Origin of hydrous fluids at seismogenic depth: Constraints from natural and experimental fault rocks

Abstract

Fluids control the mechanical behavior of fault zones during the seismic cycle. We used geochemical, mineralogical, microstructural, hydrogen isotope compositions and Fourier Transform Infrared (FTIR) investigations to characterize the origin of hydrous fluids involved in ductile and brittle shear zones at the bottom of the seismogenic crust. Natural samples were collected from exhumed mylonitic shear zones and cataclasite–pseudotachylyte bearing faults in the northern Adamello (Italian Southern Alps), which were active at 9–11 km depth. Pseudotachylytes, solidified coseismic friction-induced melts, testify to ancient seismogenic behavior of the faults. Natural pseudotachylytes were compared with artificial pseudotachylytes produced in high velocity friction experiments simulating seismic slip.Mylonites have mineralogical, elemental and hydrogen isotope compositions (−80‰<δD<−78‰) similar to the host tonalite (−77‰<δD<−73‰), within the analytical error of ±5‰. Cataclasites have instead mineralogical (chlorite, epidote, K-feldspar, no biotite), major and trace elements (enrichment in K2O, Ba, Rb; depletion in CaO, Na2O, SiO2) and hydrogen isotope (−69‰<δD<−60‰) compositions suggesting interactions with a crustal metamorphic fluid. Pseudotachylytes are composed of high temperature minerals (plagioclase, biotite, dmisteinbergite, cordierite, and scapolite) and have elemental compositions resulting from mixing of tonalite and cataclasite. Pseudotachylytes have complex microstructures, including: (i) microlitic domains, with well crystallized micrometric biotite, which have hydrogen isotope composition (−81‰<δD<−59‰) similar to cataclasites and tonalite; and (ii) cryptocrystalline domains, with poorly crystallized biotite, which have very high water content, release water upon heating at T>50°C and have low δD value (−93‰). The hydrogen isotope composition of bulk samples is dominated by the composition of cryptocrystalline domains (−103‰<δD<−88‰), where most of the water is hosted. Their hydrogen isotope composition is compatible with adsorption of present day rainfall water (δD=−95‰). Artificial pseudotachylytes have the same hydrogen isotope compositions of the starting tonalite (−76‰<δD<−74‰) or cataclasite (−68‰<δD<−62‰), with a slight decrease of the δD values in some samples (−85‰<δD<−81‰).The first ingression of a crustal metamorphic fluid occurred in cataclastic faults. Natural pseudotachylytes, when not contaminated by present day rainfall water, have a hydrogen isotope composition similar to tonalite and cataclasite, as reproduced in dry high velocity friction experiments. The fluids dissolved in coseismic melts are most likely derived from the breakdown of hydrous minerals of cataclasite and tonalite undergone melting, and we could not identify the infiltration of an external fluid during earthquakes.