The genesis of LCT-type granitic pegmatites, as illustrated by lithium isotopes in micas

Abstract

International audienceIsotopic compositions in the Monts d’Ambazac Pegmatite Field (French Massif Central) exhibit a narrow range of mica δ7Li values, ranging from -3.6 to + 3.4‰. The value obtained in biotite from the host Saint Sylvestre granite falls within this range (δ7Li = -1.5‰). Lithium concentrations are consistent with the degree of magmatic evolution of each pegmatite type: from 630 ppm in Type II up to 13,500 ppm in the more evolved Type VI pegmatite. Although the rare-element contents e.g., Li, Cs, Ta of the micas are consistent with pegmatite differentiation, δ7Li (‰) are firstly, independent of the degree of magmatic differentiation (independent of pegmatite type) and secondly, independent of the content of Li and other flux-elements such as Be and Cs. Muscovite sampled in pegmatite V from the Chabannes locality is the only pegmatite to exhibit a δ7Li variation from intermediate unit (-1.7‰) to internal pegmatitic unit (+ 3.4‰). The nature of this δ7Li variation suggests that there was extensive fractional crystallisation during the pegmatite’s consolidation. The independence of δ7Li (‰) evolution from the degree of magmatic evolution and the presence of distinct major rare-element bearing phases throughout the pegmatite field tend to confirm that the δ7Li (‰) values recorded in mica are inherited from crustal source rocks common to the granite and pegmatite-forming melts. We propose that the distinct pegmatite subtypes (beryl columbite vs lepidolite-petalite subtypes) observed throughout the Monts d’Ambazac Pegmatite Field reflect the diverse contributions of crustal protoliths. The lack of evidence of surrounding alteration combined with the absence of increasing Li-content within the host granite tend to confirm that the δ7Li values obtained within this pegmatite field are primary, and that no Li-diffusional process and/or mixing-driven Li-isotope fractionation has overprinted these isotopic compositions. In light of these results, the process of partial melting of protoliths enriched in rare-element bearing phases, e.g., mica, garnet, seems to be more responsible for Li-isotope fractionation than Li-diffusion or fractional crystallisation at the temperature of pegmatite consolidation. Finally, we discuss the use of Li isotopic compositions to identify the most highly evolved pegmatitic systems

Handheld LIBS for Li Exploration: An Example from the Carolina Tin-Spodumene Belt, USA

“…Our new data both demonstrate the reliability of such calibrations and document the importance of having matrix matching when using a calibration. Finally, [99]; Central Galicia field, Spain [100]; Fregenda area, Spain [101]; Kenticha pegmatite, Ethiopia [102]; Monts d' Ambazac field, France [103]; Moose pegmatite, Northwest Territories, Canada [104]; Tanco pegmatite, Manitoba, Canada [105]; and Dike 1, Wekusko Lake field, Manitoba, Canada [106].…”

“…Figure 14. Comparison of muscovite K/Rb-Li (ppm) systematics for the CLP prospect with muscovite K/Rb-Li relationships for other Li-pegmatites worldwide: Bailongshan field, China[99]; Central Galicia field, Spain[100]; Fregenda area, Spain[101]; Kenticha pegmatite, Ethiopia[102]; Monts d' Ambazac field, France[103]; Moose pegmatite, Northwest Territories, Canada[104]; Tanco pegmatite, Manitoba, Canada[105]; and Dike 1, Wekusko Lake field, Manitoba, Canada[106].…”

Lithium isotopes in fluid inclusions as tracers of crustal fluids: An exploratory study

“…Potential areas where the determination of Li isotopes could be used as tracers of crustal fluids include: (i) subduction zones, where Li is an indicator of slab dehydration and metasomatic processes (Chan and & Kastner, 2000;Penniston-Dorland et al, 2012); (ii) seafloor hydrothermal systems where Li is an indicator of seawater / oceanic crust interaction (Foustoukos et al, 2004;Gao et al, 2012) and (iii) ore-forming magmatic-hydrothermal systems where Li is an indicator of magmatic and metasomatic processes but also a metal of economic interest (Deveaud et al, 2015). Finally, the analysis of fluid inclusions provides direct access to the Li isotopic composition of deep crustal fluids that are not obtainable by direct sampling, and can provide valuable information that may help understanding Li transfer between the hydrosphere, the crust and the mantle, via fluids and magmas.…”

Crystallographic control on lithium isotope fractionation in Archean to Cenozoic lithium-cesium-tantalum pegmatites

“…This difference between pegmatites associated with predecessors (and parts of) of the Laurentian craton (Tanco, Black Hills) versus pegmatites from other cratonic settings (Bikita, Greenbushes, Londonderry) apparently precludes directly linking the Li isotope compositions of contemporaneous deposits with a single tectonic event or a uniform source (Deveaud et al, 2015). The observed variation in δ 7 Li values is also in stark contrast with a largely restricted δ 7 Li range in pristine mantle xenoliths (see recent summary in Tomascak et al, 2016).…”

“…Considering that evolved LCT pegmatites may crystallize Li-bearing phases whose Li coordination varies, these rocks are a natural target to study the impact of such changes on Li isotope fractionation. The large variations in δ 7 Li (per mil difference to L-SVEC reference material; see Methods and results) of pegmatites have variously been ascribed to: (i) extensive crystal-melt (magmatic) fractionation (Barnes et al, 2012;Teng et al, 2006), (ii) fractionation between the melt and exsolving fluid (Maloney et al, 2008), (iii) differences in crustal sources (Deveaud et al, 2015), or (iv) the 7 Li affinity to occupy stronger (i.e., shorter) Li-O bonds (Wunder et al, 2011). Because the coordination of Li in the crystal structures of pegmatite minerals varies and Li-φ bond distances (φ = O, OH, F) -expressed as weighted mean distance (parameters developed for quantitative characterization of deformed polyhedra by Nespolo et al, 2001; see Supplementary information for further details) -may differ by >0.5Å (Å = 10 -10 m), pegmatites are an ideal target to explore the last hypothesis.…”