High-precision apatite δ37Cl measurement by SIMS with 1012 Ω amplifier Faraday cup

Chlorine is a redox-sensitive and fluid-mobile element, and is involved in many geological processes. Apatite, a ubiquitous accessory mineral in mafic to felsic rocks, is the most-studied mineral in chlorine...

The spatial association of accessory minerals with biotite in granitic rocks from the South Mountain Batholith, Nova Scotia, Canada

We use mineral liberation analysis (MLA) to quantify the spatial association of 15,118 grains of accessory apatite, monazite, xenotime, and zircon with essential biotite, and clustered with themselves, in a peraluminous biotite granodiorite from the South Mountain Batholith in Nova Scotia (Canada). A random distribution of accessory minerals demands that the proportion of accessory minerals in contact with biotite is identical to the proportion of biotite in the rock, and the binary touching factor (percentage of accessory mineral touching biotite divided by modal proportion of biotite) would be ~1.00. Instead, the mean binary touching factors for the four accessory minerals in relation to biotite are: apatite (5.06 for 11,168 grains), monazite (4.68 for 857 grains), xenotime (4.36 for 217 grains), and zircon (5.05 for 2876 grains). Shared perimeter factors give similar values. Accessory mineral grains that straddle biotite grain boundaries are larger than completely locked, or completely liberated, accessory grains. Only apatite-monazite clusters are significantly more abundant than expected for random distribution. The high, and statistically significant, binary touching factors and shared perimeter factors suggest a strong physical or chemical control on their spatial association. We evaluate random collisions in magma (synneusis), heterogeneous nucleation processes, induced nucleation in passively enriched boundary layers, and induced nucleation in actively enriched boundary layers to explain the significant touching factors. All processes operate during the crystallization history of the magma, but induced nucleation in passively and actively enriched boundary layers are most likely to explain the strong spatial association of phosphate accessories and zircon with biotite. In addition, at least some of the apatite and zircon may also enter the granitic magma as inclusions in grains of Ostwald-ripened xenocrystic biotite.

Empirical constraints on progressive shock metamorphism of magnetite from the Siljan impact structure, Sweden

Little is known about the microstructural behavior of magnetite during hypervelocity impact events, even though it is a widespread accessory mineral and an important magnetic carrier in terrestrial and extraterrestrial rocks. We report systematic electron backscatter diffraction crystallographic analysis of shock features in magnetite from a transect across the 52-km-diameter ca. 380 Ma Siljan impact structure in Sweden. Magnetite grains in granitoid samples contain brittle fracturing, crystal-plasticity, and lamellar twins. Deformation twins along {111} with shear direction of <112> are consistent with spinel-law twins. Inferred bulk shock pressures for the investigated samples, as constrained by planar deformation features (PDFs) in quartz and shock twins in zircon, range from 0 to 20 GPa; onset of shock-induced twinning in magnetite is observed at >5 GPa. These results highlight the utility of magnetite to record shock deformation in rocks that experience shock pressures >5 GPa, which may be useful in quartz-poor samples. Despite significant hydrothermal alteration and the variable transformation of host magnetite to hematite, shock effects are preserved, which demonstrates that magnetite is a reliable mineral for preserving shock deformation over geologic time.

Tracking and Evaluating the Concentrations of Natural Radioactivity According to Chemical Composition in the Precambrian and Mesozoic Granitic Rocks in the Jangsu-gun Area, Central Southwestern South Korea

