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.

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

Methods for bulk-rock Lu-Hf, Sm-Nd and Li isotope analysis and a data table.<br>

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

Methods for bulk-rock Lu-Hf, Sm-Nd and Li isotope analysis and a data table.<br>

Need for Seismic Hydrology Research with a Geomicrobiological Focus

Earthquakes cause deformation in previously stable groundwater environments, resulting in changes to the hydrogeological characteristics. The changes to hydrological processes following large-scale earthquakes have been investigated through many physicochemical studies, but understanding of the associated geomicrobiological responses remains limited. To complement the understanding of earthquakes gathered using hydrogeochemical approaches, studies on the effects of the Earth’s deep crustal fluids on microbial community structures can be applied. These studies could help establish the degree of resilience and sustainability of the underground ecosystem following an earthquake. Furthermore, investigations on changes in the microbial community structure of the Earth’s deep crustal fluids before and after an earthquake can be used to predict an earthquake. The results derived from studies that merge hydrogeochemical and geomicrobiological changes in the deep crustal fluids due to the effect of stress on rock characteristics within a fault zone can be used to correlate these factors with earthquake occurrences. In addition, an earthquake risk evaluation method may be developed based on the observable characteristics of fault-zone aquifers.

Microbial population dynamics are dominated by stochastic forces in a low biomass subseafloor habitat

The subseafloor is a vast global habitat that supports microorganisms that have a global scale impact on geochemical cycles. Much of the subseafloor contains endemic microbial populations that consist of small populations under growth-limited conditions. For small population sizes, the impacts of stochastic evolutionary events can have large impacts on intraspecific population dynamics and allele frequencies. These conditions are fundamentally different than those experienced by most microorganisms in surface environments, and it is unknown how small population sizes and growth-limiting conditions influence evolution and population structure. Using a two-year, high-resolution environmental time-series, we examine the dynamics of 10 microbial populations from cold, oxic crustal fluids collected from the subseafloor site North Pond, located near the mid-Atlantic ridge. The 10 microbial populations were divided into groups with distinct patterns of population dynamics based on abundance, nucleotide diversity, and changes in allele frequency. Results reveal rapid allele frequency shifts linked to different types of population interactions, including sweeps, dispersal, and clonal expansion. Dispersal plays an important role in structuring the most abundant populations in the crustal fluids. Microbial populations in the subseafloor of North Pond are highly dynamic and evolution is governed largely by the stochastic forces of dispersal and drift.ImportanceThe cold, oxic subseafloor is an understudied habitat that is difficult to access, yet important to global biogeochemical cycles and starkly different compared microbial habitats on the surface of the Earth. Our understanding of microbial evolution and population dynamics has been largely molded by studies of microbes living in surface habitats that can host 10-1,000 times more microbial biomass than has been observed in the subsurface. This study provides an opportunity to observe evolution in action within a low biomass, growth-limited environment and reveals that while microbial populations in the subseafloor can be influenced by changes in selection pressure and small-scale gene sweeps, the stochastic forces of genetic drift and dispersal have an important impact on the evolution of microbial populations. Much of the microbial life on the planet exists under growth-limited conditions and the subseafloor provides a natural laboratory to explore these fundamental biological questions.

Hydrogen isotopes in phlogopite indicate crustal fluids in the UG2 chromitite layer, Bushveld Complex

&lt;p&gt;The Upper Group 2 (UG2) chromitite layer in the upper Critical Zone of the Bushveld Complex, South Africa, is the world&amp;#8217;s largest PGE orebody. The UG2 is typically 0.5 to 1.5 m thick and it consists of 75&amp;#8211;90 modal % chromite with interstitial silicate and sulfide minerals. Although a minor component, phlogopite is important because it is a hydrous phase. It has been noted that the UG2 chromitite contains more abundant phlogopite (locally &gt; 1 modal %) than the surrounding pyroxenite layers (Mathez and Mey, 2005). More and more studies suggest that water plays an important role in the UG2 chromite formation and in PGE enrichment or remobilization (e.g., Li et al., 2004; Mathez and Mey, 2005; Schannor et al., 2018). The source of the water is controversial, and this motivated our ongoing study of hydrous minerals in the UG2.&lt;/p&gt;&lt;p&gt;We determined the chemical composition and hydrogen isotope ratio of phlogopite from the chromitite layer and from the surrounding pyroxenite in drill cores from two sites the eastern and western Bushveld (Nkwe and Khuseleka, respectively). The &amp;#948;D values of phlogopite in chromitite from the eastern site are -38.2 to -25.5&amp;#8240; (mean = -29.7&amp;#8240;, n = 6). The corresponding values from the western site are similar, with -34.6 to -31.6&amp;#8240; (mean = -33.2&amp;#8240;, n = 6). The &amp;#948;D values of phlogopite from pyroxenite are more variable, ranging from -43.1 to -26.1&amp;#8240; for the eastern site (mean = -32.9&amp;#8240;, n = 4) and from -38.7 to -26.1&amp;#8240; for the western site (mean = -31.7&amp;#8240;, n = 3).&lt;/p&gt;&lt;p&gt;Published whole-rock &amp;#948;D values for silicate cumulate rocks in the upper Critical Zone are -93 to -55&amp;#8240; (Mathez et al., 1994), which are similar to mantle values (-70&amp;#177;10%; Boettcher and O'neil, 1980) and are interpreted as magmatic. &amp;#160;In comparison, our &amp;#948;D values of phlogopite from UG2 are much higher and suggest a significant contribution of crustal fluids. Harris and Chaumba (2001) estimated a &amp;#948;D value of -22&amp;#8240; for paleo-meteoric water in the Bushveld Complex. Given the relative homogeneity of the phlogopite &amp;#948;D data in both sites of the complex, and the primary appearance of the grains in thin section, we argue that the crustal fluids were incorporated in the magma before the crystallization of the UG2 layer. Triple oxygen isotopes will test our hypothesis further.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;References: Boettcher &amp; O'neil (1980) Amer. Jour. Sci. 280A, 594&amp;#8211;621. Harris &amp; Chaumba (2001) J. Petrol. 42, 1321&amp;#8211;1347. Li et al. (2004) Econ. Geol. 99, 173&amp;#8211;184. Mathez et al. (1994) Econ. Geol. 89, 791&amp;#8211;802. Mathez &amp; Mey (2005) Econ. Geol. 100, 1616&amp;#8211;1630. Schannor et al. (2018) Chem. Geol. 485, 100&amp;#8211;112.&lt;/p&gt;