The pargasosphere hypothesis, proposed by István J. Kovács and colleagues (primarily in their 2021 Global and Planetary Change paper), reframes the lithosphere-asthenosphere boundary (LAB) and related mantle processes around the stability of pargasitic amphibole (a hydrous mineral) in the shallow upper mantle.

The “pargasosphere” refers to the region of the upper mantle (typically < ~100 km depth) where pargasite (a type of amphibole) is stable. Even at very low bulk water contents (a few hundred ppm wt. or less), pargasite can form and exert strong control over the rheology (viscosity/deformation behavior), melting behavior, and physical properties of the shallow mantle.

Pargasite acts as a “water buffer”: it incorporates hydrogen (as hydroxyl) and can influence the presence of small amounts of partial melt or fluids.

Mechanism for CO₂ Degassing (Relevant to the 2026 Paper)

In the context of geological CO₂ emissions, the hypothesis explains diffuse, non-volcanic mantle-derived CO₂ release in intraplate or extensional settings (e.g., the Pannonian Basin) through cooling of the asthenosphere beneath relatively young/thin lithosphere:

1. Cooling Asthenosphere: Beneath young or thinned lithosphere (oceanic or continental extensional basins), the underlying asthenosphere cools gradually. This cooling occurs without active volcanism or nearby magma chambers.
2. Incipient Partial Melting and Volatile Behavior: The shallow upper mantle contains trace volatiles (H₂O and CO₂). As the asthenosphere cools, small fractions (<1 vol%) of H₂O- and CO₂-bearing basaltic silicate melts or supercritical fluids form or persist. Pargasite stability influences the solidus (melting temperature), keeping conditions near the point where tiny amounts of melt can exist.
3. Crystallization and CO₂ Enrichment: During cooling and crystallization of these incipient melts:
   • CO₂ has low solubility in the crystallizing silicate minerals/melts under these conditions.
   • It becomes concentrated and enriched in the residual fluids, grain boundaries, or fluid inclusions.
4. Migration and Degassing: These CO₂-rich fluids are highly mobile and buoyant. They migrate upward along favorable pathways such as:
   • Lithospheric deformation zonesExtensional structuresFaults or permeable grain networks
   This produces measurable diffuse surface CO₂ emanations even far from volcanoes. Fluxes can reach levels comparable to quiescent volcanic systems (e.g., 10³–10⁵ mol km⁻² yr⁻¹ in the Pannonian Basin examples).

The process can operate over millions of years, allowing the lithospheric mantle to accumulate and episodically release large CO₂ reservoirs (e.g., estimates of ~10³ Gt over 10 Myr in studied areas during post-rift thickening and thermal relaxation).

Broader Context in the Hypothesis

LAB Formation (Young/Thin Lithosphere): The pargasite dehydration solidus (~1100°C) creates a rheological contrast. Above it (shallower, cooler): melt-free, stronger lithosphere. Below it: small amounts of partial melt or fluids make the asthenosphere weaker. This explains geophysical observations (seismic velocity drops, conductivity changes) better than purely dry or thermal models.

MLDs (Old/Thick Cratons): In colder, thicker lithosphere, pargasite breaks down at higher pressures (~3 GPa, sub-solidus), releasing water-rich fluids that may create mid-lithospheric weak zones.

Key Implications

• It provides a geochemical mechanism for why mantle degassing isn’t limited to volcanic arcs or hotspots.
• It highlights under-sampled intraplate fluxes, relevant to long-term carbon cycle modeling (though it does not challenge modern anthropogenic dominance).
• Testable via xenoliths, noble gas/CO₂ isotopic studies, and geophysical imaging in regions like the Pannonian Basin or Vrancea zone.

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On Geological CO2 Emissions
What we know and what we don’t know about

There is a short 2026 commentary/opinion-style piece (3 pages) by István János Kovács (HUN-REN Institute of Earth Physics and Space Science, Sopron, Hungary) titled “On Geological CO₂ Emissions: What we know and what we don’t know about”, published in Science of Climate Change Vol. 6.2, pp. 29–31.

The 2026 piece is a concise, pointed commentary (~3 pages) that uses the pargasosphere framework to highlight systemic under-sampling of intraplate, non-volcanic mantle-derived CO₂ fluxes. It does not dispute that anthropogenic emissions (~35+ Gt/yr) dwarf current volcanic estimates (0.18–0.44 Gt/yr), but argues that diffuse geological sources—especially in extensional or post-rift continental settings—represent a poorly quantified “background” that matters for long-term carbon cycle models, pre-industrial baselines, and millennial-to-Myr attribution.

