大洋地幔橄榄岩-铬铁矿中的金刚石和深地幔再循环

全球多地蛇绿岩型地幔橄榄岩和铬铁矿中发现微粒金刚石,并在中国西藏南部和俄罗斯乌拉尔北部的蛇绿岩铬铁矿中发现原位产出的金刚石,认为是地球上金刚石的一种新的产出类型,不同于金伯利岩型金刚石和超高压变质型金刚石。它们与呈斯石英假象的柯石英、高压相的铬铁矿和青松矿等高压矿物以及碳硅石和单质矿物等强还原矿物伴生,指示蛇绿岩中的这些矿物组合形成于深度150~300 km或者更深的地幔。金刚石具有很轻的C同位素组成（δ13C-18‰~-28‰）,并出现多种含Mn矿物和壳源成分包裹体。研究认为它们曾是早期深俯冲的地壳物质,达到>300 km深部地幔或地幔过渡带后,经历了熔融并产生新的流体,后者在上升过程中结晶成新的超高压、强还原矿物组合,通过地幔对流或地幔柱作用被带回到浅部地幔,由此建立了一个俯冲物质深地幔再循环的新模式。蛇绿岩型地幔橄榄岩和铬铁矿中发现金刚石等深部矿物,质疑了蛇绿岩铬铁矿形成于浅部地幔的已有认识,引发了一系列新的科学问题,提出了新的研究方向。      

Carbon and nitrogen isotope,and mineral inclusion studies on the diamonds from the Pozanti–Karsanti chromitite,Turkey

The Pozanti–Karsanti ophiolite (PKO) is one of the largest oceanic remnants in the Tauride belt, Turkey. Micro-diamonds were recovered from the podiform chromitites, and these diamonds were investigated based on morphology, color, cathodoluminescence, nitrogen content, carbon and nitrogen isotopes, internal structure and inclusions. The diamonds recovered from the PKO are mainly mixed-habit diamonds with sectors of different brightness under the cathodoluminescence images. The total δ¹³C range of the PKO diamonds varies between ??18.8 and ??28.4‰, with a principle δ¹³C mode at ??25‰. Nitrogen contents of the diamonds range from 7 to 541 ppm with a mean value of 171 ppm, and the δ¹⁵N values range from ??19.1 to 16.6‰, with a δ¹⁵N mode of ??9‰. Stacking faults and partial dislocations are commonly observed in the Transmission Electron Microscopy foils whereas inclusions are rather rare. Combinations of (Ca_0.81Mn_0.19)SiO₃, NiMnCo-alloy and nano-sized, quenched fluid phases were observed as inclusions in the PKO diamonds. We believe that the ¹³C-depleted carbon signature of the PKO diamonds derived from previously subducted crustal matter. These diamonds may have crystallized from C-saturated fluids in the asthenospheric mantle at depth below 250 km which were subsequently carried rapidly upward by asthenospheric melts.     

Carbon and Nitrogen Isotopic,and Mineral Inclusion Studies on the Diamonds from the Pozanti‐Karsanti Chromitite,Turkey

He Pozanti‐Karsanti ophiolite (PKO) is one of the largest oceanic remnants in the Tauride belt, Turkey. Micro‐diamonds were recovered from the podiform chromitites, and these were investigated based on morphology, color, cathodoluminescence, nitrogen content, carbon and nitrogen isotopes, internal structure and inclusions. The diamonds recovered from the PKO are mainly mixed‐habit diamonds with sectors of different brightness under the cathodoluminescence images. The total δ¹³C range of the PKO diamonds ranges between ?18.8 ‰ and ?28.4 ‰, with a principle δ¹³C mode at ?25 ‰. Nitrogen contents of the diamonds range from 7 to 541 μg/g with a mean value of 171 μg/g, and the δ¹⁵N values range from ?19.1 ‰ to 16.6 ‰, with a δ¹⁵N mode of ?9 ‰. Stacking faults and partial dislocations are commonly observed in the Transmission Electron Microscopy foils whereas inclusions are rather rare. Combinations of (Ca_0.81Mn_0.19)SiO₃, NiMnCo‐alloy and nano‐size, quenched fluid phases were observed as inclusions in the PKO diamonds, confirming a natural origin of these diamonds. We believe that the δ¹³C‐depleted carbon signature of the PKO diamonds is a remnant of previously subducted crustal matter. These diamonds may have crystallized in metal‐rich melts in the asthenospheric mantle at depth below 250 km which were subsequently carried rapidly upward by asthenospheric melts/fliuds. We concluded that diamond‐bearing asthenospheric melts were likely involved in the formation of the Pozanti‐Karsanti podiform chromitite.     

