High paleointensities of the geomagnetic field from thermomagnetic studies on Rift Valley pillow basalts from the Mid‐Atlantic Ridge

Abstract

Nineteen pillow basalts dredged within the rift valley of the Mid‐Atlantic Ridge at36.8°N were studied by the Thellier stepwise heating method in order to determine the paleointensity of the geomagnetic field when they erupted on to the sea floor. Previously reported fission track ages are 2,000 to 6,000 years for the youngest rocks (mainly olivine basalts) and 10,000 to 100,000 years for the others (mainly plagioclase basalts and pyroxene basalts). All but three pillow basalts meet the conditions commonly considered as indicative of quite reliable paleointensity estimates: stability of the direction of NRM during its thermal demagnetization, constant ratio of NRM/TRM (natural remanent magnetization to thermoremanent magnetization) over 50% or more of the original NRM intensity (80 to 94% for 11 specimens), and reproducibility of low‐temperature partial TRM (PTRM). However, strong field thermomagnetic measurements indicate that 11 of these 16 samples display a significant increase in Curie temperature (15 to 80°C) during the paleointensity experiments below 250°C, notwithstanding the linearity of the NRM‐TRM plot in this temperature interval. This alteration, probably due to low‐temperature oxidation of the specimens, seems typical of young pillow basalts and may result in paleointensity estimates which are too high. This result shows that excellence of remanence tests (NRM‐TRM linearity and PTRM stability) does not ensure the absence of chemical changes during the Thellier experiments and therefore the validity of the paleointensity obtained. The assumption that reliable paleointensities are obtained when the Curie point increase is 10°C or less led to the selection of five of our specimens, all from the youngest group. Their mean paleointensity is 64.2±20.5 μT (standard deviation) and the corresponding virtual axial dipole moment (VADM) is 11.5±3.7× 1022 A m2. Given the variations of the VADM of the Earth's magnetic field over the last 6,000 years, as established from archeomagnetic studies, our paleointensity results suggest that the latest eruptions on the inner floor of the Rift Valley at 36.8°N occurred 1,500±1,000 years ago.

Paleomagnetic and rock-magnetic survey of Brunhes lava flows from Tancitaro volcano, Mexico

“…One sample from each site were heated up to about 630°C at a heating rate 20°C/ min and then cooled at the same rate. Curie temperature was determined by the Prévot et al's (1983) method. Alternatively, low-temperature (from about -185°C to room temperature) susceptibility was recorded using the same apparatus.…”

Early cretaceous absolute geomagnetic paleointensities from Córdoba Province (Argentina)

“…Additionally, we estimated the viscosity index according to procedures described in Prévot et al (1983). Determination of the viscosity index (Thellier and Thellier, 1944) allows to estimate the capacity of a sample to acquire a viscous remanent magnetization, and is therefore useful to obtain information about the sample's paleomagnetic stability.…”

Pre‐Columbian mural paintings from Mesoamerica as geomagnetic field recorders

“…The hypothesis of viscous magnetization (VRM) is hardly tenable, because secondary components are minor and ChRM is stable against AF treatment up to 50 to 70 mT. Moreover, the capability of samples to acquire a VRM is limited, as shown by the low values (usually less than 8%) of the viscosity indexes, evaluated according the method of Prévot et al [1983]. Lastly, there is no evidence for a secondary, chemical magnetization since the studied murals show sign neither of weathering nor of other kind of alteration.…”