Constraints on Earth's Thermal Evolution from the Heavy Noble Gas Content of the Mantle.
Abstract
The heavy noble gas content of the mantle is strongly dependent on the input and output of these gases through subduction and volcanism, respectively. The amount of each noble gas that reaches the mantle interior from the subducting slab is governed by the thermal regime of the subduction zone. In hot subduction zones, such as Cascadia, lighter gases such as Ne and Ar are preferentially lost through slab dehydration in comparison to the heavier gases, Kr and Xe; these lighter gases are returned to the atmosphere through arc volcanism, whereas more of the heavier gases remain ad become incorporated into the mantle. Alternatively, in cold subduction zones such as Honshu, there is no preferential loss in Ne and Ar compared to Kr and Xe, and each species ends up in the mantle in the same relative proportions to one another as they are in the oceanic crust. Cold subduction is the dominant thermal regime of subduction on the present-day Earth, and measurements of current noble gas abundances in the mantle are also consistent with cold subduction. This indicates that cold subduction is responsible for setting the heavy noble gas content of the mantle.
A critical question is then what thermal and tectonic evolution scenarios for the Earth are compatible with present day noble gas abundances of the mantle. We model the cycling of noble gases between the surface and mantle coupled with the mantle's thermal evolution to answer this question. We consider the classic case of mantle convection where plate tectonics is active throughout Earth history the heat flux scales with the Rayleigh number to the 1/3 power. Our results indicate the mantle noble gas content comes to equilibrium in only 1 Ga at modern mantle conditions. As a result, only 1 Ga of cold subduction is needed to produce noble gas abundances observed in the mantle. Cold subduction prevailing for the past 1 Ga is well supported by the rock record, as metamorphic rock from high pressure, low temperature environments date back to 1.5 Ga. We thus conclude that the mantle noble gas content is unaffected by processes prior to 1 Ga and therefore is compatible with a wide range of early Earth tectonic and thermal evolution scenarios; that is, it does not provide a strong constraint on Archean tectonics.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.V11G0091R
- Keywords:
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- 1009 Geochemical modeling;
- GEOCHEMISTRYDE: 1038 Mantle processes;
- GEOCHEMISTRYDE: 1041 Stable isotope geochemistry;
- GEOCHEMISTRYDE: 1060 Planetary geochemistry;
- GEOCHEMISTRY