Volume 103, Issue 12

Solid-state sintering is a process wherein atomic diffusion along grain boundaries converts unconsolidated, crystalline aggregates into dense composites. It is a process that has largely been overlooked as significant to volcanic systems. Here, we present a preliminary suite of hot isostatic pressing experiments performed on naturally occurring crystalline dacite powders that demonstrate the efficacy of solid-state sintering at elevated pressures (40, 70 MPa) and temperatures (700–900 °C) over short timescales (2.5 days). The experimental products are dense, low-permeability rocks, supporting the hypothesis that solid-state sintering may be an important process that acts on timescales relevant to magma rise and eruption. We use the experimental data to constrain a preliminary model for the extent of densification as a function of temperature, confining pressure and time. Last, we present sintering maps relevant to the time-dependent loss of porosity and permeability in granular volcanic materials. Solid-state sintering is a densification process with the capacity to heal fluid-flow pathways in volcanic systems within months to years.

Raman spectroscopy provides information on the residual strain state of host-inclusion systems that, coupled with the elastic geobarometry theory, can be used to retrieve the P-T conditions of inclusion entrapment. In situ Raman measurements of zircon and coesite inclusions in garnet from the ultrahigh-pressure Dora Maira Massif show that rounded inclusions exhibit constant Raman shifts throughout their entire volume. In contrast, we demonstrate that Raman shifts can vary from the center to the edges and corners of faceted inclusions. Step-by-step polishing of the garnet host shows that the strain in both rounded and prismatic inclusions is gradually released as the inclusion approaches the free surface of the host. More importantly, our experimental results coupled with selected numerical simulations demonstrate that the magnitude and the rate of the strain release also depend on the contrast in elastic properties between the host and the inclusion and on the inclusion crystallographic orientation with respect to the external surface. These results allowed us to give new methodological guidelines for determining the residual strain in host inclusion systems.

Earth analogs are indispensable to investigate mineral assemblages on Mars because they enable detailed analysis of spectroscopic data from Mars and aid environmental interpretation. Samples from four sites in the Iberian Pyrite Belt (El Villar, Calañas, Quebrantahuesos, and Tharsis) were investigated using mineralogical, chemical, and spectroscopic techniques, with a focus on clay minerals and alteration environments. They represent Earth analogs of areas on Mars that underwent acidic alteration. X‑ray diffraction and transmittance mid-infrared data indicate that the rocks were subjected to several degrees of acid alteration corresponding to assemblages characterized by the following mixtures: (1) illite, chlorite, interstratified chlorite-vermiculite, kaolinite-smectite, and kaolinite; (2) illite, kaolinite, and alunite; and (3) jarosite and goethite. According to mineral stability data, these three assemblages correspond to pH values 7–5, 5–3, and <3, respectively. The lack of goethite in the illite-kaolinitealunite assemblage suggests an alteration in reducing conditions. Illite was progressively dissolved by acidic alteration but is sufficiently resilient not to be diagnostic of the intensity of the alteration. Illite and kaolinite were the two most abundant phyllosilicate minerals observed, and the main reaction involving phyllosilicates was the alteration of illite to kaolinite. Mixed-layer phases appeared mainly in the mildest degree of acid alteration, with few exceptions. This suggests a transition from a mechanism dominated by transformation to a mechanism dominated by dissolution-precipitation as the intensity of the acid alteration increases. Our results highlight the sparse kaolinite–alunite occurrences on Mars as worthy of specific investigation. Acid alteration on Mars is expected to be patchy and/or consisting of fine alteration rims. Alunite occurrences on Mars in the absence of goethite may indicate an acid alteration in reducing conditions. Kaolinite produced through acid alteration on Mars is expected to exist mainly as an end-member phase of low crystallinity, which would enhance IR absorption and increase its visibility.

