Raimbourg, Hugues, Rajič, Kristijan, Moris‐Muttoni, Benjamin, Famin, Vincent, Palazzin, Giulia, Fisher, Donald, Morell, Kristin, Erdmann, Saskia, Di Carlo, Ida, and Montmartin, Clément
In several examples of subduction zones, we compared pairs of quartz veins formed either at the lower temperatures of the seismogenic zone (260°C or below), or at the higher temperatures of its downdip limit (∼330°C). All the veins analyzed here are mode I cracks that formed contemporaneously with the host‐rock main stage of deformation at peak burial conditions. Lower‐temperature veins show examples of quartz crystals with euhedral shapes and growth rims, while higher‐temperature veins contain crack‐seal microstructures. In the lower‐temperature realm, quartz growth rims have alternatingly either: (1) high cathodoluminescence (CL), CL‐blue color and high concentration in trace elements and fluid inclusions, or (2) low luminescence, CL‐brown color and low concentration in trace elements and fluid inclusions. In contrast, the quartz from higher‐temperature samples is homogeneously low luminescent and CL‐brown, except for very restricted domains of the crack‐seal microstructures where patches of CL‐blue quartz are present. The highly luminescent quartz contains high concentrations of aluminum and lithium, up to 3,000 and 400 ppm, respectively. Variations in Al and Li correlate well, so that Li appears as the main charge‐compensating cation for Al. We propose that the incorporation of Al and Li reflects the amplitude of the fluid pressure variations, which control crystal growth rates. Quartz geochemistry might therefore unravel the contrast between the seismogenic zone, where large fluid pressure variations are present, and its downdip limit, where fluid pressure variations are much more limited in amplitude. Plain Language Summary: We have examined several examples of veins that formed at large depths in collision and subduction zones. These veins formed as a result of the opening of a crack‐shaped cavity and were filled by the precipitation of quartz from dissolved silica. These veins thus have characteristics that reflect the water that was present in the pores and cracks of the rock at depth. The samples we have analyzed come from either the seismogenic zone (for temperatures of the order of ∼260°C or lower), i.e., the depth domain where large earthquakes are generated, or its downdip limit (for temperatures of the order of ∼330°C), i.e., the depth domain below which deformation is principally non seismic. The chemistry of quartz varies strongly between these two depth domains. At temperatures of the order of ∼260°C, the quartz contains either a large concentration in impurities, or is relatively pure. At higher temperatures, the quartz is essentially pure. We interpret the domains of quartz rich in impurities as having formed as a result of earthquakes, by rapid growth in disequilibrium conditions, when the pressure of the fluid dropped strongly and the fluid became suddenly highly oversaturated in dissolved silica, which then precipitated. Key Points: Variations in Al and Li concentration in hydrothermal quartz span more than two orders of magnitude and are up to 3,000 and 400 ppmGrowth rims with large Al and Li concentrations are present only at seismogenic depth and might reflect large fluid pressure dropsCrack‐seal microstructures formed at the downdip limit of the seismogenic zone might be associated with smaller fluid pressure variations [ABSTRACT FROM AUTHOR]