Subcritical Phase Separation and Occurrence of Deep-Seated Brines at the NW Caldera Vent Field, Brothers Volcano: Evidence from Fluid Inclusions in Hydrothermal Precipitates

Table 4

Characteristics of coexisting hydrothermal end-member vapor-brine pairs used in our best-fit isenthalpic mixing model. Temperature, pressure, and salinities (, in wt.% NaCl eq.) were derived from the L+V surface of the H₂O-NaCl system [57]. and (mass fractions in the corresponding vapor and brine pairs) were calculated by the lever rule and by assuming a source fluid with seawater salinity. The Sr concentrations (mmol kg^-1) of the vapor-brine pairs were calculated by assuming Sr partitioning behavior identical to that of salt and using a seawater Sr concentration of 0.09 mmol kg^-1; the ⁸⁷Sr/⁸⁶Sr value of 0.7044 () for the hydrothermal end-member was derived from 10 hydrothermal fluid samples by back calculating to [47].

Mixing model	(°C)	bar	^87Sr/^86SrEM	(wt.%)	(wt.% NaCl eq.)	SrEM,Vapor (mmol kg^-1)	(wt.%)	(wt.% NaCl eq.)	SrEM,Brine (wt.%)
1	354	170	0.70440	31	0.009	0.00025	69	5.25	0.1477
2	425	340	0.70440	77	1.55	0.0436	23	8.65	0.2432