To further our knowledge of ore genesis in one of Australia’s preeminent ore districts, we have completed a comprehensive geochemical study of ore-related porphyritic intrusions from the Archaean Kanowna Belle and Sunrise Dam gold deposits (both >10 Moz), Eastern Goldfields, Western Australia. Zircon samples (including samples from the newly developed Velvet mine) with ages ranging from 2.8 to 2.2 Ga, were investigated for O-OH isotopic signatures, trace element abundance, and U-Th-Pb compositions to elucidate the nature of the magmatic source and ore-related fluid. These intrusions have similarly high Sr/Y and La/Yb ratios to adakites from the Aleutian and Cook Islands, but lower Mg# values and higher K 2 O contents, suggesting they were derived from partial melts in a thickened crust. The modern analogs are post-collisional, high-Sr/Y granitoid porphyries in southern Tibet. Magmatic zircons have intermediate δ 18 O values (+5‰ to +6.3‰), and estimated magmatic crystallization temperatures (Ti-in-zircon) in between 660–760 °C. They are interpreted as having crystallized from positive δ 18 O magmas during water-fluxed melting of juvenile lower crust. Hydrothermal fluid modified zircons are texturally indistinguishable from magmatic zircons, but their trace element, OH, and isotopic compositions are distinct. The involvement of hydrothermal fluid in zircon growth is evidenced by a negative correlation between OH content and δ 18 O. In addition, the studied hydrothermal fluid modified zircons are characterized by high La contents, flat rare earth element patterns, weak Ce anomalies, and high Eu/Eu* ratios, suggesting they were related to a high-temperature, Zr-saturated, high-Eu, Cl-rich, and low-pH hydrothermal fluid. Such fluids are common in eastern Yilgarn gold camps.

A unique phase, belonging to an orthorhombic crystal system ( Pbca , Z = 8), is proposed in AlOOH using crystal structure searches based on an evolutionary genetic algorithm method, combined with density functional theory. This phase features a nonlinear asymmetric doubly covalent hydrogen-bond and metal cations that are sixfold oxygen coordinated. Unlike the earlier proposed monoclinic phase, the stability region of Pbca (166–189 GPa) lies well below the pressure of decomposition to Al 2 O 3 +ice X (287 GPa). In GaOOH the Pbca -type phase is not energetically favorable at any pressure. In the course of evaluating the breakdown of GaOOH to its constituent oxides, we have found a new phase of Ga 2 O 3 (U 2 S 3 -type). In InOOH, Pbca is energetically favorable over a narrow pressure interval (12–17 GPa). Also in InOOH, we find a new tetragonal structure ( P 4̄2 1m , Z = 4) stable above 51 GPa. This phase has nonlinear asymmetric hydrogen-bonds and metal cations that are sevenfold oxygen coordinated. Phonon calculations confirm the vibrational stability of the new phases and show that the high-pressure polymorphs of AlOOH are likely to be important carriers of water into the deep lower mantles of Earth and rocky super-Earths.

Nuwaite (Ni 6 GeS 2 , IMA 2013-018) and butianite (Ni 6 SnS 2 , IMA 2016-028) are two new chalcogenide minerals, occurring as micrometer-sized crystals with grossular, Na-bearing melilite, heazlewoodite, and Ge-bearing Ni-Fe alloys in veins and as mono-mineralic crack-filling material in igneous diopside in the Type B1 Ca-Al-rich inclusion (CAI) ACM -2 from the Allende CV3 carbonaceous chondrite. The chemical composition of type nuwaite is (wt%) Ni 65.3, S 10.3, Ge 8.2, Te 7.9, Sn 5.1, and Fe 1.7, with a sum of 98.5 and an empirical formula of (Ni 5.95 Fe 0.16 )(Ge 0.60 Sn 0.23 )(S 1.72 Te 0.33 ). The simplified formula is Ni 6 (Ge,Sn)(S,Te) 2 , leading to an end-member of Ni 6 GeS 2 . The chemical composition of type butianite is (wt%) Ni 62.1, Sn 8.9, Te 10.3, S 8.9, Ge 5.3, Fe 1.3, sum 99.1, giving rise to an empirical formula of (Ni 5.93 Fe 0.13 )(Sn 0.52 Ge 0.41 )(S 1.56 Te 0.45 ). Butianite’s simplified formula is Ni 6 (Sn,Ge) (S,Te) 2 and the end-member formula is Ni 6 SnS 2 . Both nuwaite and butianite have an I 4/ mmm inter-growth structure with a = 3.65 Å, c = 18.14 Å, V = 241.7 Å 3 , and Z = 2. Their calculated densities are 7.24 and 7.62 g/cm 3 , respectively. Nuwaite and butianite are the first known meteoritic minerals with high Ge and Sn concentrations. Nuwaite and butianite are very late-stage, vapor-deposited, alteration products, filling in pores within preexisting grossular-rich alteration veins and cracks in igneous Al,Ti-diopside. These phases and associated heazlewoodite and Ge-bearing alloys are observed only within the Ca-,Al-rich inclusion (CAI) and not outside it or at the inclusion-matrix interface. As only sections in one half of ACM -2 contain nuwaite/butianite, they were probably derived through a relatively low f O2 - f S2 sulfidation process, in which a highly localized, low-temperature Ge-, Sn-bearing fluid interacted with a portion of the host CAI. It is likely that the fluid became relatively more Sn- and Te-enriched with time and that crack fillings post-date vein fillings, possibly due to a late remobilization of vein sulfides.

