<p>Fluid&#8211;rock interactions link mass and energy transfer with large-scale tectonic deformation, drive the formation of mineral deposits, carbon sequestration, and rheological changes of the lithosphere. While spatial evidence indicates that fluid&#8211;rock interactions operate on length scales ranging from the grain boundary to tectonic plates, the timescales of regional fluid&#8211;rock interactions remain essentially unconstrained, despite being critically important for quantifying the duration of fundamental geodynamic processes. Here we show that reaction-induced transiently high permeability significantly facilitates fast fluid flow through low-permeability rock of the mid-crust. Using observations from an exceptionally well-exposed fossil hydrothermal system to inform a multi-element advective&#8211;diffusive&#8211;reactive transport model, we show that fluid-driven reaction fronts propagate with ~10 cm year<sup>-1</sup><sub>,</sub> equivalent to the fastest tectonic plate motion and mid-ocean ridge spreading rates. Consequently, in the presence of reactive fluids, large-scale fluid-mediated rock transformations in continental collision and subduction zones occur on timescales of tens of years, implying that natural carbon sequestration, ore deposit formation, and transient and long-term petrophysical changes of the crust proceed, from a geological perspective, instantaneously.</p>
Lithium has proven a powerful tracer of weathering processes and chemical evolution of seawater. Skeletal components of marine calcifying organisms, and in particular brachiopods, present promising archives of Li signatures. However, Li incorporation mechanisms and potential influence from biological processes or environmental conditions require a careful assessment. In order to constrain Li systematics in brachiopod shells, we present Li concentrations and isotope compositions for 11 calcitic brachiopod species collected from six different geographic regions, paralleled with data from culturing experiments where brachiopods were grown under varying environmental conditions and seawater chemistry (pH–pCO2, temperature, Mg/Ca ratio). The recent brachiopod specimens collected across different temperate and polar environments showed broadly consistent δ7Li values ranging from 25.2 to 28.1‰ (with mean δ7Li of 26.9 ± 1.5‰), irrespective of taxonomic rank, indicating that incorporation of Li isotopes into brachiopod shells is not strongly affected by vital effects related to differences among species. This results in Δ7Licalcite–seawater values (per mil difference in 7Li/6Li between brachiopod calcite shell and seawater) from −2.9‰ to −5.8‰ (with mean Δ7Licalcite–seawater value of −3.6‰), which is larger than the Δ7Licalcite–seawater values calculated based on data from planktonic foraminifera (~0‰ to ~−4‰). This range of values is further supported by results from brachiopods cultured experimentally. Under controlled culturing conditions simulating the natural marine environment, the Δ7Licalcite–seawater for Magellania venosa was −2.5‰ and not affected by an increase in temperature from 10 to 16 °C. In contrast, a decrease in Mg/Ca (or Li/Ca) ratio of seawater by addition of CaCl2 as well as elevated pCO2, and hence low-pH conditions, resulted in an increased Δ7Licalcite-seawater up to −4.6‰. Collectively, our results indicate that brachiopods represent valuable archives and provide an envelope for robust Li-based reconstruction of seawater evolution over the Phanerozoic.
Abstract Black shales may serve as an important source of metals such as Co, Ni, or As, largely due to anoxic to euxinic conditions in association with high concentrations of sulfur leading to efficient scavenging and transport of metals from seawater into the seafloor sediment. We report on an unusual type of Au mineralization newly discovered in Ediacaran trench-slope black shales in the Bohemian Massif, Czech Republic. The Au enrichment is related to the formation of a quartz–sulfide vein system and a progressive evolution of ore-forming fluids with decreasing temperature, from Sb- to As-rich to final precipitation of native gold from silica and Au-bearing low-temperature hydrothermal colloidal solutions. The hydrothermal nature of these solutions is also documented by Li contents and isotope compositions which differ markedly between barren black shales and those carrying significant late-stage quartz-rich veins. The structural relationships and orientation of the associated quartz veins point to a close connection between vein emplacement and high heat flow in response to Ordovician rifting, and breakup of the northern margin of Gondwana, and opening of the Rheic Ocean. This triggered metal and sulfur remobilization, including Au, from the associated Neoproterozoic–Cambrian volcanosedimentary successions. The documented Au mineralization and its association with the Ordovician rift-related magmatic activity is different from the widespread Variscan Au occurrences in the Bohemian Massif. Our study thus provides a new genetic model potentially important for future exploration of Au also in other terrains underlain by a rifted Cadomian basement.
