Quartz Vein Geochemistry Records Deformation Processes in Convergent Zones

In several examples of subduction zones, we compared pairs of quartz veins formed either at the lower temperatures of the seismogenic zone (260 degrees C or below), or at the higher temperatures of its downdip limit (similar to 330 degrees 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.

Automated summary

The study compared quartz veins formed at different temperatures in subduction zones, revealing distinct differences in the characteristics such as quartz crystal shapes, luminescence, and trace element concentrations. These differences may reflect the influence of fluid pressure variations on crystal growth rates.