Abstract
The high Fe‐Ti eclogites with exsolved lamellar in 530–600 m depths from the Chinese Continental Scientific Drilling (CCSD) main hole in the Sulu Ultra‐High Pressure (UHP) metamorphic belt, eastern China, record an anomalously high susceptibility (κ), natural remanent magnetization (NRM) and Köenigsberger ratio Q (NRM/Ji, Ji is induced magnetization). This provides us with a good opportunity to study the effects of magnetic minerals and exsolution lamellae on the magnetic properties of deep rocks. In this paper, we have measured systematically magnetic properties and mineral assemblage and structure for three special samples (No. 83, No.86 and No.89). Results show that these Fe‐Ti‐rich eclogites are the result of the fine‐grained (titano)‐magnetite exsolution in pyroxene and lamellar structure consisting of finely interlayered ilmenite and hematite in exsolved hemo‐ilmenite. We found that the dominant Fe‐bearing oxide minerals in samples studied are ilmenite, hematite (Hem + Ilm up to 25%), little (titano)magnetite and pyrite. The ferromagnetic susceptibility is mainly controlled by fine grained (titano)magnetite and NRM is closely related to the exsolved lamellar structure. We hence propose that the anomalous magnetism of these eclogites observed in our samples is the result of exsolution from homogenous pyroxene and ilmeno‐hematite during cooling and decompression processes. These Fe‐Ti‐rich eclogites might be one of the sources of high‐magnetic anomalies observed in the Sulu subduction zone, eastern China.
Highlights
[3] From a large number of previous magnetic measurements of crustal rocks, it is known that the magnetite is the main source of magnetism in deep crustal rocks [Frost and Shive, 1986], but recently, the exsolved hematite-ilmenite solid solution has been shown to carry a strong and stable natural remanane magnetism (NRM) [McEnroe and Brown, 2000; McEnroe et al, 2001b; Kletetschka et al, 2002; Robinson et al, 2006; McEnroe et al, 2007, 2009]
Robinson et al [2002, 2004] have postulated that this type of natural remanent magnetization (NRM) is caused by a ‘contact layer’ between antiferromagnetic hematite and ilmenite within the highly exsolved hemo-ilmenite or ilmeno-hematite and called it lamellar magnetism [Robinson et al, 2002; Kasama et al, 2004]
In this paper we study the magnetic Fe-Ti-rich eclogites collected from the Chinese Continental Scientific Drilling (CCSD) main hole in the Sulu ultrahigh pressure (UHP) metamorphic belt, eastern China
Summary
[2] The magnetic properties of deep crustal rocks are mainly controlled by minerals of the magnetite ― ulvöspinel (xFe22+Ti4+O4.(1 À x)Fe3+(Fe2+Fe3+)O4), hematite ― ilmenite (xFe2+Ti4+O3. (1 À x) Fe23+O3), and pyrrhotite (Fe1ÀXS) solid solutions [Frost and Shive, 1986; Banerjee, 1991; Frost, 1991a, 1991b; Q. Liu et al, 2007, 2008, 2009]. [3] From a large number of previous magnetic measurements of crustal rocks, it is known that the magnetite is the main source of magnetism in deep crustal rocks [Frost and Shive, 1986], but recently, the exsolved hematite-ilmenite solid solution has been shown to carry a strong and stable natural remanane magnetism (NRM) [McEnroe and Brown, 2000; McEnroe et al, 2001b; Kletetschka et al, 2002; Robinson et al, 2006; McEnroe et al, 2007, 2009]. In this paper we study the magnetic Fe-Ti-rich eclogites collected from the CCSD main hole in the Sulu UHP metamorphic belt, eastern China These rocks exhibit a very strong NRM and could help us understand the sources of magnetic anomalies in high-pressure regimes
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