Mona-Liza C. Sirbescu

Lithium isotopic systematics of granites and pegmatites from the Black Hills, South Dakota

Abstract To study Li isotopic fractionation during granite differentiation and late-stage pegmatite evolution, Li isotopic compositions and concentrations have been measured for the S-type Harney Peak Granite, the spatially associated Tin Mountain pegmatite, and possible metasedimentary source rocks in the Black Hills, South Dakota. The Harney Peak Granite is isotopically heterogeneous, with δ7Li varying from -3.1 to +6.6. The δ7Li values of Proterozoic metasedimentary rocks that are possible sources of the Harney Peak Granite range from -3.1 to +2.5 and overlap with post- Archean shales and the Harney Peak Granite. For the granite suite, there is no correlation between δ7Li and elements indicative of degrees of granite differentiation (SiO2, Li, Rb, etc.). The Li isotopic composition of the Harney Peak Granite, therefore, appears to reflect the source composition. Minerals from the zoned Tin Mountain pegmatite have extremely high Li contents and heavier Li isotopic compositions than the granite or surrounding Black Hills metasedimentary rocks. The heavier compositions may reflect Li isotopic fractionation resulting from extensive crystal-melt fractionation. Lithium concentrations decrease in the order: spodumene (~3.7 wt%), muscovite (0.2 to 2.0 wt%), plagioclase (100.1100 ppm), quartz (30.140 ppm). Plagioclase, muscovite, and spodumene in all zones display a relatively narrow range in δ7Li of +7.9 to +11.4. In contrast, quartz is isotopically heavier and more variable (+14.7 to +21.3), with δ7Li showing an inverse correlation with Li concentration. This correlation reflects the mixing of isotopically heavy Li in quartz and lighter Li in fluid inclusions, as documented by fluid inclusion compositions (δ7Li = +8.1 to +13.4 and Li of 280 to 3960 ppm). Extrapolation of this trend to an estimated intrinsic Li concentration in quartz of <30 ppm, yields an inferred δ7Li for fluid inclusion-free quartz of >+21. The large difference in δ7Li between quartz and other minerals may reflect 7Li preference for less highly coordinated sites, which have higher bond-energies (i.e., the two- or fourfold site in quartz vs. higher coordination number sites in other minerals). Comparison of the Li isotopic composition of fluid inclusions with that of the wall zone of the Tin Mountain pegmatite suggests ~4‰ isotopic fractionation during fluid exsolution, which agrees with the results derived from studies of hydrothermal alteration of basalts.

Lithium and its isotopes in tourmaline as indicators of the crystallization process in the San Diego County pegmatites, California, USA

In the lithium-cesium-tantalum-type pegmatite dikes of San Diego County, California, USA, tourmaline is the main reservoir for Li, except in the cores and the pockets of the dikes where other Li-bearing minerals also occur. Tourmaline from three subhorizontal dikes was analyzed for bulk Li concentrations and Li isotope ratios. The bottom portion of each dike includes rhythmically layered aplite called line-rock. Above the aplite is the lower pegmatite zone that crystallized upward whereas the hanging pegmatite zone crystallized downward. The lower and hanging pegmatite zones are joined at the core zone. Pockets that were once fluid-filled occur in the core zone. Tourmaline in the line-rocks and the upper border zones has 22–70 ppm Li and in the pegmatite zones 53–450 ppm Li. Large tourmaline blades in the cores have 174–663 ppm Li. Elbaite rims on prismatic tourmaline in the pockets have up to 5075 ppm Li. The progressive enrichment in Li from the wall-zones to the pockets is attributed to inward fractional crystallization of the dikes. The line-rock in each dike appears to have crystallized until the melt reached fluid saturation, at which point the melt and the fluid began to unmix to form the pegmatite zones and the pockets. The estimated initial Li concentration in the magma that produced the dikes is ~ 630 ppm. At this low concentration, Li has had much smaller effect on crystallization of the dikes than H 2 O. δ 7 Li in tourmaline in the line-rocks, the cores, and the pockets ranges from +11.2 to +16.1 ‰ with no systematic difference between these textural zones. However, in radial tourmalines δ 7 Li is > 19 ‰. The very elevated δ 7 Li may reflect Li isotope fractionation between the melt and the exsolving fluid at the time of crystallization of these tourmalines, with 7 Li preferring the more strongly-bonded occupancy in the silicate melt over a hydrated ion occupancy in the fluid. Alternatively, the elevated δ 7 Li may also have been caused by preferential accumulation of the slower-diffusing 7 Li ahead of the rapidly-growing radial tourmalines. The overall elevated δ 7 Li values of the dikes may have been acquired by Li isotope exchange with wall-rocks during passage of the pegmatite melts from their sources.

