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Ultramafic pseudotachylytes have been regarded as earthquake fossils formed at mantle depths (i.e., >30 km). Here we show that pseudotachylytes hosted by ultramafic rocks from three localities have distinct magnetic properties. Fresh host peridotites contain only small amounts of coarse‐grained magnetite. In contrast, the ultramafic pseudotachylytes contain variable amounts of significantly finer magnetite that formed coseismically through melting. Among each locality, magnetite abundance in the pseudotachylytes ranges over several orders of magnitude (4–2,000 ppm), and magnetic grain size varies considerably (from single domain to multidomain). Because the host peridotites are compositionally similar, the pseudotachylyte magnetic properties are interpreted to primarily reflect the physical and cooling conditions prevailing during seismic slip. Further, the examination of laboratory‐produced ultramafic pseudotachylytes shows that quenching does not produce superfine magnetite. We hypothesize that the magnetic properties of ultramafic pseudotachylytes are controlled by fO2 and in consequence vary systematically with depth of formation. Therefore, these properties can be used to assess if the ruptures producing the earthquakes that these pseudotachylytes represent nucleated at actual mantle depths or at shallow depths during exhumation of mantle rocks.

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