Gray E. Bebout

Lithium is of great interest as a tracer of meta-morphic reactions and related fluid-mineral interactions because of its potential to isotopically fractionate during inter- and intracrystalline diffusional processes. Study of its transfer through subduction zones, based on study of arc volcanic and metamorphic rocks, can yield insight regarding ocean-to-mantle chemical cycling. We investigated major- and trace-element concentrations and δ⁷Li in garnet in ultrahigh-pressure (UHP) Lago di Cignana metasedimentary rocks, relating these observations to reconstructed prograde devolatilization history. In all garnet crystals we studied, heavy rare earth elements (HREEs), Y, and Li showed strong zoning, with elevated concentrations in cores (15–50 ppm Li) and marked high-concentration anomalies (up to 117 ppm Li, 5500 ppm Y; little or no major-element shift) as growth annuli, in which some crystals showed subtle elevation in δ⁷Li greater than analytical error of ~3‰ (2σ). Rutile inclusions appeared abruptly at annuli and outward toward rims, accompanied by inclusions of a highly zoned, Ca- and rare earth element–rich phase and decreased Nb concentrations in garnet. These relationships are interpreted to reflect prograde garnet-forming reaction(s), in part involving titanite breakdown to stabilize rutile, which resulted in delivery of more abundant Y and HREEs at surfaces of growing garnet crystals to produce annuli. Co-enrichments in Li and Y + REEs are attributed to mutual incorporation via charge-coupled substitutions; thus, increased Li uptake was a passive consequence of elevated concentrations of Y + REEs. The small­scale fluctuations in δ⁷Li (overall range of ~9‰) observed in some crystals may correlate with abrupt shifts in major- and trace-element concentrations, suggesting that changes in reactant phases exerted some control on the evolution of δ⁷Li. For one garnet crystal, late-stage growth following partial resorption produced deviation in major- and trace­element compositions, including Li concentration, accompanied by a 10‰–15‰ negative shift in δ⁷Li, perhaps reflecting a change in the mechanism of incorporation or source of Li. These results highlight the value of measuring the major- and trace-element and isotope compositions of garnets in high-pressure and UHP metamorphic rocks in which matrix mineral assemblages are extensively overprinted by recrystallization during exhumation histories. Lithium concentrations and isotope compositions of the garnets can add valuable information regarding prograde (and retrograde) reaction history, kinetics of porphyroblast growth, intracrystalline diffusion, and fluid-rock interactions. This work, integrated with previous study of devolatilization in the Schistes Lustrés/Cignana metasedimentary suite, indicates retention of a large fraction of the initially subducted sedimentary Li budget to depths approaching those beneath volcanic fronts, despite the redistribution of this Li among mineral phases during complex mineral reaction histories.

Understanding the Earth’s geological nitrogen (N) and carbon (C) cycles is fundamental for assessing the distribution of these volatiles between solid Earth (core, mantle and crust), oceans and atmosphere. This Special Communication about the Earth’s N and C cycles contains material that is relevant for researchers who are interested in the Topical Collection on planetary evolution “Reading Terrestrial Planet Evolution in Isotopes and Element Measurements”. Variations in the fluxes of N and C between these major reservoirs through geological time influenced the evolution and determined the unique composition of the Earth’s atmosphere. Here we review several key geological aspects of the N and C cycles of which our understanding has significantly advanced during the last decade through field-based, experimental and theoretical studies. Subduction zones are the most important pathway of both N and C from the Earth’s surface into the deep Earth. A key question in the flux quantification is how much of the volatile elements is stored in the downgoing slab and introduced into the mantle and how much is returned back to the surface and the atmosphere through arc magmatism. For N, the retention of N as $\text{NH}_{4}^{+}$ NH 4 + in minerals has a major influence on fluxes between reservoirs. The temperature-dependent stability of $\text{NH}_{4}^{+}$ NH 4 + -bearing minerals determines whether N is predominantly retained in the slab to mantle depths (in subduction zones with a low geothermal gradient) or devolatilized (in subduction zones with a high geothermal gradient). Several lines of evidence suggest that the mantle is regassing with respect to N due to a net influx of subducted N over time, but this issue is highly debated and evidence to the contrary also exists. Nevertheless, there is consensus that the majority of the planetary N budget is stored in the Earth’s mantle, with the continental crust also constituting a significant N reservoir. For C, release from the subducting slab occurs through decarbonation reactions, dissolution and formation of carbonatitic liquids, but reprecipitation of C in the slab or the forearc mantle wedge may limit the effectiveness of direct return of C into the atmosphere. Carbon release through regional metamorphism in collision zone orogens also has potentially profound effects on C release into the atmosphere and consensus has emerged that such orogens are sources rather than sinks of atmospheric CO₂. On shorter timescales, contact metamorphism through interaction of mantle-derived magmas with C-bearing country rocks, and the resulting release of large quantities of CH₄ and/or CO₂, has been linked to global warming events.

