Garnet Inclusions In Diamonds Research Articles

Ancient Metasomatism in the Lithospheric Mantle, Eastern North China Craton: Insights from In-Situ Major and Trace Elements in Garnet Xenocrysts, Mengyin District

Mantle metasomatism refers to the interaction between mantle melt, fluid, and mantle rock. It not only affects the physical and chemical properties of the lithospheric mantle but also plays an important role in the process of metal and gem mineralization. In order to explore the nature and evolution of metasomatism in the lithospheric mantle of the Mengyin area in the eastern part of the North China Craton, this paper combines the previous data of garnet inclusions in diamonds and analyzes the major and trace elements of garnet xenocrysts in the Shengli No. 1 kimberlite pipe from the EPMA and LA-ICP-MS experiments. The experiments show that the garnet xenocrysts of the Shengli No. 1 kimberlite pipe are mainly lherzolitic and harzburgitic garnets. The content of Zr and TiO2 in some garnets are low, which are the characteristics of depleted garnets. Conversely, another group of garnets display high Zr and TiO2 contents, indicative of high-temperature melt metasomatism. When comparing the Ti/Eu ratio of the depleted garnets to that of the primary mantle, a significantly lower value is observed. Additionally, the (Sm/Er)N value undergoes minimal changes, while the Zr/Hf value exceeds that of the primary mantle. These characteristics are indicators of carbonatite melt metasomatism. Garnets that are affected by high-temperature melt metasomatism exhibit low (Sm/Er)N content, a significant variation in the Ti/Eu ratio, and a Zr/Hf value greater than that of the primary mantle. These characteristics indicate the influence of kimberlite melt metasomatism. Garnets impacted by carbonatite melt metasomatism display a strong sinusoidal distribution pattern of rare earth elements (REE) and are often found as lherzolitic garnet xenocrysts and garnet inclusions in diamond. On the other hand, garnets influenced by kimberlite melt metasomatism exhibit a slight sinusoidal REE distribution pattern in harzburgitic garnets and a slight sinusoidal REE distribution or a flat pattern from medium rare earth elements (MREEs) to heavy rare earth elements (HREEs) in lherzolitic garnet xenocrysts. Based on these findings, it is evident that there are at least two types of metasomatism occurring in the lithospheric mantle of the Mengyin area in the eastern part of the North China Craton. The first type involves the metasomatism of early carbonatite melt to the mantle peridotite. Garnets formed under this condition exhibit high Sr and LREE contents, as well as low Zr, Hf, Ti, Y, and HREE contents, indicating depletion characteristics. The second type entails the metasomatism of late kimberlite melts affecting the mantle peridotite. Garnets formed under this process display high Zr, Hf, Ti, Y, and HREE contents.

A new pyrope-based mineralogical-petrological method for identifying the diamond potential of kimberlite/lamproite deposits

• Fluid compositions can be additional indicator of the carbon formation in petrology. • Optimum conditions for diamond growth and preservation correspond to water-rich fluid. • Estimated fluid of the main part of the high diamond grade pipes corresponds to water-rich fluid. • Calculated fluid compositions from EMP pyrope analyses can be used in the prospecting of diamond deposits. P-T- Oxygen fugacity (fO 2 ) conditions and fluid compositions were estimated for the formation conditions of pyrope garnet inclusions in diamonds and xenocrysts from diamond-bearing and diamond-free kimberlites using their total chemical analyses and single oxythermobarometry. Our data indicate that optimal conditions for diamond growth and preservation occur in the presumed water-rich mantle fluids containing the lowest abundance of free atomic carbon. The majority of the calculated C-H-O fluid compositions for diamond formation in peridotite xenoliths from high diamond grade kimberlites correspond to a high hydrogen and low carbon and oxygen atomic fluid percents, while those from the majority of peridotite xenoliths in the low grade diamond kimberlites corresponds to the low hydrogen, high carbon and oxygen atomic percent fluids. This new approach defines the conditions of diamond formation for kimberlitic deposits. It better characterizes diamond grades in kimberlites in comparison to the previous empirical mineralogical Ca-Cr methods and can be used as a more precise mineralogical-petrological method for prospecting for kimberlitic diamond deposits.