Refined Mechanism of the Pargasosphere Hypothesis for CO₂ Degassing

The hypothesis (detailed in Kovács et al., 2021, Global and Planetary Change) centers on pargasitic amphibole (a hydrous Ca-amphibole) as a key phase controlling the shallow upper mantle (< ~100 km, roughly the lithosphere-asthenosphere boundary or LAB region) at low water contents (tens to hundreds of ppm wt. H₂O).

Key physical-chemical controls:

• Pargasite has a distinctive dehydration solidus (~1100°C at relevant pressures). This creates a rheological and melting contrast: above the solidus (shallower/cooler), the mantle is largely melt-free and stronger (lithosphere); near/below it, trace hydrous phases or incipient melts weaken the asthenosphere.
• Even at very low bulk water, pargasite acts as a volatile buffer, incorporating H₂O while rejecting much of the CO₂ present in the system.

CO₂ release during cooling/lithospherization (the core process invoked for intraplate degassing):

1. In young or thinned lithosphere (e.g., post-rift basins like the Pannonian), hot asthenospheric mantle upwells or resides close to the LAB.
2. As the system cools and the lithosphere thickens (“lithospherization”), asthenospheric peridotite crosses the pargasite stability field. Small fractions of volatile-bearing incipient melt or fluid (<1 vol%) crystallize.
3. CO₂ incompatibility: CO₂ has very low solubility in the crystallizing silicates and pargasite. It is strongly partitioned into residual fluids, grain boundaries, or fluid inclusions, becoming enriched.
4. These buoyant, mobile CO₂-rich fluids migrate upward via permeable networks, especially along deformation zones, faults, or extensional structures — even without active magmatism or nearby volcanoes.
5. Result: Diffuse surface emanations with mantle isotopic signatures (e.g., noble gases, C isotopes) at fluxes of 10³–10⁵ mol km⁻² yr⁻¹, overlapping quiescent volcanic areas.

This process operates over millions of years. In the Bakony–Balaton Highland Volcanic Field (BBHVF), xenolith data show progressive lithospheric thickening post-rift (~10 Ma to present). One modeled scenario estimates ~10³ Gt CO₂ accumulated and potentially available for release over ~10 Myr in that relatively small area (~25–40 km scale), depending on density and CO₂ concentration assumptions (e.g., ~2000 ppm scenarios).

This is episodic and tectonically modulated: fluids can be trapped in the growing SCLM (subcontinental lithospheric mantle) and released during later thermal/tectonic disturbances.

Strengths and Testable Aspects

• Integrates petrology (xenoliths, mineral stability), geophysics (seismic LAB, conductivity), and geochemistry (fluid inclusions, gas seeps in Pannonian Basin).
• Explains observations of mantle-derived CO₂ in “non-volcanic” or post-volcanic settings better than purely magmatic or metamorphic models.
• Supported by recent Pannonian work (e.g., Randazzo et al. 2025 on deep fluid degassing).

Limitations and Open Questions (Deeper Critique)

Scale and Global Integration:

Local estimates (Pannonian/BBHVF) are compelling for that extensional basin, but extrapolating to global intraplate flux is challenging. How many equivalent settings exist? How much overlaps with or is additional to mid-ocean ridge, arc, and plume fluxes?

Modern vs. Geological Relevance:

The paper explicitly notes this does not change today’s anthropogenic dominance. However, for paleoclimate and carbon cycle models (e.g., over 10⁴–10⁶ yr), adding a dynamic, tectonically variable intraplate component could shift natural baselines and require re-evaluation of source-sink balances.

Quantification Uncertainties: Accumulation rates depend on assumed CO₂ contents, densities, and thickening rates. Xenolith sampling may have biases; fluid migration efficiency, trapping, and re-dissolution in the crust add complexity (as noted in the paper with noble gas and C-He systematics).

Broader Context:

Mainstream carbon cycle literature has long recognized non-volcanic geological CO₂ (e.g., Kerrick 2001 reviews), but diffuse intraplate fluxes remain harder to measure than point sources. Kovács’ group emphasizes under-sampling and calls for more fieldwork/geophysical integration.

Overall Assessment:

This is a specialized, hypothesis-driven contribution from a research group with strong regional data (Pannonian xenoliths and gas studies). It usefully highlights knowledge gaps in deep carbon cycling without overstating impacts on contemporary climate.

The mechanism is mechanistically grounded in mineral physics and petrology, offering a plausible pathway for “invisible” mantle CO₂ release tied to lithospheric evolution. Further validation would come from expanded global xenolith/gas surveys, better flux measurements in other post-rift basins, and incorporation into coupled geodynamic-carbon models.

