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Trace element composition and cathodoluminescence properties of southern African kimberlitic zircons
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Zircon frequently occurs as a minor mineral in kimberlites, and is recognised as a member of a suite of mantle-derived megacryst minerals. Cathodoluminescence (CL) microscopy and laser ablation ICPMS analysis were used to study the internal structure and chemical composition of zircon crystals from southern African kimberlites. Zoning revealed by CL ranges from fine oscillatory to broad homogeneous cores and overgrowths. The ICPMS data show that kimberlite zircons have distinctive trace element contents, with well defined ranges for REE, Y, U, Th, P and some other trace elements. Both low REE contents (REE < 50 ppm), and distinctive chondrite-normalised REE patterns with low and flat HREE are characteristic of kimberlite zircons. Samples or zones with yellow CL have higher Th, U, Y, and REE than those with blue-violet CL. Variations in the concentrations of a range of trace elements lead to different amounts of lattice defects, creating the possibility for different levels of direct excitation of luminescence centres, and therefore different CL colours. The distinctive CL and compositional features described here can rapidly identify kimberlite zircons in prospecting samples taken during exploration for kimberlite bodies.
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... Zircon is a minor phase in all mineral concentrates but only those from TRS3 are classified as mantle-derived (see Belousova et al., 1998Belousova et al., , 2001Belousova et al., , 2002 and show no zoning (Fig. SD2). The trace-element contents and patterns of all grains are very similar ...
... Users of U-Pb analyses of zircon must recognize that zircon does not crystallize from ultramafic magmas, and therefore must be xenocrystic (Griffin et al., 2000). Trace-element data suggest that many zircons are closely related to their host magmas (Belousova et al., 1998(Belousova et al., , 2002, but some kimberlite pipes clearly carry mantle-derived zircons from earlier episodes of kimberlite magmatism (e.g., Kinney et al., 1989;Belousova et al., 2001). Therefore, zircon U-Pb ages must in principle be regarded as maximum estimates of the intrusion ages, although the age difference between the zirconcrystallizing magma and the host kimberlite may be small. ...
... Zircon from anorthosite has the following values: 176 Lu/ 177 Hf from 0.000923 to 0.003735, 176 Hf/ 177 Hf from 0.282701 to 0.282858, and ε Hf (T) = 2.0- Fig. 6. Th-U composition of zircon from the mafic-ultramafic massifs of the Khangai Mountains as compared with fields of zircon from different types of rocks: Permian Ni-Cu deposits of China [Xie et al., 2022], granitoids of Tuva [Rudnev et al., 2020а], gabbro-dolerites of the Irkut block , peridotite [Konzett et al., 2000;Liati et al., 2004;Giuliani et al., 2014;Wang et al., 2020], and kimberlite [Kinny et al., 1989;Belousova et al., 1998;Griffin et al., 2000]. 1 -Oortsog-Uul, B13057; 2 -Nomgon, B13088; 3 -Yamaat-Uul, Sh105-14; 4 -Yamaat-Uul, Sh220-14/2. 8.4. ...
We present Sm–Nd and Rb–Sr isotope composition data on mafic–ultramafic massifs in the Khangai Mountains of Western Mongolia: Oortsog-Uul, Nomgon, and Yamaat-Uul. The U–Pb age of zircon and its Lu–Hf isotope and trace-element compositions were determined by LA–ICP–MS. New and previous geochronological data obtained by SIMS and LA–ICP–MS support the Permian age of the studied gabbros. The trace-element composition of zircon, characterized by strong HREE enrichment ((Lu/Gd)n > 7) and cerium positive (Ce/Ce* > 6.6) and europium negative (Eu/Eu* = 0.16–0.49) anomalies, indicates its magmatic genesis and the possibility of using isotope characteristics to assess the origin of mafic magmas. The formation of zircon from a residual mafic melt is inferred from the enrichment of zircon in U and Th with increasing Th/U, reflecting the accumulation of these highly incompatible elements in the residual melt, and from the crystallization temperature of zircon (810–880 °С). The geochemical characteristics of the rocks, their isotopic composition, the absence of xenogenic ancient zircons, and the lack of correlation between εNd(T) and major indices of crustal contamination indicate that crustal contamination did not influence the composition of the gabbros. Isotopic data on rocks and zircon indicate the involvement of two mantle sources in the formation of the mafic–ultramafic massifs of the Khangai Mountains: (a) depleted, predominant for the Nomgon and Yamaat-Uul massifs (εHf = 16.1–2.0; εNd = 4.5–0.0; and ISr = 0.70385–0.70537), and (b) enriched, predominant for the second phase of the Oortsog-Uul massif (εHf = 1.4–0.2; εNd = –3.6… –5.7; and ISr = 0.70704–0.70933).
