Geochemical heterogeneity of polychronous relict zircons from restitogenic ultramafic rocks of the Shaman massif (Eastern Transbaikal region)

Research subject. The distribution patterns of rare earth elements (REE), as well as Y and Th, in the grains of polychromous zircons from the restitogenic ultramafic rocks of the Shaman massif (Eastern Transbaikalia). This massif is a steeply inclined protrusion that is part of the eastern branch of the Baikal-Muya ophiolite belt.Materials and methods. 31 zircon grains 100–150 μm in size were isolated from a composite sample of harzburgites and dunites with a total weight of 4 kg for their subsequent U-Pb isotope dating. These analyzes were performed by the LA-ICP-MS method by scanning along straight profiles on the plane of sections of representative zircon grains.Results. All zircon grains from the general collection are characterized by a rounded shape, a rough surface, microfracturing, a weak cathodoluminescent glow to a complete absence, and an irregular oscillatory zoning. In some grains, microinclusions of epigenetic minerals, such as quartz, mica, etc. were found. It was previously determined that, within the entire collection of zircon grains, the values of their age, as well as U and Th contents, vary across rather wide intervals (3049–502 Ma), the reasons for which are the subject of discussions. The LA-ICP-MS scanning over the profiles of representative zircon grains from the general collection showed that REE, Th, and Y are distributed highly unevenly, occasionally showing signs of zoning. It is assumed that the zircons found in the ultramafic rocks of this massif are a relict phase and appeared as a result of the transformation of very ancient (more than 3 billion years old) juvenile crystals of this mineral, which had been originally located in the upper mantle protolith.Conclusions. Transformations of juvenile zircons and their transformation into a relict phase occurred in the process of partial melting of the protolith, during which they underwent thermal action (annealing), chemical resorption, as well as disturbances in their U-Pb systems, which caused uneven “rejuvenation” of their isotopic age. It is also assumed that the revealed geochemical heterogeneity of relict zircons was mainly due to the later redistribution of trace elements with the simultaneous formation of microinclusions of epigenetic minerals in the process of infiltration along microcracks into ultramafic rocks, precipitated by acidic melts.

Vibrational spectroscopic study of three Mg–Ni mineral series in white and greenish clay infillings of the New Caledonian Ni-silicate ores

This study presents and discusses infrared spectroscopic data of well characterised, naturally occurring trioctahedral layer silicates of the serpentine (Srp), talc (Tlc), and sepiolite (Sep) mineral groups, which are found in reactivated faults and sequences of white and green clay veins (deweylite and garnierite) of the New Caledonian Ni-silicate ores. Bands assigned to the OH stretching vibrations of these 1:1 and 2:1 layer silicates in both the fundamental and first overtone regions of mid- and near-infrared (MIR and NIR) spectra, respectively, are compared to those reported in the literature for synthetic Mg–Ni series of the Srp and Tlc mineral groups. They are also presented according to the sequences of infillings recognised in the white and green veins of the Ni-silicate ores. The study reveals that serpentine-like (SL) minerals of the first sequences of clay infillings are residues of larger crystals of serpentines (lizardite, chrysotile, and antigorite) and that the newly formed talc-like (TL) minerals and Sep are the main Ni-bearing carriers of the Ni-silicate ores. Decreasing crystal size and order in serpentine species have major effects on vibrational bands. They favour the broadening of the OH stretching bands, the degradation of the signals assigned to the interlayer OH, and the enhancement of the signal related to weakly bound water molecules. The replacement of Mg by Ni in octahedral sites of the 2:1 layer silicates (TL, Sep) of the greenish clay infillings can be traced by specific OH stretching bands related to the Mg3OH, Mg2NiOH, MgNi2OH, and Ni3OH configurations in the fundamental (MIR) and first overtone (NIR) regions of the spectra. The dominance of the Mg3OH and Ni3OH configurations with respect to mixed configurations in the Mg–Ni mineral series of the clay infillings (mostly in the dominant TL minerals) suggests that Mg and Ni segregation is related to separate Mg-rich and Ni-rich mineral phases rather than to a cationic clustering within the individual particles. This segregation of Mg and Ni in discrete mineral phases is related to Mg–Ni oscillatory zoning patterns (banded patterns) and is reproduced at the scale of the Ni-silicate ores between the white (deweylite) and greenish (garnierite) veins of the reactivated faults.