Diamond in Oceanic Peridotites and Chromitites: Evidence for Deep Recycled Mantle in the Global Ophiolite Record

Diamonds have been discovered in mantle peridotites and chromitites of six ophiolitic massifs along the 1300 km‐long Yarlung‐Zangbo suture (Bai et al., 1993; Yang et al., 2014; Xu et al., 2015), and in the Dongqiao and Dingqing mantle peridotites of the Bangong‐Nujiang suture in the eastern Tethyan zone (Robinson et al., 2004; Xiong et al., 2018). Recently, in‐situ diamond, coesite and other UHP mineral have also been reported in the Nidar ophiolite of the western Yarlung‐Zangbo suture (Das et al., 2015, 2017). The above‐mentioned diamond‐bearing ophiolites represent remnants of the eastern Mesozoic Tethyan oceanic lithosphere. New publications show that diamonds also occur in chromitites in the Pozanti‐Karsanti ophiolite of Turkey, and in the Mirdita ophiolite of Albania in the western Tethyan zone (Lian et al., 2017; Xiong et al., 2017; Wu et al., 2018). Similar diamonds and associated minerals have also reported from Paleozoic ophiolitic chromitites of Central Asian Orogenic Belt of China and the Ray‐Iz ophiolite in the Polar Urals, Russia (Yang et al., 2015a, b; Tian et al., 2015; Huang et al, 2015). Importantly, in‐situ diamonds have been recovered in chromitites of both the Luobusa ophiolite in Tbet and the Ray‐Iz ophiolite in Russia (Yang et al., 2014, 2015a). The extensive occurrences of such ultra‐high pressure (UHP) minerals in many ophiolites suggest formation by similar geological events in different oceans and orogenic belts of different ages. Compared to diamonds from kimberlites and UHP metamorphic belts, micro‐diamonds from ophiolites present a new occurrence of diamond that requires significantly different physical and chemical conditions of formation in Earth's mantle. The forms of chromite and qingsongites (BN) indicate that ophiolitic chromitite may form at depths of >150‐380 km or even deeper in the mantle (Yang et al., 2007; Dobrthinetskaya et al., 2009). The very light C isotope composition (δ13C ‐18 to ‐28‰) of these ophiolitic diamonds and their Mn‐bearing mineral inclusions, as well as coesite and clinopyroxene lamallae in chromite grains all indicate recycling of ancient continental or oceanic crustal materials into the deep mantle (>300 km) or down to the mantle transition zone via subduction (Yang et al., 2014, 2015a; Robinson et al., 2015; Moe et al., 2018). These new observations and new data strongly suggest that micro‐diamonds and their host podiform chromitite may have formed near the transition zone in the deep mantle, and that they were then transported upward into shallow mantle depths by convection processes. The in‐situ occurrence of micro‐diamonds has been well‐demonstrated by different groups of international researchers, along with other UHP minerals in podiform chromitites and ophiolitic peridotites clearly indicate their deep mantle origin and effectively address questions of possible contamination during sample processing and analytical work. The widespread occurrence of ophiolite‐hosted diamonds and associated UHP mineral groups suggests that they may be a common feature of in‐situ oceanic mantle. The fundamental scientific question to address here is how and where these micro‐diamonds and UHP minerals first crystallized, how they were incorporated into ophiolitic chromitites and peridotites and how they were preserved during transport to the surface. Thus, diamonds and UHP minerals in ophiolites have raised new scientific problems and opened a new window for geologists to study recycling from crust to deep mantle and back to the surface.     

