White Paper Title: 
Volatiles in hydrothermal fluids help constrain phase separation and water rock reactions

B. Love (Western Washington University), M. Lilley (University of Washington) 

Volatiles in hydrothermal fluids are important drivers of the location and diversity of biological communities.  They are indicators of magmatic linkages, as well as tracers of phase separation and water rock reactions.  When these processes are all considered, it becomes clear that straightforward explanations of fluid chemistry are limited, and multiple factors must usually be considered.  The limited coverage of fluid samples in space and time further complicate this task.  These factors make integration of additional sources of information necessary for the best interpretation of these data.  Synthesis of numerical and geochemical models, sensor and geophysical data, broadens and deepens the interpretation of the fluid data.   

A series of samples which were collected at Sully in the Main Endeavour Field over the course of several days provide one example of how volatile data support modeling effort.  This suite of volatile data can help constrain the possible phase separation scenarios active at the time the samples were collected.  These samples show that fluid composition on a single day had a markedly different composition (more brine-like) than the fluid samples collected before and after, which are more vapor dominated.  If the two fluids are assumed to be closely related by mixing or phase separation, the volatile concentrations and chloride content rule out mixing scenarios and simple changes in the conditions of phase separation.  The data indicate that the fluids were probably produced by a two step process: brine condensation, followed by a near-critical phase separation event.  This interpretation meshes well with recent efforts by Coumou et al. which describe this type of phase separation as an unstable mode predicted by numerical modeling.  This analysis is only possible because of the high frequency with which samples were collected, which provides additional motivation for the development of more and better in-situ sensors. 

There are several other areas where volatile data can be enhanced through integration efforts. For instance, hydrogen and hydrogen sulfide data from MEF can be helpful in discerning details of the water rock reactions in the subsurface.  These field data complement modeling work especially well.  Modeling results are needed to make meaningful interpretations of the field data, and the field data are needed to constrain the models.  We hope to develop collaborations in this area.  In addition, the MEF volatile data, including isotopic values for selected samples, can further address questions such as magmatic involvement in the activity between 2000 and 2005, which has been approached through geophysical methods by Hooft et al.  Some of these efforts will build on work that is already published, but some can benefit from collaboration during the analysis stage as well.