Evolution and emplacement of high fluorine rhyolites in the Mesoproterozoic Gawler silicic large igneous province, South Australia

The Gawler Range Volcanics (GRV) and the Hiltaba Suite (HS) of South Australia form a silicic-dominated large igneous province (the Gawler SLIP) emplaced in an intracontinental setting during the Mesoproterozoic. Emplacement of the GRV lasted for a short period of time (~2 Ma), and can be separated into two main phases. The first phase (lower GRV) is composed of thick (≤3 km) sequences erupted from distinct centres, and includes small to moderate volume (up to >150 km3) felsic lavas, ignimbrites, and minor mafic and intermediate lavas. The upper GRV include extensive felsic lavas that are up to >1000 of km3 in volume and >200 km across. Using well preserved, quartz-hosted melt inclusions, we investigated the composition of the lower GRV, including major, trace, and volatile elements. The results indicate high concentrations of K2O (≤7–8 wt.%), rare earth and high field strength elements, and low concentrations of Ca, Mg, Ni, Cr, Sr and Ba in comparison with felsic continental crust. Overall, melt inclusion compositions match whole-rock geochemical characteristics. We demonstrate that the GRV magma was F-rich (≤1.3 wt.%), and had high temperature for a silicic magma. High F concentrations and high temperature would have resulted in lower than usual polymerisation of the melt and relatively low viscosity. These characteristics help explain how very voluminous felsic magma was erupted effusively and emplaced as lavas. Other intracontinental SLIP contain extensive felsic lavas and ignimbrites which appear to share similar geochemical characteristics. We also show that selective alteration caused depletion of whole-rock compositions in some trace elements, namely Pb, U, and Sn

Magma chamber dynamics in a silicic LIP revealed by quartz: The Mesoproterozoic Gawler Range Volcanics

Silicic-dominated large igneous provinces (SLIP) represent vast amounts of magma (≥ 105 km3) erupted onto the Earth's surface or injected into the crust over short time spans, and are important components of the continental crust. The conditions of formation and evolution of these large magmatic provinces and their magma chambers are still poorly constrained. In this contribution, we examine cathodoluminescence textures and trace element (Al, Ti, Fe) zoning of quartz in a Mesoproterozoic SLIP, the Gawler Range Volcanics (GRV), South Australia. We describe intra-granular textures such as truncation of growth textures and reverse zoning (rimwards increase of Ti content). These characteristics of quartz, together with remelting of already crystallised portions of the magma chamber (felsic enclaves), suggest a complex history of crystallisation and resorption, and fluctuating magma temperature. Titanium-in-quartz geothermometry indicates that adjacent quartz zones record temperature variations (ΔT) up to 70 °C in volcanic units. We also report contrasting (non-correlatable) zoning patterns amongst quartz crystals, each indicating different crystallisation conditions. The juxtaposition of quartz crystals with contrasting zoning patterns is consistent with a dynamic regime (convection, stirring, overturning) of the GRV magma chamber. These results point to pulsating magmatic conditions, compatible with a non-linear evolution of the GRV magma chamber. Heat, necessary to explain both intra-granular and infra-granular textural variations, may have been provided in different pulses by underplating of mafic magma

PIXE mapping on multiphase fluid inclusions in endoskarn xenoliths of AD 472 eruption of Vesuvius (Italy)

In this work we report a microthermometric and proton-induced X-ray emission (PIXE) mapping investigation on multiphase fluid inclusions hosted within nepheline and clinopyroxene of endoskarn xenoliths present in the deposits of the AD 472 eruption of Vesuvius. PIXE mapping on magmatic fluid inclusions repesents a useful tool for the characterization of the composition of magma derived fluids, exsolved from active magma chambers. In fluid inclusions we observed the occurrence of widespread solid phases formed by Fe, Pb, Zn, As ± Cu ± Mn, suggesting the good metal transport capability of Vesuvius magmatic fluids, which interacted with carbonate country rocks leading to the formation of endoskarn

Crystallisation of magmatic topaz and implications for Nb-Ta-W mineralisation in F-rich silicic melts - The Ary-Bulak ongonite massif