Carbon isotope heterogeneity in metamorphic diamond from the Kokchetav UHP dolomite marble,northern Kazakhstan

We analysed isotopic compositions of metamorphic microdiamond secondary ion mass spectrometry. Typical microdiamonds in this dolomite marble show star-shaped morphologies (S-type) consisting of single-crystal cores and polycrystalline rims. Four S-type microdiamonds and two R-type microdiamonds (single crystals with rugged surfaces) were analysed using a 5 μm diameter ion beam. S-type microdiamonds have heterogeneous carbon isotopic compositions even in a single grain. Analysis of a typical S-type microdiamond (no. xx01-1-13) revealed clear difference in δ¹³C between core and rim. The rim shows lighter isotopic compositions ranging from??17.2‰ to??26.9‰, whereas the core is much heavier, with δ¹³C ranging from??9.3‰ to??13.0‰. The δ¹³C values of R-type microdiamonds fall into narrow ranges from??8.3‰ to??14.9‰ for no. xx01-1-10 and from??8.3‰ to??15.3‰ for no. xx01-1-16. These δ¹³C values are similar to those of the S-type microdiamond cores. The R-type probably formed at the same stage as the core of the S-type, whereas rim growth at a second stage did not occur or occurred very weakly in R-type microdiamonds. These carbon isotopic data support the two-stage growth of microdiamonds in the Kokchetav ultrahigh-pressure host rock. To explain the second stage growth of S-type microdiamonds, we postulate a simple fluid infiltration of light carbon from neighbouring gneisses into the dolomite marble.     

西藏雅鲁藏布江缝合带的普兰地幔橄榄岩中发现金刚石:蛇绿岩型金刚石分类的提出

西藏罗布莎蛇绿岩的地幔橄榄岩和铬铁矿中发现金刚石和特殊矿物群引发了新的问题,罗布莎地幔橄榄岩含特殊地幔矿物是不是一个孤立的特殊现象,或这是一个普遍存在的规律?显然,这是一个至关重要的问题.本文报道在雅鲁藏布江缝合带西段,距离罗布莎1000km以远的普兰蛇绿岩的地幔橄榄岩中发现与罗布莎类似的金刚石和特殊地幔矿物群.普兰的地幔橄榄岩体主体为方辉橄榄岩,含少量的纯橄岩和二辉橄榄岩,研究表明,属典型MOR型亏损地幔橄榄岩.通过分选,在657kg的地幔橄榄岩大样中发现了金刚石和碳硅石等30余种矿物的特殊矿物群,包括自然铬、自然铁和自然锌等强还原条件下形成的单质元素矿物.该矿物群与罗布莎地幔橄榄岩和铬铁矿中发现的特殊矿物群十分相似,表明罗布莎的地幔橄榄岩不是雅鲁藏布江缝合带中的一个特例.结合在俄罗斯乌拉尔Ray-Iz铬铁矿中发现类似的矿物群,以及世界其他地区的有关阿尔卑斯型地幔橄榄岩中金刚石的报道,认为蛇绿岩地幔橄榄岩中可能普遍含有金刚石,并将蛇绿岩地幔橄榄岩中产出的金刚石归为一种新的金刚石产出类型,即蛇绿岩型金刚石,不同于金伯利岩型金刚石和超高压变质带中产出的变质金刚石类型.     

Some specific features of genesis of microdiamonds of octahedral and cubic habit from kimberlites of the Udachnaya pipe (Yakutia) inferred from carbon isotopes and main impurity defects

Microdiamonds (crystals smaller than 1 mm) of octahedral and cubic habit from Udachnaya kimberlite pipe (Yakutia) have been compared in order to distinguish genetic features inferred from carbon isotopic composition and impurity defects. Microdiamonds of cubic habit from the Udachnaya kimberlite pipe have a fibrous internal structure and a high content of nitrogen impurity (400–3000 ppm). Octahedral microdiamonds from the same deposit are distinguished by a low nitrogen content of 0 to 500 ppm and zoning structure. The isotopic composition of carbon (δ¹³C is –4.7‰ for octahedra and –4.5‰ for cuboids) suggests a common source of carbon for these morphologic groups. The studied characteristics can be due to crystallization of octahedra from carbon dissolved in the melt, and cuboids, under the conditions of the hampered diffusion of carbon.     

