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Cerro Machín, a volcano located in the northern segment of the Andes, is considered one of the most dangerous volcanoes in Colombia with an explosive record that involves at least five plinian events. Prior studies focused on the last dome-building eruption have suggested the presence of a water-rich mid-crustal magma reservoir. However, no direct volatile measurements have been published and little work has been completed on the explosive products of the volcano. Here, we study the largest known eruption of Cerro Machín volcano which occurred 3600 years BP producing dacitic pyroclastic fall deposits that can be traced up to 40 km from the vent. Lapilli pumice clasts have a mineral assemblage of plagioclase, amphibole, quartz, and biotite phenocrysts, with accessory olivine, Fe–Ti oxides, and apatite. The occurrence of Fo89–92 olivine rimmed by high Mg# amphibole and the established high-water contents in the magma imply the presence of magma near or at water saturation at pressures > ~ 500 MPa. Measurements of up to 10.7 wt % H2O in melt inclusions hosted in plagioclase and quartz in the 3600 years BP eruption products support the idea that Cerro Machín is a remarkably water-rich volcanic system. Moreover, this is supported by measurements of ~103 to 161 ppm H2O in plagioclase phenocrysts. The application of two parameterizations of water partitioning between plagioclase and silicate melt allows us to use our water in plagioclase measurements to estimate equilibrium melt water contents of 5 ± 1 wt % to 11 ± 2 wt % H2O, which are in good agreement with the water contents we measured in melt inclusions. Results of amphibole geobarometry are consistent with a magma reservoir stored in the mid-to-lower crust at a modal pressure of 700 ± 250 MPa, corresponding to a depth of ~25 km. Minor crystallization in the shallow crust is also recorded by amphibole barometry and calculated entrapment pressures in melt inclusions. Amphibole is present as unzoned and zoned crystals. Two populations of unzoned amphibole crystals are present, the most abundant indicate crystallization conditions of 853 ± 26°C (1 se; standard error), and the less abundant crystallized at an average temperature of 944 ± 24°C (1 se). Approximately 18% of the amphibole crystals are normally or reversely zoned, providing evidence for a minor recharge event that could have been the trigger mechanism for the explosive eruption. Plagioclase crystals also show normal and reverse zoning. The moderate Ni concentrations (<1600 μg/g) in the high-Fo olivine xenocrysts suggest that Cerro Machín primary magmas are generated by inefficient interaction of mantle peridotite with a high-silica melt produced by slab melting of basaltic material. Some sediment input is also suggested by the high Pb/Th (>2.2) and Th/La (0.3–0.4) ratios. Whole rock chemistry reveals heavy rare earth element (HREE) depletion and Sr enrichment that likely formed during the crystallization of garnet and amphibole in the upper part of the mantle or lower portion of the crust, promoting the formation of water-rich dacitic magma that was then injected into the middle-to-lower crust. Textural and compositional differences in the crystal cargo that erupted during dome-building and plinian events support the idea that large volumes of magma recharge lead to effusive eruptions, while only small recharge events are needed to trigger plinian eruptions at Cerro Machín. [ABSTRACT FROM AUTHOR]

Porosity generated during fluid–rock reaction can facilitate fluid transport and metasomatism in low permeability high-pressure metamorphic rocks. Evidence for reaction-induced porosity is found in an eclogite-facies clinopyroxene + apatite vein in an undeformed eclogitized Fe–Ti metagabbro from the Monviso Ophiolite (W. Alps) with a distinct garnet-rich selvage. Vein-forming fluids were sourced from adjacent metagabbros and reaction with the host rock removed Ca and P from the selvage and added Fe, REE, Pb and Cr. Textures at the selvage–host rock interface and in the host rock record local heterogeneity in reactivity and porosity during metasomatism linked to variable initial lawsonite abundance. These features reflect a hierarchy of pervasive-to-channelized porosity structures that facilitated widespread metasomatism of the host rock. Development of this metasomatic system in response to locally derived fluids suggests large-scale externally derived fluid transport is not required to drive extensive fluid–rock exchange. The production of porosity during metasomatic reactions could be important in facilitating further fluid–rock reaction and fluid transport in subducting slabs where permeability is low. [ABSTRACT FROM AUTHOR]

