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1. Enhanced metamorphic CO2 release on the Proterozoic Earth

   https://doi.org/10.1073/pnas.2401961121  

   Stewart, E M; Penman, Donald E (October 2024, Proceedings of the National Academy of Sciences)

   Rock metamorphism releases substantial CO2 over geologic timescales (>1 My), potentially driving long-term planetary climate trends. The nature of carbonate sediments and crustal thermal regimes exert a strong control on the efficiency of metamorphic CO2 release; thus, it is likely that metamorphic CO2 degassing has not been constant throughout time. The Proterozoic Earth was characterized by a high proportion of dolomite-bearing mixed carbonate-silicate rocks and hotter crustal regimes, both of which would be expected to enhance metamorphic decarbonation. Thermodynamic phase equilibria modeling predicts that the metamorphic carbon flux was likely ~1.7 times greater in the Mesoproterozoic Era compared to the modern Earth. Analytical and numerical approaches (the carbon cycle model PreCOSCIOUS) are used to estimate the impact this would have on Proterozoic carbon cycling and global atmospheric compositions. This enhanced metamorphic CO2 release alone could increase pCO2 by a factor of four or more when compared to modern degassing rates, contributing to a stronger greenhouse effect and warmer global temperatures during the expansion of life on the early Earth. 

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2. Carbonation and decarbonation reactions: Implications for planetary habitability

   https://doi.org/10.2138/am-2019-6884  

   Stewart, E.M.; Ague, Jay J.; Ferry, John M.; Schiffries, Craig M.; Tao, Ren-Biao; Isson, Terry T.; Planavsky, Noah J. (October 2019, American Mineralogist)

   Abstract The geologic carbon cycle plays a fundamental role in controlling Earth's climate and habitability. For billions of years, stabilizing feedbacks inherent in the cycle have maintained a surface environment that could sustain life. Carbonation/decarbonation reactions are the primary mechanisms for transferring carbon between the solid Earth and the ocean–atmosphere system. These processes can be broadly represented by the reaction: CaSiO3 (wollastonite) + CO2 (gas) ↔ CaCO3 (calcite) + SiO2 (quartz). This class of reactions is therefore critical to Earth's past and future habitability. Here, we summarize their significance as part of the Deep Carbon Obsevatory's “Earth in Five Reactions” project. In the forward direction, carbonation reactions like the one above describe silicate weathering and carbonate formation on Earth's surface. Recent work aims to resolve the balance between silicate weathering in terrestrial and marine settings both in the modern Earth system and through Earth's history. Rocks may also undergo carbonation reactions at high temperatures in the ultramafic mantle wedge of a subduction zone or during retrograde regional metamorphism. In the reverse direction, the reaction above represents various prograde metamorphic decarbonation processes that can occur in continental collisions, rift zones, subduction zones, and in aureoles around magmatic systems. We summarize the fluxes and uncertainties of major carbonation/decarbonation reactions and review the key feedback mechanisms that are likely to have stabilized atmospheric CO2 levels. Future work on planetary habitability and Earth's past and future climate will rely on an enhanced understanding of the long-term carbon cycle. 

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3. Pervasive subduction zone devolatilization recycles CO2 into the forearc

   https://doi.org/10.1038/s41467-020-19993-2  

   Stewart, E. M.; Ague, Jay J. (December 2020, Nature Communications)

   Abstract The fate of subducted CO₂remains the subject of widespread disagreement, with different models predicting either wholesale (up to 99%) decarbonation of the subducting slab or extremely limited carbon loss and, consequently, massive deep subduction of CO₂. The fluid history of subducted rocks lies at the heart of this debate: rocks that experience significant infiltration by a water-bearing fluid may release orders of magnitude more CO₂than rocks that are metamorphosed in a closed chemical system. Numerical models make a wide range of predictions regarding water mobility, and further progress has been limited by a lack of direct observations. Here we present a comprehensive field-based study of decarbonation efficiency in a subducting slab (Cyclades, Greece), and show that ~40% to ~65% of the CO₂in subducting crust is released via metamorphic decarbonation reactions at forearc depths. This result precludes extensive deep subduction of most CO₂and suggests that the mantle has become more depleted in carbon over geologic time. 