The Jangsu-gun area in the central Southwestern South Korea consists of a well-preserved Middle Paleoproterozoic gneissic basement, as well as the Late Triassic and Early Jurassic granitic rocks. Here, we present the detailed zircon U-Pb age data and whole-rock chemical compositions, including radioactive elements (e.g., U and Th) and activity concentrations of 226Ra, 232Th and 40K for the Middle Paleoproterozoic gneisses, and Late Triassic and Early Jurassic granitic rocks of the Jangsu-gun area. The Middle Paleoproterozoic gneissic basement, and the Late Triassic and Early Jurassic granitic rocks have ages of ca. 1988 Ma and 1824 Ma, 230 Ma and 187–189 Ma, respectively. Geochemically, the Middle Paleoproterozoic orthogneiss, Late Triassic granites and Early Jurassic granitic rocks show typical arc-related metaluminous to weakly peraluminous fractionated granite features with ASI (aluminum saturation index) values of 0.92 to 1.40. The mean values of U (ppm) and Th (ppm) of the Middle Paleoproterozoic orthogneisses (6.4 and 20.5, respectively), Late Triassic granites (1.5 and 10.9), and Early Jurassic granites (3.5 and 16.5) were similar to those (5 and 15) of the granitic rocks in the Earth’s crust. The mean 226Ra (Bq/kg), 232Th (Bq/kg), and 40K (Bq/kg) activity concentrations and radioactivity concentration index (RCI) are 62, 71, 1,214 and 0.96 for the Middle Paleoproterozoic orthogneisses; 16, 39, 1,614 and 0.78 for the Late Triassic granites; and 56, 70, 1031 and 0.88 for the Early Jurassic granitic rocks, respectively. The U, Th, 226Ra, 232Th, 40K, and RCI of the Middle Paleoproterozoic biotite paragneisses are similar to those of the Middle Paleoproterozoic orthogneisses. The trend of 226Ra, 232Th, and 40K activity concentrations, and the composition of U and Th from the Precambrian and Mesozoic rocks in the Jangsu-gun area indicates that monazite is the main accessory mineral controlling the concentration of natural radioactivity. Based on a detailed examination of the natural radioactivity in the rocks of the Jangsu-gun area, the Middle Paleoproterozoic orthogneisses and paragneisses, and Late Triassic and Early Jurassic granitic rocks show average high mean RCI values of 0.88−0.96, such that 32% of the rocks exceeded the recommended value of one in the guidelines for the RCI in South Korea. Especially, the RCI is closely related to the radon levels in the rocks. As a result, the Jangsu-gun area in South Korea is a relatively high radiological risk area, which exhibits higher indoor radon levels in the residences, compared with residences in the other areas in South Korea.

Uranium incorporation in fluorite and exploration of U–Pb dating

The age of ore deposits constitutes a decisive element in understanding their formation. Deciphering their precise chronology may be a challenge in the absence of mineral phases that can be dated by conventional geochronometers. Fluorite is very common either as the major or accessory mineral in a wide variety of ores and may provide information regarding the origin and timing of mineralizing fluid flows. In this contribution, we explore U–Pb dating on fluorite crystals from the world-class carbonate strata-bound fluorite ore of Pierre-Perthuis in Burgundy (Morvan massif, France). The uranium distribution within fluorite is mapped using induced fission-track and synchrotron radiation X-ray fluorescence nano-imaging, showing that higher U content is measured in an overgrowth of fluorite (Flog) as a discrete band. Preservation of a micrometer-thick zonation in U, associated with other substituted elements such as Sr, Y, Fe and Zr, implies that neither solid-state diffusion nor dissolution–recrystallization occurred. These U-bearing external fluorite overgrowths contain solid inclusions of about 30 µm globular pyrite crystals with a mean δ34S of −23.6 ± 0.4 ‰V-CDT. We propose that the U incorporation in the fluorite lattice results from the development of a redox front during bacterial sulfate reduction. Flog generation sampled and analyzed by laser ablation–inductively coupled plasma mass spectrometry (LA-ICP-MS) on four different crystals provides identical U–Pb ages within the limits of analytical uncertainty. Considered altogether, these four crystals yield an age estimate of 40.0 ± 1.7 Ma, not corrected for matrix-related elemental fractionation. Our results show that fluorite LA-ICP-MS U–Pb geochronology has potential for dating distinct crystal growth stages, although further research should be conducted to evaluate its accuracy.

REE Minerals as Geochemical Proxies of Late-Tertiary Alkalic Silicate ± Carbonatite Intrusions Beneath Carpathian Back-Arc Basin

The accessory mineral assemblage (AMA) of igneous cumulate xenoliths in volcanoclastic deposits and lava flows in the Carpathian back-arc basin testifies to the composition of intrusive complexes sampled by Upper Miocene-Pliocene basalt volcanoes. The magmatic reservoir beneath Pinciná maar is composed of gabbro, moderately alkalic to alkali-calcic syenite, and calcic orthopyroxene granite (pincinite). The intrusive complex beneath the wider area around Fiľakovo and Hajnáčka maars contains mafic cumulates, alkalic syenite, carbonatite, and calc-alkalic granite. Both reservoirs originated during the basaltic magma underplating, differentiation, and interaction with the surrounding mantle and crust. The AMA of syenites is characterized by yttrialite-Y, britholite-Y, britholite-Ce, chevkinite-Ce, monazite-Ce, and rhabdophane(?). Baddeleyite and REE-zirconolite are typical of alkalic syenite associated with carbonatite. Pyrochlore, columbite-Mn, and Ca-niobates occur in calc-alkalic granites with strong peralkalic affinity. Nb-rutile, niobian ilmenite, and fergusonite-Y are crystallized from mildly alkalic syenite and calc-alkalic granite. Zircons with increased Hf/Zr and Th/U ratios occur in all felsic-to-intermediate rock-types. If rock fragments are absent in the volcanic ejecta, the composition of the sub-volcanic reservoir can be reconstructed from the specific AMA and zircon xenocrysts–xenolith relics disintegrated during the basaltic magma fragmentation and explosion.

