Basement heating by a cooling lava: Paleomagiietic constraints

Abstract

Paleomagnetism can provide quantitative, empirical data on basement heating by cooling lava flows, thereby constraining theoretical models of heat transport processes. We used paleomagnetism to study heating in a Frenchman Springs (FS) lava by the overlying cooling Roza flow. The flows belong to the Miocene Columbia River Basalt (CRB) group, and they recorded very different primary paleomagnetic directions: FS has normal polarity, and Roza is transitional. The solid base of Roza typically rests on a few meters of the brecciated and vesicular top of FS, which is otherwise solid. Thermal demagnetization of FS specimens was usually successful at separating the overprinting due to Roza from the primary FS direction and for establishing limits on the unblocking temperatures of the overprinting. The unblocking temperatures agree well between sites separated by up to 70 km, irrespective of Roza thickness at the sites, which vary from 35-62 m. In specimens from 0.3 to 1 m below the contact, the overprinting was unblocked in the laboratory between 600 and 300 øC and close to 200 øC at 4-m depth. No overprinting was apparent below 6 m. The influence of longer heating times in the field on the unblocking temperatures was estimated by viscous remanence acquisition at elevated temperatures. The results suggest typical reductions of the unblocking temperatures by a few tens of degrees. Our observations imply much less heating of the basement than predicted by simple conductive thermal models. Accounting for the low conductivity breccia in the contact zone results in a better agreement with the experimental temperature profile, but unrealistically low conductivities are needed to sufficiently reduce the absolute temperature. The observed heating is effectively explained by postulating a wet basement at the time of Roza extrusion, as well as groundwater, to maintain a low-temperature isotherm (-100 øC) a few meters below the contact. The presence of water during the time of Roza extrusion and some other CRB flows has been suggested by field observations. the primary and secondary remanences in the rock are determined principally by heating temperature, spectrum of the blocking temperatures, relative intensities of the external fields, and the time the rock spent at and near the maximum temperature. If the rock is heated above its highest blocking temperature, it will be totally remagnetized, but if the maximum heating temperature is within the blocking spectrum, it will usually be only partially remagnetized.Ndel [1949] showed explicitly that for an assemblage of identical noninteracting single-domain (SD) particles cooling from above their Curie temperature (Tc), the magnetic moments are blocked in a narrow temperature range, the blocking temperature, Tb, determined by composition-dependent magnetic properties, grain volumes and shapes, and the associated magnetic anisotropies. The magnitude of the external field and time spent at elevated temperatures will also influence T b. Thus, blocking temperatures of a PTRM produced in ...

Geomagnetic fluctuations during a polarity transition

“…Therefore, initial cooling of the Roza flow appears to have been approximately symmetrical, governed by comparable cooling rates from the base and top, with the isotherms migrating inward at nearly equal rates. This contrasts with simple conductive cooling models of lavas [e.g., Audunsson and Levi, 1988] where isotherms below 400øC are predicted to migrate predominantly downward from the top surface, producing asymmetrical cooling. The temperature distributions for these distinct cooling models are shown in Figure 9, and the thermal parameters are given in Table 2 (see also Audunsson [1989]).…”

“…The conclusion was that simple conductive thermal models are not adequate to describe its thermal history. Audunsson and Levi [1988] observed only limited (_< 5 m) basement heating caused by Roza in drillcores DC 12, DC2 and in the outcrop in Frenchman Coulee. It was inferred that groundwater effectively maintained low temperatures (~100øC) within a few meters below the base of Roza.…”

Single-domain chemical, thermochemical and thermal remanences in a basaltic rock

“…Because they generally provide nice linear NRM–TRM diagrams, baked contacts are often considered as an ideal material for palaeointensity studies. However, considering how moderate is the temperature reached during basement heating by a cooling lava or dyke (Audunsson & Levi 1988; Camps et al 1995; Kristjansson 1985) and the long duration of this process, there are good reasons to have some doubts about the actual nature of NRM. Volcanic glasses constitute another material of debatable reliability for palaeointensity studies.…”

Revisiting the initial sites of geomagnetic field impulses during the Steens Mountain polarity reversal

“…The value of baking temperature induced by a specific lava flow in nature is somewhat uncertain. The reference thermal model of Jaeger ( 1967) provides temperature estimates in the substratum which have been found to be much too high compared to the baking temperatures deduced from palaeomagnetic data (Audunsson & Levi 1988). An absence of any significant baking zone has even been reported for flows as thick as 100m (Nyblade et a/.…”