Elastic properties of majoritic garnet inclusions in diamonds and the seismic signature of pyroxenites in the Earth's upper mantle

AbstractMajoritic garnet has been predicted to be a major component of peridotite and eclogite in Earth's deep upper mantle (>250 km) and transition zone. The investigation of mineral inclusions in diamond confirms this prediction, but there is reported evidence of other majorite-bearing lithologies, intermediate between peridotitic and eclogitic, present in the mantle transition zone. If these lithologies are derived from olivine-free pyroxenites, then at mantle transition zone pressures majorite may form monomineralic or almost monomineralic garnetite layers. Since majoritic garnet is presumably the seismically fastest major phase in the lowermost upper mantle, the existence of such majorite layers might produce a detectable seismic signature. However, a test of this hypothesis is hampered by the absence of sound wave velocity measurements of majoritic garnets with relevant chemical compositions, since previous measurements have been mostly limited to synthetic majorite samples with relatively simple compositions. In an attempt to evaluate the seismic signature of a pyroxenitic garnet layer, we measured the sound wave velocities of three natural majoritic garnet inclusions in diamond by Brillouin spectroscopy at ambient conditions. The chosen natural garnets derive from depths between 220 and 470 km and are plausible candidates to have formed at the interface between peridotite and carbonated eclogite. They contain elevated amounts (12–30%) of ferric iron, possibly produced during redox reactions that form diamond from carbonate. Based on our data, we model the velocity and seismic impedance contrasts between a possible pyroxenitic garnet layer and the surrounding peridotitic mantle. For a mineral assemblage that would be stable at a depth of 350 km, the median formation depth of our samples, we found velocities in pyroxenite at ambient conditions to be higher by 1.9(6)% for shear waves and 3.3(5)% for compressional waves compared to peridotite (numbers in parentheses refer to uncertainties in the last given digit), and by 1.3(13)% for shear waves and 2.4(10)% for compressional waves compared to eclogite. As a result of increased density in the pyroxenitic layer, expected seismic impedance contrasts across the interface between the monomineralic majorite layer and the adjacent rocks are about 5–6% at the majorite-eclogite-interface and 10–12% at the majoriteperidotite-boundary. Given a large enough thickness of the garnetite layer, velocity and impedance differences of this magnitude could become seismologically detectable.

Major and trace element partitioning between majoritic garnet, clinopyroxene, and carbon dioxide-rich liquid in model carbonated peridotite at 10 GPa and interpretations of the element chemistry of majoritic garnet inclusions in diamonds from the subcontinental mantle of Brazil and Guinea

Experimentally determined major and trace element partition coefficients between majoritic garnet, clinopyroxene, and carbon dioxide-rich liquid are reported at 10 GPa and 1800 °C in a model carbonated peridotite composition in the system CaO-MgO-Al2O3-SiO2-CO2. Besides majoritic garnet, the liquid coexists with forsterite, orthopyroxene, and clinopyroxene, making melting phase relations invariant at fixed pressure and temperature conditions. Partition coefficients span a wide range of values - for instance, Sr, Nb, Ba, La, and Ce are highly incompatible in majoritic garnet, while Ca, Y, Nb, and Ho are moderately incompatible, and Lu, Si, Al, and Mg are compatible. Strong fractionation of light rare earth elements (e.g., La, Ce, Nd, Sm) and high field strength elements (e.g., Nb, Ta, Zr, Hf, Th) is seen between majoritic garnet and liquid. The experimentally determined partitioning values are used to calculate compositions of melts in equilibrium with majoritic garnet inclusions in diamonds from select localities in Brazil and Guinea. The calculated melts largely straddle those between documented carbonatites, kimberlites, and alkali basalts, low-degree mantle melting products from carbonated peridotite. This resemblance firmly suggests that majoritic garnet inclusions in diamonds from Brazil and Guinea can simply be interpreted as precipitates from such melts, thereby offering an alternative to the hypothesis that the element chemistry of such inclusions in diamonds can largely, and sometimes only, be ascribed to subducted oceanic crust, and further that, fusion of this crust may limit the terrestrial 'carbon recycling' at depths much beyond corresponding to those of Earth's transition zone.

Parageneses of Garnet Inclusions in Diamonds From Yakutia Kimberlites Based on Raman and IR Spectroscopy Data

Diamonds with garnet inclusions have been studied from the Udachnaya, Zapolyarnaya, Komsomolskaya-Magnitnaya, Yubileynaya, Komsomolskaya, Aikhal, Mir, and Mayskaya kimberlite pipes of the Yakutia diamondiferous province. A positive correlation has been revealed between the positions of the main peaks in the Raman spectra of garnets and their chemical composition. A method for determining the paragenetic association of garnet inclusions by the Raman spectroscopy data is described. Diamonds with garnet inclusions of eclogite paragenesis are characterized by a high total nitrogen content and medium degree of aggregation, whereas diamonds with garnet inclusions with dunite–harzburgite and lherzolite parageneses are distinguished by a low nitrogen concentration and a wide variation in the degree of nitrogen aggregation. Dependences have also been revealed between the thermal–temporal parameters of diamond formation (according to IR spectroscopy of diamonds) and type of rocks (according to Raman spectroscopy of garnet inclusions).

