Zircon Inclusions Research Articles

Petrogenesis of Montana, USA Sapphires Inferred from Oxygen Isotopes and Zircon Inclusions

Abstract Montana hosts the largest sapphire deposits in the US, but the genesis of and connection among the various secondary and primary sapphire occurrences remains cryptic. In situ SIMS measurements of oxygen isotopes in sapphires and zircon inclusions in sapphires provide an opportunity to study the isotope and trace element geochemistry in order to understand sapphire-forming protoliths (i.e. crustal setting and alteration). Sapphire from Montana was transported as xenocrysts in carrier (host) magmas that resorbed sapphire exteriors during transport. The timing and nature of sapphire genesis is elucidated by SIMS measurements of trace elements and U–Pb from discrete zones in zircon inclusions with rims that are interpreted to be syngenetic with host sapphire. Montana sapphires exhibit a large range of δ18O values, from −3‰ to +12‰ VSMOW. However, all but two anomalous crystals fall in the range of 0‰ to 8‰. There is significant crystal-to-crystal variability yet averages at most deposits are consistent with high-temperature equilibration with the mantle (δ18O(Crn) = 4.4‰ to 5.7‰), with the exception of the commercial sapphire deposits at Rock Creek that average 2.7‰. Ruby analyses are limited, but typically have lower δ18O values compared to sapphires from the same detrital localities. Homogeneity within individual crystals (avg. 2 s = ±0.2‰) indicates the absence of isotopically distinct fluid or melt during crystallization. But intercrystalline δ18O ranges by up to 7‰ at a single locality, suggesting sapphire variability at a deposit reflects heterogeneity in the original protolith. Oxygen isotope fractionations between zircon rims and surrounding sapphire suggest comagmatic zircon inclusions and corundum equilibrated at high temperature. No correlation is seen for the degree of radiation damage and alteration of δ18O(Zrc) when zircon inclusions are surrounded and armored by sapphire. U–Pb ages and trace elements were measured in a small subset of syngenetic zircon inclusions in Dry Cottonwood Creek sapphires, revealing a Proterozoic (1778 ± 9 Ma) age for the protolith of sapphires at this locality and a likely polygenetic history. Previous work has suggested formation of these sapphires through partial melting of anorthosites and several anorthosites occur locally and match the age of zircon inclusion cores—the Boehls Butte anorthosite (~180 km NW of Rock Creek) and the Bitterroot anorthosite (~55 km W of Rock Creek) could correlate with Al-rich protoliths at depth. Proterozoic U–Pb ages of zircon from the Boehls Butte anorthosite (1787 ± 2 Ma) match well with the age of zircon inclusion cores in Dry Cottonwood Creek sapphires and suggest genesis in these or similar protoliths. Zircon rims with Tera-Wasserburg lower intercept ages of 110 ± 9 Ma are consistent with previous observations of a xenocrystic relationship to the ~50 Ma Eocene volcanic rocks. Corundum that formed over 50 Ma prior to being scavenged by Eocene magmas likely originated by the anatexis of Precambrian anorthosites and possibly other aluminum-rich rocks at depth.

Improved precision at high-spatial resolution of strontium isotope analysis in apatite and apatite inclusions by LA-MC-ICP-MS with 1013 Ω amplifiers

Recent advancements in in situ analytical techniques have generated new interest in measuring strontium (Sr) isotopes in rock-forming apatite and apatite inclusions in zircon, because they can bring new insights into the primary signatures of magmatic sources. This paper introduces a novel approach based on laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), which allows to achieve higher precision in situ analysis of Sr isotopes in small volumes of samples. The use of 1013 Ω current amplifiers for the acquisition of the signal on m/z 83, 83.5, 84, 85, 85.5, 86, 86.5, 87 and 88, along with a specific data reduction protocol, enhance both the internal precision and the external reproducibility of the 87Sr/86Sr ratio by a factor of four, presenting a significant improvement over the conventional use of 1011 Ω amplifiers. An internal precision better than 0.45‰ (i.e. 450 ppm; 2 s.e.) on the 87Sr/86Sr ratio can be achieved on the Durango apatite standard with a 13 μm square-shaped laser beam and a laser ablation pit of 60 μm depth. With a 5 μm beam, the precision is 2.2‰ on average with a 10 μm ablation pit depth for this standard. This analytical procedure was applied to investigate Sr isotopes in apatite and apatite inclusions within zircons from seven high BaSr magmatic rocks in the Northern Highland Terrane, Scotland. For these well-characterised, geochemically undisturbed samples, the data reveals similar Sr isotope compositions between apatite inclusions armoured in zircon, matrix apatites, and the bulk rock. This highlights the ability of apatite inclusions to faithfully record primary signatures of host magmas, elucidating their utility as reliable indicators in petrological studies of ancient samples.

