0.04 wt%–0.08 wt%) and high MgO (8.22 wt%–16.72 wt%), Cr (1337–2065 μg/g) and Ni (192–507 μg/g) contents. Their CaO, Al2O3 and Na2O contents vary from 13.56 wt% to 15.78 wt%, 14.98 wt% to 21.98 wt%, and 0.71 wt% to 1.87 wt%, respectively. In the SiO2 versus Zr/TiO2∗0.0001 diagram, the samples plot in the sub‐alkaline basalt field but show calc‐alkaline affinities in the FeOT/MgO vs. SiO2 diagram. The gabbros contain low REE contents (ΣREE = 4.71–7.71), and their chondrite normalized REE patterns show very weak depletion in light REE (LREE, (La/Yb)N = 0.34–0.50) and flat heavy REE (HREE, (Gd/Yb)N = 1.02–1.15) profiles, with slightly positive Eu anomalies (Eu/Eu∗ = 1.55–1.76) suggesting magmatic accumulation of plagioclase. Furthermore, all samples are depleted in Nb and Th, and enriched in Ba, Pb and Sr in the primitive mantle‐normalized trace element diagram. Geochemical features, such as high values of Ba/La (30–38) and low Th/Yb (0.02–0.03), medium values of (Ta/La)PM (0.30–0.35) and (Hf/Sm)PM (0.58–0.80), indicate enrichment by slab‐derived fluids and subduction‐related metasomatism. These isotropic gabbros are geochemically comparable with those of the Kermanshah and Neyriz ophiolites of Iran (Monsef et al., 2018) that are thought to have originated in a SSZ environment. Therefore, we suggest that the isotropic gabbros in the Moa‐Baracoa massif crystallized from early Cretaceous SSZ magmas that were most likely derived from a mantle source metasomatized by slab fluids in a suprasubduction zone environment.

Geochemical constraints on the genesis of the Ekou banded iron formation, Shanxi Province, North China

The Ekou banded iron formation (BIF) in the Wutai area is hosted within the late Archean Baizhiyan formation. The mineral assemblage is used to identify oxide and silicate facies. The oxide facies is composed of magnetite and quartz, and the silicate facies is characterized by the presence of silicate minerals. A geochemical analysis shows that the major elemental compositions are dominantly SiO2 and Fe2O3T, with very little Al2O3 and TiO2 and minor abundances of incompatible elements and transition elements. These results indicate that negligible terrigenous materials were involved in the BIF deposition. The rare-earth elements (REEs), normalized by post-Archean Australian shale, exhibit the characteristics of light REE (LREE) depletion, heavy REE (HREE) enrichment, and positive La, Y, and Eu anomalies. The Y/Ho ratios are superchondritic. These results indicate that the material that formed the Ekou BIF originated from the mixing of seawater and submarine hydrothermal fluids. A Ce anomaly deficiency and heavy Fe isotope enrichment indicate that the Ekou BIF formed in an anoxic marine environment. The δ30SiNBS-28 values for quartz in the Ekou BIF are similar to those of other BIFs with distribution ranges of modern sinters and black smokers. The δ18OV-SMOW distribution is similar to that of hydrothermal sedimentary siliceous rocks. These results suggest that the formation of the Ekou BIF was closely connected to submarine volcanic exhalation activity.

Petro-geochemistry, Sr[sbnd]Nd isotopes and 40Ar/39Ar ages of fractionated alkaline lamprophyres from the Mount Girnar igneous complex (NW India): Insights into the timing of magmatism and the lithospheric mantle beneath the Deccan Large Igneous Province