Metasomatic Processes in the Lithospheric Mantle Beneath the No. 30 Kimberlite (Wafangdian Region, North China Craton)

AbstractThis paper presents the first major and trace element compositions of mantle-derived garnet xenocrysts from the diamondiferous No. 30 kimberlite pipe in the Wafangdian region, and these are used to constrain the nature and evolution of mantle metasomatism beneath the North China Craton (NCC). The major element data were acquired using an electron probe micro-analyzer and the trace element data were obtained using laser ablation inductively coupled plasma-mass spectrometry. Based on Ni-in-garnet thermometry, equilibrium temperatures of 1107–1365 °C were estimated for peridotitic garnets xenocrysts from the No. 30 kimberlite, with an average temperature of 1258 °C, and pressures calculated to be between 5.0 and 7.4 GPa. In a CaO versus Cr2O3 diagram, 52% of the garnets fall in the lherzolite field and 28% in the harzburgite field; a few of the garnets are eclogitic. Based on rare earth element patterns, the lherzolitic garnets are further divided into three groups. The compositional variations in garnet xenocrysts reflect two stages of metasomatism: early carbonatite melt/fluid metasomatism and late kimberlite metasomatism. The carbonatite melt/fluids are effective at introducing Sr and the light rare earth elements, but ineffective at transporting much Zr, Ti, Y, or heavy rare earth elements. The kimberlite metasomatic agent is highly effective at element transport, introducing, e.g., Ti, Zr, Y, and the rare earth elements. Combined with compositional data for garnet inclusions in diamonds and megacrysts from the Mengyin and Wafangdian kimberlites, we suggest that these signatures reflect a two-stage evolution of the sub-continental lithospheric mantle (SCLM) beneath the NCC: (1) early-stage carbonatite melt/fluid metasomatism resulting in metasomatic modification of the SCLM and likely associated with diamond crystallization; (2) late-stage kimberlite metasomatism related to the eruption of the 465 Ma kimberlite.

Making Cratonic Lithospheric Mantle

AbstractThe origin of cratonic lithospheric mantle has been attributed to either high‐pressure (5–7 GPa) melting in hot mantle plumes or low‐pressure (<5 GPa) melting in mid‐ocean ridges or suprasubduction zones. To resolve this long‐standing debate, it is necessary to confirm under what depths the incipient cratonic mantle melted. Compared with most cratonic mantle xenoliths and xenocrysts, which commonly experienced metasomatic modification after melt extraction, diamond inclusions with predominantly harzburgitic compositions can better track the compositional signature of pristine cratonic mantle. This paper presents thermodynamic calculations performed with THERMOCALC software to establish a quantitative relationship between garnet and cratonic peridotite compositions. Along the normal cratonic geotherm (40 mW/m2), the XCa [atomic Ca/(Ca + Mg + Fe2+)] and Cr# [atomic Cr/(Cr + Al) × 100] values in garnet depend mainly on the bulk CaO/Al2O3 and Cr# values, respectively. Furthermore, mantle melting modeling shows that the Cr# of the residue displays a negative correlation with melting pressure at melt fractions of greater than ~15%. Therefore, the high Cr# values (mostly 12–36) of global garnet inclusions in diamond support a low‐pressure (<4 GPa) origin for cratonic lithospheric mantle. More importantly, the incipient cratonic mantle calculated from the garnet inclusions exhibits similar bulk CaO/Al2O3 and Cr# compositions to oceanic lithosphere but different values from arc lithosphere with higher CaO/Al2O3 and Cr# values. These results imply that cratonic mantle was initially formed by the extensive melting of hot ambient mantle in the Archean ocean ridge environments. The shallow oceanic lithosphere was subsequently stacked to generate the thick and stable cratonic lithospheric mantle.

Парагенезисы включений гранатов в алмазах из кимберлитов Якутии по данным рамановской и ИК-спектроскопии

Парагенезисы включений гранатов в алмазах из кимберлитов Якутии по данным рамановской и ИК-спектроскопии

The depth of sub-lithospheric diamond formation and the redistribution of carbon in the deep mantle