... The selected zircon grains are colorless, transparent-to-pale yellow, and euhedral, with a typical elongated prismatic shape ( Figure 3). The Th/U ratios of the grains are greater than 0.1 (Table 2), indicating a magmatic origin [41]. Their magmatic origin is further supported by their typical magmatic oscillatory zoning under cathodoluminescence (CL) images ( Figure 3). ...
The Aolunhua Mo deposit is a typical porphyry deposit, which is located in the middle southern section of the Da Hinggan Range metallogenic belt. Here, we report LA-ICP-MS zircon U-Pb age data from the Mo-associated granitoid, together with the element geochemistry of the zircons, discussing the source material of the ore-forming rock of the deposit. The zircon data constrain the crystallization age of the granite porphyry as 135.0 ± 1.0 Ma, correlating it with the widespread Yanshanian intermediate–felsic magmatic activity. The Th/U ratio of the zircon is greater than 0.1, with a significant positive Ce anomaly (Ce* = 1.72–188.71) and a negative Eu anomaly (Eu* = 0.05–0.57). The zircons show depleted LREE and enriched HREE patterns, as well as low La and Pr contents, suggesting crystallization from crust-derived magmas. Based on the geology of the ore deposit and the age data, in combination with the regional geodynamic evolution, we infer that the Aolunhua Mo deposit was formed near the peak stage of Sn poly-metallic metallogenesis in the Da Hinggan Range region at around 140 Ma, associated with a tectonic setting, characterized by the transition from compression to extension. Based on a comparison with the newly found Mo deposits along the banks of the Xilamulun River, we propose that the Tianshan–Linxi is an important Mo-metallogenic belt. It also suggests an increased likelihood for the occurrence of Mo along the north bank of the Xilamulun River.
... For the in situ analysis of zircon, fragments of isotopically well-characterised zircon megacrysts are sought-after. Zircon megacrysts are exceptionally large crystals ( [ 5 mm) that are mainly associated with mantle-derived kimberlites, carbonatites, alkali basalts, syenitic pegmatites or marbles (e.g., Hoskin and Schaltegger 2003, Belousova et al. 1998, Piilonen et al. 2018, Sutherland et al. 2016, Sinh et al. 2019, Schaltegger et al. 2015b, Cavosie et al. 2011. Since they form during relatively short time spans in magmatic, hydrothermal and metamorphic environments, zircon megacrysts are commonly used as reference material for U-Pb geochronology (e.g., Mud Tank, Gain et al. 2019, Penglai, Li et al. 2010, 91500, Wiedenbeck et al. 1995, M257, Nasdala et al. 2008, GZ7 and GZ8, Nasdala et al. 2018, BB zircons, Santos et al. 2017, M127, Nasdala et al. 2016, CZ3, Pidgeon et al. 1994, GJ1, Jackson et al. 2004, Ple sovice, Sl ama et al. 2008. ...
We characterise zircon megacrysts from Kawisigamuwa carbonatite as a new potential reference material for laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) U‐Pb and Hf isotope analysis. We studied ten 0.5‐4 cm long, brown megacrysts that consist of oscillatory zoned (OZ) and nearly featureless cathodoluminescence‐bright recrystallised (RX) zircon domains. The zircons have low to moderate radiation damage (total α‐dose <0.5 x 10 ¹⁸ events/g), tested by the measured FWHM of the ν 3 (SiO 4 ) Raman band < 5 cm ‐1 . An ID‐TIMS weighted mean ²⁰⁶ Pb/ ²³⁸ U age of 532.39 ± 0.66 Ma (2s uncertainty) was determined for OZ zircon domains of three grains. While Hafnium content varies within and between crystals (6630‐9960 μg/g) the in‐situ mean ¹⁷⁶ Hf/ ¹⁷⁷ Hf ratios of ten crystals overlap within uncertainty. We recommend a mean ¹⁷⁶ Hf/ ¹⁷⁷ Hf of 0.282003 ± 0.000020 (2s) as preliminary working value for OZ domains. Two OZ megacrysts yield indistinguishable δ ¹⁸ O VSMOW of 12.1 ± 0.4‰ and 12.2 ± 0.4‰ (2s). Elevated Th/U (>1.5), Zr/Hf (59‐75) and low Hafnium contents in OZ domains are similar to those of zircons from carbonatites of mantle origin, while high δ ¹⁸ O and low εHf i (‐15.8 to ‐17.1) indicate a crustal contribution. The zircons probably grew from crustal‐derived carbonate melts under high grade metamorphic conditions.
