ALBERT

Description: The Wentworth Pluton in the Eastern Cobequid Highlands consists principally of metaluminous to peralkaline A-type granite (~362 Ma), a large part of which was remelted by a major gabbro intrusion (~357 Ma). Magmatic minerals like allanite-(Ce), chevkinite-(Ce), zircon, and hingganite-(Y) and post-magmatic mineral phases, such as REE-bearing epidote, samarskite-(Y), aeschynite-(Y), fersmite, thorite-like phases, and hydroxylbastnäsite-(Ce), were identified. The presence of fluorine in the parental magma, indicated by whole-rock geochemical data and the presence of fluorite, increased the solubility of monazite and xenotime and thus facilitated retainment of rare metals in the magmatic system. Fractionation of allanite-(Ce) and chevkinite-(Ce) led to a melt enriched in heavy rare earth elements (HREE), from which hingganite-(Y) crystallized during late magmatic stages. The remelting of the early granite led to fluorine and sulfur release in volatile phases, which circulated with hydrothermal fluids, thus mobilizing the REE and other rare metals. Reduction of fluorine activity during the late to post-solidus crystallization resulted in the precipitation of HREE and rare metals in samarskite-(Y), thereby enriching the residual hydrothermal fluids in light rare earth elements (LREE). Post-magmatic LREE minerals, such as hydroxylbastnäsite-(Ce), either replaced earlier minerals or precipitated from these hydrothermal fluids. Carbonate fluids involved in a late regional hydrothermal circulation event along the Cobequid-Chedabucto fault (320–315 Ma) promoted Ti mobility and the formation of TiO 2 minerals and probably of aeschynite-(Y). This mineralogical diversity, in addition to the complex geological history of the pluton, provides a unique opportunity to correlate the formation of individual rare-metal minerals to different stages of pluton evolution and thus provide an insight to the formation conditions of these minerals.

Description: 〈span〉〈div〉Abstract〈/div〉This study investigates Ti mobility in the presence of halogens, as shown by the hydrothermal alteration of magmatic rutile in syenite. The syenite pegmatite studied intrudes gabbro, is preserved as a tectonic block in a major strike-slip fault zone, and formed in a back-arc environment in which there was widespread A-type granite plutonism. Rutile was studied by SEM and Raman spectroscopy, trace elements were analyzed by LA-ICP-MS, and age was determined by in situ U-Pb analysis. Magmatic rutile in the syenite forms millimetric-scale crystals rimmed by magmatic titanite and magnetite and also occurs as smaller interstitial crystals. Hydrothermal alteration occurred preferentially along crystal margins and fractures by a layer-by-layer dissolution-reprecipitation process resulting in high Zr contents (~5000 ppm) in the rutile, together with enrichment in U and depletion in high field strength elements. The magmatic emplacement age of the syenite waŝ360 Ma (dated rutile G) and no younger than 353.9 ± 5.7 Ma (mean Concordia age of interstitial rutile). The syenite was synchronous with the later phases of regional A-type granite plutonism. Most magmatic rutile has REE patterns either (1) with 1–50 times chondrite enrichment, LREE 〉 HREE and a Eu anomaly, resulting from felsic melt inclusions, or (2) flat patterns with 0.1–10 times chondrite enrichment, present in ilmenite exsolution lamellae or inclusions. Later hydrothermal halogen-rich fluids, derived from dissolution of halite, produced widespread metasomatic scapolite in the syenite. These fluids also leached Ti and other HFSE, together with REE, from large fractured rutile crystals. Such fluids resulted in local dissolution-reprecipitation of Ti and Zr and resetting of the U-Pb system in the altered rutile, at 337.4 ± 3.5 Ma. Normalized REE abundances in the hydrothermal rutile show a U-shaped pattern, with the greatest depletion in the MREE. Variations in dissolution and transport of Zr and Ti by halogen-rich fluids affect the Zr in rutile geothermometer, which yields unrealistic temperatures when applied in this study. More generally, the complexities of rutile chemistry in this hydrothermal setting could be reproduced in deeper subduction settings as a result of variations in halogen content of fluids released by prograde metamorphism.〈/span〉