Most diamonds form in the Earth's lithosphere but a small proportion contain Si-rich majoritic garnet inclusions that indicate formation in the deeper mantle. The compositions of syngenetic garnet inclusions can potential yield information on both the depth and mantle lithology in which the diamonds formed. Pressure dependent changes in garnet compositions have been calibrated using the results of experiments conducted in a multi-anvil apparatus at pressures between 6 and 16 GPa and temperatures of 1000 to 1400 °C. Using the results of these experiments a barometer was formulated based on an empirical parameterisation of the two major majoritic substitutions, referred to as majorite (Maj; Al3+=Mg2++Si4+), and Na-majorite (Na-Maj; Mg2++Al3+=Na++Si4+). Moreover, previously published experimental garnet compositions from basaltic, kimberlite, komatiite and peridotite bulk compositions were included in the calibration, which consequently covers pressures from 6 to 20 GPa and temperatures from 900 to 2100 °C. Experimental pressures are reproduced over these conditions with a standard deviation of 0.86 GPa.The barometer is used to determine equilibration pressures of approximately 500 reported garnet inclusions in diamonds from a range of localities. As the majority of these inclusions are proposed to be syngenetic this allows a detailed picture of diamond formation depths and associated source rocks to be established using inclusion chemistry. Geographic differences in diamond source rocks are mapped within the sub-lithospheric mantle to over 500 km depth. Continuous diamond formation occurs over this depth range within lithologies with eclogitic affinities but also in lithologies that appear transitional between eclogitic and peridotitic bulk compositions, with an affinity to pyroxenites. The geographic differences between eclogitic and pyroxenitic diamond source rocks are rationalised in terms of diamond formation within downwelling and upwelling regimes respectively. Macroscopic diamond formation in rocks with pyroxenite compositions are likely facilitated in the deep mantle by higher average oxidation states and low mineral H2O solubility compared to the surrounding mantle, which aid the mobility of C–O–H volatile species. The apparent lack of inclusions with a peridotite affinity may result from generally low oxygen fugacities in such lithologies, which reduces carbon mobility, and the lack of a suitable oxidising agent to allow diamonds to form from CH4. This glimpse of deep carbon cycle processes implies that heterogeneities in the carbon content, redox state and chemical composition of the mantle may be strongly coupled.

From phosphates to silicates and back: An experimental study on the transport and storage of phosphorus in eclogites during uplift and exhumation

High P-T experiments have shown that in major rock types such as MORBs, peridotites, and pelitic sediments, increasing P (and T ) leads to a gradual transfer of P from phosphates mostly represented by apatite to silicates with garnet as most important silicate P carrier. This is due to the formation of a Na 3 Al 2 (PO 4 ) 3 phase component in garnet via [ 8 ] Na [ 4 ] P [ 8 ] M − 1 2 + [ 4 ] Si − 1 that is strongly P -, and to a lesser extent T -dependent and creates garnets with significant P and Na at P as low as 2–3 GPa. Based on this experimental evidence, one would expect to routinely find P-Na-rich garnets in UH P -rocks with a wide range in composition. With very few exceptions, however, this is not the case. This discrepancy indicates that both P and Na are effectively released from garnet and re-distributed within the garnet matrix during uplift and exhumation. To explore the mechanisms of this P-Na release, P-Na-rich garnet pre-synthesized at 7 GPa and 1200 °C, containing 0.7 wt% P 2 O 5 and 0.3 wt% Na 2 O, was exposed to P-T conditions of 2 GPa and 800–1000 °C in a simplified, H 2 O-bearing, model eclogitic bulk composition. The experiments show that at subsolidus temperatures of 850–975 °C, and in the presence of a hydrous fluid, apatite quickly forms from garnet breakdown involving consumption of coexisting quartz and clinopyroxene. The apatites usually appear as rounded to lath-shaped isolated grains scattered in the garnet + clinopyroxene ± orthopyroxene + quartz + rutile matrix. More rarely, single apatite inclusions, or clusters of inclusions, may form in clinopyoxene or garnet. The observed apatite grain size is in the range ≤ ~1 × 1 to 24 × 6 μm with the largest grains occasionally containing clinopyroxene inclusions. Combined garnet breakdown and neo-formation, using pre-existing garnet as a nucleation site, may form zoned garnets with Na-P-depleted and Ti-enriched rims that represent an approach to a garnet composition typical for mid- to shallow crustal P-T conditions. Partial melting experiments indicate that eclogites containing P-rich garnet may produce P-rich and apatite-undersaturated melts for moderately SiO 2 -rich melt compositions. These melts can crystallize abundant apatite during solidification and, thus, would be effective agents for P-extraction during partial melting. Due to their very low apatite saturation concentration, the P-transport and storage capacity of granitic melts would be much more limited. An unexpected finding of this study are the substantial P and Mg contents in kyanite with 0.17–0.20 wt% P 2 O 5 and 0.20–0.56 wt% MgO, respectively. The combined P-Mg incorporation into kyanite is consistent with the coupled substitution [4] Si 4+ + [6] Al 3+ = [4] P 5+ + [6] Mg 2+ and with a strong preference of P for orthosilicate structures. The results of this study suggest that some to even all of the apatite now present in eclogites that underwent deep subduction formed by chemical adjustment of the eclogite garnet to decreasing Na 3 Al 2 (PO 4 ) 3 solubility during uplift and exhumation. This results in the appearance of apatite as a new phase in a hitherto apatite-free assemblage. Rapid transport to the surface and/or a lack of suitable reactants can suppress this re-equilibration and explain the occasionally high P- and Na-contents of eclogitic garnet-inclusions in diamond or garnets from diamondiferous eclogites sampled by kimberlites. Similarly, concurrent decreasing Ti-solubility in garnet may lead to rutile-saturation in eclogites that did not contain this phase under peak- P conditions as evidenced by the joint occurrence of (oriented) rutile and apatite inclusions in the eclogitic garnets. For the application of geothermobarometry, this delayed Ti (+P) saturation is important to be kept in mind.

