Textural and compositional data for magnetite from nine iron skarn deposits in Canada, Romania, and China show that most samples have reequilibrated by dissolution and reprecipitation, oxy-exsolution, and/or recrystallization. The dissolution and reprecipitation processes are most extensive and are present in most magnetite samples examined, whereas the oxy-exsolution occurs only in high-Ti magnetite, forming exsolution lamellae of Fe-Ti-Al oxides. Electron microprobe analysis indicates that the reequilibration processes have significantly modified the minor and trace element compositions of magnetite, notably Si, Mg, Ca, Al, Mn, and Ti, whereas oxy-exsolution is effective in decreasing the Ti content of high-Ti magnetite. Many analyses of magnetite grains from the skarn deposits plot variably in the banded iron formations (BIF), iron oxide-copper-gold (IOCG), or porphyry Cu fields using the Ti + V versus Ca + Al + Mn discrimination diagram. This pattern suggests that trace element data for magnetite that has unusual composition and/or reequilibrated cannot be reliably used as a petrogenetic indicator. Mixing of externally derived saline fluids with Fe-rich magmatic-hydrothermal solutions, an increase in temperature, and local decreasing pressure and fO 2 are considered the most important causes for the dissolution and reprecipitation, or recrystallization, of the magnetite; increasing fO 2 and decreasing temperature may facilitate oxy-exsolution of Fe-Ti-Al oxides in high-Ti magnetite. Results presented here highlight the importance of detailed textural characterization prior to in situ chemical analysis of magnetite grains so that mineral compositions can be properly evaluated in terms of the genesis and evolution of iron skarn deposits.

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The Jumping Brook Metamorphic Suite in the western Cape Breton Highlands of Nova Scotia is part of an inverted Barrovian sequence that formed during a Late Silurian–Early Devonian promontory–promontory collision in the Canadian Appalachians. In this study, systematic discrepancies between geochemical observations and thermodynamic model predictions led to the discovery of a systematic relationship linking the style of garnet core isopleth intersection (GCII) to the pyrophanite (MnTiO3) component of co‐existing ilmenite. Samples that yielded tight GCIIs at or near the garnet‐in curve were found to contain ilmenite with negligible pyrophanite components, whereas samples yielding GCIIs far removed (up to 105°C) from the garnet‐in curve were found to contain ilmenite with significant pyrophanite and/or ecandrewsite (ZnTiO3) components. Based on petrographic and geochemical observations, Mn(±Zn)‐rich ilmenite are interpreted to have sequestered Mn throughout prograde metamorphism due to sluggish intracrystalline diffusion. The amount of reactive Mn input into the thermodynamic models from whole‐rock analyses were, in some cases, overestimated, resulting in garnet‐in curve topologies that extend to erroneously low P–T conditions. Modifications to the whole‐rock chemistry that account for Mn sequestration into ilmenite, however, yielded robust model results. Our results show that, in addition to uncertainties in thermodynamic data sets and phenomenon related to reaction kinetics, Mn‐rich ilmenite may superimpose additional complexities related to the interpretation of predicted equilibria involving garnet. Numerical simulations of garnet crystallization were used to infer P–T paths of metamorphism for one sample from the garnet zone (Mn corrected) and two samples from the staurolite zone (Mn uncorrected) of the inverted sequence. Model results are remarkably similar among the three samples and indicate that garnet crystallization occurred along relatively steep (31–37°C/km) clockwise P–T paths. The peak conditions of garnet crystallization and metamorphism (560–590°C, 7.4–8.0 kbar) are interpreted to have been attained approximately simultaneously, such that the paths are characterized by tight prograde‐to‐retrograde transitions. The hairpin nature of the P–T paths is interpreted to represent the onset of thrust‐related exhumation and isograd inversion along ductile shear zones, consistent with available field and geochronological constraints.

