Mantellic degassing of helium in an extensional active tectonic setting at the front of a magmatic arc (central Mexico)

The physicochemical and isotopic characteristics of groundwater and dissolved gas of central Mexico provide valuable information about the geologic
and tectonic context of the area. Low–high-enthalpy manifestations (up to 98 °C
in springs and more than 100 °C in geothermal wells) are distributed within the
San Juan del Río, Querétaro, and Celaya hydrologic basins, located at the boundary between the current Mexican magmatic arc and an extensional continental
area with intraplate volcanism called Mesa Central Province. Groundwaters in
the study area represent a mixture between the cold water end-member with a
Ca2+-Mg2+-HCO3- composition and a hydrothermal end-member enriched in Na+ , K+ , SO4 2−, and Cl-. Cold and hot groundwaters δ2
H and δ18O plot along the same evaporation lines and do not exhibit a magmatic input. Dissolved and free gas do not
show a typical volcanic composition signature. He and Ne isotope composition
provide evidence of an important contribution of non-atmospheric noble gases.
Although helium composition mainly has a crustal origin (21–83%), the mantellic
contribution (1–39%) is higher than expected for an area lacking recent volcanism.
A volatile-rich magma aging at depth was discarded as the source of this mantellic helium signature but points out a recent mantellic contribution. Thus, we
propose that mantellic helium comes from the sublithospheric mantle into the
shallow crust through the highly permeable tectonic boundaries between the
geologic provinces, namely the N−S Taxco−San Miguel de Allende and ChapalaTula fault systems. Mantellic helium flow rates through these fault systems were
estimated to have values ranging from 0.1 m/yr to 2.9 m/yr. This He flux range
implies that aside from subduction, mantle volatile degassing enhanced by
crustal fault systems is the main degassing process in the region studied.