Possible TCRM acquisition of the Kilauea 1960 lava, Hawaii: failure of the Thellier paleointensity determination inferred from equilibrium temperature of the Fe−Ti oxide

Abstract

Natural rock samples may not always be the ideal material for the Thellier-type method as they occasionally result in high paleointensities. The Kilauea 1960 lava, Hawaii, is one such example. Several previous studies have suggested that one of the possible causes for this undesirable behavior is an acquisition of thermochemical remanent magnetization (TCRM) during lava formation. In order to examine this possibility quantitatively, equilibrium temperatures of titanomagnetite grains, which are associated with samples previously subjected to Thellier experiments, are estimated by a Fe-Ti oxide geothermometer. The results show that two specimens from the rock magnetic group giving relatively ideal Thellier paleointensities have clustered equilibrium temperatures of about 800-900 and 700-800• C. In contrast, two swarmed temperatures around 300 and 700• C were observed for the specimen from a group yielding high paleointensities. Although these are semi-quantitative estimates, when the time scales of Fe-Ti interdiffusion and lava cooling are taken into consideration, the last specimen could have acquired the TCRM during its formation. For such specimens, simple calculation predicts that TCRM/TRM (thermoremanent magnetization) ratios could be 1.19-1.72 for the blocking temperature range of 400-480• C, assuming a grain-growth model. The extent of this overestimation (20-70%) is comparable to the magnitude of the observations. It is therefore suggested that attention be paid to titanomagnetite grains with well-developed ilmenite lamellae, as these could be potential sources of overestimations of Thellier paleointensities of up to a few tenths of percentage points.

Low-temperature magnetic properties of andesitic rocks from Popocatepetl stratovolcano, Mexico

“…Indeed, at least some of our susceptibility curves resemble curves measured on synthetic samples of this composition (Ishikawa et al, 1985). Similar χ -T curves have been also reported for some Hawaiian basalts (Kontny et al, 2003;Yamamoto, 2006) where presence of (hemo) ilmenite was demonstrated by microscopic observations. On the other hand, Kosterov (2001) did not observe peaks attributable to (hemo) ilmenite is basalts from Karoo, Parana, and North Atlantic Large Igneous Provinces (LIPs), despite the fact that abundant exsolved titanomagnetite grains, where hemoilmenite should be present in lamellae (cf.…”

“…Kontny et al (2003) have measured susceptibility and hysteresis loops as a function of temperature for Hawaiian basalts, also of hotspot origin. Additional low-temperature susceptibility data for Hawaiian volcanics have been reported by Yamamoto (2006). The present study thus extends the low-temperature magnetic measurements both in rock composition (andesites rather than basalts) and in tectonic setting (subduction-related volcanics).…”

“…This preference is a consequence of limited applicability of traditional high-temperature magnetic techniques in studies of the magnetic mineralogy of sediments, because of chemical alteration during heating. At the same time, relatively little has been reported on the magnetic properties of volcanic rocks at low temperatures and, moreover, these studies have been so far limited to basalts (Radhakrishnamurty et al, 1981;McElhinny, 1981, 1982;UrrutiaFucugauchi et al, 1984;Thomas, 1993;Shau et al, 2000;Kosterov, 2001;Kontny et al, 2003;Yamamoto, 2006).…”

The intensity of the geomagnetic field in the late-Archaean: new measurements and an analysis of the updated IAGA palaeointensity database

“…The potential for biasing caused by the remanence carriers undergoing chemical changes below their Curie temperature during the original cooling of the unit (and therefore retaining a chemical or thermochemical remanence) is, as for many studies, not so well constrained. The nature of such effects and the extent to which they occur is presently an area of active research (Smirnov and Tarduno, 2005;Draeger et al, 2006;Yamamoto, 2006). Here, we merely point out that the results of the rock magnetic analysis are consistent with the units having acquired a purely thermal remanence.…”

Paleomagnetism of the Datong monogenetic volcanoes in China: paleodirection and paleointensity during the middle to early Brunhes Chron

“…Coe et al, 1984;Mochizuki et al, 2004), (3) acquisition of thermochemical remanent magnetizations (TCRM) during natural cooling (e.g. Yamamoto et al, 2003;Yamamoto, 2006), and (4) differences in coolingrate between natural and laboratory conditions (e.g. Fox and Aitken, 1980;Chauvin et al, 2005;Bowles et al, 2005;Morales et al, 2006).…”