Open-system behaviour of detrital zircon during weathering: an example from the Palaeoproterozoic Pretoria Group, South Africa

Detrital zircon in six surface samples of sandstone and contact metamorphic quartzite of the Magaliesberg and Rayton formations of the Pretoria Group (depositional age c. 2.20–2.06 Ga) show a major age fraction at 2.35–2.20 Ga, and minor early Palaeoproterozoic – Neoarchaean fractions. Trace-element concentrations vary widely, with Ti, Y and light rare earth elements (LREEs) spanning over three orders of magnitude. REE distribution patterns range from typical zircon patterns (LREE depletion, heavy REE enrichment, well-developed positive Ce and negative Eu anomalies) to patterns that are flat to concave downwards, with indistinct Ce and Eu anomalies. The change in REE pattern correlates with increases in alteration-sensitive parameters such as Ti concentration and (Dy/Sm) + (Dy/Nd), U–Pb discordance and content of common lead, and with a gradual washing-out of oscillatory zoning in cathodoluminescence images. U and Th concentrations also increase, but Th/U behaves erratically. Discordant zircon scatters along lead-loss lines to zero-age lower intercepts, suggesting that the isotopic and chemical variations are the results of disturbance long after deposition. The rocks sampled have been in a surface-near position (at least) since Late Cretaceous time, and exposed to deep weathering under intermittently hot and humid conditions. In this environment, even elements commonly considered as relatively insoluble could be mobilized locally, and taken up by radiation-damaged zircon. Such secondary alteration effects on U–Pb and trace elements can be expected in zircon in any ancient sedimentary rock that has been exposed to tropical–subtropical weathering, which needs to be considered when interpreting detrital zircon data.

Complementary Textural, Trace Element, and Isotopic Analyses of Sulfides Constrain Ore-Forming Processes for the Slate-Hosted Yuhengtang Au Deposit, South China

Yuhengtang is a representative slate-hosted Au deposit in the Jiangnan orogenic belt, South China, with a reserve of ~55 t Au and an average grade of ~3.9 g/t. Gold mineralization is characterized by veinlet and disseminated ores comprising native gold, auriferous pyrite, and arsenopyrite. Paragenesis of the Yuhengtang deposit can be divided into three stages. Pre-ore stage 1 is composed of bedding-parallel layers of pyrite in slate of the Neoproterozoic Banxi Group. Main ore stage 2 represents the Au mineralization stage, and two distinct types of mineralization can be distinguished: visible Au-arsenopyrite-pyrite in quartz veinlets and auriferous arsenopyrite-pyrite disseminated within altered slate. Post-ore stage 3 consists of quartz-pyrite-calcite-ankerite veins. In this study, we integrate electron microprobe, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and high-resolution ion microprobe (SHRIMP) analyses to document textural, isotopic, and compositional variation among texturally complex pyrite and arsenopyrite assemblages in veinlet and disseminated ores. Additionally, LA-ICP-MS sulfur isotope mapping of pyrite highlights the covariation behavior between trace elements and sulfur isotopes at the grain scale, thus allowing the factors controlling sulfur isotope fractionation in hydrothermal Au deposits to be constrained. Pyrite, of sedimentary origin (stage 1), hosts negligible Au (<1.6 ppm) but is enriched in δ34S (15.6–25.8‰). Pyrite and arsenopyrite from stage 2 veinlet mineralization both display porous and dissolution-reprecipitation textures, have low Au concentrations (<4 and <78 ppm, respectively), and show a large variation in δ34S (–2.7 to 14.7‰ and –10.3 to 12.1‰, respectively). Pyrite and arsenopyrite from disseminated mineralization are, in contrast, characterized by oscillatory zoning textures and homogeneous appearance in backscattered electron (BSE) images, respectively, and are obvious by their relatively high contents of invisible Au (up to 90 and 263 ppm, respectively) and restricted range of δ34S values (0–5.3‰). These data suggest that magmatic-hydrothermal fluids contribute most of the Au and S budget in the Yuhengtang Au deposit. The major differences between veinlet and disseminated mineralization in terms of texture, trace element concentrations, and δ34S signatures of pyrite and arsenopyrite reflect contrasting mechanisms of Au precipitation and an evolution of physicochemical parameters of the ore-forming processes, particularly fO2 and the intensity of fluid-rock interaction. Pyrite from stage 3 appears homogeneous in BSE images yet displays a wide variation in δ34S values (1.2–31.4‰), further highlighting the controlling role played by physicochemical condition (i.e., pressure) on the δ34S signature of sulfides. Results of the coupled LA-ICP-MS sulfur and trace element mapping reveal that some zoned pyrite grains from stage 2 formed via overgrowth of Au-rich, light δ34S (2.4‰) hydrothermal rims onto Au-poor, heavy δ34S (18.1–18.5‰) sedimentary cores. All results support that multiple depositional mechanisms within a dynamic mineral system were responsible for Au concentration and define the specific textural, compositional, and sulfur isotope signatures of sulfides in coexisting vein/veinlet and disseminated mineralization. The new data highlight the ore-forming processes-based interpretation for ore genesis and underpin the importance of performing complementary in situ mineralogical analyses to elucidate the source and evolution of ore-forming fluids and enable correct interpretation of the architecture of the hydrothermal Au system.

