The Permian (293 ± 2 Ma) Sebago Pluton is a homogeneous, two-mica granite situated in the Oxford pegmatite field, southwestern Maine. Surrounding the pluton is an area designated as the Sebago Migmatite Domain (SMD) dominated by metapelitic migmatites and diatexites with subordinate intrusions of heterogeneous, two-mica and biotite granites, pegmatitic leucogranites and granitic pegmatites. The Sebago Pluton plus the SMD formerly defined the extent of the Sebago Batholith. Most of the granitic pegmatites and bodies of pegmatitic leucogranites occur either within, or barely outside, the margins of the SMD. The pegmatitic leucogranite facies displays units typical of fertile granites (granites that produce granitic pegmatites) and include: megacrystic graphic K-feldspar, sodic aplite and potassic pegmatite pods hosting rare tourmaline, beryl and columbite-tantalite. Over 100 granitic pegmatite bodies (Sebago Pegmatite Group -SPG) intrude the outer portions of the SMD and neighboring granitoids and metasedimentary rocks. The pegmatite population includes mineralogically simple to complexly zoned pegmatites that are characterized by a LCT-type (Li, Cs and Ta) geochemical signature, extensive replacement of primary zones and gem-bearing miarolitic cavities. Evidence that supports the pegmatitic leucogranites as the likely parent to the SPG includes the close spatial distribution of the pegmatites to the leucogranite bodies, texturally and mineralogically similar units observed within the leucogranite and the neighboring pegmatites plus gradual, yet overlapping, rare-element fractionation from the leucogranites to the associated pegmatites. A few pegmatites (e.g., the Lord Hill pegmatite and amethyst-bearing pegmatites) show NYF tendencies unlike any other pegmatites of the SPG. Differences in fractionation degree, evolution and/or relation to another fertile granite-pegmatite system may be responsible for this apparently anomalous group of pegmatite dikes.

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The crystallization history of the Black Mountain granitic pegmatite, near Rumford, western Maine, is evaluated using compositions of rock-forming and accessory minerals. The pegmatite is strongly zoned; the zonation developed from the consolidation of a rare-element-enriched melt. Field and trace-element data show that the wall zone crystallized first from an initially Benriched melt. Subsequent crystallization of three intermediate zones reflects changes in melt composition, which include a general decrease in levels of Mg, Ca, and Fe, and increases in levels of Be, Nb, Ta, Sn, and P. Significant enrichment in Li, Rb, Cs and F occurred during the latest stage of pegmatite crystallization prior to core development. Late albite-dominant units enriched in B, Nb>Ta, Sn and Zr replace primary zones; their chemical trends deviate from normal trends of fractionation (e.g., late enrichment in Fe). SOMMAIRE La cristallisation progressive de la pegmatite granitique de Black Mountain, près de Rumford, dans l'ouest du Maine, fait l'objet d'une évaluation fondée sur la composition des minéraux principaux et accessoires. Le massif de pegmatite est fortement zoné; cette zonation s'est développée par la consolidation d'un magma enrichi en éléments rares. Les relations de terrain et les concentrations des éléments traces montrent que c'est la zone externe, près de la paroi, qui a cristallisé d'abord à partir d'un bain fondu enrichi en bore. Par la suite, la cristallisation de trois zones intermédiaires témoigne de changements dans la composition du liquide, par exemple une diminution générale en concentration de Mg, Ca et Fe, et une augmentation en concentration de Be, Nb, Ta, Sn et P. Un enrichissement important en Li, Rb, Cs et F marque le stade final de cristallisation magmatique avant la formation du noyau de quartz. Des unités tardives à dominance d'albite enrichies en B, Nb>Ta, Sn et Zr remplacent ensuite les zones primaires; leurs tendances chimiques dévient du tracé évolutif normal d'un fractionnement magmatique, montrant par exemple un enrichissement tardif en fer.(Traduit par la Rédaction)

In 1980, the compositions of a set of microbeam reference materials were published [1] and made available to the world. These mostly mineral and natural glasses were characterized by classical wetchemical analysis by staff of the Department of Mineral Sciences of the Smithsonian Institution (SI) and named the Smithsonian Microbeam Standards (SMS). Additional carbonates, synthetic rare-earth element phosphates and trace element doped synthetic glasses were added to the collection bringing the total number of currently available samples to 57. To date more than 1300 requests have been filled with over 20,000 individual samples distributed to laboratories worldwide free of charge. Whereas a few of these are no longer available because of very limited quantity, most have quantities sufficient for distribution for many decades at the current demand in aliquots of sufficient size for electron beam microanalysis reference mounts. However, in the last ten years there has been about a threefold increase in the number of SMS distributed (see figure 1). The reason for this increase is not clear. Many requests are for labs with new instrumentation including more frequently for quantification using energy dispersive x-ray spectroscopy. More requests are also coming from laser ablation labs for which only some of the SMS are in sufficient quantity to be appropriate. Laboratories with heavily used mounts in need of replacement are encouraged to request replacement samples. The Corning glass archaeological reference samples, doped with selected trace elements, exist in reasonably large enough quantities to be suitable for destructive analytical methods.