How to cite: Yang, A. Y., Zhao, S., Zhang, S., Langmuir, C., Zhou, Z., Tamura, Y., Ma, J., Xu, Y., and Zhao, T.: The role of slab serpentinite and lower crust in deep water cycles: Insights from B-Sr isotopes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5047, https://doi.org/10.5194/egusphere-egu25-5047, 2025.

Serpentinization, a low-temperature hydrothermal alteration of ultramafic rocks, strongly influences the physical and chemical properties of the oceanic lithosphere. Complete serpentinization significantly decreases the density, yield strength, and seismic velocities of ultramafic rocks, which may result in a great increase in volumes.  Serpentinites are enriched in H₂O, up to 13.5 wt%, and they contain much higher amounts of volatiles (such as carbon, sulfur, and nitrogen) compared to olivine. In particular, serpentine minerals can be stable at great depths, >150 km, suggesting that serpentinization may play an important role for the transferring of H₂O and other volatiles into great depths in subduction zones.

In spite of the significance of serpentinization, the proportions of serpentine in subduction zones still remain poorly constrained. Serpentinization kinetics are important parameters for quantifying the percentage of serpentine in subduction zones. The kinetic data obtained by Martin and Fyfe (1970) have been used to estimate the proportions of serpentine, <5% in subduction zones, which suggests that the distribution of serpentine in subduction zones is strongly affected by serpentinization kinetics. The kinetic data of Martin and Fyfe (1970) were derived from experiments using synthetic forsterite. In recent years, serpentinization experiments were performed using natural olivine, which has rates of serpentinization around 1-2 orders of slower compared to synthetic forsterite (Malvoisin et al., 2012; McCollom et al., 2016).

We have experimentally studied the kinetics of peridotite and olivine serpentinization at temperatures of 300-500 °C and pressures of 3.0-20 kbar. Compared to olivine, peridotite is serpentinized at much faster rates, reflecting the effect of pyroxene and spinel. At relatively high temperatures, e.g., 400-500 °C and 3.0 kbar, the kinetics of olivine serpentinization are sluggish, due to positive Gibbs energies of olivine serpentinization under such T-P conditions. In contrast, the kinetics of peridotite serpentinization under these T-P conditions are much faster. At 500 °C and 20 kbar, complete serpentinization can be achieved within a short period (e.g., 20 days). This suggests that the proportions of serpentine in subduction zones may be mainly controlled by the presence of H₂O rather than the rates of serpentinization. With the presence of enough H₂O, the percentage of serpentine in subduction zones may be much higher than previously proposed.

References:

Huang, R. F. et al. (2017) Journal of Geophysical Research: Solid Earth, 122, doi:10.1002/2017JB014231.

Huang, R. F. et al. (2023). Journal of Geophysical Research: Solid Earth, https:doi.org/10.1029/2022JB025218.

Malvoisin, B. et al. (2012). Journal of Geophysical Research: Solid Earth, 117(B4), B04102, doi: 10.1029/2011JB008842

Martin, B. & Fyfe, W. S. (1970). Chemical Geology, 6, 185‒202.

McCollom, T. M. et al. (2016). Geochimica et Cosmochimica Acta, 181, 175-200.

How to cite: Huang, R., Sun, W., Li, W., and Shang, X.: Effect of temperature, pressure, and chemical compositions of fluids on the rates of olivine and peridotite serpentinization, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5559, https://doi.org/10.5194/egusphere-egu25-5559, 2025.

Natural hydrogen, or white/gold hydrogen, can potentially be a key factor in the energy transition to mitigate climate change and many exploration efforts are underway in various part of the globe. Yet, in continental settings where its exploitation would be easiest, the hydrogen cycle is poorly constrained. Most of the hydrogen produced naturally is through the oxidation of Fe in silicates due to the hydration of iron-rich rocks at depth, either ultramafic rocks or banded iron formation (BIF). Nevertheless, the fluid responsible for the alteration of such rocks is usually assumed to be pure water, which is an oversimplification of fluid-rock interaction processes. Here I present the results of reaction path thermodynamic modeling of successive interactions of an initial meteoritic fluid with different rocks that would compose the upper crust in a continental setting, i.e., granites, quartzites, carbonates and evaporites, which is then reacted with ultramafic rocks.

