Cratonic Lithosphere Discontinuities and the Deep Volatile Cycle
Abstract
Cratons represent modern Earth's thickest and coldest lithospheres, typically reaching deep into the upper mantle. Seismic studies reveal the presence of both ubiquitous mid-lithospheric discontinuities (MLDs) and of rare discontinuities at the lithosphere-asthenosphere boundary (LAB).
Despite its strong initial depletion and attendant loss of highly incompatible carbon, the continental lithospheric mantle constitutes a significant present-day carbon reservoir [1]. Thus, carbon - and water - must have been reintroduced over time from small-volume volatile-rich melts typically forming beneath thick cratonic LABs. This led to gradual re-oxidation and refertilisation, and to the eventual deposition of MLD-producing metasomes consisting of carbonates and hydrous minerals. Such metasomes, sometimes interpreted as failed kimberlite intrusions, are much more rarely identified in xenolith suites worldwide than the small-volume potassic magmas derived from them, which testify to their existence at least from the Palaeoproterozoic onwards. The rarity of cratonic LAB discontinuities (LADs), which may require small volumes of partial melt to be present, reflects the difficulty to stabilise and maintain such melt below deep present-day intact cratonic roots ( 180-260 km), except if sufficiently warm and/or volatile-rich and/or oxidising mantle is advected. If temperatures, degree of hydration and seismic velocity gradient can be independently constrained, the presence of LADs may place lower bounds on the amount of volatiles that must be present in the mantle volume, whereas their absence places upper bounds. Despite the rarity of LADs, kimberlite activity on all cratons indicates that small-volume melts were repeatedly formed and extracted in the past. These melts eventually intruded deeply into lithospheric cores causing rheological weakening and densification and leading to reworked deep cratonic roots, and to meta-cratonisation where deep roots were entirely lost. Meta-cratonic LAB depths (mainly at 80 to 150 km) strikingly overlap those of MLDs in intact neighbouring continental mantle. These MLDs plausibly provided lithospheric weak zones along which the deep root could be removed during continent collision, flat subduction or rifting. [1] Foley & Fischer 2017 Nature Geosci- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T33C0419A
- Keywords:
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- 7205 Continental crust;
- SEISMOLOGYDE: 8103 Continental cratons;
- TECTONOPHYSICSDE: 8110 Continental tectonics: general;
- TECTONOPHYSICSDE: 8120 Dynamics of lithosphere and mantle: general;
- TECTONOPHYSICS