Available data (Sr, Nd and Pb isotope systematics) for the Proterozoic Samalpatti–Sevattur (Tamil Nadu, India) carbonatite complexes indicate ultimately a mantle origin of these carbonatites. However, for both intrusions various degrees of mixing between mantle and crustal components of the parent carbonatite fluid have been evidenced based on stable C–O and noble gas (He, Ne, Ar) isotope compositions. An integrated petrographic, noble gas (He, Ne, Ar), clumped isotope and trace element geochemical study has been performed to further constrain their genesis and provide additional constraints on their evolution. Oriented texture of calcite, formation of subgrains, grain boundary bulging as well as sheared and inclined calcite twins indicate dynamic deformation; whereas triple junctions and polygonal mosaic texture indicate static recrystallization of the carbonatites. Although the metamorphism may have had significant effect on the ordering of C and O atoms, clumped isotope analyses (expressed as D 47 ) indicate temperatures < 250°C for Sevattur carbonatites. The obtained low crystallization temperatures are in line with the compatible/incompatible element ratios. High Ho/Y, U/Th and Y/Ce ratios support the carbothermal origin of the carbonatites and the fractionation of the fluids from the footwall syenitic rocks. Plots of Ba/Mn vs. Nb/Th and Ba/La vs. Nb/Pb, however, do not support their fractionation or immiscibility from the host pyroxenite rocks. Crustal-like R/R A values (~0.05) of Sevattur are slightly different from the Samalpatti ratios (~0.2) and imply a higher (2-3% vs 0.1%) mantle contribution to the magma formation of the Samalpatti carbonatites. Two samples from Samalpatti have mantle-plume-like 20 Ne/ 22 Ne (10.5 and 11.8) and 21 Ne/ 20 Ne (0.030 and 0.035) ratios. Model calculations prove that the high 4 He and 21 Ne concentrations cannot be explained with decay of the U and Th hosted by the apatite and calcite crystal lattice, but they were trapped from the mineralizing fluids in the fluid inclusions. High ΣREE (>1000 ppm) and Ba concentrations of Sevattur compared to Samalpatti as well as the low 87 Sr/ 86 Sr ratios (~0.0705) and high Ba/Th ratios (1000/10000) suggest the melting of an enriched Subcontinental Lithospheric Mantle (SCLM) which was previously enriched in incompatible LILE elements deriving from the hydrothermally altered ocean floor (sediments). Although volumetrically not comparable, most of the geochemical parameters and hence the magma genesis makes the Sevattur carbonatite comparable with the world-class REE deposits in China. In turn, the low ΣREE (<100 ppm), the high 87 Sr/ 86 Sr ratios (0.706–0.708), the average Ba concentrations (~110) and the low Ba/Th ratios (~100) suggest the melting of the SCLM or mantle which was enriched in sedimentary carbonates. Noble gases indicate that compared to the Sevattur complex the fraction of the mantle (-plume) derived magma could have been higher compared to the Sevattur complex. Our study has proven, that in spite of the close spatial proximity (2 km) the neighboring carbonatite complexes have experienced basically contrasting syn- and post emplacement history and have different magma sources.
The Miocene Kaiserstuhl volcanic complex in the Rhine graben rift is known for simultaneously exposing both intrusive and erupted (pyroclastic) calciocarbonatites. This makes Kaiserstuhl a promising candidate for studying the field and genetic relations between intrusive calciocarbonatite and its eruptive equivalent, and the processes enabling eruption of the calciocarbonatite at the surface in particular. Eruptive calciocarbonatites in Kaiserstuhl are represented by carbonatite tuff and lapillistone beds covering a debrite fan on the western flank of the volcano. The debrites are interpreted as lahar (debris flow) and possibly also debris-avalanche deposits. Based on the observed textures, the debris flows were most likely derived by water dilution from debris avalanches resulting from edifice failure, which occurred in the central part of the Kaiserstuhl volcanic complex. The edifice failure ultimately exposed the intrusive system, and the carbonatite pyroclasts (lapilli and ash) were ejected from narrow vents represented by open-framework tuff-breccias aligned along the detachment scarp. Since the Ca-carbonates break down rapidly at high temperatures and low pressures, calciocarbonatites are unlikely to form surface lavas. On the other hand, the presence of the calciocarbonatite pyroclastic deposits suggests that some geological process faster than the high-temperature breakdown of Ca-carbonate may facilitate calciocarbonatite eruption. We suggest that the sudden exposure and decompression of a suprasolidus high-level carbonatite intrusion by edifice collapse may be a suitable scenario enabling calciocarbonatite eruption. The absence of edifice failures on alkaline volcanoes, where carbonatite intrusion is either supposed or exposed, may explain the overall scarcity of erupted calciocarbonatites.