Carbon isotopic composition of diamonds from the Archangelsk diamond province

The first data are reported on the carbon isotopic composition of diamond crystals from the Grib pipe kimberlite deposit of the Archangelsk diamond province (ADP). The δ¹³C value of the crystals ranges from ?2.79 to ?9.61‰. The isotopic composition of carbon was determined in three zoned crystals (δ¹³C of ?5.8 ?6.96 ‰, ?5.64/ ?5.85 ‰, and ?5.94/ ?5.69 ‰), two “diamond in diamond” samples (diamond inclusion with δ¹³C of ?4.05 and ?6.34 ‰ in host diamond crystals with δ¹³C of ?8.05 and ?7.54 ‰, respectively), and two samples of coated diamonds (cores with δ¹³C of ?6.98 and ?6.78‰ and coats with δ¹³C of ?7.51 and ?8.01 ‰, respectively). δ¹³C values were obtained for individual diamond crystals from bort-type aggregates (δ¹³C of ?4.24/ ?4.05 ‰, ?6.58/ ?7.48 ‰, and ?5.48/ ?6.08 ‰). Correlations were examined between the carbon isotopic composition of diamonds and their crystal morphology; the color; the concentration of nitrogen, hydrogen, and platelet defects; and mineral inclusions content. It was supposed that the observed δ¹³C variations in the crystals are most likely related to the fractionation of carbon isotopes rather than to the heterogeneity of carbon sources involved in diamond formation. The isotopic characteristics of diamonds from the Grib pipe were compared with those of previously investigated diamonds from the Lomonosov deposit. It was found that diamonds from these relatively closely spaced kimberlite fields are different; this also indicates the existence of spatially localized peculiarities of isotope fractionation in processes accompanying diamond formation.     

On the possibility of a kinetic fractionation of nitrogen stable isotopes during natural diamond growth

In a diamond from New South Wales (Australia), cubic and octahedral growth sectors, as identified by cathodoluminescence (CL), show slight differences in N-contents of 29 and 42 ppm respectively but no significant differences in either δ¹³C, δ¹⁵N and nitrogen aggregation state with values at +1.96‰, +19.4‰, and 25% Type IaAB aggregation, respectively.Two gem cubes from the Orapa kimberlite (Botswana) were studied by CL revealing a nonfaceted cubic growth. Accordingly, nine other gem cubes were combusted and yielded δ¹³C-values from -5.33‰ to -6.63‰, δ¹⁵N from -1.0‰ to -5.5‰, and nitrogen contents from 914 to 1168 ppm, with nitrogen aggregation state being only Type IaA (zero % B). The gem cubes show striking similarities to fibrous/coated diamonds, not only in both δ¹³C ranges (less than 3‰ from -5 to -8‰), but also in the high levels of nitrogen (≈ 1000 ppm), suggesting that the two diamond types are related. Additionally, no δ¹⁵N variation was detected between the cube and octahedral growth sectors of the Australian diamond, in the cube sectors of the nine gem cubes from Botswana, nor in fibrous/coated diamonds previously studied. These analyses contrast with an earlier study on a synthetic diamond, which reported a strong kinetic fractionation of N-isotopes of about 40‰ between cube and octahedral growth. The present evidence, therefore, suggests that kinetic fractionation of N-isotopes does not operate during natural diamond formation.     

The application of C isotope measurements to the identification of the sources of C in diamonds: a review

In the past decade, the isotopic compositions of C in > 600 inclusion-bearing diamonds have been determined. Such analyses have revealed the following isotopic characteristics: (1) peridotitic diamonds, which typically contain garnet, chromite, olivine and/or orthopyroxene inclusions with refractory compositions (high Mg, Cr), have δ¹³C values predominantly between −10 and −1‰, with a sharp peak in the distribution near −5‰; (2) eclogitic diamonds, which commonly contain inclusions of omphacitic clinopyroxene, Cr-poor pyrope, and/or eclogitic accessory minerals such as rutile, kyanite, coesite or sanidine, have δ¹³C values between −34 and +3‰, with a smaller peak near −5‰; (3) the isotopic compositions obtained for suites of diamonds from individual occurrences are, in general, unique and do not resemble the range and distribution obtained by amalgamating the diamond isotope data from a number of localities; (4) isotopic zoning patterns and heterogeneities are found in some diamonds; cores of coated diamonds tend to be depleted in¹³C relative to the rims, and within single octahedral diamonds δ¹³C variations of nearly 6‰ have been reported.Because expected C isotope fractionations at mantle temperatures are small, attempts to model the full range of diamond isotope values through fractionating a homogeneous mantle C source have been unsuccessful. Nevertheless, fractionation is probably responsible for some of the observed variation in δ¹³C values. Two other models have also been proposed to account for the diamond characteristics outlined above. The “primordial model” suggests that the range and distribution of C isotope compositions are inherited from primordial C in the mantle which has an inhomogeneous isotopic composition, such as that found in meteorites. The “subduction model” suggests that subducted, crustal C is the source of C in diamonds, as organic and inorganic C compounds in the crust exhibit a range of δ¹³C values similar to that observed in diamonds. This paper reviews the C isotope characteristics of diamonds and compares the models which have been proposed to explain the origins of these characteristics.     