... Generally, REEs contents can reach 0.9 wt.% in natural zircon crystals (Poller et al., 2001); however, this level can be exceeded in some igneous samples (>2 wt.%; Ni et al., 2020). Furthermore, REEs-containing zircon crystals typically contain P and other elements (Romans et al., 1975;Speer, 1982;Hinton and Upton, 1991;Belousova et al., 1998;De Hoog et al., 2014), and P concentrations can vary from tens to thousands of ppm in natural zircon (Hoskin et al., 2000;Ballard et al., 2002;Ni et al., 2020). Therefore, REEs (e.g., Dy) and P can both significantly affect the Raman spectra ( Fig. 5b; Fig. 7c and d). ...
... Most grains display clear oscillatory and concentric zoning, and many grains show convoluted and truncated zoning. These features, together with their high Th/U ratios (mostly > 0.4), indicate a magmatic origin [51][52][53][54][55][56]. To ensure the accuracy of the ages, analysis points were selected within the oscillatory zones. ...
Late Paleozoic–early Mesozoic intrusive rocks are distributed widely along the northern margin of the Qaidam Basin in the northern Tibetan Plateau. To constrain the tectonic evolution, we carried out petrological, chronological, and geochemical studies of the Chahannuo gabbros. LA-ICP-MS Zircon U–Pb dating yields an age of 255.0 ± 0.9 Ma for the gabbros, which confirms the existence of Indosinian tectono-magmatic activity on the northern margin of the Qaidam Basin. The Chahannuo gabbros have low whole-rock SiO2, Fe2O3 contents, and high Al2O3 contents, which suggests a calc-alkaline affinity. In addition, the gabbros have high MgO, Cr, and Ni contents and Mg#, similar to those predicted of the regional basaltic melts, and indicating that they were affected mainly by fluid from the subducted slab. The Chahannuo gabbros are characterized by arc-like trace element patterns, with enrichment in LREE and LILE, and depletion in HREE and HFSE. No obvious negative Eu anomalies also indicate that no significant magmatic differentiation has occurred. The low Nb/La ratio and Ti content in gabbros samples suggests that the Chahannuo gabbros were partially contaminated by the crust during their formation. The Chahannuo gabbros have high incompatible element ratios (Rb/Sr, Th/Nd, and Th/La), which are closer to the category of enriched mantle. Combing our data with previous data from contemporaneous magmatism in the region, we suggest that the Chahannuo gabbros formed in a continental arc environment related to the northward subduction of the Paleo-Tethyan oceanic plate.
Large volumes of Early Mesozoic intermediate to basic igneous rocks related to the evolution of the Paleo-Tethys Ocean are exposed in the East Kunlun Orogenic Belt (EKOB). The petrography, geochemistry, and results of zircon U-Pb dating of Defusheng intermediate to basic rocks from the eastern segment of the EKOB are presented in this report. Zircon U–Pb dating of the intermediate to basic rocks yields ages of 239–245 Ma (Middle Triassic). Defusheng intermediate to basic rocks have low TiO2 contents (0.80–1.47 wt.%) and widely varying MgO (3.14–6.08 wt.%), and are enriched in large ion lithophile elements and light rare earth elements, having a geochemical composition similar to that of island arc basalts. The variation diagrams of major elements indicate that the Defusheng intermediate to basic rocks underwent fractional clinopyroxene and olivine crystallization. Depletion of the high-field-strength elements Nb, Ta, and Ti may have been caused by the mantle wedge having been infiltrated by fluids derived from the subducted slab. The Defusheng intermediate to basic rocks represent magmatic records of the Early Mesozoic oceanic crust subduction in Eastern Kunlun. This indicates that the final closure of the Paleo-Tethyan Ocean and the beginning of collisional orogeny occurred after the Middle Triassic.
