Isotopic evidence for the infiltration of mantle and metamorphic CO2–H2O fluids from below in faulted rocks from the San Andreas Fault system

Abstract

To characterize the origin of the fluids involved in the San Andreas Fault (SAF) system, we carried out an isotope study of exhumed faulted rocks from deformation zones, vein fillings and their hosts and the fluid inclusions associated with these materials. Samples were collected from segments along the SAF system selected to provide a depth profile from upper to lower crust. In all, 75 samples from various structures and lithologies from 13 localities were analyzed for noble gas, carbon, and oxygen isotope compositions. Fluid inclusions exhibit helium isotope ratios ( 3 He/ 4 He) of 0.1-2.5 times the ratio in air, indicating that past fluids percolating through the SAF system contained mantle helium contributions of at least 35%, similar to what has been measured in present-day ground waters associated with the fault . Calcite is the predominant vein mineral and is a common accessory mineral in deformation zones. A systematic variation of C-and O-isotope compositions of carbonates from veins, deformation zones and their hosts suggests percolation by external fluids of similar compositions and origin with the amount of fluid infiltration increasing from host rocks to vein to deformation zones. The isotopic trend observed for carbonates in veins and deformation zones follows that shown by carbonates in host limestones, marbles, and other host rocks, increasing with increasing contribution of deep metamorphic crustal volatiles. At each crustal level, the composition of the infiltrating fluids is thus buffered by deeper metamorphic sources. A negative correlation between calcite δ 13 C and fluid inclusion 3 He/ 4 He is consistent with a mantle origin for a fraction of the infiltrating CO 2 . Noble gas and stable isotope systematics show consistent evidence for the involvement of mantle-derived fluids combined with infiltration of deep metamorphic H 2 O and CO 2 in faulting, supporting the involvement of deep fluids percolating through and perhaps weakening the fault zone. There is no clear evidence for a significant contribution from meteoric water, except for overprinting related to late weathering.

Orogenic gold: Common or evolving fluid and metal sources through time

“…The latter geometry would lead to a narrow depth range for release of fluids and metals at shallow levels, which were channeled into an eventual frozen and non-convecting mantle wedge that was itself devolatilized during eventual sinking of the slab. This is again a hypothetical model that cannot be proven using any unequivocal analytical data available, although radiogenic isotope, halogen, and noble gas data are taken as indicative of such in many studies (see above) and studies of these components in many crustalscale fault zones, such as the San Andreas fault system (Kennedy and van Soest, 2007;Pili et al, 2011) and Karakorum fault (Klemperer, 2013), indicate that mantle fluids reach the shallow crust. The attraction of such a model as was proposed for the Jiaodong deposits is that orogenic gold deposits inevitably form due to heating of new crust and such crust is always being subducted in the tectonic setting defined for orogenic gold, suggesting a fundamental connection.…”

Mass removal and clay mineral dehydration/rehydration in carbonate‐rich surface exposures of the 2008 Wenchuan Earthquake fault: Geochemical evidence and implications for fault zone evolution and coseismic slip

“…[53] If fluid entering the fault zone is assumed to have had constant isotopic composition [Hausegger et al, 2010;Molli et al, 2010], the relative position of individual data points in the d 13 C-d 18 O diagram (Figure 7) may relate to variations in the amount of fluid interacting with the sample [Kirschner and Kennedy, 2001;Pili et al, 2002Pili et al, , 2011. Large breccia fragments with isotopic values near the protolith interacted with small quantities of fluid.…”

“…Using bulk rock chemical analysis, Goddard and Evans [] concluded that fluids passing through fault zones react with the fault rock, dissolving and transporting soluble cations away. Such an effect can change fault rock volume, insoluble content and isotopic composition, as well as transport and mechanical properties [ Labaume et al ., ; Chen et al ., ; Molli et al ., ; Pili et al ., , ]. Similarly, fluid‐assisted mineralogical changes may cause fault weakening and reactivation [e.g., Chester et al ., ], particularly when clay minerals are formed [e.g., Wintsch et al ., ; Vrolijk and van der Pluijm , ].…”

Deep Carbon Emissions from Volcanoes

“…Separating the relative proportions of mantle and crustal carbon is possible through investigation of the isotopic composition of emitted carbon (e.g., Chiodini et al 2011) and is increasingly important given that during eruptions magmatic intrusions may interact with crustal material, strongly enhancing the CO 2 output of the volcanic system (Troll et al 2012), at least temporarily. The magnitude of diffuse mantle CO 2 can also be identified isotopically in mixed metamorphic and magmatic gases using Carbon (Chiodini et al 2011) or Helium isotopes as a proxy for deep mantle sources in both major fault systems (Pili et al 2011) and crustal tectonic structures (Crossey et al 2009).…”