How Two Unassuming Elements, Re and Os, Assumed Acclaim in the Geosciences

<p>Re and Os (rhenium and osmium) are chalcophile-siderophile elements (transition metals) with a unique position in isotope geochemistry.  Unlike other commonly used decay schemes for radiometric dating, these metals take residency in resource-related media, for example, sulfide minerals, the organic component in black shales, coals, and bitumens and oils.  In sum, the reducing environment is their haven whereas under oxidizing conditions, Re and Os become unmoored and the radiometric clock becomes compromised.  The clock is not temperature sensitive, and its applicability spans Early Archean to Pleistocene. </p><p>This Bunsen Medal lecture will explore and review the challenges in bringing Re-Os from the meteorite-mantle community into the crustal environment.  At the center of it all is our ability to turn geologic observation into a thoughtful sampling strategy.  The potential to date ore deposits was an obvious application and molybdenite [Mo(Re)S<sub>2</sub>], rarely with significant common Os and rarely with overgrowths, became an overnight superstar, yielding highly precise, accurate, and reproducible ages.  Yet, molybdenite presented our first sampling challenge with recognition of a puzzling parent-daughter (<sup>187</sup>Re-<sup>187</sup>Os) decoupling in certain occurrences.  A strategic sampling procedure was employed.  From there, the diversity of applications spread, as molybdenite is also an accessory mineral in many granitoids, and can be a common trace sulfide in metamorphic rocks.  Whether conformable with and/or crosscutting foliation, molybdenite ages define the timing of deformational events.  Pyrite and arsenopyrite can also be readily dated. </p><p>Applications jumped from sulfides to organic matter.  The hydrogenous component from organic matter in black shales gives us Re-Os ages in the sedimentary record for the Geologic Time Scale.  This led to construction of an Os isotope seawater curve – an ongoing process.  Unlike the well-known Sr seawater curve, the short residence time of Os in the oceans creates a high-definition time record with unambiguous high-amplitude swings in <sup>187</sup>Os/<sup>188</sup>Os.  Re-Os puts time pins into the biostratigraphic record, and we have even directly dated fossils.  Re-Os opened the door for a new generation of paleoclimate studies to evaluate seawater conditions at the time of organic blooms and organic sequestration in bottom mud.  Uplift and continental erosion can be balanced with hydrothermal input into oceans based on changes in the Os isotope composition of seawater.  The timing and connectivity of opening seaways can be determined, and the timing of glaciation and deglaciation events can be globally correlated.  The timing and instigators of mass extinctions are carried in the Re-Os record.  A major meteorite impact places an enormous scar in the Os isotope record as seawater drops toward mantle values but recovers in just a few thousand years.  Most recently, Re-Os has transformed our understanding of the events and fluids involved in construction of whole petroleum systems. </p><p>Looking to the future, what kinds of data sets will be explored and what are the interdisciplinary skill sets needed to interpret those data?  Re-Os will continue to provide us with new ways to dismantle geologic media for new scientific understanding of processes that have shaped our lithosphere, biosphere and hydrosphere, recording their intersection and exchange. </p>