A lab-on-a-chip approach integrating in-situ characterization and reactive transport modelling diagnostics to unravel (Ba,Sr)SO4 oscillatory zoning

The co-precipitation of sulphate minerals such as celestine and barite is widely studied because their formation is ubiquitous in natural and anthropogenic systems. Co-precipitation in porous media results in crystallization of solid solutions yielding characteristics such as oscillatory zoning that are rarely observed in bulk solution or in batch experiments. In the past, the precipitation of compositionally-zoned (Ba,Sr)SO4 crystals was observed post-mortem in macroscopic silica gel counter-diffusion experiments. Their formation was originally explained by the difference in the solubility products of the end-members combined with diffusion-limited transport of solutes to the mineral-fluid interface, while a later study favored the idea of kinetically controlled reactions. With recent advances combining in-operando microfluidic experiments and reactive transport modelling, it is now possible to verify hypotheses on the driving forces of transport-coupled geochemical processes. We developed a “lab on a chip” experiment that enabled the systematic study of the nucleation and growth of oscillatory-zoned (Ba,Sr)SO4 crystals in a microfluidic reactor. The compositions of the solid solutions were determined by in-situ Raman spectroscopy. Our investigation shows (1) that the composition of the nucleating phases can be approximated using classical nucleation theory, (2) that the oscillatory zoning is not solely controlled by the limited diffusional transport of solutes, and (3) that nucleation kinetics plays a major role in the switch between different stoichiometric compositions. The zoning phenomena is governed by the complex interplay between the diffusion of reactants and the crystallization kinetics as well as other factors, e.g. surface tension and lattice mismatch.

Crystal Specific Constraints on Subvolcanic Processes Preceding Eruptions at Mt Taranaki, New Zealand

<p>Andesitic magmas are the product of a complex interplay of processes including fractional crystallisation, crystal accumulation, magma mixing and crustal assimilation. Recent studies have suggested that andesitic rocks are in many cases a complex mixture of a crystal cargo and melts with more silicic compositions than andesite. In situ glass- and mineral-specific geochemical techniques are therefore key to unravelling the processes and timescales over which andesitic magmas are produced, assembled and transported to the surface. To this end, this thesis presents a detailed in situ glass- and mineral-specific study of six Holocene eruptions (Kaupokonui, Maketawa, Inglewood a and b, and Korito) at Mt Taranaki to investigate the petrogenetic processes responsible for producing these sub-plinian eruptions at this long-lived (130 000 yr) andesitic volcano. Mt Taranaki is an andesitic stratovolcano located on the west coast of New Zealand’s North Island and as such it is distinct from the main subduction related volcanism. Crystal-specific major and trace element data were combined with textural analysis and quantitative modelling of intensive magmatic parameters and crystal residence times to identify distinct mineral populations and constrain the magmatic histories of the crystal populations. Least-squares mixing modelling of glass and phenocryst compositions demonstrates that the andesitic compositions of bulk rock Mt Taranaki eruptives results from mixing of a daciticrhyolitic melt and a complex crystal cargo (plagioclase, pyroxene, amphibole) that crystallised from multiple melts under a wide range of crustal conditions. Magma mixing of compositionally similar end members that mix efficiently also occurred beneath Mt Taranaki, and as such only produced prominent disequilibrium textures in a small proportion of the minerals in the crystal cargo. The chemistry of the earliest crystallising amphibole indicates crystallisation from an andesitic-dacitic melt at depths of ca. 20-25 km, within the lower crust. Magmas then ascended through the crust relatively slowly via a complex magmatic plumbing system. However, most of the crystal cargo formed by decompression-driven crystallisation at depth so 6-10 km, as is indicated by the dominance of oscillatory zoning and the equilibrium obtained between mineral rims and the host glasses. Taranaki magmas recharge on timescales of 1000-2000 yrs. The eruptions investigated here provide a snapshot of the end of one cycle and the beginning of another. The younger Kaupokonui and Maketawa eruptions (ca. 2890 - <1950 yr BP) are the least evolved magmas, record a stronger mixing signal in the crystal cargo, and are volumetrically smaller than the earlier Inglewood a and b and Korito eruptions (ca. 4150-3580 yr BP). The Kaupokonui and Maketawa eruptions may reflect arrival of a new pulse of magma from the lower crust, or that these are early eruptions within a recharge sequence, which have not had as much time to further differentiate and evolve as the earlier Inglewood a and b and Korito eruptions that represent the end of a magma recharge cycle. One of the six investigated eruptions was identified to come from Fantham’s Peak on the basis of its distinctive glass and mineral chemistry and petrology. Glass trace element data indicate that this eurption’s magmatic system was distinct from that of the other main vent Holocene eruptions investigated in this study. Crystal residence times were investigated using Fe-Mg interdiffusion in clinopyroxene and indicate that magma bodies stall in upper crustal storage chambers for timescales of a few months to years. The younger eruptions of the least evolved magmas with the strongest mixing signal return the shortest residence times, which may indicate that magma mixing events occurring a few months before eruption may have been the trigger for these eruptions at Mt Taranaki. Amphibole geospeedometry for these eruptives reveal rapid magma transport from depths of 6-10 km to the surface on timescales of < 1 week.</p>