Evidence for Low‐Pressure Crustal Anatexis During the Northeast Atlantic Break‐Up

AbstractWhile basaltic volcanism is dominant during rifting and continental breakup, felsic magmatism may be a significant component of some rift margins. During International Ocean Discovery Program (IODP) Expedition 396 on the continental margin of Norway, a graphite‐garnet‐cordierite bearing dacitic unit (the Mimir dacite) was recovered in two holes within early Eocene sediments on Mimir High (Site U1570), a marginal high on the Vøring Transform Margin. Here, we present a comprehensive textural, petrological, and geochemical study of the Mimir dacite in order to assess its origin and discuss the geodynamic implications. The major mineral phases (garnet, cordierite, quartz, plagioclase, alkali feldspar) are hosted in a fresh rhyolitic, vesicular, glassy matrix that is locally mingled with sediments. The major element chemistry of garnet and cordierite, the presence of zircon inclusions with inherited cores, and thermobarometric calculations all support an upper crustal metapelitic origin. While most magma‐rich margin models favor crustal anatexis in the lower crust, thermobarometric calculations performed here show that the Mimir dacite was produced at upper‐crustal depths (<5 kbar, 18 km depth) and high temperature (750–800°C) with up to 3 wt% water content. In situ U‐Pb analyses on zircon inclusions give a magmatic crystallization age of 54.6 ± 1.1 Ma, consistent with emplacement that post‐dates the Paleocene‐Eocene Thermal Maximum. Our results suggest that the opening of the Northeast Atlantic was associated with a phase of low‐pressure, high‐temperature crustal anatexis preceding the main phase of magmatism.

Petrogenesis of a TTG intrusive suite: the La Dauversière pluton, Abitibi greenstone belt, Canada

Insights into the source and partial melting conditions of tonalite–trondhjemite–granodiorite (TTG) intrusive suites provide essential constraints on the tectonic regimes prevailing before 2.5 Ga. As case studies on individual suites increase, the TTG group becomes more heterogeneous, offering detailed insights into the evolution of the Archean crust. This study focuses on the La Dauversière pluton, a TTG suite in the Chibougamau area, Abitibi greenstone belt, Canada, to unravel complex differentiation processes in a pluton with a relatively limited volume. According to whole-rock and zircon chemistry, the La Dauversière pluton is a TTG suite that contains less Na and light rare earth elements (LREE) than other TTG suites of the Abitibi greenstone belt. Whole-rock chemistry and melt inclusions in zircon also point to an atypical fractional crystallization process that favors K enrichment, contrasting with the Na enrichment trends observed in most TTG suites globally. Magma hybridization, with late magma pulses interacting with early phases partially crystallized at depth, likely explains the chemistry of the La Dauversière pluton. The source of these magmas differs from the basaltic source that gave rise to most TTG suites of the Abitibi greenstone belt, possibly because older basement occurs in the Chibougamau area and is lacking in the rest of the greenstone belt. The La Dauversière pluton underscores the importance of interpreting the petrogenesis of individual TTG suites before inferring global source processes and geodynamic settings.

Melt inclusions in zircon: a window to understanding the structure and evolution of the magmatic system beneath the Laguna del Maule volcanic field

Melt inclusions in zircon: a window to understanding the structure and evolution of the magmatic system beneath the Laguna del Maule volcanic field

Petrologically controlled oxygen isotopic classification of cogenetic magmatic and metamorphic sapphire from Quaternary volcanic fields in the Eifel, Germany

Gem sapphire is commonly retrieved from primary and secondary deposits associated with alkali basaltic fields, but its source rocks are rarely preserved. The Eifel (Rhenish Massif, western Germany), although not producing gem sapphire, shares many petrologic and geochemical similarities with such fields worldwide. Due to the young age of volcanic deposits and active quarrying, sapphire-bearing rocks are readily accessible, along with detrital sapphire from modern sediments. Here, oxygen isotope and trace element compositions are reported for 223 sapphire grains, and rutile and zircon inclusions in sapphire were dated indicating crystallization synchronous with Paleogene–Quaternary volcanism. Endmembers in δ18O range are sapphires from syenites representing mantle-derived differentiated melts with minor crustal contamination (~4–6‰) and contact metamorphic mica schists (>10‰) as purely crustal source rocks. Intermediate values between ~6 and 10‰ require variable degrees of mantle-crust hybridization. Lower crustal granulite sources are dismissed based on their oxygen isotopic compositions being lower than most sapphire crystals. Diffusion modelling of sharp oxygen isotopic zonation in compositionally zoned crystals precludes crystal residence at >900 °C over the lifetime of evolved magma reservoirs in the Eifel (c. 50 ka). This argues against direct mantle or lower crustal sapphire origins. Instead, low temperature residence is consistent with sharp δ18O gradients, coexisting andalusite, and fluid inclusion barometry. Hence, Eifel sapphire crystallization is attributed to contact metamorphic aureoles around upper crustal (5–7 km) magma bodies where phonolite, trachyte, and carbonatite melts differentiated from mafic parental magmas, and reacted with metasedimentary wall rocks.