Kovács (2026) vs. Climate/Carbon Cycle Models: Key Comparison

Kovács’ short commentary explicitly critiques current climate-carbon system models (including those used in IPCC assessments) for under-representing or omitting diffuse intraplate geological CO₂ fluxes, particularly mantle-derived contributions in non-volcanic or post-volcanic continental settings. He acknowledges that anthropogenic emissions (~35+ Gt CO₂/year) vastly exceed conventional volcanic estimates (0.18–0.44 Gt/year), but argues that gaps in natural geological fluxes matter for long-term (millennial to Myr-scale) carbon cycle dynamics.

What Mainstream Models (e.g., IPCC AR6) Typically Include

Climate and Earth System Models (ESMs) in CMIP6/IPCC frameworks treat geological CO₂ sources as follows:

Volcanic and magmatic degassing: Included as a relatively small, often quasi-steady background flux (subaerial + submarine/mid-ocean ridges ~0.3–0.4 Gt CO₂/year globally, sometimes with variability for large eruptions). This is part of the long-term “slow” carbon cycle balancing silicate weathering and burial.

Other geological sources: Metamorphic decarbonation, hydrothermal, and some tectonic degassing are sometimes parameterized, but with high uncertainty. Diffuse, non-point-source fluxes are harder to incorporate due to sparse global measurements.

Focus: Models emphasize fast carbon cycle (atmosphere-land-ocean exchanges) for centennial-scale projections. For paleoclimate and long-term balance, they include volcanic outgassing as a key source, but treat it as relatively smooth or regionally focused (arcs, ridges, hotspots). Intraplate continental degassing is not dynamically resolved at the level of Kovács’ pargasosphere mechanism.

Pre-industrial baseline: Often anchored around 1750 or Holocene values, with natural sources assumed in rough steady-state with sinks over recent millennia.

Uncertainties in deep-time carbon releases (e.g., PETM) are acknowledged, but modern-era attribution treats natural geological fluxes as negligible compared to human emissions.

Kovács’ Specific Claims on Model Shortcomings

Underestimation of intraplate fluxes:

Models are “biased toward areas of active volcanism” and lack “robust parameters” for diffuse mantle-derived CO₂ from cooling asthenosphere beneath young/thin lithosphere, lithospheric deformation zones, and extensional settings (e.g., Pannonian Basin fluxes of 10³–10⁵ mol km⁻² yr⁻¹, comparable to quiescent volcanoes).

Missing dynamic behavior:

The pargasosphere mechanism allows CO₂ accumulation (~10³ Gt over ~10 Myr in one studied area like Bakony–Balaton) in the subcontinental lithospheric mantle (SCLM), followed by episodic release tied to tectonic/thermal events. This “dynamic behaviour is almost entirely missing from current climate-carbon system models.”

Consequences:

1. Oversimplifies the natural baseline for pre-industrial atmospheric CO₂.
2. Complicates attribution on millennial+ scales (separating natural vs. anthropogenic).

Kovács does not claim this overturns modern anthropogenic dominance or short-term (decadal-centennial) model projections. He calls for better constraints to improve long-term models, paleoclimate reconstructions, and geodynamic understanding.

Assessment of the Gap

Scale:

Even if intraplate diffuse fluxes are higher than assumed, global extrapolation from regional data (Pannonian Basin) remains uncertain. Mainstream estimates already include some tectonic/hydrothermal contributions, and total geological outgassing is still ~1% or less of anthropogenic. Significant upward revision would require widespread evidence across many intraplate regions.

Timescale dependence:

For IPCC-style 21st-century projections, this has minimal impact—models are forced by prescribed emissions scenarios. For deep-time carbon cycle modeling (e.g., Phanerozoic CO₂ evolution, PETM analogs), incorporating variable intraplate + pargasosphere-style processes could refine source-sink balances.

Strength of critique:

Kovács highlights a genuine measurement gap (intraplate regions are undersampled). However, mainstream literature (e.g., Deep Carbon Observatory) has been expanding volcanic + geological flux estimates and notes uncertainties. The pargasosphere hypothesis offers a mechanistic explanation testable via xenoliths, gas isotopes, and geophysics, but it is not yet mainstream in global carbon budget models.

Bottom line:

Kovács agrees with models on today’s dominant human role but argues they undersample a variable, tectonically modulated natural geological component. This primarily affects long-term baseline and paleoclimate accuracy rather than invalidating centennial climate projections.

Published: Science of Climate Change Vol. 6.2, pp. 29–31

DOI: 10.53234/scc202603/20

Author: István János Kovács

Abstract

Geological CO₂ emissions from intraplate regions remain poorly quantified despite their relevance to the long-term carbon cycle. Observations from the Pannonian Basin indicate measurable mantle-derived degassing, highlighting the need for improved constraints on non-volcanic geological CO₂ fluxes.