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Abstract The goal of this study is to constrain the origins of layering in the seismic velocity structure within the cratonic mantle lithosphere (i.e. mid‐lithospheric discontinuities [MLDs]). For long‐lived stations in cratons worldwide, we calculated S‐to‐P converted phase receiver function stacks using time domain deconvolution and a k‐means algorithm to select robust, consistent receiver functions. Negative MLDs appear in only 50% of the receiver function stacks, indicating that negative MLDs are common but intermittent. The negative MLDs correspond to shear velocity drops of 1%–4%, which could be caused by layers of minerals created by metasomatism, although vertical layering in seismic anisotropy cannot be ruled out. In craton interiors, negative MLDs have a lower amplitude (<3% velocity drops) and can be explained by metasomatism of the original Archean mantle. Negative MLD amplitudes increase with decreasing upper mantle shear velocity (toward the outer margins of the cratons), but do not depend on the age of the craton. Thus, negative MLD amplitudes are not dominated by age‐related variations in the cratonic mantle composition, and, instead, are more strongly correlated with proximity to tectonic and metasomatic activity that occurred long after craton formation. Negative MLDs are less numerous among stations that have Paleoproterozoic and Archean thermotectonic ages, consistent with the view that shallow release of slab‐derived fluids during early “warm” subduction was less favorable for negative MLD formation. We also observe velocity gradients below 150 km at stations in craton boundaries and interiors, indicating the presence of seismic velocity changes at the cratonic lithosphere‐asthenosphere boundary and/or Lehmann discontinuity.
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Evidence for amorphous calcium carbonate originated mid-lithospheric discontinuities
The cratonic lithosphere is a vast host for deep recycled carbon, trapping up to several weight percent CO2 at depths overlapping the seismic mid-lithospheric discontinuities (MLDs). However, the role of carbonates, especially for the latest discovered amorphous calcium carbonate (CaCO3), is underestimated in the formation of MLDs. Using the pulse-echo-overlap method in a Paris-Edinburgh press coupled with synchrotron X-ray diffraction, we explored the acoustic velocities of CaCO3 under high pressure-temperature (P-T) conditions relevant to the cratonic lithosphere. Two anomalous velocity drops were observed associated with the phase transition from aragonite to amorphous phase and with the pressure-induced velocity drop in the amorphous phase around 3 GPa, respectively. Both drops are comparable with approximately 35% and 52% reductions for compressional (VP) and shear (VS) wave velocities, respectively. The VP and VS values of the amorphous CaCO3 above 3 GPa are about 1/2 and 1/3 of those of the major upper-mantle minerals, respectively. These velocity reductions caused by the presence of CaCO3 would readily cause MLDs at depths of 70–120 km dependent on the geotherm even if only 1–2 vol.% CaCO3 is present in the cratonic lithosphere.
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- Award ID(s):
- 2243184
- PAR ID:
- 10587199
- Publisher / Repository:
- Cell Press
- Date Published:
- Journal Name:
- The Innovation Geoscience
- Volume:
- 2
- Issue:
- 4
- ISSN:
- 2959-8753
- Page Range / eLocation ID:
- 100098
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.59717/j.xinn-geo.2024.100098