U–Pb Age and Mineral Inclusions in Zircon from Diamondiferous Garnet–Pyroxene Rocks of the Kumdy-Kol Microdiamond Deposit in Northern Kazakhstan

Abstract ––This paper describes the results of studying inclusions and determining the U–Pb age in zircon from the diamondiferous garnet–pyroxene rock of the Kumdy-Kol metamorphogenic diamond deposit, located in the Kokchetav subduction-collision zone. The distribution of rare earth elements in garnet and clinopyroxene is used as a basis for estimating the equilibrium pressure (5.5 ± 0.3 GPa) and temperature (993 ± 24 °C), which correspond to the diamond stability field. The composition of mineral inclusions in zircon indicates its formation at both the progressive and the regressive stage of metamorphism. The concordia diagram shows that the figurative points of zircon lie on a discordia with an upper intersection at 1953 ± 139 Ma and a lower intersection at 512 ± 4 Ma. The main peak on the graph of the probability density distribution of zircon ages corresponds to an age of 519 Ma. The presence of grossular–almandine garnet inclusions in zircon confirms the previous assumption that the basement rocks of the Kokchetav massif act as protoliths of garnet–pyroxene rocks.

Crystallization processes of quartz in a granitic magma: Implications for the magma chamber processes of Okueyama granite, Kyushu, Japan

Cathodoluminescence (CL) characterization combined with titanium concentration of quartz crystals in granite is a prevalent tool for elucidating the crystal growth of quartz in granitic magmas. The CL pattern yields to the internal structure, and the titanium concentration allows crystallization temperature determination based on TitaniQ geothermometry. Using two approaches, 1) observations of quartz crystals using thin sections and 2) observations based on multiple sections of separated quartz, we identified the overall characteristics of quartz crystals within a magma chamber and the 3D internal structure and growth characteristics of quartz grains. Thin section observations allow for the characterization of quartz containing zircon inclusions, indicating that zircon crystallization was followed by quartz crystallization; therefore, the onset of quartz crystallization occurred later than the onset of zircon crystallization in the cooling magma chamber. Such petrological restrictions permit allow for suitable geothermometer use and appropriate TiO2 activity estimation, and thus, accurate quartz crystallization temperature determination. Observations using multiple sections of separated quartz crystals provide 3D internal structures and true onset parts within the grains. Progressive variation in local activities of TiO2 in the concentration boundary layer at the growing quartz crystal interface in the magma chamber can be evaluated using a combination of the crystallization temperature and variations in titanium concentration within the grain, yielding a sequential crystallization model of oscillatory zoning quartz with local activities of TiO2 in the cooling magma chamber. The difference in CL characteristics and crystallization temperature among the lithofacies in granite contributes to clarifying the spatiotemporal magma chamber processes.

Nanoscale imaging of Fe-rich inclusions in single-crystal zircon using X-ray ptycho-tomography

We apply X-ray ptycho-tomography to perform high-resolution, non-destructive, three-dimensional (3D) imaging of Fe-rich inclusions in paleomagnetically relevant materials (zircon single crystals from the Bishop Tuff ignimbrite). Correlative imaging using quantum diamond magnetic microscopy combined with X-ray fluorescence mapping was used to locate regions containing potential ferromagnetic remanence carriers. Ptycho-tomographic reconstructions with voxel sizes 85 nm and 21 nm were achievable across a field-of-view > 80 µm; voxel sizes as small as 5 nm were achievable over a limited field-of-view using local ptycho-tomography. Fe-rich inclusions 300 nm in size were clearly resolved. We estimate that particles as small as 100 nm—approaching single-domain threshold for magnetite—could be resolvable using this “dual-mode” methodology. Fe-rich inclusions (likely magnetite) are closely associated with apatite inclusions that have no visible connection to the exterior surface of the zircon (e.g., via intersecting cracks). There is no evidence of radiation damage, alteration, recrystallisation or deformation in the host zircon or apatite that could provide alternative pathways for Fe infiltration, indicating that magnetite and apatite grew separately as primary phases in the magma, that magnetite adhered to the surfaces of the apatite, and that the magnetite-coated apatite was then encapsulated as primary inclusions within the growing zircon. Rarer examples of Fe-rich inclusions entirely encapsulated by zircon are also observed. These observations support the presence of primary inclusions in relatively young and pristine zircon crystals. Combining magnetic and tomography results we deduce the presence of magnetic carriers that are in the optimal size range for carrying strong and stable paleomagnetic signals but that remain below the detection limits of even the highest-resolution X-ray tomography reconstructions. We recommend the use of focused ion beam nanotomography and/or correlative transmission electron microscopy to directly confirm the presence of primary magnetite in the sub 300 nm range as a necessary step in targeted paleomagnetic workflows.