Thermodynamic modeling was computed at 300 °C and 5 kbar using the DEW model and the EQ3/6 package. Meteoritic fluid (initial concentrations set at 10^-6 molal (mol/kg) of water for all dissolved elements, except Cl at 0.1 molal and 5 molal for evaporites) after the first interaction had higher dissolved Na, K, Fe, Al and Si after interacting with granites than the other three lithologies, although Si was also high after interaction with quartzites. Dissolved C, Ca and Mg were higher after interaction with carbonates and evaporites. These fluids were then reacted with 861 compositions of ultramafics spanning the entire range of Ol-Opx-Cpx compositions. Maximum serpentinization degree was always achieved for peridotite composition intermediate in Ol-Opx, reaching about 45-50 vol% for fluids that interacted with granites or quartzites, while only reaching 20-25 vol% for fluids having interacted with carbonates and almost no serpentinization for fluids having interacted with evaporites. Magnetite and H₂ content were coupled for all settings but were decoupled from serpentinization degree, with highest contents of H₂ produced for low dunitic initial peridotite compositions. The maximum amount of H₂ produced was 0.1 molal for fluids interacted with granites, quartzites and carbonates, an order of magnitude higher than for fluids that interacted with evaporites.

The above results highlight the vital importance of taking into account the actual chemistry of fluids that are responsible for the serpentinization of ultramafic bodies in continental settings. Especially, the presence of evaporites in sedimentary sequences, for example in the widely studied Pyrenees could hamper the natural production of hydrogen. Thus, further exploration of areas of economic importance for natural hydrogen production should carefully map the lithologies in contact with doing fault that are believed to be the carrier of surface fluid toward peridotite bodies at depth.

How to cite: Siron, G.: The impact of the upper crust composition on the production of natural hydrogen during serpentinization in continental settings, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18059, https://doi.org/10.5194/egusphere-egu25-18059, 2025.

Serpentinization of ultramafic rocks results from relatively low-temperature metamorphic reactions and is associated with the generation of natural hydrogen. Ocean opening at magma-poor rifted margins occurs under relatively cold conditions and provides a window of opportunity for the emplacement of shallow peridotites, firstly beneath thinned continental crust in the hyperextended distal margin, and secondly in the subsequent oceanward exhumed mantle at the continent-ocean transition [COT] before oceanic spreading is established. The domain covering both the distal margin and the COT is substantial, and interpretation of seismic data suggests that a variable width band of about 100 km of exhumed mantle persists along both conjugate margins of the southern North Atlantic. This raises questions about how much hydrogen has been produced in the past, how much is currently accumulated under sedimentary successions and, finally, the current hydration state of the shallow mantle in order to assess its potential as an energy source to produce additional hydrogen via stimulation. The occurrence of serpentinization and hydrogen generation in the lherzolitic rocks at magma-poor margins depends on the presence of suitable thermodynamic conditions in terms of pressure and temperature, as well as access to water. The latter requires the embrittlement of the rifted margin crust, which also depends on the rheological properties of the mantle and continental rocks, the thermal regime, and the tectonic stresses. The sequence of mantle upwelling in the distal margin and the COT, and how it is related to the deposition of sedimentary layers is also of key importance, as sediment provides a blanket of low-permeability that may promote trapping and profoundly affects the patterns of hydrothermal circulation and so the temperature field. Temperature is a key factor as it modulates the thermodynamics of serpentinization, as well as the 120ºC isotherm, which is relevant to the limits of life that can lead to biological hydrogen consumption. Thus, extension rates in rifted margins strongly control the onset of magmatism and the serpentinization of the shallow mantle beneath the hyperextended thinned continental crust and the COT. By coupling a geodynamic model with thermodynamic calculations, we discuss the above effects, focusing on the sensitivity of serpentinization and molecular hydrogen generation in the distal margin and the COT to spreading rates as control factor. Simulations indicate that at full spreading rates of 15–20 mm.yr^-1 past hydrogen generation is likely to reach its optimal conditions, whereas spreading at 30 mm.yr^-1 has less opportunity for the presence of shallow serpentinized mantle and hydrogen production, marking the transition to faster spreading styles.

How to cite: García-Pintado, J. and Pérez-Gussinyé, M.: Tectonics, Serpentinization and Natural Hydrogen Generation at the Continent-Ocean Transition in Magma-poor Rifted Margins, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18575, https://doi.org/10.5194/egusphere-egu25-18575, 2025.

The hydrothermal alteration of the Fe²⁺- rich mantle peridotites (serpentinization) tectonically exhumed at the Earth’s surface is associated to the oxidation of Fe²⁺ to Fe³⁺ that in turns creates reducing conditions characterized by the formation of hydrogen, native metals and uncommon minerals like Fe–Ni alloys and sulfides. Opaque mineral assemblages in serpentinized peridotites offer key insights into these redox reactions, capturing changes in hydrogen/oxygen and sulfur fugacity, thus allowing to unravel the mechanisms governing fluid-rock interactions and their broader impact on geochemical cycles.