Mount Girnar is one of the most conspicuous alkaline complexes (gabbro-diorite-syenite-lamprophyre), which intrude some of the earliest erupted basalts (ca. 69 Myr) of the Deccan Large Igneous Province (Deccan LIP) in the Kathiawar plateau of NW India. Petrography, bulk-rock geochemistry, Sr and Nd isotopes and 40Ar/39Ar mineral ages of fractionated (Mg#: 36.3–43.6) lamprophyre dykes (younger intrusives) are reported from two widely separated domains from Mt. Girnar. Petrography and mineral chemistry reveal that major mineral assemblages (pargasite and kaersutite varieties of amphibole, diopside, biotite and feldspar) in the lamprophyres are pristine and devoid of alteration. The lamprophyres belong to the alkaline variety in general and the camptonite - monchiquite series in particular. The bulk-rock major and trace elements of the Girnar lamprophyres display very good correlation with each other and also with those of associated rocks (syenites, diorites and gabbros) which support their genetic relationship. Trace-element ratios do not evidence crustal contamination and reveal derivation of the lamprophyres from partial melting of a lithospheric mantle source significantly modified by interaction with asthenospheric-derived melts, and resembles other alkaline rocks from the Deccan LIP in this regard. Initial 87Sr/86Sr (0.7052–0.7053) and 143Nd/144Nd (0.5125–0.5127) of the Girnar lamprophyres and associated rocks such as syenite, diorite and gabbro are tightly clustered and further attest to their derivation from a cogenetic parental melt. Their positive εNdi values (+0.8 to +3.4) require a mantle source that has experienced moderate long-term depletion of light rare-earth elements. Neoproterozoic/Early Cambrian depleted-mantle (TDM) Nd model ages of ~414–588 Myr are closer to the timing of break-up of the Rodinia supercontinent as well as coincide with the Ediacaran-Cambrian Malagasy orogeny. 40Ar/39Ar dating of three mineral separates (amphibole and biotite) from the lamprophyres gave precise plateau ages of 65.9 ± 0.3 Myr to 66.1 ± 0.4 Myr demonstrating that the emplacement of the Mt. Girnar igneous complex was close to the Cretaceous-Paleogene boundary. The emplacement of a range (110 Myr to 68.5 Myr) of other spatially related alkaline as well as silicic plutonic complexes, such as the Mundwara, Sarnu-Dandali, and Barda complexes prior to the main flood basalt event at ca. 66.0–65.1 Myr, highlights the role of extensional events in pre-existing rift/fault zones preceding the plume-lithosphere interaction in the Deccan LIP.

Unravelling the processes controlling apatite formation in the Phalaborwa Complex (South Africa) based on combined cathodoluminescence, LA-ICPMS and in-situ O and Sr isotope analyses

The Phalaborwa world-class phosphate deposit (South Africa) is hosted by a Paleoproterozoic alkaline complex mainly composed of phoscorite, carbonatite, pyroxenitic rocks, and subordinate fenite. In addition, syenite and trachyte occur in numerous satellite bodies. New petrological and in-situ geochemical data along with O and Sr isotope data obtained on apatite demonstrate that apatite is in the principal host rocks (pyroxenitic rocks, phoscorite and carbonatite) formed primarily by igneous processes from mantle-derived carbonatitic magmas. Early-formed magmatic apatite is particularly enriched in light rare earth elements (LREE), with a decrease in the REE content ascribed to magma differentiation and early apatite fractionation in isolated interstitial melt pockets. Rayleigh fractionation favored a slight increase in δ18O (below 1%) at a constant Sr isotopic composition. Intrusion of fresh carbonatitic magma into earlier-formed carbonatite bodies locally induced re-equilibration of early apatite with REE enrichment but at constant O and Sr isotopic compositions. In fenite, syenite and trachyte, apatite displays alteration textures and LREE depletion, reflecting interaction with fluids. A marked decrease in δ18O in apatite from syenite and trachyte indicates a contribution from δ18O-depleted meteoric fluids. This is consistent with the epizonal emplacement of the satellite bodies. The general increase of the Sr isotope ratios in apatite in these rocks reflects progressive interaction with the country rocks over time. This study made it possible to decipher, with unmatched precision, the succession of geological processes that led to one of the most important phosphate deposits worldwide.

Zircon geochemistry of Edong granitoids in the Middle-Lower Yangtze Metallogenic Belt (Eastern China): Constraints on W-Cu-Fe skarn mineralization

