GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 313-1
Presentation Time: 1:35 PM


JENKINS, David M., Geological Sciences and Environmental Studies, Binghamton University, Binghamton, NY 13902-6000,

Calcium amphiboles with chlorine (Cl) contents of 1.6-1.7 anions per formula unit (apfu) have been the focus of interest for decades owing to the information they can reveal about the fluids that lead to economic deposits, or about the nature of fluids or melts in planetary crusts. In most cases the source of Cl is assumed to be NaCl; however, other salts such as CaCl2, MgCl2, and FeCl2 have been identified as being important constituents of high-temperature hydrothermal or deep-crustal metamorphic fluids. A simple comparison of the efficacy of several different halides in forming Cl-bearing amphibole is presented here.

Synthesis of calcium amphibole from mixtures with the bulk composition of hastingsite (NaCa2(Fe2+4Fe3+)(Al2Si6)O22Cl2 = Hs) was done using starting mixtures with four different chlorides or chloride combinations. These mixtures used CaCl2, 1:0.5 NaCl:CaCl2, 1:0.5 NaCl:FeCl2, or FeCl2 to provide a full 2.0 Cl apfu along with appropriate proportions of Na2CO3, CaCO3, Al2O3, SiO2, Fe, and Fe2O3, the latter two being added after the former reagents were mixed and decarbonated. Care was taken to minimize the highly hygroscopic CaCl2 from exposure to air while weighing, and each mixture was dried at 160-200°C prior to being sealed (dry) in AgPd50-capsules. Treatment was made at 700°C and 0.43 GPa for 168 h in internally-heated gas vessels using an Ar+H2 mixture as both the pressure medium and to provide a H2 fugacity of 125 bars (= -0.5 log(fO2) below Co-CoO). The products consisted of amphibole along with hedenbergite, plagioclase, fayalite, ± garnet, and a salt. Analysis by electron microprobe showed that each starting mixture produced amphiboles with nearly the same Cl content (average of 0.76 ± 0.12 Cl apfu), with the remaining Cl residing in the coexisting salt. Slightly, but not significantly, higher Cl (0.89 ± 0.14) resulted when only CaCl2 was the source, and slightly lower (0.61 ± 0.16) when NaCl+CaCl2 was used.

These results suggest the type of chloride is less important than the total Cl concentration (activity) in the starting mixture. It should be emphasized that these experiments were carried out as dry syntheses and it is not yet known if these results apply to aqueous solutions. These experiments do, however, provide a valuable reference for understanding the role of concentrated brines.