Crystal Specific Constraints on Subvolcanic Processes Preceding Eruptions at Mt Taranaki, New Zealand

<p>Andesitic magmas are the product of a complex interplay of processes including fractional crystallisation, crystal accumulation, magma mixing and crustal assimilation. Recent studies have suggested that andesitic rocks are in many cases a complex mixture of a crystal cargo and melts with more silicic compositions than andesite. In situ glass- and mineral-specific geochemical techniques are therefore key to unravelling the processes and timescales over which andesitic magmas are produced, assembled and transported to the surface. To this end, this thesis presents a detailed in situ glass- and mineral-specific study of six Holocene eruptions (Kaupokonui, Maketawa, Inglewood a and b, and Korito) at Mt Taranaki to investigate the petrogenetic processes responsible for producing these sub-plinian eruptions at this long-lived (130 000 yr) andesitic volcano. Mt Taranaki is an andesitic stratovolcano located on the west coast of New Zealand’s North Island and as such it is distinct from the main subduction related volcanism. Crystal-specific major and trace element data were combined with textural analysis and quantitative modelling of intensive magmatic parameters and crystal residence times to identify distinct mineral populations and constrain the magmatic histories of the crystal populations. Least-squares mixing modelling of glass and phenocryst compositions demonstrates that the andesitic compositions of bulk rock Mt Taranaki eruptives results from mixing of a daciticrhyolitic melt and a complex crystal cargo (plagioclase, pyroxene, amphibole) that crystallised from multiple melts under a wide range of crustal conditions. Magma mixing of compositionally similar end members that mix efficiently also occurred beneath Mt Taranaki, and as such only produced prominent disequilibrium textures in a small proportion of the minerals in the crystal cargo. The chemistry of the earliest crystallising amphibole indicates crystallisation from an andesitic-dacitic melt at depths of ca. 20-25 km, within the lower crust. Magmas then ascended through the crust relatively slowly via a complex magmatic plumbing system. However, most of the crystal cargo formed by decompression-driven crystallisation at depth so 6-10 km, as is indicated by the dominance of oscillatory zoning and the equilibrium obtained between mineral rims and the host glasses. Taranaki magmas recharge on timescales of 1000-2000 yrs. The eruptions investigated here provide a snapshot of the end of one cycle and the beginning of another. The younger Kaupokonui and Maketawa eruptions (ca. 2890 - <1950 yr BP) are the least evolved magmas, record a stronger mixing signal in the crystal cargo, and are volumetrically smaller than the earlier Inglewood a and b and Korito eruptions (ca. 4150-3580 yr BP). The Kaupokonui and Maketawa eruptions may reflect arrival of a new pulse of magma from the lower crust, or that these are early eruptions within a recharge sequence, which have not had as much time to further differentiate and evolve as the earlier Inglewood a and b and Korito eruptions that represent the end of a magma recharge cycle. One of the six investigated eruptions was identified to come from Fantham’s Peak on the basis of its distinctive glass and mineral chemistry and petrology. Glass trace element data indicate that this eurption’s magmatic system was distinct from that of the other main vent Holocene eruptions investigated in this study. Crystal residence times were investigated using Fe-Mg interdiffusion in clinopyroxene and indicate that magma bodies stall in upper crustal storage chambers for timescales of a few months to years. The younger eruptions of the least evolved magmas with the strongest mixing signal return the shortest residence times, which may indicate that magma mixing events occurring a few months before eruption may have been the trigger for these eruptions at Mt Taranaki. Amphibole geospeedometry for these eruptives reveal rapid magma transport from depths of 6-10 km to the surface on timescales of < 1 week.</p>