Partial Melting of Bimineralic Eclogite by Clinopyroxene Breakdown

<p>The generation of melt during exhumation of UHP–HP metamorphic rocks is an important variable in our full understanding of the fluid–melt-fluid evolution during the subduction cycle and the exhumation mechanism of deeply subducted continental crust (Wang et al., 2020; Sizova et al., 2012). It is generally believed that the partial melting of deeply subducted eclogite is controlled by the mineral assemblage, particularly the presence of any hydrous minerals, and the metamorphic P–T path. Here we report results from the Sulu belt, which was formed by the deep subduction of the passive margin of the Yangtze Craton under the North China Craton, with exhumation occurring during the Mesozoic (240–210 Ma). Recent studies in this belt have shown that phengite-bearing UHP eclogites can develop a solute-rich supercritical fluid or melt along grain boundaries by dehydroxylation of nominally anhydrous minerals during the early stage of decompression and/or trigger partial melting by breakdown of phengite and/or omphacite during the later stage of exhumation (Wang et al., 2014; Wang et al., 2020; Feng et al., 2021). However, the capacity of bimineralic eclogite to melt remains enigmatic due to the anhydrous mineral assemblage, indicating a low primary bulk water content, and the absence of studies reporting evidence of melting.</p><p>To determine whether bimineralic eclogite can produce melt during exhumation we undertook a comprehensive study, including petrology, microstructure and geochronology, on a bimineralic eclogite boudin within gneiss from a locality in the northeast of the Sulu belt. The margin of the eclogite boudin is extensively retrogressed, whereas the core is well preserved with distinctive garnet-rich and pyroxene-rich layers. The Ca-rich clinopyroxene in the boudin core exhibits abundant exsolved quartz formed during exhumation. In the pyroxene-rich layers micrometer-scale intergranular pockets composed of euhedral Ca-rich hornblende and Ca-rich plagioclase, and accessory barite and apatite, are interpreted as leucosome. Comparing the calculated bulk composition of the leucosome pockets, which is diorite, with the clinopyroxene, garnet and accessory mineral compositions from the host, suggests that the melt formation is dominated by the breakdown of clinopyroxene rather than garnet or the accessory minerals, based on the trace element characteristics. Symplectitic intergrowths of hornblende and plagioclase occur along boundaries between the garnet-rich and pyroxene-rich layers and extend into both. </p><p>LA-ICP–MS analysis of metamorphic zircon from the eclogite with leucosome pockets yields an age range of 230–210 Ma. Ti-in-zircon thermometry yields a wide range of temperatures from 800 to 500°C. By contrast, temperatures calculated from the rock-forming minerals yield 890–830°C (Grt–Cpx thermometry at 3 GPa), 880–820°C (Amp–Pl (from leucosome pocket) thermometry at 1 GPa), and 700–650°C (Amp–Pl (from symplectite) thermometry at 1.0–0.5 GPa). Overall, we interpret the partial melting of the bimineralic eclogite in the northeastern Sulu belt to record breakdown of clinopyroxene during decompression from UHP–HP metamorphic conditions. This represents the first detailed micro-scale study of in situ melting of UHP bimineralic eclogite.</p><p> </p>

Awaruite (Ni3Fe) as a platinum-group elements concentrator: Preliminary data

<p>The most famous of natural occurring iron-nickel alloys are kamacite, taenite and tetrataenite, forming iron meteorites. Normally, they have significant platinum-group elements (PGE) content being a result of high siderophile behaviour of the latter. In spite of native iron and nickel having been described in terrestrial rocks, the most abundant Fe-Ni mineral in Earth’s crust is awaruite (Ni<sub>3</sub>Fe). Current work represents the preliminary results of testing the ability of awaruite to concentrate PGE.</p><p>Awaruite is a widespread accessory mineral of ultramafic complexes. Its formation is usually assessed to the serpentinization of olivine which produces reductive fluid. The latter reacts with nickel sulfides and produces awaruite. Several reports of awaruite occurring together with platinum-group minerals (PGM) are present in the literature. In the Ural-Alaskan type complexes of Koryak Highlands (Far East Russia), such cases are abundant. Textural investigations of such complexes discovered a diverse array of serpentine–related mineralization, including isoferroplatinum in chlorite matrix, isoferroplatinum–amphibole intergrowths, and a wide range of PGE, Fe and Cu alloys formed in serpentine veinlets together with awaruite and base metal sulfides. This provides evidence of the relation between awaruite and platinum mineralization.</p><p>LA-ICP-MS has been used to reveal the PGE content in awaruite and coexisting sulfides. Grains from the placers related to the Galmoenan complex of Ural-Alaskan type were used for this study. The analysis revealed that sulfides may bear significant PGE admixture. Unexpectedly, the most abundant impurity is Os. Its content varies from 0.7 to 538 ppm. The shape of the time-resolved spectra of some samples indicates the possible presence of solid inclusions which concentrate Os. However, most of them, including those with 538 ppm Os, exhibit plain time-resolved spectra suggesting homogeneous Os distribution. Contents of other PGE are moderate: up to 8.3 ppm Pt, 1.4 ppm Pd, 4.3 ppm Ru, 0.25 ppm Rh and 2.6 ppm Ir.</p><p>Some awaruite grains also show relatively high Os content (up to 89 ppm), but time-resolved spectra of them exhibit clear evidence of mineral inclusions presence. In one case, Os spike coincides with the S spike, suggesting that Os is incorporated into the sulfide phase. In the case of spikeless spectra, Os content is always below the detection limit (b.d.l.). Rhodium content also is always b.d.l., while Ru content reaches 0.44 ppm, Ir – 0.08 ppm, and Pt – 0.03 ppm. The only element explicitly showing significant and homogenous presence in the awaruite is Pd, that content reaches 5.8 ppm in one analysis and 0.2–1.1 in many others.</p><p>These data indicate that in the studied case, awaruite mineralization is accompanied by the formation of PGM, while its role as a direct PGE concentrator is moderate and restricted to the first tenths ppm of Ru and Pd. Sulfides have shown much more impressive ability in concentrating PGE. Their selective enrichment in Os is a novelty and demands explanation.</p><p> </p><p>Author thanks Evgeniy Sidorov and Dima Kamenetsky for the assistance. CODES of UTAS is greatly acknowledged for the LA-ICP-MS analyses. This work was supported by the Russian Foundation for Basic Research (RFBR) grant No 20-05-00290 A.</p>

