Processes controlling lithium isotopic distribution in contact aureoles: A case study of the Florence County pegmatites, Wisconsin

Li isotopes may be useful tracers of fluid flow in a number of geological environments and case studies of contact aureoles have highlighted the very large Li isotopic fractionation that can be generated in these settings. However, the amount of isotopic fractionation and the distance that Li travels into the country rocks vary greatly from place to place. Seeking to identify the parameters that govern Li distribution in contact aureoles, we apply a combination of Li isotope analyses, 1-D diffusion and 2-D advection-diffusion modeling to two country rock profiles adjacent to Li-rich pegmatite dikes from the Florence County pegmatite field, Wisconsin. Although less than ∼3 m thick, the pegmatite sheets have a large impact on the Li budget of the country rocks (amphibolites and schists); Li is enriched in adjacent country rocks by up to a factor of 20 over more distant amphibolites and schists. Li from the pegmatite has traveled more than 50 m into the country rocks, and Li isotopes are systematically fractionated with distance from the contacts (with δ7Li varying from +6 at the contact to-7 at 30 m from the contact in one case). These observations are consistent with diffusive fractionation of Li through an advecting grain-boundary fluid. Both one-dimensional diffusion and two-dimensional advection-diffusion models fail to reproduce the exact Li distribution in the profiles, suggesting that fluid advection, coupled with heterogeneous permeability, plays an important role in determining the final Li distribution within the contact aureoles

The tetrad effect and geochemistry of apatite from the Altay Koktokay No. 3 pegmatite, Xinjiang, China: implications for pegmatite petrogenesis

In order to better constrain the evolution and petrogenesis of pegmatite, geochemical analysis was conducted on a suite of apatite crystals from the Altay Koktokay No. 3 pegmatite, Xinjiang, China and from the granitic and amphibolitic wall rocks. Apatite samples derived from pegmatite zones show convex tetrad effects in their REE patterns, extremely negative Eu anomalies and non-chondritic Y/Ho ratios. In contrast, chondritic Y/Ho ratios and convex tetrad effects are observed in the muscovite granite suggesting that different processes caused non-chondritic Y/Ho ratios and lanthanide tetrad effects. Based on the occurrence of convex tetrad effects in the host rocks and their associated minerals, we propose that the tetrad effects are likely produced from immiscible fluoride and silicate melts. This is in contrast to previous explanations of the tetrad effect; i.e. surface weathering, fractional crystallization and/or fluid-rock interaction. Additionally, we put forward that extreme negative Eu and non-chondritic Y/Ho in apatite are likely caused by the large amount of hydrothermal fluid exsolved from the pegmatite melts. Evolution of melt composition was found to be the primary cause of inter and intra-crystal major and trace element variations in apatite. Mn entering into apatite via substitution of Ca is supported by the positive correlation between CaO and MnO. Different evolution trends in apatite composition imply different crystallization environments between wall rocks and pegmatite zones. Based on the geochemistry of apatite samples, it is likely that there is a genetic relationship between the source of muscovite granite and the source of the pegmatite

Field Observations and Lab Tests of Acid Brines: Implications for Past Deposition, Diagenesis, Erosion, and Life on Mars

Geochemical and mineralogical data obtained from lithified strata of Mars indicate that acid saline waters once existed on and just below its surface [1-5]. This confirms the interpretations of acid waters on Mars by Burns [6,7] based upon spectral signatures suggestive of jarosite and schwertmannite, as well as the theoretical work of Clark [8]. Acid saline environments on Earth are good analogs for martian lithified strata, especially those at Meridiani Planum [9,10]. In addition, acid brines may have been responsible for the formation of channels and some small craters on Mars [11,12]. We propose that acid brines should be considered possible agents of chemical sedimentation, diagenesis, and sediment transportation and erosion on Mars. Terrestrial acid saline lake systems are an uncommon, but natural, type of extreme environment. Here we summarize the characteristics of modern acid saline systems in Chile, Western Australia, and Victoria, Australia, and how they compare to martian strata. We also present both physical sedimentology and waterrock interaction experiments involving acid brines. Knowledge of the geology, geochemistry, and microbiology of terrestrial acid saline systems may lead to recommendations for future investigations of Mars and better understanding of its geologic evolution and search for possible past life