A detailed rock magnetic and opaque mineralogy study of the basalts from the Nazca Plate

Abstract

The penetration of igneous basement in the Nazca Plate during DSDP Legs 16 and 34 provided samples of both fine-grained pillow-basalt and coarse-grained massive flow units. The magnetic mineral in these basalt samples is initially a titanomagnetite (Fe,Ti,O4) with a narrow range of composition of x = 0.62 f 0.05. Subsequent to formation, the titanomagnetite grains are generally subjected to low temperature oxidation to titanomaghemite with a corresponding rise in Curie temperature from the initial values of 120-150°C up to a maximum of 400°C. Both grain size and low-temperature oxidation state play important, and interrelated, roles in controlling the intensity and stability of magnetic remanence and other magnetic properties. Overall grain size can, in some cases, be related to oxidation state since some sections of the relatively impermeable massive flows can remain unoxidized for as long as 40 Myr while pillow basalts are extensively oxidized within 34 Myr. Low-temperature alteration in turn effects magnetic grain size since oxidation and subsequent Fe cation migration results in grain subdivision by the formation of shrinkage cracks. A five-stage sequence of the microscopic changes that are associated with progressive low-temperature oxidation is proposed and illustrated with photomicrographs from these basalt samples.A hierarchy in the intensity of magnetic remanence may exist with unoxidized pillow basalts having a much higher intensity and oxidized pillow basalts having a much lower intensity than the massive flow units. While pillow basalts are relatively immune to the addition of secondary components of magnetization, the coarse-grained massive flows readily acquire components of viscous remanence. Although they oxidize much more slowly than pillows, when oxidation does take place, components of chemical remanence can be acquired by the multi-domained grains in the massive flow units. 319319A 320 321

Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments

“…Easy access to the upper extrusive basalts has resulted in a knowledge bias of crustal magnetization. At this time there is general understanding and agreement regarding the importance of the extrusives and their magnetic properties [Ade-Hall et al, 1971;Bina and Henry, 1990;Johnson and Hall, 1978;Kent and Gee, 1994]. In contrast, our understanding of how the lower lithologic units, including diabase dikes, gabbros and serpentinized peridotites, relate to the anomaly signal is more uncertain.…”

“…3-11) and are therefore comparable with Group 2 (PSD) basalts . Bina et al [1990a] suggested that the ODP 648B PSD basalts experienced a low degree of maghemitization that had not yet physically divided the titanomagnetite grains into smaller sizes [Johnson and Hall, 1978] and/or longer lava cooling times that allows larger grains to form. We interpret Group 2 (PSD) basalts as having lower oxidation parameters (lower maghemization) than Group 1 (SD) basalts, from their susceptibility curves, microscopic observations, and larger oxide grain sizes in general.…”

“…The source of NRM in unaltered basalts is thermoremanent magnetization (TRM), acquired as the iron-titanium (Fe-Ti) oxides cool through their Curie temperatures. The Fe-Ti oxide titanomagnetite with ulvopsinel content x = 0.6 (TM60, Fe 3 .xTixO 4 ) is the most common magnetic carrier in basalts Johnson and Hall, 1978]. Zhou et al [1997] …”

Magnetic properties of Cenozoic ophiolites in the Hayama-Mineoka and Setogawa belts, South-Central Honshu, Japan.

“…The averaged x-value (0.79) of the present basalts samples except hydrothermally altered ones is substantially higher than that of unoxidized titanomagnetite contained in the oceanic basalts (0.65 after JOHNSON and HALL, 1978). Such higher x-values have been reported in the oxidized basalts recovered in the DSDP holes (JOHNSON and HALL, 1978;KOBAYASHI et al, 1979) and attributed to bleaching of iron from titanomagnetite to form reddish stains in cavities of titanomagnetite and the surrounding silicates.…”

“…Such higher x-values have been reported in the oxidized basalts recovered in the DSDP holes (JOHNSON and HALL, 1978;KOBAYASHI et al, 1979) and attributed to bleaching of iron from titanomagnetite to form reddish stains in cavities of titanomagnetite and the surrounding silicates. If such a bleaching proceeds at low temperature, more oxidized titanomagnetite would have larger x.…”