Oman Drilling Project Hole BA3A (International Continental Drilling Program) recovered ~300 meters of harzburgite, with subordinate dunite in the upper 180 meters. BA3A peridotites are intersected by minor gabbroic and clinopyroxenitic dikes. Alteration is pervasive and extensive, primarily focused around dikes and veins, where it forms distinct alteration halos. Serpentine is the dominant alteration mineral within the peridotites. The opaque mineral assemblage in Hole BA3A includes pentlandite ((FeNi)₉S₈), magnetite, awaruite (Ni₂Fe), heazlewoodite (Ni₃S₂), native copper, and covellite (CuS). Sulfide minerals are primarily located within the serpentine groundmass or serpentine veins and are rarely found within pyroxene. This spatial distribution suggests that most sulfides formed as secondary phases. In highly serpentinized samples, sulfides are often finely dispersed as grains smaller than 2 μm within the serpentine groundmass. Pentlandite, the most abundant sulfide mineral, is commonly associated with magnetite and awaruite, with minor occurrences of heazlewoodite. Magnetite is frequently observed within the cleavage planes of pentlandite, as narrow rims along its edges, or in association with awaruite as thin veins. Pentlandite is also closely associated with native copper. Covellite forms banded rims around pentlandite, native copper, and magnetite. Magnetite is widespread, occurring as intergrowths with pentlandite, as fine grains (<2 μm) in the centers of serpentine veins, as fine-grained veins cutting through the mesh texture, and as overgrowths in fully serpentinized samples. Thermodynamic modeling indicates that, except for covellite—formed at low aH₂ and high aH₂S—sulfides like pentlandite, heazlewoodite, native copper, and magnetite are stable under high aH₂ conditions. We propose that the observed Cu-bearing assemblages has formed at temperatures below 200°C under highly reducing conditions which is consistent with serpentinization.

How to cite: Eslami, A., Malvoisin, B., and Godard, M.: Opaque mineral assemblages indicate highly reducing conditions in Oman Drilling Project Hole BA3A, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18636, https://doi.org/10.5194/egusphere-egu25-18636, 2025.

Garnet U-Pb dating by laser ablation – multicollector – inductively coupled plasma – mass spectrometry (LA-MC-ICPMS) in metamorphic rocks allows constraining the main stages of tectonic thickening during orogenesis. Dates obtained from garnet generally record prograde growth or the baric peak, attained during the tectonic stacking. This is of great importance because accessory minerals, such as zircon, usually analysed in metamorphic rocks date thermal peaks (or retrograde cooling) rather than prograde metamorphism and commonly constrains exhumation rather than tectonic thickening. In addition, the exact significance of the geochronological information obtained from these accessory minerals, however, is difficult to determine. Within this communication, we will explore the chronological differences between garnet and zircon from several metamorphic rocks, such as UHP gneisses, eclogites or granulites. This technique has the potential of providing time constraints of the earliest stages of continental collisions and of the dynamics of arc systems. These findings have a considerable impact on unravelling the timing of orogenic tectonothermal evolutions and supercontinent assemblies.

How to cite: Albert, R., Millonig, L. J., Beranoaguirre, A., Gerdes, A., Marschall, H. R., Moreno-Martín, D., Díez Fernández, R., Sánchez Martínez, S., and Arenas, R.: In-situ garnet U-Pb dating of metamorphic rocks: Exploring geochronological differences between garnet and accessory minerals, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12008, https://doi.org/10.5194/egusphere-egu25-12008, 2025.

The Eo-Oligocene Adamello plutonic complex (Southern Alps) has been mapped and studied for over a century. The NW unit, Avio granodiorite, was recently reexamined by [1], who observed that magmatic biotite gave constant ³⁹Ar-⁴⁰Ar ages for four grainsizes between 71 and 1000 µm. We dated zircon [2] and micas from two plutonic masses in the E of the massif, near the crustal-scale Giudicarie Fault. The smallest unit, Sòstino, had been least well studied. It is the oldest pluton of the entire complex (c. 45 Ma zircon ages, [2]), intruded at about 10 km depth. Its biotite (38-39 Ma by Rb–Sr and K–Ar [3]) was sampled anew and studied in detail. We obtained electron microprobe element maps and analyzed two size fractions by ³⁹Ar-⁴⁰Ar stepheating. The "isochemical" [4] age is 38.81±0.03 Ma. In contrast to the size-independent ages of Avio biotites, the 125-250 µm sieve fraction is 0.5 Ma younger than the 250-500 µm fraction, and has a different Ca-Cl-K signature; both K contents are sub-stoichiometric. Two mica generations are inferred, with the smaller fraction lying on the high-Cl-low-age alteration trend of the large fraction. The microprobe maps confirm chloritization, Ti unmixing, local Ba enrichment, and patchy Fe/Mg heterogeneity. The neighbouring Corno Alto pluton has a higher age gap (c. 43 Ma zircon ages, [2]; 33-34 Ma Rb–Sr and K–Ar biotite ages [3]). We followed the same redundant petrochronological approach as for Sòstino. The biotite "isochemical" [4] age is 35.88±0.04 Ma. The fine grainsize is 2 Ma younger than the coarse one, its Ca-Cl-K signature is distinct, both K contents being substoichiometric. The two biotite generations are texturally very clearly identified by microprobe, with correspondingly evident Ti-Ba-Fe-Mg-K compositional differences. The larger age difference between size fractions, and the larger zircon-biotite age gap, are explained by the higher mass fraction of the secondary biotite generation in Corno Alto relative to Sostino. The fluid-controlled formation of secondary biotite may be related to the hydrothermal circulation surrounding the large, younger intrusions in the N and NW. The plutonic biotite samples are c. 20 Ma younger than the basement biotite ages near and far from the plutons.