Mount St. Helens (MSH) volcano, in the southern Washington Cascades arc, has produced dominantly dacitic to andesitic magmatic products over the last 300 ka. Basaltic to basaltic andesitic magmas erupted only during the relatively brief (ca. 2100–1800 yr B.P.) Castle Creek period from vents separated by no more than a few kilometers. They provide a unique perspective on the evolution of this volcano. Despite close temporal and spatial proximity, these mafic magmas define two distinct compositional lineages: (1) low-K tholeiites (LKT) and (2) basalts of “oceanic island” or intraplate affinity (or IPB). Both lack typical arc geochemical signatures and appear to derive from distinct mantle sources, neither of which has been significantly modified by slab-derived fluid or melt components. No true calc-alkalic basalts have erupted from MSH despite its obvious arc setting. Each lineage includes derivative lavas that range from ~7 to 5 wt% MgO and ~49 to 55 wt% SiO 2 , and both are slightly porphyritic with dominantly olivine and plagioclase, minor spinel, and trace clinopyroxene in some but not all samples. With respect to incompatible elements (e.g., K, La, Nb, Th, etc.), compositional trends for the two lineages are dramatically different and inconsistent with simple fractional crystallization processes. The data instead suggest that each lineage was produced dominantly by mixing between distinct parental LKT and IPB basaltic magmas and material of intermediate composition roughly similar to average MSH andesite. Mineralogical characteristics of macrocrysts in MSH basalts indicate that they do not represent equilibrium assemblages. Olivine compositions and textures in some samples implicate accumulation of crystals formed from multiple magmas, and evidence for magma mixing is reinforced by the rare presence of “blebs” of rhyolitic glass. These assemblages of crystals presumably are derived from different magmas and/or older MSH magmatic products (including crystal mush zones) within crustal conduit systems. Extrapolation of compositional trends (“mixing arrays”) to higher MgO content implicates the involvement of three types of parental magma: primitive LKT as well as distinct nepheline- and hypersthene-normative IPBs (or ne-IPB and hy-IPB), variants of which have erupted repeatedly from monogenetic vents in this sector of the Cascades. Such magmas are interpreted to form from distinct lherzolitic mantle sources (less fertile, with lower clinopyroxene content for LKT) at depths on the order of 80 (LKT) and 50–60 (IPBs) kilometers, under near-anhydrous conditions, in response to decompression rather than flux-melting. We also report a set of self-consistent estimates of temperature, pressure, water content, magma density, and weight fraction of “andesitic” mixing component for samples of each lineage. These parameters are highly correlated and serve to constrain the structure of the magma feeder system beneath MSH. A dynamic continuum of melt compositions is likely present, controlled principally by temperature and density gradients within the system. We envisage that during Castle Creek time the most primitive basaltic magmas formed distinct reservoirs in the deep crust, with the ne-IPB variant near 28 km depth and the LKT variant near 23 km. More silicic members of these lineages appear to have evolved at depths between ~20–15 km. We suggest that reservoir depths were controlled mainly by magma density that, in turn, is largely determined by the degree of mixing with “andesitic” components at crustal depths. This configuration implies a vertical magmatic plexus with connections extending well into the upper mantle. The sharp chemical distinction between the LKT and IPB mixing arrays suggests that the respective feeder systems were isolated and rarely interacted despite their close proximity. Finally, it appears that the presence of large, complex, and long-lived conduit systems beneath stratovolcanoes can act as “magma traps,” within which deeper-seated (mantle) inputs are prone to modification by interaction with stored magmas and their differentiation products. In contrast, the occurrence of relatively primitive basalts from monogenetic vents distal from stratovolcanoes implies that diverse basaltic magmas ascend beneath virtually the entire arc segment and that the true complexity of this “mantle wind” is locally masked by modifications within the crust.