Microcrystalline diamonds in the oceanic lithosphere and their nature

The carbon isotope composition of microdiamonds found in products of the Tolbachik Volcano eruption, Kamchatka (porous lavas and ash), was studied. The isotope composition of microdiamonds (with an average value of δ¹³C =–25.05‰) is close to that of microsized carbon particles in lavas (from–28.9 to–25.3‰). The general peculiarities of the diamond-forming environment include (1) no evidence for high pressure in the medium; (2) a reduced environment; and (3) mineralogical evidence for the presence of a fluid. The geochemical data characterizing the type of diamonds studied allow us to suggest that they were formed in accordance with the mechanism of diamond synthesis during cavitation in a rapidly migrating fluid, which was suggested by E.M. Galimov.     

The nature and origin of sideritic ironstone bands in the Tertiary Lowmead and Duaringa Basins,Queensland

Sideritic ironstones in Tertiary lacustrine oil shale from the Lowmead and Duaringa Basins in Queensland, contain two distinctive types of siderite in the ironstone bands: sphaerosiderite in the mudstone and coal, and finely crystalline siderite in the lamosite. The petrological evidence indicates that the siderite in the ironstone bands formed eogenetically by growing displacively within the soft sediment. Chemically the siderite is very pure though the sphaerosiderite sometimes shows compositional zoning. Stable oxygen and carbon isotope analyses of the siderite show a wide range of values from ‐12.8‰ to ‐2.4 %₀ δ¹⁸O (PDB) and ‐5.5‰ to +12.9‰ δ¹³C (PDB) for the Lowmead Basin; and ‐9.6‰ to ‐1.2‰ δ¹⁸O (PDB) and ‐18.6‰ to +16.4‰ δ¹³C (PDB) for the Duaringa Basin. The oxygen isotope data indicate that the siderite formed in freshwater environments but not in isotopic equilibrium with the formation waters. Kinetic factors offer the most plausible explanation for the anomalously light δ¹⁸O values of many of the siderites. The carbon isotope data show that the carbonate for the formation of the siderite originated predominantly from methanogenic fermentation processes but there was also the varying influence of bacterial oxidation processes. The different petrological and isotopic characteristics of the ironstones broadly reflect variations in their depositional environments and the variable eogenetic conditions in which the siderite formed. There is no suitable single model to explain the genesis of all the different types of ironstones other than that a synsedimentary iron‐enrichment process is involved.     

Oxygen isotope composition of syngenetic inclusions in diamond from the Finsch Mine,RSA

Oxygen isotope data have been obtained for silicate inclusions in diamonds, and similar associated minerals in peridotitic and eclogitic xenoliths from the Finsch kimberlite by laser-fluorination. Oxygen isotope analyses of syngenetic inclusions weighing 20–400 μg have been obtained by laser heating in the presence of ClF₃. ¹⁸O/¹⁶O ratios are determined on oxygen converted to CO₂ over hot graphite and, for samples weighing less than 750 μg (producing <12 μmoles O₂) enhanced CO production in the graphite reactor causes a systematic shift in both δ¹³C and δ¹⁸O that varies as a function of sample weight. A “pressure effect” correction procedure, based on the magnitude of δ¹³C (CO₂) depletion relative to δ¹³C (graphite), is used to obtain corrected δ¹⁸O values for inclusions with an accuracy estimated to be ±0.3‰ for samples weighing 40 μg.Syngenetic inclusions in host diamonds with similar δ¹³C values (−8.4‰ to −2.7‰) have oxygen isotope compositions that vary significantly, with a clear distinction between inclusions of peridotitic (+4.6‰ to +5.6‰) and eclogitic paragenesis (+5.7‰ to +8.0‰). The mean δ¹⁸O composition of olivine inclusions is indistinguishable from that of typical peridotitic mantle (5.25 ± 0.22‰) whereas syngenetic purple garnet inclusions possess relatively low δ¹⁸O values (5.00 ± 0.33‰). Reversed oxygen isotope fractionation between olivine and garnet in both diamond inclusions and diamondiferous peridotite xenoliths suggests that garnet preserves subtle isotopic disequilibrium related to genesis of Cr-rich garnet and/or exchange with the diamond-forming fluid. Garnet in eclogite xenoliths in kimberlite show a range of δ¹⁸O values from +2.3‰ to +7.3‰ but garnets in diamondiferous eclogites and as inclusions in diamond all have values >4.7‰.     

