INSIGHTS ON MAGMATIC FLUID EXSOLUTION AND EVOLUTION FROM CHLORINE THE UNDER-APPRECIATED MAGMATIC VOLATILE COMPONENT (Invited Presentation)

Melt inclusions are essential to investigations on magmatic volatile concentrations and processes of fluid exsolution and movement, fluid-melt-mineral reaction, fluid-driven eruption, and magmatic-hydrothermal mineralization. Interpretation of melt inclusion data and their application to such processes requires accurate constraints on volatile solubilities in melts as a function of composition, pressure, and temperature. Water and CO2 are the dominant magmatic volatiles, but H2O may be lost from inclusions by diffusion through host phenocrysts (magmatic abundances may not be maintained), both volatiles may be sequestered by melt inclusion bubbles, and H and C analyses of smaller melt inclusions may require SIMS analyses which tend to restrict the number of inclusions actually analyzed. In contrast, Cl is readily analyzed by EPMA, even in smaller melt inclusions, and there is little evidence that it diffuses through host phenocrysts. Currently however, we have poor constraints on potential Cl loss to inclusion bubbles. Recent experiments and empirical modeling support calculation of Cl solubilities in melts and Cl partitioning between COHSCl-bearing fluids and melts. Interestingly, Cl dissolution in melts varies strongly with system composition, so this volatile provides unique constraints on fluid behavior during progressive magma evolution. By normalizing measured Cl contents of melt inclusions to their respective Cl solubility values in melts at fixed pressure and temperature and comparing these ratios to indices of melt differentiation one can track the evolutionary stage of initial fluid exsolution and the Cl contents of fluids. For this study, we have computed Cl solubility values in melts represented by more than 3400 felsic to dacitic melt inclusions and normalized these values to measured Cl contents as a function of melt differentiation. We determine that many rhyolitic inclusions represent fluid(s)-saturated magmas and that the average Cl content of the initial fluid (with 0.2 wt% fluid content) computed for all melt inclusions is 6 wt%. Also, more than 900 of the melt inclusions are consistent with higher modeled Cl contents achieving as much as 70 wt% in the initial fluids. In summary, (measured Cl/Cl solubility) is a useful geochemical tool complementary to CO2-H2O in melt inclusions.