[1] Mittempergher et al, J Geol Soc 179 (2022) 2021-101 – [2] Favaro S., Resentini A., Tiepolo M., Malusà M.G., Zanchetta S. (2024) Abstract EGU24-14975 – [3] Del Moro et al, Mem Soc Geol It 26 (1983) 285 – [4] Müller et al, Contrib Miner Petrol 144 (2002) 57-77

How to cite: Villa, I. M., Favaro, S., Malusà, M. G., Resentini, A., and Zanchetta, S.: Post-magmatic hygrochronology of plutonic micas, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12066, https://doi.org/10.5194/egusphere-egu25-12066, 2025.

In situ geochronology using laser ablation is essential for integrating microstructural and chemical records of minerals with their geochronological information. The in situ method is especially important for potassic mica, which readily deforms, often define rock structures and fabrics, and are susceptible to chemical modification by reactive fluids. In situ ⁴⁰Ar/³⁹Ar geochronology of mica has been widely used for decades and in situ Rb/Sr geochronology has recently emerged as a prevalent method. In contrast, in situ K/Ca geochronology has been demonstrated, but the method has not been refined or utilized. Combined in situ K/Ca and Rb/Sr geochronology was developed at the Fipke Laboratory for Trace Element Research (University of British Columbia, Okanagan), allowing direct comparison of the two isotopic systems and exploring the potential of K/Ca dating. The methodology was tested using two-mica leucogranites and migmatitic paragneisses from the central Seve Nappe Complex (SNC), comprising remnants of Baltican continental crust in the Scandinavian Caledonides. The central SNC consists of continental crustal rocks hosting volumetrically-subordinate eclogites, peridotites, and pyroxenites. The crustal lithologies record partial melting beginning at c. 484-480 Ma, while the (ultra)mafic lithologies provide evidence for high-pressure metamorphism at c. 460-454 Ma, altogether reflecting subduction-exhumation cycles of the central SNC. Subsequent collision of the Baltican continent with Laurentia led to widespread deformation and metamorphism in the SNC, starting at c. 430-425 Ma. Continental anatexis produced the studied leucogranites and migmatitic paragneisses; these rocks comprise quartz, white mica, biotite, plagioclase, K-feldspar, and clinozoisite, with accessory apatite, monazite, xenotime, and zircon. The accessory phases were dated via in situ U-Pb geochronology to compare with in situ white mica and biotite K/Ca and Rb/Sr geochronology results. The dates yielded by apatite (424 ± 15 Ma, 428 ± 17 Ma), monazite (427 ± 2 Ma), and xenotime (426 ± 2 Ma) are all within uncertainty (pooled age of 426.5 ± 1.4 Ma) and record the timing of continental collision. Zircon provide a wider range of concordant U-Pb dates (473 ± 8 Ma to 419 ± 5 Ma). White mica and biotite Rb/Sr isochron dates from all four examined rocks (438 ± 5 Ma, 430 ± 4 Ma, 433 ± 4 Ma, 431 ± 5 Ma) are slightly older than the apatite/monazite/xenotime and youngest zircon U-Pb dates, but still record continental collision. The same mica volumes yielded older K/Ca isochron dates (484 ± 21 Ma, 482 ± 14 Ma, 486 ± 15 Ma, 486 ± 12 Ma), similar to the oldest concordant zircon U-Pb dates and the overall timing of partial melting in the broader central SNC crustal rocks. The different K/Ca and Rb/Sr dates indicate that these two isotopic systems are decoupled in the same mica volumes, controlled either by interactions of Ca and Sr with the mica lattice or by diffusion gradients influenced by bulk rock composition. The results presented herein demonstrate that the K/Ca isotopic system has the potential for retrieving older geologic histories in polymetamorphic terranes. Further detailed investigations and continued development of the methodology, including improved reference materials, are required.

How to cite: Barnes, C., Larson, K., Button, M., and Camacho, A.: Combined in situ K/Ca and Rb/Sr geochronology of potassic mica, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14442, https://doi.org/10.5194/egusphere-egu25-14442, 2025.