Crustal fluids cause strong Lu-Hf fractionation and Hf-Nd-Li isotopic provinciality in the mantle of continental subduction zones

Metasomatized mantle wedge peridotites exhumed within high-pressure terranes of continental collision zones provide unique insights into crust-mantle interaction and attendant mass transfer, which are critical to our understanding of terrestrial element cycles. Such peridotites occur in high-grade gneisses of the Ulten Zone in the European Alps and record metasomatism by crustal fluids at 330 Ma and high-pressure conditions (2.0 GPa, 850 °C) that caused a transition from coarse-grained, garnet-bearing to fine-grained, amphibole-rich rocks. We explored the effects of crustal fluids on canonically robust Lu-Hf peridotite isotope signatures in comparison with fluid-sensitive trace elements and Nd-Li isotopes. Notably, we found that a Lu-Hf pseudo-isochron is created by a decrease in bulk-rock 176Lu/177Hf from coarse- to fine-grained peridotite that is demonstrably caused by heavy rare earth element (HREE) loss during fluid-assisted, garnet-consuming, amphibole-forming reactions accompanied by enrichment in fluid-mobile elements and the addition of unradiogenic Nd. Despite close spatial relationships, some peridotite lenses record more intense fluid activity that causes complete garnet breakdown and high field strength element (HFSE) addition along with the addition of crust-derived unradiogenic Hf, as well as distinct chromatographic light REE (LREE) fractionation. We suggest that the observed geochemical and isotopic provinciality between peridotite lenses reflects different positions relative to the crustal fluid source at depth. This interpretation is supported by Li isotopes: inferred proximal peridotites show light δ7Li due to strong kinetic Li isotope fractionation (–4.7–2.0‰) that accompanies Li enrichment, whereas distal peridotites show Li contents and δ7Li similar to those of the depleted mantle (1.0–7.2‰). Thus, Earth’s mantle can acquire significant Hf-Nd-Li-isotopic heterogeneity during locally variable ingress of crustal fluids in continental subduction zones.

Deep Crustal Fluid Upwelling through Structural Pathways Observed from Non-active Volcanic Regions, Western Kumamoto, Japan

Natural springs containing volcanic and magmatic components occur along major volcano-seismotectonic regions over the worlds. However, features of the deep-originated waters were less documented from regions where active volcanic and magmatic activities are not distributed. To characterize the presence of deep fluids of non-volcanic origin 28 groundwater samples (~ 1,230 m deep) were collected from hot spring sites located at western coast of Kumamoto where the typical subduction related magmatisms are absent. The samples were measured for dissolved ion concentrations and stable isotope ratios (δ2HH2O, δ18OH2O, δ13CDIC and δ34SSO4) that were compared with data of 33 water samples from vicinity surface systems. The groundwaters were classified into three types based on major hydrochemistry: high Cl− fluid, low concentration fluid, and high HCO3− fluid. Our dataset suggests that the high Cl− fluid was formed by saline water mixing with aquifer waters of meteoric origin and subsequently evolved by reverse cation exchange. The low concentration fluid is identical to regional aquifer water of meteoric origin that was subjected to cation exchange. The high HCO3− fluid showed the highest HCO3− concentrations (~ 3,888 mg/l) with the highest δ13CDIC (-1.9‰). Taking recent geophysical mappings under the study area, we suggest that dissolved carbon was of mantle origin and fluids with high HCO3− generated in lower crust were transported towards surface through structural weakness under open tectonic setting. Observed δ2HH2O and δ18OH2O shifts support this scenario. The occurrence of deep crustal fluid discharges was sporadic and limited in surface in the study area. Their impacts on surface hydrological systems were minimal except few locations.