Dawsonite: An inclusion mineral in quartz from the Tin Mountain pegmatite, Black Hills, South Dakota

Abstract Dawsonite - NaAl(CO3)(OH)2- was identified in primary fluid inclusions in quartz from the Li-rich Tin Mountain pegmatite, Black Hills, South Dakota, by petrography, SEM-EDS analysis, and Raman spectroscopy. This is the first report of dawsonite as an inclusion mineral in a pegmatite. The presence of dawsonite in the inclusions is evidence for the existence of carbonate ions in the complex pegmatite melt and/or exsolved magmatic fluid. The lack of dawsonite as a macroscopic mineral is attributed to its high solubility in the late pegmatite fluids and to the small fraction of carbonate ions in the melt. However, its common occurrence, along with other carbonate and borate minerals in fluid inclusions, suggests that carbonate and borate complexes play an important role in petrogenesis of pegmatites.

Experimental Crystallization of Undercooled Felsic Liquids: Generation of Pegmatitic Texture

The crystallization kinetics of silicate liquids were studied experimentally in the system haplogranite–B–Li–H2O, at variable degrees of undercooling and variable water concentration. We investigated the kinetics of nucleation and crystallization of unseeded synthetic hydrous haplogranite with 1 wt % Li2Oþ2 3 wt % B2O3 added (composition C1) and 2 wt % Li2Oþ 4 6 wt % B2O3 added (composition C2). Compositions C1 and C2 are simplified representative bulk compositions of Lirich pegmatites and their highly differentiated cores, respectively. Starting water contents varied between 3 and 9 wt %. With few exceptions, the system remained water-undersaturated. About 86 isothermal runs of 1–60 days duration, grouped in 25 time series of constant temperature and initial H2O content, were carried out at temperatures from 400 to 700 C at 300 MPa, corresponding to variable degrees of undercooling between the liquidus and glass transition. Viscosity measurements indicate that the glass transition for both compositions is below 400 C for 3 wt % water and below 300 C for 6 5 wt % water. The melts remained virtually crystal free at 400 C, about 100 C and 120 C above the glass transition for compositions C1 and C2, respectively, in experiments up to 30 days long. This result is consistent with the existence of low-temperature, undercooled melts in the crust. At lower values of undercooling the runs crystallized partially, up to about 70% volume fraction. Undercooling and the amount of water are the main factors controlling nucleation and growth rates, and therefore textures. Minerals nucleate and grow sequentially according to mineralspecific nucleation delays. The mineral assemblage started with Li–Al stuffed quartz (in C1) and virgilite (in C2), solid-solutions between quartz and c-spodumene. The quartz-like phases were typically followed by spherulitic alkali feldspar–quartz intergrowths, euhedral petalite, and fine-grained muscovite. Nearly pure quartz formed as rims and replacement of metastable virgilite and stuffed quartz, in particular at the boronand water-rich crystallization front of large feldspar or petalite. With the exception of muscovite, all minerals nucleated heterogeneously, on the capsule wall or on pre-existing minerals, and grew inwards, towards the capsule center. Experimental textures resembled the textures of zoned pegmatites, including skeletal, graphic, unidirectional, radiating, spherulitic, massive, and replacement textures. In some cases, when fluid saturation was reached, miarolitic cavities developed containing euhedral crystals. Although unidirectional growth rates appeared to slow down in time, volumetric rates for stable graphic alkali-feldspar quartz intergrowths and petalite remained constant for up to 60 days and 70% crystallization. Metastable stuffed quartz and virgilite diminished in their growth rates in runs of 30 days or longer, were resorbed in the melt, and were partially replaced by second-generation quartz. Unobstructed, self-sustained crystal growth in conditions of very low nucleation density appears to be the dominant mechanism to form giant pegmatitic crystals, although experimental growth rates are much slower than predicted in nature based on conductive-cooling models. VC The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com 539 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2017, Vol. 58, No. 3, 539–568 doi: 10.1093/petrology/egx027 Advance Access Publication Date: 12 June 2017