FERROPARGASITE-PARGASITE JOIN AND ITS EFFECT ON CHLORINE ACCEPTANCE

High-chlorine calcium-amphiboles have been reported in the Marcy anorthosite orthogneisses of the Adirondack Mountains, NY (Morrison, 1991, Am Min), where a Cl-poor (0.05-0.47 wt% Cl) and Cl-rich (1.8-3.0 wt% Cl) series were observed; however, little is known about the fluid/melt from which these amphiboles formed. In this study we experimented with the effects of the molar Fe/(Fe+Mg) ratio (Fe#) on the uptake of chlorine in amphiboles formed along the ferropargasite (NaCa₂(Fe²⁺₄Al)(Si₆Al₂)O₂₂(OH,Cl)₂)-pargasite (NaCa₂(Mg₄Al)(Al₂Si₆)O₂₂(OH,Cl)₂) join in the presence of a 1 molal NaCl brine. This study determined the relationship between Fe# and the amount of chorine absorbed by experimentally synthesizing samples in 0.2 increments of the Fe# along this join.

Samples were synthesized from oxide mixtures in cold seal and gas vessels at conditions ranging from 650-950˚C and 2-4 kbars. Samples were put in Pt or AgPd capsules with 10 wt% brine and treated at f_O2 conditions near the magnetite-wüstite buffer. Amphibole yields were from 65% and up, with small amounts of plagioclase and pyroxene present in all of the samples.

The products were analyzed by X-ray diffractometry (XRD) to determine the type and proportions of major and minor phases in the samples. Our primary tool for analysis was the electron microprobe, which gave us information on specific crystal compositions.

The results indicated that samples synthesized with 1m NaCl reveal Mg-Cl avoidance with Cl contents increasing linearly with the Fe#. The Cl contents reached a maximum of only 0.086 wt% in end-member ferropargasite. Our data follows Morrison’s (1991) Cl-poor amphibole trend suggesting that the Cl-poor series may have formed in the presence of an aqueous brine of moderate salinity. However, our results are at least a factor of 20 less than the Cl-rich series, suggesting that either a higher salinity brine or perhaps a silicate-chloride melt formed these amphiboles. We are currently expanding the study by using a wide range of NaCl brines (0m, 1m, 20m, and 40m) for intermediate samples of 0.4 and 0.6 Fe# to see if these hypersaline conditions will reproduce the Cl-rich amphiboles observed in the Adirondacks.