Inclusions in magmatic zircon from Slavonian mountains (eastern Croatia): anatase, kumdykolite and kokchetavite and implications for the magmatic evolution

Abstract. Micro-Raman spectroscopy was used to determine the inclusions in magmatic zircon from the Late Cretaceous A-type acid igneous rocks in the Slavonian mountains (Mt. Papuk and Mt. Požeška Gora), in the southwestern part of the Pannonian Basin (Croatia). The mineral inclusions detected in the early-crystallised zircon are anatase, apatite, hematite, ilmenite and possibly magnetite. Numerous melt inclusions comprise albite, cristobalite, hematite, kaolinite, K-feldspar, kokchetavite, kumdykolite muscovite and quartz, where this mineral association is characteristic of so-called nanorocks (nanogranites), commonly found in peritectic garnets from high-grade metamorphic rocks. Here we present the first finding of kokchetavite and kumdykolite in a magmatic zircon. Together with anatase and hematite, these polymorphs are likely evidence of rapid uplift and consequent rapid cooling of hot oxidised magma generated in the lower crust and its emplacement in the upper crustal level. This finding provides further confirmation that kumdykolite and kokchetavite do not require ultra-high pressure (UHP) to form and should not be considered exclusively UHP phases. The rapid uplift was possible due to the formation of accompanying extensional deep rifts during the tectonic transition from compression to extension, associated with the closure of the Neotethys Ocean in the area of present-day Slavonian mountains in the Late Cretaceous (∼82 Ma).

Cold deep subduction of Indian continental crust and release of ultrahigh‐pressure fluid during initial exhumation: Insights from coesite‐bearing eclogite‐vein systems in Kaghan Valley, Pakistan

AbstractThe ultrahigh‐pressure (UHP) eclogites from the Kaghan Valley in Pakistan, which formed by the deep subduction of the Indian plate beneath the Asian plate in the Eocene, contain complex metamorphic vein systems (including both isolated veins and vein networks), with mineral assemblages of epidote + quartz + kyanite + phengite ± omphacite ± garnet. The investigations on the Kaghan UHP eclogite‐vein systems provide important insights into the mechanism and timing of metamorphic dehydration, fluid flow, and fluid–rock interaction in the deeply subducted Indian continental slab as well as the chemical characteristics of slab‐derived, aqueous fluids. Abundant lawsonite pseudomorphs, characterized by prismatic aggregates of epidote, kyanite, and quartz porphyroblasts, are first recognized in the Kaghan eclogites. This observation, in combination with the occurrence of coesite pseudomorphs in epidote porphyroblasts as well as the coexistence of epidote and coesite in the eclogite zircon, indicates the previous existence of UHP lawsonite in these eclogites. Petrological studies and phase equilibrium modelling reveal clockwise P–T trajectories for the Kaghan eclogites that are featured by prograde vectors in lawsonite‐stability regions with peak conditions of 3.0–3.4 GPa/650–690°C, followed by isothermal decompression and lawsonite breakdown under UHP conditions during the initial exhumation stage. The results of metamorphic evolution, together with in situ epidote and bulk Sr isotopic analyses, indicate that the fluids responsible for vein systems are most likely derived from the breakdown of UHP lawsonite in the eclogites. SIMS U–Pb dating of metamorphic zircons from the eclogites, integrated with the Raman analysis of inclusions in zircons, indicates that the UHP dehydration of eclogites occurred at 46.4 ± 1.2 and 46.8 ± 0.9 Ma. Analyses of hydrothermal zircons from the veins yielded slightly younger ages of 44.7 ± 1.0 and 44.9 ± 1.4 Ma, which represent the timing of fluid flow and/or vein crystallization during exhumation of the UHP rocks. Mass‐balance calculation results, in combination with the vein compositions, show that the fluid flow and fluid‐eclogite interaction led to the transfer of Si, Al, Ca, K, and incompatible trace elements from the eclogites into the fluids, from which the vein systems crystallized. This study indicates cold deep subduction of Indian continental crust along low geothermal gradients (6–7°C/km). The UHP fluid liberation and channelized fluid flow occurred during the initial exhumation of the cold Indian slab and are expected to induce the transfer of H2O and incompatible trace elements from the Indian slab to the Asian lithosphere, which potentially contributes to the formation of post‐collisional magmas. Moreover, we suggest that metamorphic vein systems in UHP lawsonite eclogites offer important constraints on the occurrence and timing of fast slab exhumation in continental subduction‐collision zones.