In the present study, four samples of natural melilites were characterized using electron microprobe analysis, powder X‑ray diffraction, FTIR, and Raman spectroscopy, and their thermodynamic properties were measured with a high-temperature heat-flux Tian-Calvet microcalorimeter. The enthalpies of formation from the elements were determined to be: –3796.3 ± 4.1 kJ/mol for Ca 1.8 Na 0.2 (Mg 0.7 Al 0.2 Fe 2 0.1+ )Si 2 O 7 , –3753.6 ± 5.2 kJ/mol for Ca 1.6 Na 0.4 (Mg 0.5 Al 0.4 Fe 2 0.1+ )Si 2 O 7 , –3736.4 ± 3.7 kJ/mol for Ca 1.6 Na 0.4 (Mg 0.4 Al 0.4 Fe 2 0.2+ )Si 2 O 7 , and –3929.2 ± 3.8 kJ/mol for Ca 2 (Mg 0.4 Al 0.6 )[Si 1.4 Al 0.6 O 7 ]. Using the obtained formation enthalpies and estimated entropies, the standard Gibbs free energies of formation of these melilites were calculated. Finally, the enthalpies of the formation of the end-members of the isomorphic åkermanite–gehlenite and åkermanite–alumoåkermanite series were derived. The obtained thermodynamic properties of melilites of different compositions can be used for quantitative modeling of formation conditions of these minerals in related geological and industrial processes.

Iron in ferropericlase experiences a spin crossover from a high spin to a low spin under lower mantle conditions, which generates anomalies in many properties such as the heat capacity and sound velocity. In this study, the effect of the spin crossover on thermal conductivity was evaluated by considering the effects of the spin crossover on P wave velocity and heat capacity at constant volume but ignoring the effect on the mean free path. The spin crossover completely changes the conventional pressure and temperature dependences of the thermal conductivity. The spin crossover can significantly reduce the thermal conductivity of ferropericlase. The pressure dependence of the thermal conductivity of ferropericlase will show a double-valley feature across the spin-crossover region at the appropriate temperature (e.g., 1000 K). In contrast to the conventional decrease in the thermal conductivity with temperature, the thermal conductivity of ferropericlase in the Earth’s D ″ layer may increase with temperature in some temperature regions. The unusual effect of spin crossover on the thermal conductivity can be expected in other minerals with spin crossover. The spin crossover effect needs serious consideration when estimating the thermal conductivity at the core-mantle boundary.

Planetary cores are composed mainly of Fe with minor elements such as Ni, Si, O, and S. The physical properties of Fe alloys depend on their composition. Changes in c / a ratio, center shifts, and elastic properties of Fe and Fe–Ni alloys were reported previously. However, such properties of Fe light-element alloys have not yet been extensively studied. Si is a plausible candidate as a light element in planetary cores. Therefore, we studied the electronic properties and compressional behavior of Fe-Si alloys with a hexagonal-close-packed (hcp) structure under high pressure using synchrotron Mössbauer spectroscopy (SMS) and X‑ray diffraction (XRD). Center shifts (CS) were observed at pressures of 21.4–45.3 GPa for Fe-2.8wt%Si and of 30.9–62.2 GPa for Fe-6.1wt%Si. Some of SMS and XRD measurements were performed under the same conditions using a newly developed system at the BL10XU beamline of SPring-8, which allowed simultaneous characterization of the electron information and crystal structure. Changes in the CS values were observed at 36.9 GPa in Fe-2.8wt%Si and 54.3 GPa in Fe-6.1wt%Si. The ratios of c / a in the hcp structure were measured at pressures of 21.2–49.6 GPa in Fe-2.8wt%Si and 32.9–61.4 GPa in Fe-6.1wt%Si. The c / a ratio changed at pressures of 30–45 GPa in Fe-2.8wt%Si and at 50 GPa in Fe-6.1wt%Si. Changes in the CS and c / a ratio were explained according to the electronic isostructural transition in Fe–Si alloys. In addition, the transition pressure increased with increasing Si content in metallic iron. This finding is significant as changes in seismic wave velocities due to the change in c / a ratio of Fe–Si alloys with an hcp structure might be observed if Venus has a solid inner core.