It has long been established that skarn deposits of different metals are associated with intrusions of different compositions. Intermediate-felsic granitoids in the Edong district of the Middle-Lower Yangtze Metallogenic Belt host skarn deposits with varying W-Cu-Fe proportions, yet previous whole-rock geochemical studies could not distinguish between the magmatic suites that formed the different skarn deposits. In this study, LA-ICP-MS zircon geochemical data are presented for the Ruanjiawan W skarn (60300 t WO3), Tongshankou Cu skarn (0.6 Mt Cu), Tonglushan Cu-Fe skarn (1.1 Mt Cu and 56.8 Mt Fe) and Jinshandian Fe skarn (200 Mt Fe) deposits in the Edong district. Zircons from the ore-forming intrusions of these skarn deposits are all characterized by heavy rare earth element (HREE) enrichments and light rare earth element (LREE) depletions, with positive Ce anomalies and slightly negative Eu anomalies. Tungsten contents in zircons from the W ore-forming intrusion (0.02–3.16 ppm W; avg. 1.14 ppm) are generally higher than in zircons from the Cu ore-forming intrusion (0.00–0.24 ppm W; avg. 0.03 ppm). Copper content in zircons from the Cu ore-forming intrusion is higher than in those from the W ore-forming intrusion. Fertile intrusive rocks from the four skarn deposits contain zircons with high calculated Ce4+/Ce3+ ratios (Ruanjiawan: 71.7; Tongshankou: 207.5; Tonglushan: 263.6; Jinshandian: 189.0. Fluorine content of the Ruanjiawan intrusion is higher than the Tonglushan and Jinshandian intrusions, followed by the Tongshankou intrusion. Chlorine contents show the opposite trend. We propose that the different source regions for the ore-fertile intrusions is the critical factor that controls the metal content, oxygen fugacity and volatile components. Our studies suggest that zircon geochemistry is a good indicator to distinguish tungsten causative intrusions from copper- and iron ore related intrusions (i.e. W content ≧ 1.14 ppm and Ce4+/Ce3+ ratios ≦ 207.5).

Petrology and geochemistry of mafic and ultramafic cumulate rocks from the eastern part of the Sabzevar ophiolite (NE Iran): Implications for their petrogenesis and tectonic setting

The Late Cretaceous Sabzevar ophiolite represents one of the largest and most complete fragments of Tethyan oceanic lithosphere in the NE Iran. It is mainly composed of serpentinized mantle peridotites slices; nonetheless, minor tectonic slices of all crustal sequence constituents are observed in this ophiolite. The crustal sequence contains a well-developed ultramafic and mafic cumulates section, comprising plagioclase-bearing wehrlite, olivine clinopyroxenite, olivine gabbronorite, gabbronorite, amphibole gabbronorite and quartz gabbronorite with adcumulate, mesocumulate, heteradcumulate and orthocumulate textures. The crystallization order for these rocks is olivine ​± ​chromian spinel → clinopyroxene → plagioclase → orthopyroxene → amphibole. The presence of primary magmatic amphiboles in the cumulate rocks shows that the parent magma evolved under hydrous conditions. Geochemically, the studied rock units are characterized by low TiO2 (0.18–0.57 ​wt.%), P2O5 (<0.05 ​wt.%), K2O (0.01–0.51 ​wt.%) and total alkali contents (0.12–3.04 ​wt.%). They indicate fractionated trends in the chondrite-normalized rare earth element (REE) plots and multi-element diagrams (spider diagrams). The general trend of the spider diagrams exhibit slight enrichment in large ion lithophile elements (LILEs) relative to high field strength elements (HFSEs) and positive anomalies in Sr, Pb and Eu and negative anomalies in Zr and Nb relative to the adjacent elements. The REE plots of these rocks display increasing trend from La to Sm, positive Eu anomaly (Eu/Eu∗ ​= ​1.06–1.54) and an almost flat pattern from medium REE (MREE) to heavy REE (HREE) region [(Gd/Yb)N ​= ​1–1.17]. Moreover, clinopyroxenes from the cumulate rocks have low REE contents and show marked depletion in light REE (LREE) compared to MREE and HREE [(La/Sm)N ​= ​0.10–0.27 and (La/Yb)N ​= ​0.08–0.22]. The composition of calculated melts in equilibrium with the clinopyroxenes from less evolved cumulate samples are closely similar to island arc tholeiitic (IAT) magmas. Modal mineralogy, geochemical features and REE modeling indicate that Sabzevar cumulate rocks were formed by crystal accumulation from a hydrous depleted basaltic melt with IAT affinity. This melt has been produced by moderate to high degree (~15%) of partial melting a depleted mantle source, which partially underwent metasomatic enrichment from subducted slab components in an intra-oceanic arc setting.