First evaluation of stiff-in-soft host–inclusion systems: experimental synthesis of zircon inclusions in quartz crystals

Quartz crystals with zircon inclusions were synthesized using a piston-cylinder apparatus to experimentally evaluate the use of inclusions in “soft” host minerals for elastic thermobarometry. Synthesized zircon inclusion strains and, therefore, pressures (Pinc) were measured using Raman spectroscopy and then compared with the expected inclusion strains and pressures calculated from elastic models. Measured inclusion strains and inclusion pressures are systematically more tensile than the expected values and, thus, re-calculated entrapment pressures are overestimated. These discrepancies are not caused by analytical biases or assumptions in the elastic models and strain calculations. Analysis shows that inclusion strain discrepancies progressively decrease with decreasing experimental temperature in the α-quartz field. This behavior is consistent with inelastic deformation of the host–inclusion pairs induced by the development of large differential stresses during experimental cooling. Therefore, inclusion strains are more reliable for inclusions trapped at lower temperature conditions in the α-quartz field where there is less inelastic deformation of the host–inclusion systems. On the other hand, entrapment isomekes of zircon inclusions entrapped in the β-quartz stability field plot along the α–β quartz phase boundary, suggesting that the inclusion strains were mechanically reset at the phase boundary during experimental cooling and decompression. Therefore, inclusions contained in soft host minerals can be used for elastic thermobarometry and inclusions contained in β-quartz may provide constraints on the P–T at which the host–inclusion system crossed the phase boundary during exhumation.

Ultra-deep (>5 GPa) subduction of paragneiss and metagranite in the Dabie UHP Terrane, central China: Constraints from zircon inclusions, phase equilibrium modelling and U-Pb geochronology

In the Dabie orogen, central China, the ultrahigh pressure metamorphism (UHPM) was best recorded from meta-mafic and ultramafic rocks occurring as lenses within voluminous amphibolite-facies metafelsic rocks. However, whether or not these metafelsic rocks experienced ultra-deep subduction (>5 GPa) has been rarely quantified because of the rare preservation of peak diagnostic assemblages. In this study, we reconstruct the P-T-t paths for paragneisses and metagranites from the Dabie region to better elucidate their metamorphic evolution based on detailed petrological, mineralogical and Raman micro-spectrometer analysis of zircon inclusion combined with conventional geothermobarometry, phase equilibrium modelling and U-Pb dating. At least three stages of metamorphic evolution are identified for these lithologies. The UHPM is represented by the assemblages of coesite + garnet Ia/Ib + jadeite + phengite + K-feldspar + aragonite for paragneiss and diamond + aragonite + magnesite + phengite + rutile + garnet for metagranite, respectively. The paragneiss experienced peak conditions of 4.5–6.0 GPa/690–720 °C, followed by a decompression heating to 3.5–4.6 GPa/750–850 °C. Also, the diamond + aragonite + magnesite inclusions in zircon from the metagranite provide the first direct mineralogical evidences for ultra-deep subduction. Furthermore, the minimum peak pressure was defined at 5.4 GPa based on pseudosection for metagranite. As a result, this study provides the first mineralogical and modelling evidences of ultra-deep subduction for the metafelsic rocks in the Dabie orogen. The dating results suggest that the UHPM, anatexis and amphibolite-facies overprint occurred at 236 ± 3 Ma, 226 ± 1 Ma and 195 ± 4 Ma, respectively. The estimated average exhumation rate is 5.8–7.2 km/Ma from peak UHP to amphibolite-facies stages. Moreover, a comparison of different collisional orogens suggests that the preservation of UHP minerals in metafelsic rocks strongly depends on exhumation rate.

Water contents and pressures of melts in unerupted felsic magma constrained by SEM-EDS analysis of homogenized melt inclusions in zircon

Water contents and pressures of melts in unerupted felsic magma constrained by SEM-EDS analysis of homogenized melt inclusions in zircon

Primary source of titanomagnetite ores at the Orokolo Bay placer deposit: Implications for petrogenesis