Corundum-rich dykes constraining Triassic alkaline magmatism in the Ivrea-Verbano Zone (Southern Alps): a zircon approach

<p>Zircon is a common accessory mineral in evolved magmatic rocks and its investigation can provide unevaluable geochronological and geochemical information. The lower continental crust forming the Ivrea-Verbano Zone (IVZ, Southern Alps) locally shows the discordant intrusion of swarms of felsic dykes, which petrology was poorly constrained. Corundum-rich (Crn up to 55 vol.%) felsic dykes were sampled in two different outcrops along the Sabbiola valley (central IVZ). Besides corundum, they consist mainly of sodic plagioclase (An=5-10 %), biotite-siderophyllite, ±K-feldspar and ±hercynite. These dykes intrude granulites and Permian mafic intrusives, showing either pegmatite-like or porphyroclastic textures and contain abundant zircon. Trace elements concentration, as well as the isotopic U-Pb and Lu-Hf compositions of zircons have been determined by LA-ICP-(MC)MS to unravel emplacement ages and nature of parental melts. U-Pb weighted average ages point to Norian emplacement (ca. 224 Ma). Zircons are characterized by very high concentrations in REE, Th, U, Nb and Ta. REE patterns show marked negative Eu anomaly. These data, in association with the enrichments of Na in plagioclases and of Fe in micas and oxides, suggest that the parent melts were extremely evolved differentiates. Porphyroclastic texture developed in the frame of magmatic processes due to volatiles overpressure. Strongly positive Hf<sub>(</sub><sub>t)</sub> values (+13 on average) suggest a derivation of the parental melts from depleted to mildly enriched mantle sources. This observation and the corundum saturation (evidence for low silica activity) point to limited crustal contamination, which was favored by the high eutectic temperature of the host rocks. It is proposed that studied dykes segregated from peraluminous melts produced by exsolution processes affecting volatile-rich differentiates during alkaline magmatism (Bonazzi et al., 2020).</p><p>Triassic magmatic activity is largely documented throughout the Southern Alps, being related to different tectono-magmatic cycles. Nevertheless, before this study, the evidence of Triassic magmatism in IVZ was restricted only in its northernmost tip (Finero area, e.g. Zanetti et al., 2013; Schaltegger et al., 2015). This work provides robust constraints about the transition of the geochemical affinity of Southern Alps magmatism from orogenic-like to anorogenic during Norian, linked to a regional uprising of the asthenosphere and changes of tectonic regime.</p><p> </p><p>References</p><p>Bonazzi, M.; Langone, A.; Tumiati, S.; Dellarole, E.; Mazzucchelli, M.; Giovanardi, T.; Zanetti, A. Mantle-Derived Corundum-Bearing Felsic Dykes May Survive Only within the Lower (Refractory/Inert) Crust: Evidence from Zircon Geochemistry and Geochronology (Ivrea–Verbano Zone, Southern Alps, Italy). Geosciences 2020, 10, 281.</p><p>Schaltegger, U.; Ulianov, A.; Muntener, O.; Ovtcharova, M.; Peytcheva, I.; Vonlanthen, P.; Vennemann, T.; Antognini, M.; Girlanda, F. Megacrystic zircon with planar fractures in miaskite-type nepheline pegmatites formed at high pressures in the lower crust (Ivrea Zone, southern Alps, Switzerland). Am. Miner. 2014, 100, 83–94.</p><p>Zanetti, A.; Mazzucchelli, M.; Sinigoi, S.; Giovanardi, T.; Peressini, G.; Fanning, C.M. SHRIMP U-Pb Zircon Triassic Intrusion Age of the Finero Mafic Complex (Ivrea-Verbano Zone, Western Alps) and its Geodynamic Implications. J. Pet. 2013, 54, 2235–2265.</p>