The early Paleozoic tectono-magmatic activity within the South China block, which is well illustrated by Ordovician−Devonian granites in the western Qinhang belt, was the response to closure of the Proto-Tethys Ocean and convergence of continental blocks. The spatiotemporal distribution and source characteristics of the granites provide us the opportunity to understand the processes and driving mechanisms of intracontinental orogeny. As an example, the Miaoershan-Yuechengling granite batholith in northern Guangxi, located along the western margin of the Qinhang orogenic belt, is mainly composed of quartz monzonite and monzogranite. All the granitic rocks from Miaoershan-Yuechengling batholith are composed of K-feldspar, quartz, plagioclase, biotite, and hornblende. Geochronologic dating indicates that the Miaoershan-Yuechengling batholith was emplaced during the late Silurian and Early Devonian, respectively. The rocks have high SiO2, with an average value of 73.29 wt%, and total alkalis (Na2O + K2O = 7.21−10.03 wt%), but low Al2O3 (12.96−15.51 wt%), showing characteristics of the high-potassium calc-alkaline series of S-type peraluminous granites (Al2O3/[CaO + Na2O + K2O] = 1.03−1.22). Trace elements in the Miaoershan-Yuechengling granitic rocks are characterized by enrichment of large ion lithophile elements and depletion of high field strength elements. Their rare earth element (REE) trends are characterized by relatively flat distribution patterns with weak light REE enrichment, weak heavy REE fractionation, and negative Eu anomalies. Zircons from the rocks have negative εHf(t) values ranging from −13.24 to −5.1, with crustal model ages (THf2) of 2.2−1.7 Ga. These features indicate that they are S-type granites with parental magmas originating from partial melting of sandy argillaceous sources of Paleoproterozoic lower continental crust. The thermal budget for Ordovician to Early Devonian magmatism is attributed either to crustal thickening in relation to intracontinental orogenic compression or to crustal thinning due to postorogenic tectonic extension during assembly and breakup of Greater Gondwana. This study reveals that the change in mantle convection systems during plate interactions acted as a major driving force for the early orogenic processes, late collapse of the orogenic belt, and massive syncollisional to postorogenic magmatism.
The crystallization of ZrSiO4 is generally accomplished by the addition of mineralizers into ZrO2-SiO2 binary oxides. The current investigation aimed to investigate the effect of adding calcium phosphates into ZrO2-SiO2 binary oxides on the yield of ZrSiO4. The concentration of calcium phosphate additions were varied to obtain ZrSiO4 that fetches improved mechanical and biological properties for application in hard tissue replacements. The findings highlight the significant role of Ca2+ and P5+ in triggering the ZrSiO4 formation via their accommodation at the Zr4+ and Si4+ sites. Especially, calcium phosphate additions trigger the t- → m-ZrO2 transition beyond 1000 °C, which consequently reacts with SiO2 to promote ZrSiO4 formation. Calcium phosphates are accommodated at the lattice sites of ZrSiO4 with a maximum limit of 20 mol%, beyond which the crystallization of β-Ca3(PO4)2 is noticed. The optimum amount of 20 mol% of calcium phosphates displayed a better strength than that of all the investigated specimens. More than 80% of cell viability in MG-63 cells was invariably determined in all the calcium phosphate-added ZrSiO4 systems.
The development of mineralogy, the evolutionary changes in compre hending the mineral substance of the earth are closely associated with the progress of research methods. Over a space of more than two and half centuries, from the goniometry of the mineral crystals to microscopic petrography and optical mineralogy, to crystal structure determinations, electron micros copy and electron diffraction and finally investigations into their electri cal, magnetic and mechanical properties, all this has led to the formation of the existing system of mineralogy, its notions, theories and to a proper description of minerals. However, no matter how great the variety of methods employed in mineralogy, they all come to a few aspects of substance characteristics. These are methods of determining the composition, structure and proper ties of the minerals. Thus the X-ray micro analyzer, the atom-absorption, neutron-activation, chromatographic and other analyses open up new opportunities for determining nothing else but the elementary com position of minerals.
Zircons enclosed in corundum megacrysts from basalts have a short prismatic habit with the prism slightly more developed than the pyramidal faces. The {110} prism is dominant compared with {100}, indicating crystallisation from alkaline-peralkaline environments. Rare zircon decomposition reactions suggest the possible presence of a primary SiO 2 phase in the source paragenesis for corundum megacrysts. The zircon inclusions are geochemically distinctive: high Y (up to 1.2%), U (UO 2 up to 1.7 wt%), Th (ThO 2 up to more than 2 wt%), Hf (up to 3.4 wt%), and REE ( REE up to 5000 ppm). They form a cogenetic group that is characterised by extreme REE fractionation with preference for the HREE varying from 30-100× chondritic Sm to 5000-20000× chondritic Lu. REE partitioning between zircon and silicate liquid is evaluated. The parental melts calculated for the zircon inclusions and other corundum-related zircons, using zircon/melt partition coefficients derived in the present paper, show concave-down REE patterns with a maximum Sm concentration close to 100× chondritic Sm abundance. Such REE patterns represent highly evolved melts after extensive fractionation of low-Ca feldspar prior to and throughout the crystallisation of zircon. The characteristics of zircon inclusions provide evidence that corundum megacrysts must have crystallised from alkaline and highly evolved melts under very reducing conditions, implying that the corundum is unlikely to have formed as high pressure phenocrysts in the host mafic magmas.