To better understand the transport of Mo and W in granitic melts and the formation mechanism of porphyry ore deposits, we have investigated the diffusivities of Mo and W in granitic melts with 0.04–5.1 wt% H 2 O at 1000–1600 °C and 1 GPa using a diffusion couple approach and a Mo saturation approach with Mo sheet serving as the source. The Mo and W diffusivities obtained from diffusion profiles measured by LA-ICP-MS can be described as: DMo,anhy=10−1.47±0.73exp[−(387±25)/RT],DW,anhy=10−1.28±1.05exp[−(396±35)/RT],DMo,2.7wt%H2O=10−5.37±0.52exp[−(211±18)/RT],DMo,5.1wt%H2O=10−6.87±0.69exp[−(133±20)/RT],$\begin{align}& {{D}_{\text{Mo},\text{anhy}}}={{10}^{-1.47\pm 0.73}}\exp \left[ -{\left( 387\pm 25 \right)}/{\text{R}T}\; \right], \\ & {{D}_{\text{W},\text{anhy}}}={{10}^{-1.28\pm 1.05}}\exp \left[ -{\left( 396\pm 35 \right)}/{\text{R}T}\; \right], \\ & {{D}_{\text{Mo},2.7\text{wt }\text{%}{{\text{H}}_{2}}\text{O}}}={{10}^{-5.37\pm 0.52}}\exp \left[ -{\left( 211\pm 18 \right)}/{\text{R}T}\; \right], \\ & {{D}_{\text{Mo},5.1\text{wt }\text{%}{{\text{H}}_{2}}\text{O}}}={{10}^{-6.87\pm 0.69}}\exp \left[ -{\left( 133\pm 20 \right)}/{\text{R}T}\; \right], \end{align}$ where D is diffusivity in m 2 /s (with the subscripts denoting water contents and “anhy” representing nominally anhydrous melt), R is the gas constant, T is the temperature in K, and the activation energies in the exponential are in kJ/mol. When the influence of H 2 O is incorporated, Mo diffusivity in granitic melts with <5.1 wt% H 2 O can be modeled as: logDMo=−(1.94±1.58)−(0.87±0.36)w−[(19341±2784)−(2312±620)w]/T$log {{D}_{\text{Mo}}}=-\left( 1.94\pm 1.58 \right)-\left( 0.87\pm 0.36 \right)w-{\left[ \left( 19341\pm 2784 \right)-\left( 2312\pm 620 \right)w \right]}/{T}\;$ where w is H 2 O content in the melt in wt%. The diffusion behavior (low diffusivities, high activation energies, and strong H 2 O effects) of Mo and W indicates that they exist and diffuse in the melt in the form of hexavalent cations. Their low diffusivities imply that the bulk concentrations of Mo and W in exsolved hydrothermal fluid and those in the melt are probably not in equilibrium. However, because of the large fluid-melt partition coefficients of Mo and W, they can still be enriched in the hydrothermal fluid, although to a lesser extent than equilibrium partitioning would allow. Slow Mo and W diffusion can be a significant rate-limiting step for the formation of porphyry Mo/W deposits.

We present the first equation of state and structure refinements at high pressure of single-crystal phase egg, AlSiO 3 OH. Phase egg is a member of the Al 2 O 3 -SiO 2 -H 2 O system, which contains phases that may be stable along a typical mantle geotherm (Fukuyama et al. 2017) and are good candidates for water transport into Earth’s deep mantle. Single-crystal synchrotron X‑ray diffraction was performed up to 23 GPa. We observe the b axis to be the most compressible direction and the b angle to decrease up to 16 GPa and then to remain constant at a value of ~97.8° up to the maximum experimental pressure reached. Structure refinements performed at low pressures reveal a distorted octahedron around the silicon atom due to one of the six Si-O bond lengths being significantly larger than the other five. The length of this specific Si-O4 bond rapidly decreases with increasing pressure leading to a more regular octahedron at pressures above 16 GPa. We identified the shortening of the Si-O4 bond and the contraction of the vacant space between octahedral units where the hydrogen atoms are assumed to lie as the major components of the compression mechanism of AlSiO 3 OH phase egg.

