Diamonds from the Alps reveal carbon mobility in subduction zones

Ocean Subduction Dynamics in the Alps

Elements ◽

10.2138/gselements.17.1.9 ◽

2021 ◽

Vol 17 (1) ◽

pp. 9-16

Author(s):

Philippe Agard ◽

Mark R. Handy

Keyword(s):

Transition Zone ◽

Continental Margin ◽

North Atlantic ◽

Subduction Zones ◽

Mantle Transition Zone ◽

Continental Subduction ◽

The Alps ◽

Slow Spreading ◽

Selective Preservation

The Alps preserve abundant oceanic blueschists and eclogites that exemplify the selective preservation of fragments of relatively short-lived, small, slow-spreading North Atlantic–type ocean basins whose subducting slabs reach down to the Mantle Transition Zone at most. Whereas no subducted fragments were returned during the first half of the subduction history, those exhumed afterwards experienced conditions typical of mature subduction zones worldwide. Sedimentary-dominated units were under-plated intermittently, mostly at ~30–40 km depth. Some mafic–ultramafic-dominated units formed close to the continent were subducted to ~80 km and offscraped from the slab only a few million years before continental subduction. Spatiotemporal contrasts in burial and preservation of the fragments reveal how along-strike segmentation of the continental margin affects ocean subduction dynamics.

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Diamond growth from organic compounds in hydrous fluids deep within the Earth

Nature Communications ◽

10.1038/s41467-019-12984-y ◽

2019 ◽

Vol 10 (1) ◽

Cited By ~ 5

Author(s):

Maria Luce Frezzotti

Keyword(s):

Organic Compounds ◽

Subduction Zones ◽

Diamond Growth ◽

Diamond Surfaces ◽

Molecular Precursors ◽

Carbon Saturation ◽

Carboxylate Groups ◽

Dissolved Species ◽

Lithospheric Plates ◽

The Alps

Abstract At subduction zones, most diamonds form by carbon saturation in hydrous fluids released from lithospheric plates on equilibration with mantle rocks. Although organic molecules are predicted among dissolved species which are the source for carbon in diamonds, their occurrence is not demonstrated in nature, and the physical model for crustal diamond formation is debated. Here, using Raman microspectroscopy, I determine the structure of carbon-based phases inside fluid inclusions in diamond-bearing rocks from the Alps. The results provide direct evidence that diamond surfaces are coated by sp2-, and sp3-bonded amorphous carbon and functional groups of carboxylic acids (e.g., carboxyl, carboxylate, methyl, and methylene), indicating the geosynthesis of organic compounds in deep hydrous fluids. Moreover, this study suggests diamond nucleation via metastable molecular precursors. As a possible scenario, with carbon saturation by reduction of carboxylate groups, I consider tetrahedral H-terminated C groups as templates for the growth of sp3-structured carbon.

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Subduction initiation and subsequent burial-exhumation of (ultra)high-pressure rock

10.5194/egusphere-egu21-10097 ◽

2021 ◽

Author(s):

Stefan Markus Schmalholz ◽

Lorenzo Candioti ◽

Joshua Vaughan-Hammon ◽

Thibault Duretz

Keyword(s):

High Pressure ◽

Subduction Zones ◽

Conceptual Models ◽

Passive Margin ◽

Far Field ◽

Subduction Initiation ◽

Ultra High Pressure ◽

Orogenic Wedge ◽

The Alps ◽

Marine Basin

Subduction zones are one of the main features of plate tectonics, they are essential for geochemical cycling and are often a key player during mountain building. However, several processes related to subduction zones remain elusive, such as the initiation of subduction or the exhumation of (ultra)high-pressure rocks.Collision orogens, such as the European Alps or Himalayas, provide valuable insight into long-term subduction zone processes and the larger geodynamic cycles of plate extension and subsequent convergence. For the Alps, geological reconstructions suggest a horizontally forced subduction initiation caused by the onset of convergence between the Adriatic and European plates. During Alpine orogeny, the Piemont-Liguria basin and the European passive magma-poor margin (including the Brian&#231;onnais domain) were subducted below Adria. The petrological rock record indicates burial and subsequent exhumation of both continental and oceanic crustal rocks that were exposed to (ultra)high-pressure metamorphic conditions during their Alpine burial-exhumation cycle. Moreover, estimates of exhumation velocities yield magnitudes in the range of several mm/yr to several cm/yr. However, published estimates of exhumation velocities, ages of peak metamorphic conditions and estimates for peak pressure and peak temperature vary partly significantly, even for the same tectonic unit. Consequently, many, partly significantly, contrasting conceptual models for subduction initiation (convergence versus buoyancy driven) or rock exhumation (channel-flow, diapirism, episodic regional extension, erosion etc.) have been proposed for the Alps.&#160;Complementary to the data-driven approach, mathematical models of the lithosphere and upper mantle system are useful tools to investigate geodynamic processes. These mathematical models integrate observational and experimental data with the fundamental laws of physics (e.g. conservation of mass, momentum and energy) and are useful to test conceptual models of subduction initiation and rock exhumation. Here, we present numerical solutions of two-dimensional petrological-thermo-mechanical models. The initial model configuration consists of an isostatically and thermally equilibrated lithosphere, which includes mechanical heterogeneities in the form of elliptical regions with different effective viscosity. We model a continuous geodynamic cycle of subsequent extension, no far-field deformation and convergence. During extension, the continental crust is necked, separated and mantle is exhumed, forming a marine basin bounded by passive margins. During the subsequent stage with no far-field deformation, the thermal field of the lithosphere is re-equilibrated above a convecting mantle. During convergence, subduction is initiated at one passive margin and the mantle lithosphere below the marine basin as well as the other passive margin are subducted. During progressive subduction, parts of the subducted continental upper crust are sheared-off the subducting plate and are exhumed to the surface, ultimately forming an orogenic wedge. For the convergence, we test the impact of serpentinites at the top of the exhumed mantle on orogenic wedge formation. We compare the model results with observational and experimental constraints, discuss the involved processes and driving forces and propose a model for subduction initiation and (ultra)high-pressure rock exhumation for the Alps.

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Ocean subduction dynamics in the Alps

10.5194/egusphere-egu21-5235 ◽

2021 ◽

Author(s):

Philippe Agard ◽

Mark Handy

Keyword(s):

Transition Zone ◽

Continental Margin ◽

North Atlantic ◽

Subduction Zones ◽

Mantle Transition Zone ◽

Continental Subduction ◽

The Alps ◽

Slow Spreading ◽

Selective Preservation

The Alps preserve abundant oceanic blueschists and eclogites that exemplify the selective preservation of fragments of relatively short-lived, small, slow-spreading North Atlantic-type ocean basins (here the ~400-700 km wide Alpine Tethys), whose subducting slabs reach down to the Mantle Transition Zone. Whereas none of the subducted fragments were returned during the first half of the subduction history, those exhumed afterwards experienced conditions typical of mature subduction zones worldwide. Sedimentary-dominated units were underplated intermittently, mostly at ~30-40 km depth, while mafic/ultramafic-dominated units subducted to ~80 km (In the W. Alps), whose protoliths had formed close to the continent, were offscraped from the slab only a few Ma before continental subduction. Spatiotemporal contrasts in burial and preservation of the fragments reveal how along-strike segmentation of the continental margin affects ocean subduction dynamics.

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