Magnetic properties and paleointensities as function of depth in a Hawaiian lava flow

Abstract

The outcome of paleointensity experiments largely depends on the rock-magnetic properties of the samples. To assess the relation between volcanic emplacement processes and rock-magnetic properties, we sampled a vertical transect in a 6 m thick inflated lava flow at Hawaii, emplaced in 588 AD. Its rock-magnetic properties vary as function of distance from the flow top; the observations can be correlated to the typical cooling rate profile for such a flow. The top and to a lesser extent the bottom parts of the flow cooled faster and reveal a composition of TM60 in which the magnetic remanence is carried by finegrained titanomagnetites, relatively rich in titanium, with associated low Curie and unblocking temperatures. The titanomagnetite in the slower cooled central part of the flow is unmixed into the magnetite and € ulvospinel end-members as evidenced by scanning electron microscope observation. The remanence is carried by coarse-grained magnetite lamella ( TM0) with high Curie and unblocking temperatures. The calibrated pseudo-Thellier results that can be accepted yield an average paleointensity of 44.1 6 2.4 lT. This is in good agreement with the paleointensity results obtained using the thermal IZZI-Thellier technique (41.6 6 7.4 lT) and a recently proposed record for Hawaii. We therefore suggest that the chance of obtaining a reliable paleointensity from a particular cooling unit can be increased by sampling lavas at multiple levels at different distances from the top of the flow combined with careful preliminary testing of the rockmagnetic properties.

Demise of the rapid-field-change hypothesis at Steens Mountain: The crucial role of continuous thermal demagnetization

“…For (ii), indications from thermomagnetic curves of nearly pure magnetite near the top and bottom and a nearly unoxidized titanomagnetite phase in the interior is consistent with enhanced access to oxygen from the outside. Note, however, that de Groot et al (2014) have documented the opposite trend in a much thicker (6 m) flow in Hawaii, where degree of oxidation appears to have been controlled instead by cooling rate. The high-Curie-temperature phases that dominate at the margins were likely formed by oxy-exsolution at high-temperature, implying further reduction in effective magnetic grain size, consistent with the hysteresis results.…”

“…Considerable attention has been devoted over the years to basalts that exhibit negative interactions between the partial TRMs of two magnetic phases with different Curie temperatures (e.g., Nagata and Ozima, 1955;Havard and Lewis, 1965;Creer and Petersen, 1969;Creer et al, 1970;Heller and Petersen, 1982;Krása et al, 2005;Draeger et al, 2006). The relevance of these studies for this paper is not the negative interactions themselves, which are relatively weak in Steens flow B51 and not responsible for the failure of conventional thermal demagnetization to isolate the primary TRM direction.…”

Nonheating methods for absolute paleointensity determination: Comparison and calibration using synthetic and natural magnetite‐bearing samples

“…Based on our experimental investigation of the synthetic and natural samples, we feel that ARM may be a better analogue of TRM than SIRM for the purposes of calibrated absolute paleointensity determination. Although the ARM and pseudo‐Thellier methods are characterized by intrinsic uncertainty associated with the calibration procedure, both methods have resulted in reasonable paleointensity estimates from natural and model samples in this and other studies [e.g., Yu , ; de Groot et al ., , , , ; Paterson et al ., ]. However, our results emphasize that when possible and practical, the values of calibration factors f ARM and f PT should be individually selected based on the available information about the magnetic grain size and domain state.…”

“…The pseudo‐Thellier method has been applied more recently to determine absolute paleointensity estimates from igneous rocks and synthetic samples [ Yu et al ., ; de Groot et al ., , , , ; Paterson et al ., ]. In the experiments on a model TRM in synthetic and natural samples, Yu et al .…”

“…[] suggested to use this range as one of the acceptance criteria for the pseudo‐Thellier method. Although the linear calibration dependence described by equation does not pass through the origin, the formula yielded reasonable results from a historical Hawaiian lava flow [ de Groot et al ., ] and lava flows from the Canary and Terceira Islands [ de Groot et al ., , ].…”