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| Titel |
Physico-chemical constraints on cratonic lithosphere discontinuities |
| VerfasserIn |
Sonja Aulbach, Stéphane Rondenay, Ritske Huismans |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250097174
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| Publikation (Nr.) |
 EGU/EGU2014-12725.pdf |
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| Zusammenfassung |
| The origins of the mid-lithospheric discontinuity (MLD) and lithosphere-asthenosphere
boundary (LAB) have received much attention over the recent years. Peculiarities of
cratonic lithosphere construction - compositional and rheological stratification due to
thickening in collisional settings or by plume subcretion, multiple metasomatic
overprints due to longevity - offer a variety of possibilities for the generation of
discontinuities.
Interconnected small degrees of conductive partial melt (carbonate-rich melts, such as
carbonatites and kimberlites, or highly alkaline melts) at the cratonic LAB, which produce
seismic discontinuities, may be generated in the presence of volatiles. These depress the
peridotite solidus sufficiently to intersect the mantle adiabat at depths near the cratonic
LAB at ~160-220 km, i.e. above the depth of metal saturation where carbonatite
becomes unstable. The absence of agreement between the different seismic and
magnetotelluric estimates for the depth of the LAB beneath Kaapvaal may be due to
impingement of a plume, leading to a pervasively, but heterogeneously metasomatised
(“asthenospherised”) hot and deep root. Such a root and hot sublithosphere may yield
conflicting seismic-thermal-geochemical depths for the LAB. The question arises whether the
chemical boundary layer should be defined as above or below the asthenospherised part of the
SCLM, which has preserved isotopic, compositional (non-primitive olivine forsterite content)
and physical evidence (e.g. from teleseismic tomography and receiver functions) for a
cratonic heritage and which therefore is still distinguishable from the asthenospheric
mantle. If cratonic lithosphere overlies anomalously hot mantle for extended periods
of time, the LAB may be significantly thinned, aided by penetration of relatively
high-degree Fe-rich partial melts, as has occurred beneath the Tanzanian craton.
Xenoliths from the deep Slave craton show little evidence for “asthenospherisation”.
Its root was penetrated by cooler carbo-silicate melts (kimberlites) between ~60
and 350 Ma ago, which are expected in the absence of excess TP, and this may
produce discontinuities rather than gradual changes in physical properties near the
LAB.
MLDs have been detected in continental lithosphere globally at depths from ~60 to 150
km. This may relate to melt infiltration fronts beneath Tanzania and Kaapvaal. However,
the global presence of melts or fluids at MLDs is unlikely, and the spinel-garnet
transition in typically Cr-rich cratonic lithosphere occurs over too large a pressure
interval. This discontinuity is therefore suggested to represent “proto-LABs” that
relate to craton construction. Dipping discontinuities are frequently explained with
craton thickening during subduction-accretion, but can likewise reflect thinning
and erosion of cratonic lithosphere at its edges prior to root thickening by plume
subcretion. Anisotropy can then result from deformation in upwelling mantle that flows
laterally after impinging upon a lithospheric lid. The greater strength of the MLD
signature compared to the LAB may relate to the sharpness of the discontinuity rather
than the size of the seismic velocity anomaly. Thinned edges may be preferred
pathways of kimberlites and their mapping may aid in the discovery of potentially
diamondiferous kimberlites, provided the “diamond window” was not consumed by erosion. |
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