The pyroxenite-diamond connection

Pieces of the Earth’s mantle occurring either as tectonic fragments or xenoliths in volcanic rocks are dominantly peridotites, assemblages of olivine, ortho- and clinopyroxene with minor garnet and/or spinel. They frequently contain pyroxene-rich inclusions which have compositions intermediate between peridotite and basalt. These pyroxenites typically contain varying amounts of more iron-rich (than peridotite) clinopyroxene, orthopyroxene, garnet and/or spinel and are commonly compositionally layered. Surprisingly, despite their subordinate abundance in mantle fragments, pyroxenitic compositions appear be the dominant sources of majoritic garnet inclusions in diamonds, the principal window into the mineralogy of the deep upper mantle and the transition zone (Kiseeva et al., 2013a). In this study we show that the pyroxenite-diamond association is a consequence of the interaction between basaltic and peridotitic compositions in the presence of carbonate melt and that layering of the pyroxenites is a natural consequence of this interaction. Reduction of carbonate to carbon at high pressures is responsible for the genetic connection between pyroxenite and diamond and the abundance of pyroxenitic inclusions reflects this connection rather than a high abundance of this rock type in the mantle.

Garnet inclusions in diamond: the role of elastic properties

Garnet inclusions in diamond: the role of elastic properties

Diamond–garnet geobarometry: The role of garnet compressibility and expansivity

We report P–V and T–V Equations of State (EoS) for synthetic single crystal pyrope (Py, Mg3Al2Si3O12) and the P–V EoS for synthetic single crystals of almandine (Alm, Fe3Al2Si3O12) as well as an intermediate composition (Py60Alm40) as measured by in-situ high-pressure and high-temperature X-ray diffraction experiments. The unit-cell volumes of the three samples were measured at room temperature and different pressures and up to about 8GPa in a diamond-anvil cell. The high-temperature experiment was carried out using a micro-furnace. The pressure-volume data were fitted to a third order Birch–Murnaghan EoS giving the following coefficients: V0=1506.15(16)Å3, KT0=163.7(1.7)GPa and K′=6.4(4) for pyrope, V0=1533.52(10)Å3, KT0=172.6(1.5)GPa and K′=5.8(5) for almandine and V0=1516.32(13)Å3, KT0=167.2(1.7)GPa and K′=5.6(5) for the intermediate Py60Alm40 composition. The unit-cell volume along the pyrope–almandine join changes linearly within the error of measurement indicating ideality in the volume of mixing behaviour for the solid solution. The first pressure derivative, K′, for all three garnets is similar within experimental uncertainty with an average value of K′=6.0(4). The thermal expansion parameters for end-member pyrope, as described using to the Kroll-type EoS, with the Einstein temperature, θE, fixed to 320K, are α(303K, 1bar)=2.543(5)∗10−5K−1 and V0=1504.64(4)Å3. The determined thermoelastic parameters were combined in a thermal-pressure type EoS (thermal-P type EoS) to calculate the entrapment pressures, Pe, for natural garnet inclusions in diamonds at mantle temperatures. A calculated pressure of 5.8GPa at T=1500K using our newly determined thermoelastic parameters appears to be more consistent with diamond-forming conditions compared to the higher pressures that range up to 6.8GPa at T=1500K, which are obtained using thermoelastic data in the literature.

Ancient mantle metasomatism recorded in subcalcic garnet xenocrysts: Temporal links between mantle metasomatism, diamond growth and crustal tectonomagmatism

