Melt inclusions in phenocrysts are a potentially powerful tool in petrological research that can provide the only direct information available on the physical parameters ( P, T and melt composition) of crystallisation at various stages in the evolution of magmatic systems. However, melt inclusions also differ in principle from other parts of the magmatic system in that their composition, after trapping, may be controlled by the composition of the host phenocryst and therefore the direct application of our understanding of macro-scale magmatic processes to the interpretation of melt inclusion data can lead to erroneous conclusions. Our results indicate that the compositions of melt inclusions in early formed phenocrysts (olivine, pyroxene, plagioclase and spinel), often of most interest in petrological studies, can be affected by processes such as volatile dissociation, oxidation and/or partial re-equilibration with their host, both during natural cooling and homogenisation experiments. In particular, melt inclusions in all minerals are prone to hydrogen diffusion into or out of the inclusions after trapping and prior to eruption, and during homogenisation experiments. If not taken into account, this can significantly affect the crystallisation temperatures derived from the homogenisation experiments. Melt inclusions in highmagnesian olivine phenocrysts commonly have lower Fe contents compared to the initially trapped composition due to reequilibration with the host at lower temperatures. This often leads to the appearance of sulphide globules and in some cases high-magnesian clinopyroxene daughter crystals, and may cause an increase in the oxidation state of the inclusions. Homogenised melt inclusions in plagioclase phenocrysts in MORB usually have lower Ti and Fe, and higher Si contents compared to the melt composition at the moment of trapping. However, homogenisation experiments can provide reliable estimates of trapping temperature and the MgO, Al 2 O 3 , CaO, Na 2 O, and K 2 O contents of the host magma at the moment of trapping. Some of these processes can be identified by observing the behaviour of melt inclusions during homogenisation experiments using low-inertia visually controlled heating stages, and their effects can be minimised by using appropriate experimental conditions as determined by kinetic experiments, ideally completed for each phenocryst type in every sample. We also discuss general aspects of melt inclusion studies aimed at recovering H 2 O content of primary mantle-derived magmas and demonstrate that, in cases of low-pressure crystallisation, it is important to identify the 0009-2541/02/$ -see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 -2 5 4 1 ( 0 1 ) 0 0 3 6 9 -2

Brothers caldera volcano is a submarine volcano of dacitic composition, located on the Kermadec arc, New Zealand. It hosts the NW caldera vent field perched on the steep slope of the caldera walls and includes numerous, active, high-temperature (max 302°C) chimneys and a greater amount of dead, sulfide-rich spires. Petrographic studies of these chimneys show that three main zones can occur within the chimneys: a chalcopyrite-rich core, surrounded by a sulfate-dominated zone, which is in turn mantled by an external rind of Fe oxides, calcite, and silicates. Four chimney types are identified based on the relative proportions of the chalcopyrite and sulfate layers and the presence or absence of anhydrite. Two are Cu rich, i.e., chalcopyrite-sulfate and chalcopyrite-bornite chimneys, and two are Zn rich, i.e., sphalerite-barite and sphalerite-chalcopyrite.Chimney growth begins with the formation of a sulfate wall upon which sulfides precipitate. Later, zone refining results in a chalcopyrite-rich core with pyrite/marcasite and sphalerite occurring predominantly near the outer margins. In chalcopyrite-bornite chimneys, the chalcopyrite core rapidly loses permeability and limits the thickness of the surrounding sulfate layer. In these chimneys, bornite, chalcocite, and covellite form along the outer margin of the chalcopyrite zone as a result of oxidation by seawater. Zinc-rich chimneys display a more vertical zonation and their growth involves an upward-advancing barite cap followed by chalcopyrite deposition (if present) nearer the base. The vertical zonation and lack of anhydrite in these chimneys also implies that larger chalcopyrite and anhydrite deposits may exist subsea floor. The different chimney types are related to subsea-floor permeability, the amount of fluid mixing that occurs prior to venting, and heterogeneous fluid compositions.The occurrence of specular hematite and Bi or Au tellurides associated with chalcopyrite are consistent with magmatic contributions to the NW caldera vent site. These tellurides are the first gold-bearing phase to be identified in these chimneys, and the Bi-Au association suggests that gold enrichment up to 91 ppm is due to scavenging by liquid bismuth. The presence of tellurides in Brothers chimneys have implications for other telluride-bearing deposits, such those in the Urals. Likewise, other aspects of the mineralogy (i.e., textures) and zonation, including the implied subsea-floor deposition, presented here from an active, undeformed environment can aid in understanding ancient volcanogenic massive sulfide (VMS) deposits that have undergone various degrees of metamorphism.

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A new model of sulfur solubility in mafic and/or ultramafic silicate magmas, which accounts for the effects of pressure, temperature, oxygen fugacity, major element, and Ni contents in the silicate melt and the coexisting sulfide liquid, is presented in this paper. The model postulates the existence of positively charged Fe-Ni sulfide complexes in the melt of a general formula (FeyNi1-y)zS 2(z-1)+ , which are formed as a result of complexation reactions between the sulfide-forming ions (Fe 2+ , Ni 2+ , S 2− ) and (Fe,Ni)S species in the silicate liquid. The new model can explain both the anomalously high S solubility in iron-enriched silicate systems and the "parabolalike" dependence of S contents in silicate melts on their Fe content. The proposed mechanism of sulfide solubility was calibrated on a dataset of 213 anhydrous experimental glasses (both Ni free and Ni bearing) and 53 S-saturated MORB glasses, and incorporated into a new version of the COMAGMAT (v. 5) magma crystallization model. The COMAGMAT-5 model can estimate sulfur concentration at sulfide saturation (SCSS) in a wide range of experimental and natural compositions, including Fe/Ni variations in silicate melts and coexisting sulfides. Despite relatively low concentrations, nickel is shown to have a pronounced effect on S solubility, causing significant variations in the onset of sulfide immiscibility in melts with otherwise similar major element compositions. An application example of the new SCSS model to "B-1 magma" proposed as parent for the Lower and Lower Critical zones of the Rustenburg Layered Suite, Bushveld Complex, is discussed.

We have developed a high-resolution 3D model of the Alberton-Mathinna section of the “Main Slide,” northeast Tasmania. This geological model expresses a new synthesis based on mapping and structural interpretation on multiple cross sections. We have refined this model by 3D geophysical inversion constrained by gravity and magnetic survey data coupled with drilling and rock physical property databases. Our modeling incorporates statistically generated sensitivity characterization metrics into 3D model products that map confidence in the geometry of geological units at depth. The results include a granitoid surface that is considerably more detailed than earlier versions based on 2D modeling. Among the new features to emerge is a cupola 1.6 km below and slightly west of the Mathinna goldfield. At the Ringarooma United deposit located within the Alberton goldfield, we seethat the fault network underpinning the deposit was intruded by granite to a depth of approximately 400 m. Ore-forming solutions for both deposits have been interpreted as metamorphic in origin, but our results suggest the possibility of a role for magmatic fluids (i.e., granite related) in the gold-mineralizing system, particularly for the Ringarooma United deposit.

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