AbstractCoastal and river sand sequences of Holocene age at Orokolo Bay in Papua New Guinea are host to secondary titanomagnetite ore deposits. The resource developer (Mayur Resources) declares a measured orebody of 139 million tons at grades of 11.35% Fe, 1.94% Ti, and 712 ppm Zr. Orokolo Bay is located near the boundary between the Papuan Thrust and Fold Belt and the Aure Fold Belt. This region is characterized by sedimentary rocks of the Papuan Basin, which are intruded and overlain by a diverse array of magmatic intrusive and volcanic rocks, ranging from mafic alkaline to felsic calc‐alkaline types related to arc–continent collisions in the Cenozoic. The Vailala and Purari rivers located to the east and west of Orokolo Bay, respectively, are the main sediment pathways. Earlier provenance studies have suggested that the deposited sediments mainly come from the erosion of volcanic rocks found within the catchments of these two rivers. However, these studies did not specifically identify the primary source of the Fe‐Ti ores within this region. Here, we report on a potential primary source of the Fe‐Ti ores by examining the occurrence and chemical compositions for detrital titanomagnetites, and associated pyroxene and amphibole minerals. The purpose of this was to discern specific attributes of the primary source for its identification. Lithic sediments comprised of magnesio‐hornblende (Mhbl), tschermakaite (Tsr) and magnesio‐hastingsite (Mhst) amphiboles, clinopyroxene (Cpx), and Fe‐Ti oxide minerals dominate the heavy sands. Feldspar is more abundant than quartz, and Cr‐spinel is rare. Two primary titanomagnetite types were categorized based on their homogeneous grain textures and TiO2 content, Tmt1 (average 28 wt% TiO2) and Tmt2 (average 7 wt% TiO2). Pleonaste exsolutions and zircon inclusions distinguish Tmt1 from Tmt2, while apatite and quenched glass inclusions occur in both. A portion of both primary titanomagnetites exhibits hydrothermal overprinting, this subset was categorized as Tmt3 (average 12.5 wt% TiO2). Fe‐Ti oxides are associated with Cpx and amphiboles as inclusions, and as pseudomorphs in opacitic amphiboles. Tmt1 coexists predominantly with Mhst‐Tsr but also appears less frequently with Mhbl and Cpx. Tmt2 occurs almost exclusively with Mhbl and Cpx. Titanomagnetite chemistry is low in Cr, Mg, and V content suggesting crystallization from evolved melts. Tmt3 is shows enriched Si, Ca, Al, and Mg concentrations that are characteristic of late‐stage fluids derived from calcareous rocks. Amphibole and Cpx chemistry indicate they were derived from a subalkaline source magma of arc affinity. Fe‐Ti thermobarometry reveals Tmt1 (990 to 967°C, −0.51 to −0.18 ΔNNO) formed at higher temperatures and in less oxidized conditions than Tmt2 (834 to 689°C, 0.33 to 3.2 ΔNNO). Amphibole thermobarometry reveals Mhst‐Tsr and Mhbl formed under similar conditions as Tmt1 and Tmt2, respectively. The lack of Cr‐spinel, olivine, and the abundant occurrence of zircon and Mhbl indicate an evolved subalkaline source characterized by felsic rocks related to arc magmatism. The only volcanic source which aligns with these characteristics is Mount Yelia within the Vailala River catchment. Compositional variation between Tmt1 and Tmt2 titanomagnetites and associated silicate minerals is attributed to heterogeneous temperature and ƒO2 conditions within the melt during crystallization. The provenance of Ti‐rich titanomagnetite ores from sources related to arc magmatism indicates the potential of other areas associated with arc activity in Papua New Guinea to host secondary deposits rich in Fe‐Ti ores.

The Singhbhum Craton (India) records a billion year of continental crust formation and modification

The petrogenesis of continental crust from its ultimate mantle source can be reconstructed from the element abundances and radiogenic isotope compositions of ideally pristine igneous rocks. The initial isotope compositions of igneous rocks provide geochemical constraints on the age, composition and evolution of their source(s). Determining initial isotope ratios for rock samples can be challenging, especially in rocks with a long and protracted thermal history. The Rb-Sr system is highly sensitive to parent-daughter element fractionation during magma differentiation. This makes the Rb-Sr isotope systematics ideal to trace the precursor composition of Archean felsic crust and constrain the time of element fractionation during the formation and subsequent modification of continental crust. Initial isotope compositions can be obtained directly from minerals that strongly prefer the daughter element and effectively exclude the parent element of the radio-isotope system of interest. Apatite, having a near zero Rb/Sr ratio, is ideal for preserving its initial 87Sr/86Sr and zircon records initial 176Hf/177Hf compositions. Combined modelling of Sr and Hf isotope data from granitoids of the Archean Singhbhum Craton, indicates that the older Paleoarchean granitoids, emplaced between 3.53 Ga and 3.44 Ga, were derived from a mafic precursor (∼52–54 wt% SiO2) sourced from a depleted mantle at ∼3.71 Ga. Initial 87Sr/86Sr isotope signatures of matrix apatite and apatite inclusions in zircon from the younger Paleoarchean granitoids (3.4–3.2 Ga) of the Singhbhum Craton indicate these younger granitoids were produced by mixing of magma generated from an older mafic source and partial melts derived from the older granitoids. The combined Sr-Hf isotope modelling links the timing of mantle extraction of the precursor material for Paleoarchean Singhbhum granitoids with a known mafic crust extraction event at ∼3.71 Ga. In combination, the new Sr isotope data from apatite combined with whole rock and zircon Hf isotope data from the literature reveal a ∼1 Ga protracted crustal growth and differentiation history of the nucleus of the Singhbhum Craton. By combining radio-isotope systems like 87Rb-87Sr and 176Lu-176Hf, the petrogenesis of Archean felsic crust from the extraction of mafic material from the mantle to reworking in an orogenic cycle to emplacement can be reconstructed. This approach can be applied to other greenstone-gneiss terranes to quantify the spatio-temporal and compositional evolution of voluminous felsic crust and the formation of cratons in the Archean.