Discovery and Significance of Diamonds and Moissanites in Chromitite within the Skenderbeu Massif of the Mirdita Zone Ophiolite, West Albania

In recent years diamonds and other unusual minerals(carbides,nitrides,metal alloys and native elements) have been recovered from mantle peridotites and chromitites(both high-Cr chromitites and high-Al chromitites) from a number of ophiolites of different ages and tectonic settings.Here we report a similar assemblage of minerals from the Skenderbeu massif of the Mirdita zone ophiolite,west Albania.So far,more than 20 grains of microdiamonds and 30 grains of moissanites(SiC) have been separated from the podiform chromitite.The diamonds are mostly light yellow,transparent,euhedral crystals,200~300 μm across,with a range of morphologies;some are octahedral and cuboctahedron and others are elongate and irregular.Secondary electron images show that some grains have well-developed striatums.All the diamond grains have been analyzed and yielded typical Raman spectra with a shift at ~1325 cm~(-1).The moissanite grains recovered from the Skenderbeu chromitites are mainly light blue to dark blue,but some are yellow to light yeUow.All the analyzed grains have typical Raman spectra with shifts at 766 cm~(-1),787 cm~(-1),and 967 cm~(-1).The energy spectrums of the moissanites confirm that the grains are composed entirely of silicon and carbon.This investigation expands the occurrence of diamonds and moissanites to Mesozoic ophiolites in the Neo-Tethys.Our new findings suggest that diamonds and moissanites are present,and probably ubiquitous in the oceanic mantle and can provide new perspectives and avenues for research on the origin of ophiolites and podiform chromitites.     

Lithium enrichment and isotopic variation in minerals from peridotite xenoliths from northwestern Ethiopian plateau

We report Lithium (Li) concentrations and isotopic compositions for co-existing olivine, orthopyroxene (opx), and clinopyroxene (cpx) mineral separates from depleted and metasomatised peridotite xenoliths hosted by basaltic lavas from northwestern Ethiopian plateau (Gundeweyn area). The peridotites contain five lherzolites and one harzburgite and are variably depleted and enriched in LREE relative to HREE. In both depleted and enriched lherzolites, Li is preferentially incorporated into olivine (2.4-3.3 ppm) compared to opx (1.4-2.1 ppm) and cpx (1.4-2.0 ppm) whereas the Li contents of olivines (5.4 ppm) from an enriched harzburgiteare higher than those of lherzolites. Olivines from the samples show higher Li abundances than normal mantle olivines (1.6-1.9 ppm) indicating the occurrence of Li enrichments through melt-preroditite interaction. The average δ⁷ Li values range from +2.2 to +6.0‰ in olivine, from -0.1 to +2.0‰ in opx and from -4.4 to -0.9‰ in cpx from the lherzolites. The Li isotopic composition (3.5‰) of olivines from harzburgite fall within the range of olivine from lherzolites but the opxs show low in δ⁷Li (-2.0‰). Overall Li isotopic compositions of olivines from the peridotites fall within the range of normal mantle olivine, δ⁷Li values of ~+4±2‰ within uncertainty, reflecting metasomatism (enrichment) of the peridotites by isotopically heavy Li-rich asthenospheric melt. Li isotope zonation is also observed in most peridotite minerals. Majority of olivine grains display isotopically heavy cores and light rims and the reverse case is observed for some olivine grains. Orthopyroxene and clinopyroxene grains show irregular distribution in δ⁷Li. These features of Li isotopic compositions within and between grains in the samples reflect the effect of diffusion-driven isotopic fractionation during meltperidotite interaction and cooling processes.     