We have identified carbonatitic melts as the main agent for metasomatism in cpx-free garnet harzburgites from the Kaapvaal craton. Substantial overlap of the Ti/Eu and Zr/Hf ratios of subcalcic garnets from xenoliths and garnet inclusions in diamonds corroborate previous findings that mantle metasomatism and the growth of diamonds are connected processes. The key process involves the interaction of a carbonatitic melt with depleted garnet harzburgite leading to dissolution and regrowth of the constituent minerals and the growth of diamonds by redox reactions. Model calculations show that only small amounts of a carbonatite melt, between 0.3 to 3%, are needed to generate the range of sinusoidal Rare Earth Element patterns in the garnets from harzburgites and the inclusions in diamonds. The εHf values of the xenolith garnets range from extremely positive (∼+1000) to extremely negative values (−65) with accompanying εNd values varying from +38 to −41. It can be shown that the negative εHf and εNd values correspond to the initial ratios of the metasomatizing agent. They can only stem from a very old (early Archean or Hadean) crustal component. Very negative εNd values have been found previously in subcalcic garnets from xenoliths and inclusions in diamonds. They yield early Archean model ages for the growth of diamonds. In these studies, very high, unsupported 87Sr/86Sr ratios were found as well. The combined evidence suggests that the metasomatic agents are derived from an old source with a high Rb/Sr ratio, typical for pelitic crustal rocks. Metasomatism in the mantle, the formation of diamonds and tectonomagmatic events in the crust occurred contemporaneously at least within the time span between the early Archean and the middle Proterozoic as can be seen from the comparison of crustal ages, Lu–Hf isochron ages from subcalcic garnets and Sm–Nd and Re–Os ages of inclusions in diamonds.

Metapyroxenite in the mantle transition zone revealed from majorite inclusions in diamonds

Research Article| August 01, 2013 Metapyroxenite in the mantle transition zone revealed from majorite inclusions in diamonds Ekaterina S. Kiseeva; Ekaterina S. Kiseeva 1Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia *Current address: Department of Earth Sciences, University of Oxford, OX1 3AN Oxford, UK. Search for other works by this author on: GSW Google Scholar Gregory M. Yaxley; Gregory M. Yaxley 1Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar Aleksandr S. Stepanov; Aleksandr S. Stepanov 1Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia2ARC Centre of Excellence in Ore Deposits and School of Earth Sciences, University of Tasmania, Hobart, TAS 7001, Australia Search for other works by this author on: GSW Google Scholar Hrvoje Tkalčić; Hrvoje Tkalčić 1Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia Search for other works by this author on: GSW Google Scholar Konstantin D. Litasov; Konstantin D. Litasov 3V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Science, Novosibirsk 630090, Russia Search for other works by this author on: GSW Google Scholar Vadim S. Kamenetsky Vadim S. Kamenetsky 2ARC Centre of Excellence in Ore Deposits and School of Earth Sciences, University of Tasmania, Hobart, TAS 7001, Australia Search for other works by this author on: GSW Google Scholar Geology (2013) 41 (8): 883–886. https://doi.org/10.1130/G34311.1 Article history received: 12 Dec 2012 rev-recd: 20 Mar 2013 accepted: 01 Apr 2013 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Ekaterina S. Kiseeva, Gregory M. Yaxley, Aleksandr S. Stepanov, Hrvoje Tkalčić, Konstantin D. Litasov, Vadim S. Kamenetsky; Metapyroxenite in the mantle transition zone revealed from majorite inclusions in diamonds. Geology 2013;; 41 (8): 883–886. doi: https://doi.org/10.1130/G34311.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The transition zone of the Earth’s mantle (the depth interval between two major seismic discontinuities at 410 km and 660 km) is critical to understanding our planet’s evolution. Some diamonds are thought to have originated in the transition zone and the inclusions found in them are the only samples of material directly extracted from this depth range. By comparing natural majorite garnet inclusions in diamonds with the compositions of experimentally crystallized majorite garnets, we determine two major compositional trends, the pure metabasitic (or eclogitic) trend and the combined metaperidotitic and metapyroxenitic trend, that are strongly correlated with their preferred substitution mechanisms during majorite formation. Based on these trends, we demonstrate that the majority of the reported majorite inclusions in natural diamonds formed neither in a pure metabasite nor in a metaperidotite lithology, but in fact crystallized from a wide range of compositions intermediate between conventional basaltic and peridotitic, referred to here as metapyroxenitic. Given the dominance of metapyroxenite-type majorite diamond inclusions and their inferred syngenetic origin, we argue that a significant fraction of metapyroxenite rock is present within Earth’s transition zone and is important in the diamond-forming process. This is in agreement with recent self-consistent seismological and/or mineral physics studies that support models of a lithologically heterogeneous transition zone. From trace element and carbon isotope features, we infer a crustal origin for these rocks. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Diamond from recycled crustal carbon documented by coupled δ18O–δ13C measurements of diamonds and theirinclusions