Crustal fluid pressure gradients and permeability evolutions estimated from metamorphic fluid-rock reaction zones (Sør Rondane Mountains, East Antarctica)

<p>Fluid activity in the crust is a key process controlling the generations of earthquakes, magmas, ore deposition formation and deep geothermal activities. Although high pore fluid pressure has been recognized by geophysical observations and geological observations of mineral filled fractures, the actual fluid pressure, their durations and associated permeability are controversial and remain largely unknown. Here we propose a new methodology estimating the duration, fluid pressure gradients and permeability recorded in fluid-rock reaction zones, by utilizing thermodynamic analyses in conjunction with halogen (Cl, F) profiles along the reaction zones.</p><p>We have analyzed exceptionally well-exposed crustal fluid–rock reaction zones at Sør Rodane mountains, East Antarctica. The thermodynamic analyses of granitic dike–granulite-facies crust reaction zone at 0.5 GPa, 700°C (Uno et al., 2017) and amphibolite-facies hydration reaction zones around mineral-filled fractures at ~0.3 GPa, 450°C (Mindaleva et al., 2020) reveals extremely high fluid pressure gradients of ~100 MPa/10cm or ~1 MPa/mm across the reaction zones. The reactive transport analysis suggest that fluid activity lasted for 100–250 days and ~10 hours, respectively. These extremely high fluid pressure gradients represent the low permeability of the intact amphibolite and granulite host rocks without fractures. The estimated permeabilities of the host rocks are 10<sup>−20</sup>–10<sup>−22</sup> m<sup>2</sup>, and are several orders smaller than the widely accepted crustal permeability model (~10<sup>−18</sup> m<sup>2</sup>; e.g., Ingebritsen and Manning, 2010). On the other hand, permeability along the fractures are estimated as high as 10<sup>−11</sup>–10<sup>−16</sup> m<sup>2</sup>, which is analogous to the permeability estimated for the hypocenter migration for the crustal earthquake swarms (~10<sup>−14–15</sup> m<sup>2</sup>; e.g., Nakajima and Uchida, 2018). Our observation supports that low permeability of intact crust promotes fluid accumulation and subsequent fracturing in the crust and/or underlying plate boundaries.</p><p> </p><p>[References]</p><p>Nakajima, J., Uchida, N., 2018. Nature Geoscience <strong>11</strong>, 351–356.</p><p>Ingebritsen, S.E., Manning, C.E., 2010. Geofluids <strong>10</strong>, 193–205.</p><p>Uno, M., Okamoto, A., Tsuchiya, N., 2017. Lithos <strong>284–285</strong>, 625–641.</p><p>Mindaleva, D., Uno, M., Higashino, F. et al., 2020. Lithos <strong>372–373</strong>, 105521.</p>

Radon degassing triggered by tidal loading before an earthquake

The presence of anomalous geochemical changes related to earthquakes has been controversial despite widespread, long time challenges for earthquake prediction. Establishing a quantitative relationship among geochemical changes and geodetical and seismological changes can clarify their hidden connection. Here we determined the response of atmospheric radon (222Rn) to diurnal tidal (K1 constituent) loading in the reported 11-year-long variation in the atmospheric radon concentration, including its anomalous evolution for 2 months before the devastating 1995 Kobe earthquake in Japan. The response to the tidal loading had been identified for 5 years before the occurrence of the earthquake. Comparison between these radon responses relative to crustal strain revealed that the response efficiency for the diurnal K1 tide was larger than that for the earthquake by a factor of 21–33, implying the involvement of crustal fluid movement. The radon responses occurred when compressional crustal stress decreased or changed to extension. These findings suggest that changes in radon exhaled from the ground were induced by ascent flow of soil gas acting as a radon carrier and degassed from mantle-derived crustal fluid upwelling due to modulation of the crustal stress regime.

Poly-cataclasites: Implications to the Seismic Cycle

Poly-cataclasites are rocks that have undergone multiple episodic deformational histories. These rocks retained the microstructures developed from older deformational events. They are a common occurrence in the Median Tectonic Line Japan and can be used to evaluate the changes in rock deformational processes throughout the earthquake cycle. Based on the description of mineralogical changes and the microstructures of the cataclasitic clasts, we are able to establish two main deformational events Microstructures of the co-seismic phase relates to the randomly oriented clast developed through fluidisation. Clast that are foliated formed during the aseismic phase through creeping accompanied by the precipitation of phyllosilicate minerals and the consumption of feldspathic minerals. We propose that the presence of crustal fluid circulation is essential in governing the poly-cataclasites deformational cycle providing insights into the underlying deformational processes during the earthquake cycle.