A dual-beam focused ion-beam -scanning electron microscope (FIB-SEM) instrument was used for detailed textural and chemical analysis of polyphase inclusions in spodumene. Our 3D representation of an inclusion (GB-38) was constructed from SEM images of serial cross-sections exposed by milling laterally through a selected inclusion with a 30 keV beam of Ga ions. The volume of the inclusion and daughter phases was determined from the sum of the volume of parallel slices; the volume of each slice was determined by multiplying the measured slice area by its thickness. Semiquantitative energy-dispersive X-ray (EDX) analysis and element mapping were performed on each cross-section to facilitate the identification of all solids within the inclusion. FIB-SEM analysis of spodumene-hosted inclusions from the Tanco and Greenbushes occurrences of granitic pegmatite are presented to demonstrate the advantages of 3D analysis for petrographic investigations of polyphase inclusions. The highresolution 3D reconstructions afforded by the FIB-SEM method significantly extend our ability to study and interpret complex solid-rich fluid and melt inclusions in igneous and in high-pressure metamorphic rocks. SOMMAiRENous nous sommes servis d'un instrument à deux faisceaux, dont un faisceau focalisé d'ions et un microscope électronique à balayage (FIB-SEM), afin de faire une analyse texturale et compositionnelle détaillée d'inclusions polyphasées dans le spodumène. Nous avons construit notre représentation d'une inclusion (GB-38) en trois dimensions à partir d'images effectuées au microscope électronique d'une série de coupes transversales préparées en taillant latéralement une inclusion choisie avec un faisceau de 30 keV d'ions de Ga. Le volume de l'inclusion et des phases secondaires a été déterminé à partir de la somme des volumes des tranches parallèles. On obtient le volume de chaque tranche en multipliant son aire par son épaisseur. Une analyse semiquantitative par dispersion d'énergie (EDX) et une répartition des éléments, effectuées sur chaque tranche, ont facilité l'identification de toutes les phases solides au sein de l'inclusion. L'analyse par la technique FIB-SEM d'inclusions piégées dans le spodumène provenant des pegmatites granitiques de Tanco et Greenbushes démontre les avantages d'une analyse tri-dimensionnelle pour les études pétrographiques d'inclusions polyphasées. Les reconstructions tri-dimensionnelles qui en résultent augmentent sensiblement notre abilité d'étudier et d'interpréter les complexités des inclusions fluides ou de reliquats magmatiques riches en phases solides dans les roches ignées et les roches métamorphiques de haute pression. (Traduit par la Rédaction)Mots-clés: inclusions polyphasées, inclusions fluides, reliquats magmatiques, pétrographie des inclusions, approche FIB-SEM à deux faisceaux, sections en série, reconstruction 3D, pegmatite granitique, spodumène. §

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P-T-t conditions of Grenvillian metamorphism were derived from mineral assemblages, growth zoning in garnet and monazite geochronology from metapelites of the Flinton Group in southeastern Ontario. Initial conditions of garnet growth determined through isopleth thermobarometry yielded 3.7 kbar and 515°C and geothermal gradients of 40°C km -1 . Phase equilibria modeling revealed an increase from 615 to 715°C and 5.6 to 7.9 kbar across the study area. Forward modeling of growth zoning in a population of garnet porphyroblasts with the THERIA_G software revealed a clockwise path over the P-T interval 3.7 to 5.9 kbar and 513 to 615°C. Diffusional relaxation of growth zoning in relatively small porphyroblasts of the population provided an average heating rate of 2°C Ma -1 . In situ U-Pb monazite geochronology revealed a major age population at 977 ± 4 Ma, interpreted to represent monazite growth at the expense of allanite near the peak conditions of garnet growth and metamorphism.

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A review of K-feldspar compositions from miarolitic pegmatites shows that in most pegmatites the pocket K-feldspars are enriched in Rb and Cs relative to exopocket K-feldspar within the same body. Rayleigh modeling of simultaneous crystallization of K-feldspar from a melt and coexisting aqueous solution predicts that the Cs content of K-feldspar falls to nil, which implies that rare-alkali enriched (up to 2190 ppm Cs) pocket K-feldspar must have crystallized from a fluid-undersaturated pegmatite melt. However, most petrologists contend that miarolitic cavities develop after exsolution of an aqueous phase from a pegmatite melt. To investigate the process responsible for the high uptake of Cs and Rb in pocket K-feldspar we determined the rare-alkali content of synthetic K-feldspars that crystallized at 500 °C from a supercritical aqueous fluid in a granitic melt + fluid system. The K/Cs ratio of the synthetic K-feldspar was compared to modeled K/Cs ratios for K-feldspars formed from a water-saturated melt in which the initial Cs concentration (Co) of the melt was identical to the starting glass used in experiments. Our results show that the K/Cs ratios of synthetic K-feldspar are orders of magnitude lower than that predicted using the Rayleigh fractionation model. We attribute the high uptake of Rb and Cs in K-feldspar to kinetic effects associated with rapid crystal growth in an undercooled water-saturated melt. Therefore, we propose that Rb- and Cs-rich K-feldspars that line the pockets of natural miarolitic pegmatites are the products rapid growth in an aqueous fluid that coexists with a highly fractionated residual melt.