This book describes the composition of the present upper crust, and deals with possible compositions for the total crust and the inferred composition of the lower crust. The question of the uniformity of crustal composition throughout geological time is discussed. It describes the Archean crust and models for crustal evolution in Archean and Post-Archean time. The rate of growth of the crust through time is assessed, and the effects of the extraction of the crust on mantle compositions. The question of early pre-geological crusts on the Earth is discussed and comparisons are given with crusts on the Moon, Mercury, Mars, Venus and the Galilean Satellites.
More than 50 years ago, when the minerals of the kimberlites were being studied, a problem arose that remains in dispute in many ways. It amounts in essence to: “Were the minerals formed along with the binding matrix in which they now occur, or were they xenogenic… ?” (Sarsadskikh and Rovsha, 1960). This question is totally applicable to the zircons.
Ilmenite megacrysts in the Monastery kimberlite occur both as discrete monomineralic crystals and intergrown with all the other phases of the Cr-poor megacryst suite (cpx, opx, garnet, Fe-rich olivine, phlogopite and zircon). The ilmenites show systematic variations in trace element content which are interpreted in terms of a fractional crystallization model. Covariation of major and trace elements in the ilmenites with respect to their Nb content defines smooth curves, with breaks in the trends corresponding to changes in the inferred cumulate assemblage. Nb apparently behaved as an incompatible element in the megacryst magma throughout its crystallization history, and the Nb content of ilmenite serves as a useful fractionation index. After the appearance of ilmenite, the crystallization sequence of the Monastery megacryst suit is ilmenite+cpx+garnet+opx, followed by ilmenite+phlogopite, then ilmenite+zircon±phlogopite and finally by ilmenite+zircon+olivine+phlogopite.The incompatible behaviour of Nb indicates that ilmenite was overall never a predominant phase in the cumulate assemblage. Elevated Cr contents in late-stage ilmenites cannot be explained by the simple fractional crystallization model, and may require another process such as magma mixing or magma reaction with wall rock. The parent megacryst magma must have been highly magnesian and enriched in incompatible trace elements and in this respect may have been similar to meimechite. The late differentiate of this magma cannot be kimberlite, but must be undersaturated and high in iron, potassium, titanium and incompatible elements.
A UV laser ablation microprobe coupled to an ICPMS has been used to determine trace element concentrations in solids with a spatial resolution of 50 microns and detection limits ranging from 2 μg/g for Ni to 50 ng/g for the REE, The, and U. Experiments designed to optimize laser operating conditions show that pulse rates of 4 Hz produce a steady state signal with less inter-element fractionation per unit time than higher pulse rates (10–20 Hz). Comparisons of laser microprobe analyses of garnets and pyroxenes using the NIST 610 and 612 glasses as calibration standards, with proton microprobe, solution ICPMS, INAA and XRF data show no significant matrix effects. Laser microprobe analyses of the NIST 610 and 612 glasses have a precision and accuracy of 2–5%, and error analysis shows that counting statistics and the precision on the internal standard concentration accounts for the analytical uncertainty. The NIST glasses appear to be useful calibration materials for trace element analysis of geological materials by laser microprobe.
Kimberlitic zircons from 16 intrusions in Lesotho, the Republic of South Africa, and Tanzania have been investigated. The following characteristics distinguish the kimberlitic zircons from the zircons derived from most other sources: 1. Rounded to subrounded grains predominate; euhedral zircons are very rare. 2. Perfect parting in several directions, mainly (100) and (111). 3. Lattice distortions occur (decreasing molar volume). 4. High abundancy of fluid inclusions, occupying healed fractures. 5. Uranium contents less than 30 ppm U. Low contents of Th, Y, P, REE. 6. Whitish alteration rims composed of monoclinic and tetragonal zirconia in varying proportions are common. — The use of zircons in order to identify and trace kimberlite intrusives is discussed, as there may be a possible connection between some properties of the kimberlitic zircons and the diamond grade of their host rocks.
Existing data for thorium and uranium in zircons from granitic rocks are discussed, and new data for thorium and uranium in zircons from kimberlites are presented. The new data show that thorium and uranium are at very low levels (from 1.8 to 7.2 ppm for thorium, and from 7.0 to 28.0 ppm for uranium) in kimberlitic zircons, but that the Th:U ratio is just as low (from 0.23 to 0.70) as for granitic zircons. The Th:U ratio in zircons is then discussed by comparison with the ratio in igneous rocks, and possible reasons for the low ratio in zircons are suggested.