Diamond growth from oxidized carbon sources beneath the Northern Slave Craton, Canada: A δC-N study of eclogite-hosted diamonds from the Jericho kimberlite

Diamonds from high- and low-MgO groups of eclogite xenoliths from the Jericho kimberlite, Slave Craton, Canada were analyzed for carbon isotope compositions and nitrogen contents. Diamonds extracted from the two groups show remarkably different nitrogen abundances and δ¹³C values. While diamonds from high-MgO eclogites have low nitrogen contents (5-82 ppm) and extremely low δ¹³C values clustering at ∼−40‰, diamonds from the low-MgO eclogites have high nitrogen contents (>1200 ppm) and δ¹³C values from −3.5‰ to −5.3‰.Coupled cathodoluminescence (CL) imaging and SIMS analysis of the Jericho diamonds provides insight into diamond growth processes. Diamonds from the high-MgO eclogites display little CL structure and generally have constant δ¹³C values and nitrogen contents. Some of these diamonds have secondary rims with increasing δ¹³C values from −40‰ to ∼−34‰, which suggests secondary diamond growth occurred from an oxidized growth medium. The extreme negative δ¹³C values of the high-MgO eclogite diamonds cannot be produced by Rayleigh isotopic fractionation of average mantle-derived carbon (−5‰) or carbon derived from typical organic matter (∼−25‰). However, excursions in δ¹³C values to −60‰ are known in the organic sedimentary record at ca. 2.7 and 2.0 Ga, such that diamonds from the high-MgO eclogites could have formed from similar organic matter brought into the Slave lithospheric mantle by subduction.SIMS analyses of a diamond from a low-MgO eclogite show an outer core with systematic rimwards increases in δ¹³C values coupled with decreases in nitrogen contents, and a rim with pronounced alternating growth zones. The coupled δ¹³C-nitrogen data suggest that the diamond precipitated during fractional crystallization from an oxidized fluid/melt from which nitrogen was progressively depleted during growth. Model calculations of the co-variation of δ¹³C-N yielded a partition coefficient (K_N) value of 5, indicating that nitrogen is strongly compatible in diamond relative to the growth medium. δ¹³C values of diamond cores (−4‰) dictate the growth medium had higher δ¹³C values than primary mantle-derived carbon. Therefore, possible carbon sources for the low-MgO eclogite diamonds include oxidized mantle-derived (e.g. protokimberlite or carbonatite) fluids/melts that underwent some fractionation during migration or, devolatilized subducted carbonates.     

Organic geochemical record of increased productivity in Lake Naukuchiyatal, Kumaun Himalayas, India

Organic geochemical proxies have been studied in a 45-cm-long core retrieved from Lake Naukuchiyatal in Kumaun Himalayas, India. Increase in TOC, N, hydrocarbons and pigments concentration from bottom to surface sediments of the core indicates increase in the lake productivity. Stable isotopes (δ¹³C and δ ¹⁵ N), biomarkers (TAR, CPI and n-ΣC_15,17,19) and C/N atomic (between 9 and 12) suggest dominance of algal derived organic matter in these sediments. Decrease in organic δ¹³C values (between ?27 and ?31‰) in surface sediments indicate influence of sewage and land runoff in shifting organic δ¹³C values, whereas low (between ?0.23 and 2.2‰) δ¹⁵N values together with high pigment concentrations (zeaxanthin and echinenone) represent dominance of cyanobacteria in the lake.     

Geochemistry of Triassic Carbonates: Exploration Guide to Pb–Zn Mineralization in North Tunisia

The Triassic carbonate rocks in Northern Tunisia (Nappes, Domes, Jurassic Mountains zones), consist of massive carbonates, clays and gypsum with authigenic minerals. These are associated with several Pb–Zn deposits and occurrences. At Jebel Ichkeul, Bechateur and Oum Edeboua, these Triassic carbonates exhibit enrichment in Pb (0.32 to 228 ppm), Zn (17 to 261 ppm), Cd (5 to 6 ppm) and Co (0.3 to 89.5 ppm), with respect to their average contents in crustal carbonates. The enrichment is more pronounced at Oum Edeboua (near the ore zone). Permeability is one of the most effective factors of dispersion of metallic trace elements, causing the development of geochemical halos. The genetic relationship of the Triassic carbonate rocks with the ore deposits was controlled by diapirism and tectonic movements, which favored mineralization along the Triassic‐cover contact as well as the remobilization of metals from the mineralized rocks. Analysis of metallic trace elements in Triassic rocks provides clues to the presence of possible mineral deposits. These could be effectively used for both geochemical interpretation and mineral exploration. Carbon and O‐isotope data (– 9.3‰ < δ¹³C < +3‰; +21.9 < δ¹⁸O < +31‰) suggest that the Triassic carbonates of all study areas have marine carbonates as their origin; some of them show significantly lower δ¹⁸O values indicating some exchange with hydrothermal fluids. Calcites associated with mineralization at Oum Edeboua have δ¹³C of –6.2‰ to –8.22‰ and δ¹⁸O of +24.88‰ to +25‰. The C‐isotope compositions of these calcites are ¹³C depleted, indicating an organic origin.     