Shallow magmatic processes revealed by cryptic microantecrysts: a case study from the Taupo Volcanic Zone

Arc magmas typically contain phenocrysts with complex zoning and diverse growth histories. Microlites highlight the same level of intracrystalline variations but require nanoscale resolution which is globally less available. The southern Taupo Volcanic Zone (TVZ), New Zealand, has produced a wide range of explosive eruptions yielding glassy microlite-bearing tephras. Major oxide analyses and textural information reveal that microlite rims are commonly out of equilibrium with the surrounding glass. We mapped microlites and microcrysts at submicron resolution for major and trace element distributions and observed three plagioclase textural patterns: (1) resorption and overgrowth, (2) oscillatory zoning, and (3) normal (sharp) zoning. Pyroxene textures are diverse: (1) resorption and overgrowth, (2) calcium-rich bands, (3) hollow textures, (4) oscillatory zoning, (5) sector zoning, (6) normal zoning and (7) reverse zoning. Microlite chemistry and textures inform processes operating during pre-eruptive magma ascent. They indicate a plumbing system periodically intruded by short-lived sub-aphyric dykes that entrain microantecrysts grown under diverse physico-chemical conditions and stored in rapidly cooled, previously intruded dykes. Changes in temperature gradients between the intrusion and the host rock throughout ascent and repeated magma injections lead to fluctuations in cooling rates and generate local heterogeneities illustrated by the microlite textures and rim compositions. Late-stage degassing occurs at water saturation, forming thin calcic microcryst rims through local partitioning effects. This detailed investigation of textures cryptic to conventional imaging shows that a significant proportion of the micrometre-sized crystal cargo of the TVZ is of antecrystic origin and may not be attributed to late-stage nucleation and growth at the onset of volcanic eruptions, as typically presumed.

Addressing Molybdenite187 Re Parent- 187 Os Daughter Intra-Crystalline Decoupling In Light of Recent In-Situ Micro-Scale Observations

Every so often, an analytical advancement or challenging geological occurrence necessitates re-evaluation of well-established geochronological methods. Rhenium-osmium (Re-Os) dating of sulfide minerals, especially molybdenite, by isotope dilution-negative thermal ionization mass spectrometry (ID-NTIMS) yields demonstrably accurate, robust ages. Difficulty in determining an age is caused by either 1) geological complexity including intra-crystalline heterogeneity or 2) inadequate analytical capabilities. Established, systematic methods overcome most of these difficulties, with exceptions. With respect to molybdenite, geological complexity is encountered as macro-scale polyphase overgrowths. Micro-scale complexity is observed as primary Re oscillatory zoning and secondary intra-granular 187Re parent-187Os daughter decoupling. Macro- and micro-scale geologic heterogeneities are overcome by systematic, targeted sampling protocols built upon careful hand sample and microscopic observations linking molybdenite crystallization to the host rock's geologic history.Advances in instrumentation permit more accurate and precise determination of elemental and isotopic compositions at the micro-scale, which ultimately reveals geological complexity and heterogeneity. Yet, newly produced high-resolution data must be carefully scrutinized and interpreted correctly. Two new micro-scale analytical techniques have been proposed: (1) to use NanoSIMS imaging to reveal heterogeneities that could preclude micro-scale geochronology (spot dating) methods, and (2) enable micro-scale geochronology using LA-ICP-MS/MS to distinguish the interfering masses of parent Re and daughter Os isotopes. We turn first to NIST Reference Material 8599, Henderson molybdenite, to compare its Re-Os characteristics and ID-NTIMS results with the newly published Re-Os molybdenite data. We identify the "best possible results" by quantifying the homogeneity of the NIST molybdenite, including model age variability, precision of ID-NTIMS analyses, and statistical treatment. Subsequently, we examine the limitations of the two newly proposed methods. We demonstrate that parent-daughter decoupling cannot be precluded by visual inspection of NanoSIMS isotopic maps. In addition, we prove mathematically that the quantifying the contribution of 187Os to the total measured mass 187 by LA-ICP-MS/MS is far too imprecise to achieve high-precision geochronology. Statistical data treatment and reporting can also result in misguided conclusions and applications.