Devolatilization and fluid-rock interaction processes along subduction interfaces, in particular at depths where episodic tremor and slip events (ETS) are inferred, are evidenced by the occurrence of metamorphic veins in exhumed metamorphic terranes. We investigate the late Cretaceous lawsonite blueschist-facies Seghin complex, part of the Zagros suture zone (Iran), a well-preserved paleo-subduction mélange composed of an antigorite-rich matrix wrapping foliated metatuffs and minor carbonate-bearing metasediments. We first focus on characterizing the relative chronology, conditions of deformation and potential fluid source(s) of Lws+Cpx+Gln veins and aragonite-filled explosive hydraulic breccias. Petrological, geochemical as well as O-C and Sr-Nd isotopic systematics of silicate-rich veins suggest formation mostly from internal devolatilization. This stage is followed at near peak burial conditions by pervasive, externally-derived fluid influx events, with fluids characterized by REE enrichments, and geochemical signatures indicating mixing between metasedimentary-derived fluids and far-traveled mafic-ultramafic-derived fluids. Our geochemical and petrological observations suggest that a host rock-buffered isotopic homogenization occurred between the infiltrating fluids and the rock matrix.The high pore fluid pressures that enabled the formation of these deep veins also enabled the formation of shallower fault-related rocks including breccias, foliated cataclasites and fluidized ultracataclasites, intimately associated with extensional Gln-bearing veins and Lws+Gln+Ph+Ab fluid-filled pockets. Mineral assemblages reveal that this faulting occurred upon exhumation throughout the lawsonite blueschist-facies (i.e. 35 to 20 km depth). Crosscutting relationships among multiple generations of fluidized ultracataclasites and extensional veins show that episodic seismic faulting and hydrofracturing were contemporaneous processes. Mechanical modelling confirms that the studied fault-related features can only form under nearly lithostatic pore fluid pressure conditions, maintaining the system in a critically unstable regime that promotes recurrent seismic faulting. We propose a large-scale tectonic model in which deeply produced H2O-rich fluids are transported as highly pressurized “pulses” over tens of km parallel to the subduction interface, triggering episodic hydrofracturing and host rock-buffered isotopic homogenization within the ETS region. The mechanical consequence of these events is the triggering of unstable slip within the seismogenic window, as deduced in this unique record of blueschist-facies crustal paleo-earthquakes. These results shed a new light on the physical nature of the numerous moderate magnitude events (Mw=3-6) that are extensively recorded nowadays in Mariana-type plate boundary systems.

The primary stratigraphic fabric of a chaotic rock unit in the Zermatt Saas ophiolite of the Western Alps was reworked by a polyphase Alpine tectonic deformation. Multiscalar structural criteria demonstrate that this unit was deformed by two ductile subduction-related phases followed by brittle-ductile then brittle deformation. Deformation partitioning operated at various scales, leaving relatively unstrained rock domains preserving internal texture, organization, and composition. During subduction, ductile deformation involved stretching, boudinage, and simultaneous folding of the primary stratigraphic succession. This deformation is particularly well-documented in alternating layers showing contrasting deformation style, such as carbonate-rich rocks and turbiditic serpentinite metasandstones. During collision and exhumation, deformation enhanced the boudinaged horizons and blocks, giving rise to spherical to lozenge-shaped blocks embedded in a carbonate-rich matrix. Structural criteria allow the recognition of two main domains within the chaotic rock unit, one attributable to original broken formations reflecting turbiditic sedimentation, the other ascribable to an original sedimentary mélange. The envisaged geodynamic setting for the formation of the protoliths is the Jurassic Ligurian-Piedmont ocean basin floored by mostly serpentinized peridotites, intensely tectonized by extensional faults that triggered mass transport processes and turbiditic sedimentation.