Metamorphic titanite–zircon pseudomorphs after igneous zirconolite

Abstract. The formation of metamorphic zircon after baddeleyite is a well-known reaction that can be used to date the metamorphism of igneous silica-undersaturated rocks. By contrast, metamorphic minerals formed after igneous zirconolite have rarely been reported. In this paper, we document metamorphic titanite + zircon pseudomorphs formed from the metamorphic breakdown of igneous zirconolite in syenodiorite and syenite, in the southeastern Sveconorwegian Province, Sweden. Water-rich fluid influx during tectonometamorphism in epidote–amphibolite-facies metamorphic conditions caused the release of silica during a metamorphic reaction involving igneous feldspar and pyroxene and the simultaneous breakdown of igneous Zr-bearing phases. Typical titanite + zircon intergrowths are elongated or platy titanite crystals speckled with tiny inclusions of zircon. Most intergrowths are smaller than 15 µm; some are subrounded in shape. Locally, bead-like grains of titanite and zircon are intergrown with silicate minerals. The precursor igneous zirconolite was found preserved only in a sample of near-pristine igneous syenodiorite, as remnant grains of mainly < 2 µm in size. Two somewhat larger crystals, 8 and 12 µm, allowed semiquantitative confirmation using microprobe analysis. Analogous with zircon pseudomorphs after baddeleyite, titanite + zircon pseudomorphs after zirconolite potentially offer dating of the metamorphic reaction, although the small size of the crystals makes dating with today's techniques challenging. The scarcity of reports of zirconolite and pseudomorphs reflects that they are either rare or possibly overlooked.

UHP metamorphism, decompression anatexis and retrogression of garnet-bearing metagranite and granitic gneiss from the Dabie orogen during continental collision

Metamorphosed granitic rocks are commonly volumetrically dominant in continental collisional terranes, which are crucial targets for resolving the metamorphic evolution of deeply subducted continental crust. However, their metamorphic processes in ultrahigh pressure (UHP) belts are often poorly understood, because they are commonly intensely retrogressed and presently dominated by amphibolite-facies minerals with very rare petrological evidences of UHP metamorphism. It is known that, in these lithologies, zircon can potentially preserve evidence of the UHP history; therefore, the P-T-t path of metafelsic rocks may be estimated using a multidisciplinary approach which combines zircon microanalysis with phase equilibria modelling. In the central Dabie UHP metamorphic zone (CDZ) (China), two types of metamorphosed garnet-bearing granitic rocks, i.e. metagranite and granitic gneiss, whose massive vs. gneissic structure reflects a different degree of deformation, are closely associated with eclogites. Opposite to the eclogites, their detailed metamorphic evolution is poorly understood. In this study, a three-stage metamorphic evolution is reconstructed for these lithologies, based on field observation, bulk-rock elemental geochemistry, sensitive high-resolution ion microprobe zircon UPb dating, Raman micro-spectroscopy analysis of zircon inclusion, mineral chemistry, conventional thermobarometry and phase equilibria modelling. The first UHP stage is represented by the assemblage coesite-rutile-garnet-phengite preserved in the metamorphic domains of zircon and by garnet core in the matrix, revealing minimum peak pressures of 44 kbar and 30 kbar for granitic gneiss and metagranite, respectively, at 231 ± 4 Ma. The second stage is represented by anatexis at 18–20 kbar, 725–870 °C and 222 ± 3 Ma, evidenced by leucosomes at the outcrop scale and by quartzofeldspathic films with low dihedral angles, melt inclusions preserved in peritectic garnet and quartz/K-feldspar/plagioclase- garnet intergrowth at the micro-scale. The third stage is represented by amphibolite-facies overprint at 590–700 °C, 9–14 kbar, at 214–219 Ma, recorded by biotite-plagioclase-quartz symplectites developed at the expenses of phengite, re-equilibration of the peritectic garnet rim, and amphibole-quartz (biotite-titanite) inclusions in zircon overgrowth rims. The resulting P-T-t path overlaps with previous results estimated from other CDZ UHP rocks. Besides, the protoliths of both metagranite and granitic gneiss were emplaced during the mid-Neoproterozoic (683–786 Ma) period, as already documented throughout the Dabie orogen. Compared with previously published data for meta-granitic rocks in the CDZ, the weakly deformed metagranite does not appear to represent low-grade metamorphic rocks. Instead, this study (i) suggests that the two groups of lithologies experienced a coeval metamorphic evolution; (ii) reconstructs a multistage P-T-t history based on zircon microanalysis and pseudosections; (iii) reveals that anatexis, retrogression and deformation facilitate the elimination of (U)HP mineralogical records of metagranitoids. Overall, these results provide new constraints on the metamorphic evolution and exhumation processes of deeply subducted granitic rocks during continental collision.