The atomic arrangements of eight synthetic samples along the fluorapatite-hydroxylapatite join were examined using X‑ray crystallographic techniques; the results of those refinements demonstrate that the incorporation of both F and OH in the apatite anion column, mimicking the human apatite system as modified by fluoridation, is complex. The compositions of the anion columns in the phases ranged from [F 0.40 (OH) 0.60 ] to [F 0.67 (OH) 0.33 ], and the high-precision structure refinements yielded R 1 values from 0.0116 to 0.0140. The apatite structure responds to the variable content of the anion columns. Counterintuitively, the OH groups in the anion column move monotonically closer to the mirror planes at z = 1⁄4, 3⁄4 with increasing F content, despite the decreasing size of the triangle of Ca2 atoms to which the column anions bond and the increasing overbonding of the hydroxyl oxygen. In the structure the F atoms are underbonded and have zero degrees of positional freedom in the (0,0,1⁄4) special position to relieve that underbonding; the bonding deficiency of the anion column is relieved by the overbonding of the O(H) atom in the anion column, overbonding that increases with increasing content of underbonded F in the anion column. Together the underbonded F and the overbonded OH meet the formal bond valence (1.0 v.u.) required by the anion column occupants. The changes in bonding from the individual anion column occupants to the surrounding Ca2 atoms with composition induce bond length changes principally in the irregular Ca2 polyhedron and also affect the a lattice parameter in the apatites. The bond valence values imparted on the F, OH column anions, when extrapolated to end-member compositions, suggest that different column anion arrangements may exist near the F and OH end-member compositions, as is also seen along the apatite Cl-OH join. These values have implications for the incorporation of fluoride in human teeth during the fluoridation process.

Diamond-anvil cell (DAC) experiments focusing on the solubility of carbonates and aqueous carbon speciation at subduction zones require pressure monitoring with sensitive, chemically inert sensors. Commonly used pressure indicators are either too insensitive or prone to contaminate pressure-transmitting media due to their increased solubility at high pressure and/or temperature ( P-T ). Here, the P - and T -induced frequency shifts of the Raman vibrational modes of natural crystalline carbonate minerals aragonite, calcite, dolomite, magnesite, rhodochrosite, and siderite have been calibrated for application as Raman spectroscopic P and T sensors in DACs up to 500 °C and 6 GPa. The shifts of all modes are quasi-constant over the observed P and T ranges and are generally less prominent for internal modes than for external modes. Our method provides a sensitive and robust alternative to traditional pressure calibrants, and has three principal advantages: (1) higher sensitivity (for particular Raman vibrational modes), (2) monitoring P-T -induced shifts of several modes allows even more accurate P-T determination, and (3) no contamination of pressure-transmitting media by foreign materials can occur. Additionally, the isobaric and isothermal equivalent of the Grüneisen parameter and the anharmonic parameter for each of the traced modes have been determined.