The origin of variability in δ13C values of cratonic diamonds is controversial, particularly for diamonds associated with eclogitic source rocks. The variability may be due to distinct primordial reservoirs, high-temperature isotope fractionation, recycling of crustal carbon, or any combination of the three. In contrast, the interpretation of variability in δ18O values of silicate minerals in eclogite xenoliths and eclogitic inclusions in diamonds is less contentious—the hypothesis that they represent rocks that were exposed to weathering and hydrothermal alteration at the surface of Earth is broadly, but not universally, accepted. Here, we report high-precision SIMS oxygen isotope measurements of 15 eclogitic garnet inclusions in diamond from the Damtshaa mine, which comprises four kimberlites within the Orapa cluster of kimberlites. The results demonstrate a link between δ18O values of inclusions and δ13C values of their host diamonds. The δ18OVSMOW values range from +4.7‰ to +8.8‰ and have a median value of +5.7‰, similar to the distribution exhibited by cratonic eclogite xenoliths worldwide, at the nearby Orapa Mine, and of oceanic crust. Contrary to previous suggestions, there is no evidence for a unique oxygen isotope distribution for inclusions in diamond. The oxygen isotope ratios do not correlate with garnet compositions; they have, however, a strong negative correlation with the δ13C values of their host diamonds. We interpret this correlation to have geochemical significance. It cannot be due to primordial heterogeneities or high-temperature isotope fractionation, but is most likely due to an association between recycled near-surface crustal rocks and recycled carbon. We suggest that protoliths to the eclogites at Damtshaa that were more strongly affected by low-temperature seafloor weathering (recorded by δ18O≥∼6‰) also have concentrations of primary organic carbon, and, therefore, low-δ13C Damtshaa diamonds are associated with recycling of crustal carbon.

Garnet-bearing ultramafic rocks from the Dominican Republic: Fossil mantle plume fragments in an ultra high pressure oceanic complex?

Ultra high pressure (UHP) garnet-bearing ultramafic rocks from the Dominican Republic may represent the only known example where such rocks were exhumed at an ocean–ocean convergent plate boundary, and where the protolith crystallized from a UHP magma (>3.2GPa, >1500°C). This study focuses on the petrology and geochemistry of one of the ultramafic lithologies, the pegmatitic garnet-clinopyroxenite (garnet+clinopyroxene+spinel+corundum+hornblende). Three distinct types of garnet were recognized: Type-1 garnet (low Ca, high Mg) is interpreted as near magmatic (P>3.2GPa, >1500°C). Type-1′ garnet (high Ca, low Mg) is interpreted as having formed approximately isochemically from magmatic high-Al clinopyroxene. Type-2 garnet (intermediate Ca, high Mg, and low Fe+Mn) formed together with hornblende as a result of late, low-pressure retrograde hydration. Clinopyroxene is close to diopside–hedenbergite (Mg# ~88) and metasomatized by arc-related fluids. Spinel and corundum occur as microinclusions in type-1 and type-1′ garnets in the only reported natural occurrence of coexisting garnet+spinel+corundum, indicative of very high pressure. Chondrite-normalized REEs (rare earth elements) of the garnets show humped or weakly sinusoidal patterns, typically associated with garnet inclusions in diamond and garnet in kimberlite that crystallized at UHP conditions. These humped to weakly sinusoidal REE patterns developed as the result of interaction with a light REE-enriched metasomatic fluid. Partitioning of REEs between type-1′ and type-1 garnets is consistent with the former having inherited its REEs from a high-Al clinopyroxene predecessor. The partitioning preserves a record of near-solidus temperatures (~1475°C). Petrology and phase relationships independently suggest near-solidus conditions >1500°C (the highest temperature conditions reported in a UHP orogenic setting), providing evidence for an origin in a mantle plume. Therefore, the Dominican ultramafic rocks may represent the only example of exhumed “fossil fragments” of mantle plume in an orogenic setting (oceanic or continental).

Ancient metasomatism recorded by ultra-depleted garnet inclusions in diamonds from DeBeers Pool, South Africa

Peridotitic garnet inclusions in diamonds from the DeBeers Pool mines represent samples from an extremely depleted mantle source. A large number of garnet inclusions have CaO contents of less than 1.8 wt.% and olivine inclusions are characterized by very high Mg-numbers (average: 94.6). These observations suggest that initial melt depletion in the protolith for the cratonic root beneath Kimberley (Kaapvaal Craton) exceeded orthopyroxene exhaustion (i.e. ≥ 50% melt extraction) and hence went farther than typically observed for cratonic peridotites worldwide. This signature of extreme depletion is not readily apparent from the study of mantle xenoliths from the same area and strongly underlines the unique role that diamond may play as a time capsule. Analyses of rare earth and other trace elements show that DeBeers Pool garnet inclusions, despite their ultra-depleted major element composition, were affected by metasomatic enrichment processes similar to those recorded by inclusions in diamonds worldwide. The DeBeers Pool garnet inclusions have a wide range of REE concentrations with variable sinuosity of their chondrite normalized abundance patterns. Sinuosity of garnet REE N patterns generally decreases with increasing fertility (i.e. higher Ca Ti, Fe, Y, and Zr); however, a distinction between the REE N signatures of harzburgitic and lherzolitic garnets is not possible. The REE signatures indicate that the source rocks of these garnets were affected by two phases of metasomatism. LREE–MREE enrichment is preserved by all garnets and indicates interaction with a fluid of highly fractionated character. The variable LREE–MREE concentrations of the garnet inclusions result from progressive evolution of the fluid to higher LREE/HREE ratios during continued equilibration with or precipitation of garnet. This metasomatic fluid was pervasive and affected the entire depth section of the lithosphere represented by the DeBeers Pool inclusions. A second phase of enrichment by HREE- and other HFSE-carrying melts, that also transported major elements (Ca and Fe), affected only a portion of the diamond source region and was restricted to comparatively shallower lithosphere at depths < 180 km. Diamond precipitation was likely associated with both styles of metasomatic re-enrichment. The similarity of lherzolitic and harzburgitic REE N patterns and their similarly variable REE concentrations suggests diamonds of both parageneses were formed concurrently during both phases of metasomatic re-enrichment.