Textural, mineralogical and geochemical data on F-rich rhyolite (ongonite) from the Ary-Bulak massif of eastern Transbaikalia help constrain the formation of magmatic topaz. In these rocks, topaz occurs as phenocrysts, thus providing compelling evidence for crystallisation at the orthomagmatic stage. Cathodoluminescence images of topaz and quartz reveal growth textures with multiple truncation events in single grains, indicative of a dynamic system that shifted from saturated to undersaturated conditions with respect to topaz and quartz. Electron microprobe and Raman analyses of topaz indicate near-pure F composition [Al2SiO4F2], with very limited OH replacement. Laser ablation ICP-MS traverses revealed the presence of a large number of trace elements present at sub-ppm to hundreds of ppm levels. The chemical zoning of topaz records trace element fluctuations in the coexisting melt. Concentrations of some trace elements (Li, Ga, Nb, Ta and W) are correlated with cathodoluminescence intensity, thus suggesting that some of these elements act as CL activators in topaz.The study of melt inclusions indicates that melts with different F contents were trapped at different stages during formation of quartz and topaz phenocrysts, respectively. Electron microprobe analyses of glass in subhedral quartz-hosted melt inclusions indicate F ≤ 1.2 wt.%, whereas irregular-shaped melt inclusions hosted in both topaz and quartz have F ≤ 9 wt.%. Cryolithionite [Na3Li3Al2F12] coexists with glass in irregular inclusions, implying high Li contents in the melt. The very high F contents would have increased the solubility of Nb, Ta and W in the melt, thus allowing progressive concentration of these elements during magma evolution. Crystallisation of Nb–Ta–W-oxides (W-ixiolite and tantalite–columbite) may have been triggered by separation of cryolithionite, which would have caused F and Li depletion and consequent drop in the solubility of these elements

Silicate-sulfide liquid immiscibility in modern arc basalt (Tolbachik volcano, Kamchatka): Part I. Occurrence and compositions of sulfide melts

Silicate-sulfide liquid immiscibility plays a key role in the formation of magmatic sulfide ore deposits but incipient sulfide melts are rarely preserved in natural rocks. This study presents the distribution and compositions of olivine-hosted sulfide melt globules resulting from silicate-sulfide liquid immiscibility in primitive arc basalts. Abundant sulfide droplets entrapped in olivine from primitive basalts of the 1941 eruption and pre-historic eruptive cone “Mt. 1004” of the Tolbachik volcano, Kurile-Kamchatka arc. Inclusions range from submicron to 250 μm in size, coexist with sulfur-rich glass (≤ 1.1 wt% S), and, in some cases, with magmatic anhydrite. Saturation in sulfide occurred early in the evolution of a water- and sulfur-rich magma, moderately oxidized (QFM + 1 to +1.5), which crystallized high-Mg olivine (Fo₈₆ˍ₉₂), clinopyroxene and Cr-spinel. The process developed dense “clouds” of sulfide in relatively small volumes of magma, with highly variable abundances of chalcophile metals. The low degree of sulfide supersaturation promoted diffusive equilibration of the growing droplets with the melt in Ni and Cu, resulting in high concentrations (≈ 38 mol%) of CuS and NiS in the earliest sulfide liquids. The Tolbachik samples provide a glimpse into deep arc processes not seen elsewhere, and may show how arc magmas, despite their oxidized nature, saturate in sulfide.This study was supported by the Russian Science Foundation grant # 16-17-10145. This is CRPG contribution #253

The fluorine link between a supergiant ore deposit and a silicic large igneous province

Olympic Dam is a supergiant Fe oxide Cu-U-Au-Ag ore deposit (~9 × 109 t) that is also enriched in rare earth elements (REEs) and fluorine (F). The immediate host to the ore is hydrothermal breccia within granite and volcanic rocks of a Mesoproterozoic silicic large igneous province. Analyses of melt inclusions in quartz phenocrysts in rhyolite show that the silicic magmas of this province were unusually rich in F (up to 1.3 wt%). Fluorite and other F-rich minerals that crystallized from these magmas provided a gigantic reservoir of F. As a result, the Olympic Dam ore-forming fluid was F-rich and had exceptional capacity to transport diverse elements. Further, we infer that hydrofluoric acid, the most corrosive acid known, contributed to hydrothermal breccia formation by dissolution that in turn increased permeability and accelerated the rate of fluid-rock interaction. It is no accident that the world's largest hydrothermal ore deposit occurs in an F-rich silicic large igneous province

Arrival of extremely volatile-rich high-Mg magmas changes explosivity of Mount Etna

The volcanic hazard potential of Mount Etna volcano is currently nourished by long-lasting, powerful eruptions of basaltic magmas coupled with increased seismicity and ground deformation, and the world's largest discharge of volcanic gases. The current evolutionary cycle of Mount Etna activity is consistent with subduction-related chemical modifications of the mantle source. Arrival of a new mantle-derived magma batch beneath the volcano has been hypothesized, but is still elusive among the erupted products. Here we demonstrate petrological and geochemical affinities between the magmas supplying modern eruptions and high-Mg, fall-stratified (FS) basalts ejected violently 4 k.y. ago. The FS primitive magmas (13 wt% MgO) are characteristically volatile enriched (at least 3.8 wt% H2O and 3300 ppm CO2), and bear a trace element signature of a garnet-bearing, metasomatized source (high Gd/Yb, K/La, U/Nb, Pb/Ce, Ca/Al). They started crystallizing olivine (Fo91), clinopyroxene (Mg# 92.5), and Cr spinel deep in the plumbing system (>5 kbar), contributing to the cumulate piles at depth and to differentiated alkaline basalt and trachybasalt magmas in the shallow conduit. Continuous influx of mantle-derived, volatile-rich magmas, such as those that supplied the FS fallout, provides a good explanation for major compositional and eruptive features of Mount Etna