Application of stable carbon isotopes for reconstructing salt‐marsh floral zones and relative sea level,New Jersey,USA

We investigated use of δ¹³C in bulk organic sediment to define the botanical origin of samples preserved in coastal sediment as a means to reconstruct relative sea level in New Jersey, USA. Modern transects at three sites demonstrated that low and high salt‐marsh floral zones dominated by C₄ species (Spartina alterniflora and Spartina patens) were associated with sediment δ¹³C values between ?18.9‰ and ?15.8‰ and occurred from mean tide level (MTL) to mean higher high water (MHHW). Brackish transitional settings vegetated by Phragmites australis with Iva fructescens and Typha sp. (C₃ species) and freshwater upland samples (C₃ species) were characterized by bulk sediment δ¹³C values of ?27.0‰ to ?22.0‰ and existed above MHHW. Parallel transects at one site suggested that intra‐site variability was not discernible. The utility of δ¹³C values for reconstructing relative sea level in New Jersey is limited by an inability to differentiate between brackish sediments related to sea level and freshwater upland samples. To facilitate this distinction in a 4.4 m core, we used a multi‐proxy approach (δ¹³C values with presence or absence of agglutinated foraminifera) to recognize indicative meanings for four sample types. Sediment with δ¹³C values greater than ?18.9‰ was derived from a vegetated salt‐marsh and formed between MTL and MHHW. Sediment with δ¹³C values less than ?22.0‰ and containing agglutinated foraminifera formed in a brackish transitional zone between MHHW and highest astronomical tide (HAT). This is the narrowest elevational range of the four sample types and most precise sea‐level indicator. Sediment with δ¹³C values less than ?22.0‰ and lacking foraminifera can only constrain the upper bound of former sea level. Samples with intermediate values (?22.0‰ to ?18.9‰) formed between MTL and HAT. Using these indicative meanings and radiocarbon dates, we suggest that a transition from brackish to salt‐marsh δ¹³C values recorded in the core took approximately 350 years (from 1800 to 1450 cal. a BP). Copyright © 2011 John Wiley & Sons, Ltd.     

Microscale Simultaneous Measurement of Carbon and Nitrogen Isotopes on Natural Diamond

Simultaneous analysis of carbon and nitrogen isotope ratios by SIMS was applied for the first-time to a natural diamond from the Kelsey Lake kimberlite, State Line Distinct, Colorado (UWD-1). This in situ procedure is faster, reduces sample size for analysis, and measures both isotope ratios from a single ~ 10 μm diameter pit, a critical advantage for zoned diamonds. The carbon isotope ratio (expressed as δ¹³C_VPDB) of the bulk UWD-1 crystal, determined by the conventional combustion method in the present study, is -5.9‰ ± 0.2‰ (VPDB, 2s). Nitrogen mass fraction ([N]) and isotope ratio (expressed as δ¹⁵N_Air) were determined by stepwise combustion and gas-source mass-spectrometry, resulting in 553 ± 64 μg g^-1 and -6.7‰ ± 1.1‰ (Air, 2s), respectively. Secondary ions of ¹²C₂^-, ¹²C¹³C^-, ¹²C¹⁴N^-, and ¹²C¹⁵N^- were simultaneously measured by SIMS using three Faraday cups and one electron multiplier. The spot-to-spot reproducibility of δ¹³C and δ¹⁵N values for the UWD-1 (178 spots on sixteen chips, 10 μm spots), were 0.3‰ and 1.6‰, respectively (2s). While ¹²C¹⁴N^-/¹²C₂^- ratios, which are an indicator for [N], varied up to 12% among these sixteen chips, such variation did not correlate with either δ¹³C or δ¹⁵N values. We propose that UWD-1 is a suitable reference sample for microscale in situ analysis of δ¹³C and δ¹⁵N values in diamond samples.