Apatite evidence for a fluid-saturated, crystal-rich magma reservoir forming the Quellaveco porphyry copper deposit (Southern Peru)

Large volume, intermediate-felsic magma reservoirs are the source of melt and mineralising fluids which generate porphyry copper deposits. Cooling and crystallisation of hydrous magmas drives the exsolution and expulsion of a magmatic volatile phase—a process which remains challenging to constrain in porphyry Cu systems where the record of magma volatile compositions is rarely preserved. Here, we use the halogen compositions of apatite inclusions shielded as inclusions within zircon to constrain volatile evolution in magma reservoirs which pre-date and are synchronous with porphyry Cu mineralisation at Quellaveco, Southern Peru. Geochemical and textural data confirm that the zircon-included apatites escaped re-equilibration with hydrothermal fluids, unlike apatites found in the groundmass of the same rocks. We, therefore, recommend that future studies attempting to reconcile magmatic volatile budgets using apatite in porphyry Cu systems should focus on apatite inclusions in zircon. By combining the apatite inclusion data with numerical modelling, we find evidence that the magma reservoir sourcing porphyry Cu mineralisation remained fluid-saturated for the entire period recorded by apatite crystallisation. By contrast, the pre-mineralisation batholith shows more variable, potentially fluid-undersaturated behaviour. Our modelling suggests that in order to attain the porphyry melt volatile compositions inferred from apatite, the magma reservoir must have exsolved a large proportion of its volatile budget, consistent with having been held at high crystallinity (40–60% crystals). This crystallisation interval coincides with peak chlorine and copper extraction from intermediate-felsic magmas, and would have permitted efficient fluid migration and accumulation at the roof of the system. We suggest that the storage of large-volume, long-lived, crystal-rich magma reservoirs in magmatic arcs may be a critical step in generating world-class porphyry copper deposits.

Effect of chemical composition on zircon radiation damage dating: Implications for low-temperature thermochronology

Zircon radiation damage dating is a low-temperature thermochronological method that can reveal the cooling histories of magmatic intrusions and discriminate sedimentary provenance in combination with other dating methods. This method has broad application prospects because of its advantages of non-destructive, high efficiency, and capable of double (or multiple) dating, involving only multiple measurements by Raman spectrometer and laser ablation-inductively coupled plasma-mass spectrometry. However, several factors, such as zircon chemical composition and the non-uniformity of radiation damage annealing kinetics, can cause poor precision when using this method and thus restricts its wide application. This study examined the effect of chemical composition (P, Ti, Dy, Th, U, and Hf) on Raman spectra using synthetic zircon crystals grown in a lithium-molybdate flux. The results show that the full width at half-maximum (FWHM) of the ν3(SiO4) band has positive linear correlations with the concentrations of P, Ti, Dy, Th, and U in decreasing order of influence, while the FWHM is unaffected by Hf at concentrations <1 wt.% but increases at concentrations >10 wt.%. Furthermore, the Raman shift is negatively correlated with Th, U, and Dy concentrations, positively correlated with Hf, and shows no obvious correlation with Ti and P. Thus, our study shows that chemical composition is a non-ignorable factor for calculating zircon radiation damage age using Raman spectroscopy, especially for zircon with relatively high concentrations of P, rare earth elements (REEs), Th, U, and Hf. The obtained multiple linear regression equation provides a potential means for estimating the FWHM at zero dose and implication for improving the dating precision of this method. In addition, the observed effects of REEs, Th, U, and Hf on the Raman shift of the ν3(SiO4) band indicate that chemical composition in zircon might affect the estimation of the P-T conditions of geological processes when using entrapped zircon inclusions in host minerals or the field of zircon as an in situ pressure sensor in hydrothermal experiments. Our study suggests that zircon radiation damage dating, excluding geochemical effects, will be more accurate for addressing lower-temperature geological processes.