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.
Mid‐lithosphere discontinuities are seismic interfaces likely located within the lithospheric mantle of stable cratons, which typically represent velocities decreasing with depth. The origins of these interfaces are poorly understood due to the difficulties in both characterizing them seismically and reconciling the observations with thermal‐chemical models of cratons. Metasomatism of the cratonic lithosphere has been reported by numerous geochemical and petrological studies worldwide, yet its seismic signature remains elusive. Here, we identify two distinct mid‐lithosphere discontinuities at ∼87 and ∼117 km depth beneath the eastern Wyoming craton and the southwestern Superior craton by analyzing seismic data recorded by two longstanding stations. Our waveform modeling shows that the shallow and deep interfaces represent isotropic velocity drops of 2%–8% and 4%–9%, respectively, depending on the contributions from changes in radial anisotropy and density. By building a thermal‐chemical model including the regional xenolith thermobarometry constraints and the experimental phase‐equilibrium data of mantle metasomatism, we show that the shallow interface probably represents the metasomatic front, below which hydrous minerals such as amphibole and phlogopite are present, whereas the deep interface may be caused by the onset of carbonated partial melting. The hydrous minerals and melts are products of mantle metasomatism, with CO2‐H2O‐rich siliceous melt as a probable metasomatic reagent. Our results suggest that mantle metasomatism is probably an important cause of mid‐lithosphere discontinuities worldwide, especially near craton boundaries, where the mantle lithosphere may be intensely metasomatized by fluids and melts released by subducting slabs.
more » « less- NSF-PAR ID:
- 10480126
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- AGU Advances
- Volume:
- 4
- Issue:
- 6
- ISSN:
- 2576-604X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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