The Catalão II and Catalão I carbonatite complexes are Cretaceous intrusions in the northwestern part of the Alto Paranaíba Igneous Province, central Brazil, and contain various trioctahedral micas. Drill-hole sampling and mineralogical and geochemical data suggest the existence of different types of cumulate rocks as carbonatites (calcio- and ferrocarbonatites in Catalão II; magnesiocarbonatites in Catalão I), magnetitites, apatitites, and phlogopitites. In Catalão II complex the presence of ultramafic lamprophyres (phlogopite-picrites) and fenites (syenites and clinopyroxenites) are identified. Phlogopite, Fe 3+ -rich phlogopite, and tetraferriphlogopite are ubiquitous, with marked and variable pleochroism. In the Catalão II, micas from magnetitites, apatitites, and calciocarbonatites are close in composition to the tetraferriphlogopite end-member. The Mg/(Mg+ [VI] Fe tot ) average value is 0.959 for micas from magnetitites, 0.878 for micas from apatitites, and 0.875 for micas from calciocarbonatites, suggesting an enrichment of octahedral iron in crystals from calciocarbonatites. The trioctahedral micas from the fenites are intermediate in composition between phlogopite and tetraferriphlogopite, with 0.227 ≤ [IV] Fe ≤ 0.291 apfu. Micas of phlogopitites, ferrocarbonatites, and phlogopite-picrites show a significant variation in [IV] Fe, [IV] Al, [VI] Mg, and [VI] Fe contents, suggesting the existence of different mica populations. In the Catalão I magnetitites micas show a quite constant Mg content but marked differences in [IV] Fe 3+ and [IV] Al content, so there are [IV] Fe 3+ -bearing phlogopite and tetraferriphlogopite. The micas from Catalão I apatitites have variable [IV] Fe 3+ , [IV] Al, and Mg contents and are mainly tetraferriphlogopite with a minor phlogopite population. Trioctahedral micas from Catalão I magnesiocarbonatites contain crystals close to tetraferriphlogopite with [IV] Fe 3+ and a limited variation in Mg content. In these complexes, heterovalent octahedral substitutions are mainly related to Ti 4+ and, only in a few samples, to Fe 3+ . The primary mechanism regulating Ti uptake into the mica structure is the Ti-oxy [ [VI] Ti 4+[VI] (Mg,Fe) 2 – + 1 (OH) – –2 O 2 2– ] substitution. Crystal structural analysis shows that all mica crystals are 1M polytypes with the expected space group C 2/ m . The nearly equal size and mean electron count between M1 and M2 octahedral sites suggest a disorder of octahedral cations between these two sites. The low Al and the high Fe 3+ content in the tetrahedral site of these micas, as well as the high (OH) – content, reflects the general enrichment in FeO and H 2 O and the peralkaline nature of magma from which Catalão I and Catalão II micas crystallized. Micas strongly enriched in Fe 3+ and poor in Ti and Al, are used, for the first time, as indicators of crystallization temperature using published geothermometers. Temperatures range between ~800 and 558 °C for the Catalão II cumulate rocks, in high oxygen fugacity conditions, mainly over the HM condition. In contrast, the variability in temperature (703–631 and 1050–952 °C) for Catalão II phlogopite-picrites is consistent with mica crystallization in an HM-NiNiO environment. The calculated temperatures for Catalão I cumulate rocks (742–542 °C for magnesiocarbonatites, 923–540 °C for magnetitites, and 679–550 °C for apatitites) are quite similar with those of Catalão II rocks, but they generally show higher oxygen fugacity conditions (over HM condition).

We report on the oxygen isotope composition of Jurassic rhyolites from a silicic large igneous province, the Chon Aike Province (Patagonia, Argentina). Quartz is shown to have refractory behavior with respect to diffusional oxygen isotope exchange, making it a robust tracer of magmatic processes. Detailed secondary ion mass spectroscopy (SIMS) transects across 24 quartz crystals reveal homogeneous, but elevated, oxygen isotope values (10.9–12.5‰). None of the analyzed grains display distinct discontinuities in 18 O values. Late hydrothermal exchange is limited to a few tens of micrometers next to cracks, some grain boundaries, and inclusions. No correlation with igneous zoning as revealed by cathodoluminescence (CL) was found. Finally, quartz crystals display little to no inter-grain variability at a sample or outcrop scale. Zircons (7.5–10.1‰), in contrast, display significant inter–crystalline oxygen isotopic heterogeneity (>2.0‰) at a sample scale, but core-rim analyses reveal no systematic variations. This is interpreted to confirm the antecrystic nature of zircons, while quartz crystals mostly are phenocrysts. The studied quartz and zircon provide, hence, complementary information on the evolution of the magmatic system of the Chon Aike Province. Zircon likely captures information about the deeper source region, in contrast to quartz that will record the last stages of the magmatic system and thus might provide important information on the buildup and duration of magma chamber processes in the upper crust. The data illustrate that quartz—in the absence of recrystallization—can retain its magmatic signature and is thus a useful tracer of pre-eruptive magmatic processes. The high δ 18 O values of both zircon and quartz require a significant (>50%) crustal—most likely sedimentary— contribution in the melt formation process, either via assimilation or anatexis. This conclusion yields new constraints on petrological models for the Chon Aike Province.