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4. Preliminary 1:200,000 scale geologic map of the Bhaba Pass-Pin Valley region, Himachal Himalaya, NW India

   https://doi.org/10.1130/abs/2024AM-401981  

   Vlaha, Dominik; Genge, Marie C; Ganbat, Ariuntsetseg; Branton, Evon; Zuza, Andrew; Haproff, Peter J; Reyes, Francisco; Guevara, Victor; Webb, A Alexander; Singh, Birendra P (January 2024, Geological Society of America Abstracts with Programs)

   Tectonic models for the development of the Himalaya, Earth's largest active collisional mountain belt, have been developed and tested through pressure, temperature, and time (P-T-t) information collected from exposed metamorphic rocks. Inferred deep burial and subsequent exhumation of these rocks are usually justified by observable structures (e.g., Main Central thrust) and mapping relationships. However, regions where pressure estimates are at odds with field-based reconstructions are reconciled with hypothesized cryptic structures that have since been completely eroded. Such field versus thermobarometric discrepancies significantly impact interpretations on the geometry, magnitude, and distribution of deformation. Here, we conducted detailed field mapping of the Paleogene Tethyan fold-thrust belt in the Himachal Himalaya, NW India, which is the structurally highest part of the Himalayan orogen and deforms a ~10–15 km thick Neoproterozoic–Cretaceous passive margin section. In this region, P-T estimates yield 6–8 kbar and ~650°C, which suggests burial to depths of ~25–30 km. To assess the viability of this deep burial, we constructed a 1:200,000 scale geologic map of the Bhaba Pass-Pin Valley region. Geologic mapping was focused on the stratigraphy, structural configuration, and metamorphic isograds of the basal Tethyan strata. Detailed field mapping aided the construction of balanced cross sections, which guided subsequent multi-method analytical approaches that fit into a coherent structural framework. Our field observations and map relationships show no major structures, abrupt changes in metamorphic grade or composition that would suggest deep burial of the stratigraphically continuous basal Tethyan group. Balanced cross sections throughout the study area suggest moderate amounts of shortening strain (~30–36%). This contribution highlights the importance of detailed field mapping to interpret P-T estimates. Ongoing analytical methods are being conducted to constrain the thermal architecture and metamorphic history of the Tethyan fold-thrust belt. 

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5. Oligocene-Neogene lithospheric-scale reactivation of Mesozoic terrane accretionary structures in the Alaska Range suture zone, southern Alaska, USA

   https://doi.org/10.1130/B35665.1  

   Waldien, Trevor S.; Roeske, Sarah M.; Benowitz, Jeffrey A.; Twelker, Evan; Miller, Meghan S. (August 2020, GSA Bulletin)

   null (Ed.)

   Abstract Terrane accretion forms lithospheric-scale fault systems that commonly experience long and complex slip histories. Unraveling the evolution of these suture zone fault systems yields valuable information regarding the relative importance of various upper crustal structures and their linkage through the lithosphere. We present new bedrock geologic mapping and geochronology data documenting the geologic evolution of reactivated shortening structures and adjacent metamorphic rocks in the Alaska Range suture zone at the inboard margin of the Wrangellia composite terrane in the eastern Alaska Range, Alaska, USA. Detrital zircon uranium-lead (U-Pb) age spectra from metamorphic rocks in our study area reveal two distinct metasedimentary belts. The Maclaren schist occupies the inboard (northern) belt, which was derived from terranes along the western margin of North America during the mid- to Late Cretaceous. In contrast, the Clearwater metasediments occupy the outboard (southern) belt, which was derived from arcs built on the Wrangellia composite terrane during the Late Jurassic to Early Cretaceous. A newly discovered locality of Alaska-type zoned ultramafic bodies within the Clearwater metasediments provides an additional link to the Wrangellia composite terrane. The Maclaren and Clearwater metasedimentary belts are presently juxtaposed by the newly identified Valdez Creek fault, which is an upper crustal reactivation of the Valdez Creek shear zone, the Late Cretaceous plate boundary that initially brought them together. 40Ar/39Ar mica ages reveal independent post-collisional thermal histories of hanging wall and footwall rocks until reactivation localized on the Valdez Creek fault after ca. 32 Ma. Slip on the Valdez Creek fault expanded into a thrust system that progressed southward to the Broxson Gulch fault at the southern margin of the suture zone and eventually into the Wrangellia terrane. Detrital zircon U-Pb age spectra and clast assemblages from fault-bounded Cenozoic gravel deposits indicate that the thrust system was active during the Oligocene and into the Pliocene, likely as a far-field result of ongoing flat-slab subduction and accretion of the Yakutat microplate. The Valdez Creek fault was the primary reactivated structure in the suture zone, likely due to its linkage with the reactivated boundary zone between the Wrangellia composite terrane and North America in the lithospheric mantle. 

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