Application of precise 142Nd/ 144Nd analysis of small samples to inclusions in diamonds (Finsch, South Africa) and Hadean Zircons (Jack Hills, Western Australia)

146Sm– 142Nd and 147Sm– 143Nd systematics were investigated in garnet inclusions in diamonds from Finsch (S. Africa) and Hadean zircons from Jack Hills (W. Australia) to assess the potential of these systems as recorders of early Earth evolution. The study of Finsch inclusions was conducted on a composite sample of 50 peridotitic pyropes with a Nd model age of 3.3 Ga. Analysis of the Jack Hills zircons was performed on 790 grains with ion microprobe 207Pb/ 206Pb spot ages from 3.95 to 4.19 Ga. Finsch pyropes yield 100 × ɛ 142Nd = − 6 ± 12 ppm, ɛ 143Nd = − 32.5, and 147Sm/ 144Nd = 0.1150. These results do not confirm previous claims for a 30 ppm 142Nd excess in South African cratonic mantle. The lack of a 142Nd anomaly in these inclusions suggests that isotopic heterogeneities created by early mantle differentiation were remixed at a very fine scale prior to isolation of the South African lithosphere. Alternatively, this result may indicate that only a fraction of the mantle experienced depletion during the first 400 Myr of its history. Analysis of the Jack Hills zircon composite yielded 100 × ɛ 142Nd = 8 ± 10 ppm, ɛ 143Nd = 45 ± 1, and 147Sm/ 144Nd = 0.5891. Back-calculation of this present-day ɛ 143Nd yields an unrealistic estimate for the initial ɛ 143Nd of − 160 ɛ-units, clearly indicating post-crystallization disturbance of the 147Sm– 143Nd system. Examination of 146,147Sm– 142,143Nd data reveals that the Nd budget of the Jack Hills sample is dominated by non-radiogenic Nd, possibly contained in recrystallized zircon rims or secondary subsurface minerals. This secondary material is characterized by highly discordant U–Pb ages. Although the mass fraction of altered zircon is unlikely to exceed 5–10% of total sample, its high LREE content precludes a reliable evaluation of 146Sm– 142Nd systematics in Jack Hills zircons.

Diamondiferous peridotitic microxenoliths from the Diavik Diamond Mine, NT

Twenty-five diamonds recovered from 21 diamondiferous peridotitic micro-xenoliths from the A154 South and North kimberlite pipes at Diavik (Slave Craton) match the general peridotitic diamond production at this mine with respect to colour, carbon isotopic composition, and nitrogen concentrations and aggregation states. Based on garnet compositions, the majority of the diamondiferous microxenoliths is lherzolitic (G9) in paragenesis, in stark contrast to a predominantly harzburgitic (G10) inclusion paragenesis for the general diamond production. For garnet inclusions in diamonds from A154 South, the lherzolitic paragenesis, compared to the harzburgitic paragenesis, is distinctly lower in Cr content. For microxenolith garnets, however, Cr contents for garnets of both the parageneses are similar and match those of the harzburgitic inclusion garnets. Assuming that the microxenolith diamonds reflect a sample of the general diamond population, the abundant Cr-rich lherzolitic garnets formed via metasomatic overprinting of original harzburgitic diamond sources subsequent to diamond formation, conversion of original harzburgitic diamond sources occurred in the course of metasomatic overprint re-fertilization. Metasomatic overprinting after diamond formation is supported by the finding of a highly magnesian olivine inclusion (Fo95) in a microxenolith diamond that clearly formed in a much more depleted environment than indicated by the composition of its microxenolith host. Chondrite normalized REE patterns of microxenolith garnets are predominantly sinusoidal, similar to observations for inclusion garnets. Sinusoidal REEN patterns are interpreted to indicate a relatively mild metasomatic overprint through a highly fractionated (very high LREE/HREE) fluid. The predominance of such patterns may explain why the proposed metasomatic conversion of harzburgite to lherzolite appears to have had no destructive effect on diamond content.