Platinum-group elements and gold in sulfide melts from modern arc basalt (Tolbachik volcano, Kamchatka)

Sulfide melt inclusions entrapped in primitive olivine phenocrysts can be used to understand the compositions of early sulfide melts that may ultimately contribute to magmatic sulfide ore deposits. Sulfide globules hosted in olivine (86–92 mol% Fo) from the Tolbachik basalt (the 1941 eruption) are characterized in terms of their major and trace element abundances using electron microscopy and LA–ICP–MS analysis. Distribution of major elements within individual sulfide globules varies from homogeneous to heterogeneous. Phases include monosulfide solid solution (MSS) and intermediate solid solution (ISS) intergrowths and exsolved low-temperature minerals such as pyrrhotite, pentlandite, chalcopyrite and cubanite. Trace elements (platinum-group elements — PGE, Ag, Te, Au, Pb and Bi) are also present in solid solution in sulfide phases and as micron-sized particles (“nuggets”). Such nuggets of dominantly Au, Pt, Au–Pd and Pd–Te are contained randomly within sulfide matrices or, more commonly, at phase boundaries. Nuggets are also attached to outer surfaces of sulfide globules. Concentrations of PGE in sulfides follow a log normal distribution over four orders of magnitude. The highest measured noble metal concentrations in the analyzed globules (436 ppm Au + PGE) are 13.3 ppm Au, 115 ppm Pt and 299 ppm Pd, whereas 40% of globules have < 15 ppm of noble metals. Gold and PGE concentrations correlate, suggesting these elements were concentrated by the same process(es). We propose that a number of anomalous concentrations of one or several noble metals in the analyzed globules can be best explained by entrapment of Au–PGE-rich particles (solid or liquid) from the silicate melt. Although the individual Tolbachik sulfide globules have variable PGE abundances, their mean composition resembles those of major PGE-sulfide ore deposits (e.g., Norilsk, Sudbury, Platreef and Merensky Reef).This study was supported by the Russian Science Foundation grant #16-17-1014

Copper-Containing Magnesioferrite in Vesicular Trachyandesite in a Lava Tube from the 2012-2013 Eruption of the Tolbachik Volcano, Kamchatka, Russia

Cu-rich magnesioferrite was found in vesicular basaltic trachyandesite in one of lava tubes (Duplex) that formed during the 2012-2013 eruption of the Tolbachik volcano, Kamchatka. This mineral is commonly associated with hematite, tenorite, halite, sylvite, and Ca-rich silicates (mainly, esseneite and Na-rich melilite) in high-temperature (800-1000 degrees C) reactionary zones (up to 100 mu m) covering vesicular rocks and lava stalactites in the Duplex tube. The mineral relationships of this assemblage indicate the following crystallization sequence: Ca-rich silicates + hematite -> Cu-rich magnesioferrite -> tenorite -> chlorides. This formed due to the reaction of hot gases containing Cu, alkalis, and Cl with solidified lava rock. The composition of magnesioferrite varies strongly in CuO (5.8-17.3 wt %; cuprospinel end-member-15-47 mol %), whereas the contents of other oxides are minor, indicating the main isomorphic substitution is Mg2+ Cu2+. Compositions with maximal CuO content nominally belong to Mg-rich cuprospinel: (Cu0.48Mg0.41Mn0.09Zn0.02Ca0.02) (Fe1.943+Al0.03Ti0.02)O-4. Increasing CuO content of the Duplex Cu-rich magnesioferrite is reflected in Raman spectra by moderate right shifting bands at approximate to 700-710 and 200-210 cm(-1) and the appearance of an additional band at 596 cm(-1). This supports the main isomorphic scheme and may indicate a degree of inversion in the spinel structure.Peer reviewe

High-vacuum-compatible high-power Faraday isolators for gravitational-wave interferometers

Faraday isolators play a key role in the operation of large-scale gravitational-wave detectors. Second-generation gravitational-wave interferometers such as the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) and Advanced Virgo will use high-average-power cw lasers (up to 200 W) requiring specially designed Faraday isolators that are immune to the effects resulting from the laser beam absorption–degraded isolation ratio, thermal lensing, and thermally induced beam steering. In this paper, we present a comprehensive study of Faraday isolators designed specifically for high-performance operation in high-power gravitational-wave interferometers