Description: In the Scotian Basin, offshore eastern Canada, an unusual combination of high heat flow in the Cretaceous and the abundance of halite has resulted in unusual diagenetic minerals such as sphalerite. The Newburn H-23 well is the most distal well in the basin with good core samples and has two previously unknown diagenetic mineral occurrences: fluorine-rich ferroan calcite and diagenetic zircon. This study uses SEM backscattered electron images and EDS analyses, EMP WDS mineral analyses and Raman spectroscopy to determine mineral chemistry and textures to investigate the diagenetic and thermal significance of these minerals. Late diagenetic Fe-calcite contains 1–2.5 wt% fluorine, mostly from adsorption, but rarely as small fluorite crystals. Fluoride is also adsorbed on the surfaces of some framework minerals and chlorite. Fluoride was transported in highly saline formation brines derived from the Argo salt Formation. Zircon grains, 20–40 μm in size, have crystal outlines that are straight adjacent to pores, partially lobate filling porosity, and cross cutting other grains: these may be diagenetic. Some zoned detrital zircon grains show 1–3 μm wide diagenetic outgrowths. Neoformation of diagenetic zircon requires temperatures of 〉250 °C. Transport of zirconium is favored by ligands in low-pH solution, principally fluoride and phosphate anions, with zirconium mobilized during the alteration of metamict detrital zircon under low-grade metamorphic conditions. The presence of diagenetic sphalerite and the documented mid-Cretaceous thermal event in the Scotian Basin indicate conditions that could have been suitable for the formation of diagenetic zircon in this well. Suitable geological conditions for such diagenetic formation of zircon will be found in a subsiding rift basin with early evaporites that are affected by a subsequent phase of volcanism due to new rifting or subduction.

Description: 〈span〉Kimberlites are ultrabasic, Si-undersaturated, low Al, low Na rocks rich in CO〈sub〉2〈/sub〉 and H〈sub〉2〈/sub〉O. The distinctive geochemical character of kimberlite is strongly influenced by the nature of the local underlying lithospheric mantle. Despite this, incompatible trace element ratios and radiogenic isotope characteristics of kimberlites, filtered for the effects of crustal contamination and alteration, closely resemble rocks derived from the deeper, more primitive, convecting mantle. This suggests that the ultimate magma source is sub-lithospheric. Although the composition of primitive kimberlite melt remains unresolved, kimberlites are likely derived from the convecting mantle, with possible source regions ranging from just below the lithosphere, through the transition zone, to the core–mantle boundary.〈/span〉

Description: This study investigates Ti mobility in the presence of halogens, as shown by the hydrothermal alteration of magmatic rutile in syenite. The syenite pegmatite studied intrudes gabbro, is preserved as a tectonic block in a major strike-slip fault zone, and formed in a back-arc environment in which there was widespread A-type granite plutonism. Rutile was studied by SEM and Raman spectroscopy, trace elements were analyzed by LA-ICP-MS, and age was determined by in situ U-Pb analysis. Magmatic rutile in the syenite forms millimetric-scale crystals rimmed by magmatic titanite and magnetite and also occurs as smaller interstitial crystals. Hydrothermal alteration occurred preferentially along crystal margins and fractures by a layer-by-layer dissolution-reprecipitation process resulting in high Zr contents (~5000 ppm) in the rutile, together with enrichment in U and depletion in high field strength elements. The magmatic emplacement age of the syenite was ~360 Ma (dated rutile G) and no younger than 353.9 ± 5.7 Ma (mean Concordia age of interstitial rutile). The syenite was synchronous with the later phases of regional A-type granite plutonism. Most magmatic rutile has REE patterns either (1) with 1-50 times chondrite enrichment, LREE 〉 HREE and a Eu anomaly, resulting from felsic melt inclusions, or (2) flat patterns with 0.1-10 times chondrite enrichment, present in ilmenite exsolution lamellae or inclusions. Later hydrothermal halogen-rich fluids, derived from dissolution of halite, produced widespread metasomatic scapolite in the syenite. These fluids also leached Ti and other HFSE, together with REE, from large fractured rutile crystals. Such fluids resulted in local dissolution-reprecipitation of Ti and Zr and resetting of the U-Pb system in the altered rutile, at 337.4 ± 3.5 Ma. Normalized REE abundances in the hydrothermal rutile show a U-shaped pattern, with the greatest depletion in the MREE. Variations in dissolution and transport of Zr and Ti by halogen-rich fluids affect the Zr in rutile geothermometer, which yields unrealistic temperatures when applied in this study. More generally, the complexities of rutile chemistry in this hydrothermal setting could be reproduced in deeper subduction settings as a result of variations in halogen content of fluids released by prograde metamorphism.