Eight good reasons why the uppermost mantle could be magnetic

Abstract

Wasilewski et al. (1979) concluded that no magnetic remanence existed in the uppermost mantle and that even if present, such sources would be at temperatures too high to contribute to long wavelength magnetic anomalies (LWMA). However, new collections of unaltered mantle xenoliths indicate that the uppermost mantle could contain ferromagnetic minerals. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 2/39This is most easily explained as a thermoremanent magnetization acquired by pre-existing ferromagnetic minerals as xenoliths cool rapidly at the Earth's surface from magmatic temperatures, acquired during ascent. 7. Modern experimental data suggest that the wüstite-magnetite oxygen buffer and the fayalite-magnetite-quartz oxygen buffer extend several tens of km within the uppermost mantle. 8. The magnetic properties of mantle xenoliths vary consistently across tectonic settings. In conclusion, the model of a uniformly non-magnetic mantle should be revisited.

Depth to Curie temperature or bottom of the magnetic sources in the volcanic zone of la Réunion hot spot

“…This assumption is largely supported by recent studies that suggest that the upper mantle is magnetized (Ferré et al, 2014). This is also in agreement with an oceanic mantle mostly composed of peridotite, with a substantial proportion of serpentinite, and therefore highly magnetized.…”

“…However, there is growing evidence that the upper lithospheric mantle may also contribute to magnetic anomalies in some geologic environments (Ferré et al, 2014). As also demonstrated in Supplementary Online material B, we find that magnetic sources beneath La Réunion Island lie significantly deeper than the crust-mantle interface.…”

A statistical spatial power spectrum of the Earth's lithospheric magnetic field

“…Of course, they do not prove the validity of the underlying assumptions. The statistical solution does not resolve the ambiguity about the nature (induced or remanent) and the formation process of the magnetic sources nor does it exclude the presence of magnetic carrier locally down to the upper mantle (Ferré et al 2014). The regional interpretations of the magnetic anomalies in terms of susceptibility only (e.g.…”

Earthquakes in the Mantle? Insights From Rock Magnetism of Pseudotachylytes

“…For these localities, the mean magnetic coercivity ( H c ) falls in the 20 to 50 mT range, with a mean around 30 mT, in agreement with values obtained from the major hysteresis loops (Table ). These ultramafic pseudotachylytes show FORC similarities to pseudotachylytes derived from quartzo‐feldspathic/tonalitic host rocks (e.g., Ferré et al, ; Ferré, Friedman, et al, and Ferré, Geissman, et al, ). The H c parameter varies in a range typical for stoichiometric magnetite and further confirms the absence of highly oxidized phases such as hematite.…”