TY - JOUR A1 - Thomas, Rainer A1 - Webster, J. D. A1 - Rhede, Dieter A1 - Seifert, W. A1 - Rickers, Karen A1 - Förster, Hans-Jürgen A1 - Heinrich, Wilhelm A1 - Davidson, P. T1 - The transition from peraluminous to peralkaline granitic melts: Evidence from melt inclusions and accessory minerals JF - Lithos : an international journal of mineralogy, petrology, and geochemistry N2 - Fractional crystallization of peraluminous F- and H(2)O-rich granite magmas progressively enriches the remaining melt with volatiles. We show that, at saturation, the melt may separate into two immiscible conjugate melt fractions, one of the fractions shows increasing peraluminosity and the other increasing peralkalinity. These melt fractions also fractionate the incompatible elements to significantly different degrees. Coexisting melt fractions have differing chemical and physical properties and, due to their high density and viscosity contrasts, they will tend to separate readily from each other. Once separated, each melt fraction evolves independently in response to changing T/P/X conditions and further immiscibility events may occur, each generating its own conjugate pair of melt fractions. The strongly peralkaline melt fractions in particular are very reactive and commonly react until equilibrium is attained. Consequently, the peralkaline melt fraction is commonly preserved only in the isolated melt and mineral inclusions. We demonstrate that the differences between melt fractions that can be seen most clearly in differing melt inclusion compositions are also visible in the composition of the resulting ore-forming and accessory minerals, and are visible on scales from a few micrometers to hundreds of meters. KW - granite melts KW - magma evolution KW - melt inclusions KW - melt-melt immiscibility KW - peraluminosity KW - peralkalinity KW - accessory minerals Y1 - 2006 U6 - https://doi.org/10.1016/j.lithos.2006.03.013 SN - 0024-4937 VL - 91 IS - 1-4 SP - 137 EP - 149 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Thomas, R. A1 - Förster, Hans-Jürgen A1 - Rickers, Karen A1 - Webster, J. D. T1 - Origin and evolution of extremely F-rich hydrous melt fractions and hydrothermal fluids during differentiation of highly evolved tin-granite magmas Y1 - 2004 SN - 0016-7037 ER - TY - JOUR A1 - Webster, J. D. A1 - Thomas, R. A1 - Förster, Hans-Jürgen A1 - Seltmann, R. A1 - Tappen, C. T1 - Geochemical evolution of halogen-enriched granite magmas and mineralizing fluids of the Zinnwald tin-tungsten mining district, Erzgebirge, Germany N2 - We remelted and analyzed crystallized silicate melt inclusions in quartz from a porphyritic albitezinnwaldite microgranite dike to determine the composition of highly evolved, shallowly intruded, Li- and F-rich granitic magma and to investigate the role of crystal fractionation and aqueous fluid exsolution in causing the extreme extent of magma differentiation. This dike is intimately associated with tin- and tungsten-mineralized granites of Zinnwald, Erzgebirge, Germany. Prior research on Zinnwald granite geochemistry was limited by the effects of strong and pervasive greisenization and alkali-feldspar metasomatism of the rocks. These melt inclusions, however, provide important new constraints on magmatic and mineralizing processes in Zinnwald magmas. The mildly peraluminous granitic melt inclusions are strongly depleted in CAFEMIC constituents (e.g., CaO, FeO, MgO, TiO2), highly enriched in lithophile trace elements, and highly but variably enriched in F and Cl. The melt inclusions contain up to several thousand ppm Cl and nearly 3 wt% F, on average; several inclusions contain more than 5 wt% F. The melt inclusions are geochemically similar to the corresponding whole-rock sample, except that the former contain much more F and less CaO, FeO, Zr, Nb, Sr, and Ba. The Sr and Ba abundances are very low implying the melt inclusions represent magma that was more evolved than that represented by the bulk rock. Relationships involving melt constituents reflect increasing lithophile-element and halogen abundances in residual melt with progressive magma differentiation. Modeling demonstrates that differentiation was dominated by crystal fractionation involving quartz and feldspar and significant quantities of topaz and F-rich zinnwaldite. The computed abundances of the latter phases greatly exceed their abundances in the rocks, suggesting that the residual melt was separated physically from phenocrysts during magma movement and evolution. Interactions of aqueous fluids with silicate melt were also critical to magma evolution. To better understand the role of halogen-charged, aqueous fluids in magmatic differentiation and in subsequent mineralization and metasomatism of the Zinnwald granites, Cl-partitioning experiments were conducted with a F-enriched silicate melt and aqueous fluids at 2,000 bar (200 MPa). The results of the experimentally determined partition coefficients for Cl and F, the compositions of fluid inclusions in quartz and other phenocrysts, and associated geochemical modeling point to an important role of magmatic-hydrothermal fluids in influencing magma geochemistry and evolution. The exsolution of halogen-charged fluids from the Li- and F- enriched Zinnwald granitic magma modified the Cl, alkali, and F contents of the residual melt, and may have also sequestered Li, Sri, and W from the melt. Many of these fluids contained strongly elevated F concentrations that were equivalent to or greater than their Cl abundances. The exsolution of F-, Cl-, Li-, +/- W- and Sn-bearing hydrothermal fluids from Zinnwald granite magmas was important in effecting the greisenizing and alkali-feldspathizing metasomatism of the granites and the concomitant mineralization Y1 - 2004 SN - 0026-4598 ER - TY - JOUR A1 - Thomas, R. A1 - Förster, Hans-Jürgen A1 - Rickers, Karen A1 - Webster, J. D. T1 - Formation of extremely F-rich hydrous melt fractions and hydrothermal fluids during differentiation of highly evolved tin-granite magmas : a melt/fluid-inclusion study N2 - Quartz crystals from topaz-zinnwaldite-albite granites from Zinnwald (Erzgebirge, Germany) contain, in addition to primary and secondary fluid inclusions (FIs), abundant crystalline silicate-melt inclusions (MIs) with diameters up to 200 mum. These MIs represent various stages of evolution of a highly evolved melt system that finally gave rise to granite-related Sn-W mineralization. The combination of special experimental techniques with confocal laser Raman- microprobe spectroscopy and EMPA permits precise measurement of elevated contents of H2O, F, and B in re-homogenized MIs. The contents of H2O and F were observed to increase from 3 to 30 and 1.9 to 6.4 wt%, respectively, during magma differentiation. However, there is a second MI group, very rich in H2O, with values up to 55 wt% H2O and an F concentration of approximately 3 wt%. Ongoing enrichment of volatiles H2O, F, B, and Cl and of Cs and Rb can be explained in terms of magma differentiation triggered by fractional crystallization and thus, is suggested to reflect elemental abundances in natural magmas, and not boundary-layer melts. Partitioning between melt and cogenetic fluids has further modified the magmatic concentrations of some elements, particularly Sn. The coexistence of two types of MIs with primary FIs indicates fluid saturation early in the history of magma crystallization, connected with a continuous sequestration of Sn, F, and B. The results of this study provide additional evidence for the extraordinary importance of the interplay of H2O, F, and B in the enrichment of Sn during magma differentiation by decreasing the viscosity of and increasing the diffusivity in the melts as well as by the formation of various stable fluoride complexes in the melt and coexisting fluid Y1 - 2005 SN - 0010-7999 ER -