Can we track magma degassing in the crust?
This is what the authors of this peer-reviewed paper (Dr Isabelle Chambefort; Leader of Geothermal the Next Generation, and Prof John Dilles from Oregon State University) tried to answer.
The composition of New Zealand’s deep magmatic fluids, that drive the Taupō Volcanic Zone geothermal systems, is relatively unknown. The current conceptual model assumes the deep heat source of hydrothermal fluids is a partial molten zone in the mantle and/or shallow intrusions (~4 km) in the crust.
The targeting of deep supercritical or superhot geothermal resources requires knowing:
(i) the chemical composition (i.e. the unique fingerprint) of the primary exsolved fluids, and
(ii) the spatial distribution of deep-seated magma bodies.
In this study, we used rock and clay mineral compositions to track the magmatic fluid signature in geothermal systems using, in particular, its metal content. We used a 600,000 year-old buried intrusion and its hydrothermal halo at Ngatamariki Geothermal Field as a proxy for what a shallow magma in the crust could looks like. We compare its geochemistry to the rocks and clays of Rotokawa and Ohaaki geothermal fields.
We found that the compositional zoning of altered rock and illite within these present-day geothermal systems (Rotokawa, Ohaaki) is profoundly different from those of the old, high-temperature magmatic-hydrothermal altered rocks above the 600,000 year old Ngatamariki intrusive complex. If we examine the hypothesis proposed by Bertrand et al. (2015), that the highly conductive area at the brittle-ductile transition is the result of deeply circulating brine with a high conductivity and a crystalizing magma chlorine-rich fluid, this should be accompanied by transport of metals, which will be deposited in the hydrothermal halo. However, this is not observed in the active TVZ geothermal systems studied here. We conclude that most of the geothermal systems are fed by heat conducted from deeper (>6 km) basaltic and rhyolitic magmas. Metals in fluids in geothermal systems could be sourced from both basement and rare, extremely diluted, magmatic fluids.
More work on the composition of the degassing phase from various magmatic composition is on its way to constrain the contribution from deep basalt or the rhyolitic mush.