Physics
Scientific paper
Apr 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989pepi...54..169b&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 54, Issue 1-2, p. 169-179.
Physics
14
Scientific paper
Initial susceptibility k, coercive force Hc, remanent coercive force Hrc, and saturation induced and remanent magnetizations Js and Jrs, have been measured at a series of 10-12 temperature steps between ambient temperature and 340°C for three selected fine- or coarse-grained submarine basalts ranging in age from the present to 30 Ma. Possible magneto-chemical changes which could have been induced by the successive heatings were checked through repeated measurements at room temperature which showed that no significant changes of the hysteresis parameters occurred below the 300°C temperature step. Average Curie points are between 110 and 170°C but a quite large dispersion of Curie points exists in a single rock, sometimes over 400°C. At room temperature the bulk of magnetic oxides consist either of multidomain (MD) grains, in the two coarse-grained rocks (dolerites), or of small pseudosingle-domain (PSD) particles in the fine-grained rock (a pillow basalt). The temperature dependence of coercivity of the oldest coarse-grained rocks is surprisingly different from that of synthetic titanomagnetites. Coercivity is found to increase at intermediate temperatures, with a maximum in the range 250-300°C. In the most spectacular example, Hc rises from 28 to 75 Oe and Hrc from 67 to 200 Oe when temperature rises from 20 to 235° C. Correlatively, Jrs/Js increases to 0.3, indicating the presence of small PSD grains. The youngest pillow basalt sample displays similar although faint evidence of the presence of grains finer and with higher Curie points than the bulk of the titanomagnetite population. This thermal behaviour of submarine basalts is interpreted as the result of differential maghemitization within a single rock, the finest grains being more rapidly and more completely oxidized at low temperature. In our 3-Ma-old coarse-grained sample, the average oxidation index z is only 0.3 (average Curie point 110°C) while the finest grains are completely maghemitized (z ⋍ 1) with a Curie point of the order of 450° C.
Bina Matteo
Prévot Michel
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