Nappes in the southern sector of the Southern Brasilia Belt record suturing of the Paranapanema Block and Socorro Guaxupe Arc with a subducted passive margin on the western side of the Sao Francisco Craton. We report secondary ion mass spectrometry U-Pb zircon ages that for the first time constrain the age of: (1) retrograded eclogite from a block along the tectonic contact beneath the uppermost nappe in a stack of passive margin-derived nappes; (2) high-pressure granulite-facies metamorphism in the uppermost passive margin-derived nappe; (3) high-pressure granulite-facies metamorphism in the overlying arc-derived nappe. Rare zircons from a retrograded eclogite yield a [sup.206]pb/[sup.238] U age of 678 [+ or -] 29 Ma, which we interpret as most likely to date close-to-peak-P metamorphism and to provide a minimum age for detachment of the overlying passive margin-derived nappe from the subducting plate. Zircon associated with ilmenite in samples from two structural levels in the passive margin-derived high-pressure granulite nappe yields [sup.206]pb/[sup.238]U ages of 648 [+ or -] 12 and 647 [+ or -] 11 Ma, and Ti-in-zircon crystallization temperatures from c. 860[degrees]C down to c. 785[degrees]C, but skewed toward the lower part of the range. These data indicate zircon formation during cooling from around peak T to the solidus, consistent with the high-T retrograde P-T path deduced from microstructures linked to phase assemblage fields in isochemical phase diagrams. Rb-Sr multi-mineral-whole-rock isochrons from two samples from close to the bottom of this nappe date formation of a retrograde sillimanite-bearing penetrative fabric to c. 590 Ma at temperatures of c. 750[degrees]C (based on Ti-in-quartz thermometry). Rare zircons from leucosome in high-pressure granulite from the overlying arc-derived nappe yield a [sup.206]pb/[sup.238]U age of 622 [+ or -] 28 Ma and Yi-in-zircon crystallization temperatures from c. 970[degrees]C down to c. 820[degrees]C, which we interpret to record formation of zircon during cooling from peak high-pressure granulite-facies conditions. These ages indicate that the first stage of craton amalgamation in West Gondwana may have occurred earlier than previously inferred. Supplementary material: Geochemical and geochronological data are available at http://www.geolsoc.org.uk/SUP18377. doi: 10.1144/0016-76492008-140.

We have performed five separate sets of experiments to elucidate the effects of magnetite, ulvospinel-magnetite solid solution, and pyrrhotite crystallization on the budgets of Au, Cu, and Ag at magmatic conditions. The experiments were done in both hydrous and anhydrous assemblages at temperatures between 800 and 1050[degrees]C, pressures from ambient to 140 MPa, log [fo.sub.2] from NNO-0.25 to NNO, and log [fs.sub.2] from -1.5 to -3.0. Nernst-type partition coefficients ([+ or -]1[sigma]) at 800[degrees]C in a water-saturated assemblage are [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. Nernst-type partition coefficients ([+ or -]1[sigma]) at 1050[degrees]C in an anhydrous assemblage are [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. The calculated values for [D.sup.Usp/melt.sub.Au] and [D.sup.Usp/melt.sub.Cu] indicate that the addition of Ti to magnetite increases significantly the Au- and Cu-scavenging potential of ulvospinel relative to end-member magnetite. Partition coefficients for Cu and Au between pyrrhotite and melt indicate that a temperature change from 1050 to 800[degrees]C in an anhydrous and hydrous assemblage, respectively, results in no observable change in Cu partitioning. The calculated partition coefficients are used to model the effect of crystal fractionation on the concentrations of Ag, Cu, and Au. Model results suggest that the co-crystallization of magnetite and pyrrhotite sequester no more than 2% Ag, 7% Cu, and 37% Au from the melt over the first 25% solidification. If the melt reaches volatile saturation after 25% crystallization, the presence of end-member magnetite and pyrrhotite do not appear to inhibit the Cu-, Au-, and Ag-ore potential of the magma. Ulvospinel-magnetite, however, may reduce the Au concentration in the melt by approximately one-third relative to its initial value that decreases the overall Au available to partition into the volatile phase. Keywords: Metal budgets, magma evolution, element partitioning, differentiation, crystal fractionation

We report data that allow us to evaluate the method of trapping immiscible, saline aqueous fluids (i.e., vapor and brine in the NaCl-KCl-HCl-Fe[Cl.sub.2]-AuH[Cl.sub.2]-[H.sub.2]O system) as synthetic fluid inclusions in pre-fractured quartz cores in order to quantify the concentrations of Au, Fe, K, and Na, among coexisting three-phase, immiscible fluids (i.e., haplogranite melt, brine, and vapor) at magmatic conditions. Coexisting vapor and brine were trapped experimentally at 800 [degrees]C and 100-110 MPa as synthetic fluid inclusions in both quartz microfractures and quenched silicate melt (i.e., glass), and also sampled indirectly using the recovered quenched aqueous fluid. Quartz-hosted and glass-hosted brine inclusions were analyzed by laser-ablation inductively-coupled-plasma mass spectrometry (LAICPMS) and instrumental neutron activation analysis (INAA), respectively. Quenched aqueous fluid from each experiment containing a quartz core was recovered and analyzed by atomic absorption spectrophotometry (AAS). The composition of aqueous fluids trapped as quartz-hosted inclusions, glass-hosted inclusions, and those recovered after quench yield consistent and precise data, at the 2[sigma] uncertainty level, for the elements of interest. The overlapping Au, Fe, K, and Na concentrations in aqueous fluids trapped and analyzed via three entirely different instrumental techniques (i.e., LAICPMS, INAA, and AAS) suggest strongly that quartz microfractures heal on a slow enough time scale to permit entrapment of fully equilibrated aqueous fluids at our experimental PTX conditions. The data evince clearly that the chemical composition of fluids in quartz microfractures at the time of self-healing represents equilibrium conditions; hence, synthetic fluid inclusions in experiments with low thermal gradients across the charge provide a reasonable estimate of fluid composition at least at the experimental conditions examined in this study. Keywords: Analysis (chemical), fluid inclusions, experimental petrology, magmatic-hydrothermal fluids, element partitioning, immiscible fluids, igneous petrology, vapor, brine

Abstract--Oceanic incidence and spawning frequency of Chesapeake Bay striped bass (Morone saxatilis) were estimated by using microchemical analysis of strontium in otoliths. Otoliths from 40 males and 82 females sampled [...]