S U M M A R YWe have determined the directions of primary remanence in over 400 lava flows in the northwestern peninsula of Iceland (near 66.1 • N, 23.3 • W). These lavas were sampled in 16 profiles spread across 75 km, at the stratigraphic level of the oldest (approximately 15 Myr in age) of several distinctive lignite-bearing sediments in the peninsula. We find that the pattern of polarity reversals in the lava pile is broadly similar in these profiles but it is generally not possible to trace short polarity zones and excursions over more than a few kilometres laterally. Our results, which also include measurements on the lignite sediments at three sites, are consistent with previous suggestions that these sediment beds represent a time gap of the order of 0.2 Myr between the lavas above and below. The mean virtual geomagnetic pole of this collection is 'far-sided' by approximately 7 • , and the scatter of individual poles is greater than that in palaeomagnetic surveys on younger lava series in Iceland.
Drilling-induced remanent magnetization (DIRM) in drill cores can limit their use for magnetostratigraphic studies and preclude the use of secondary viscous remanence for their azimuthal orientation. DIRM was studied in a drill core of a thick Miocene basalt flow now buried at 0.45 km. Due to zonation of the magnetic properties within the flow, DIRM was observed in specimens whose remanence is controlled by grains ranging from multidomain (MD) to single domain (SD). DIRM in this drill core has the following properties: (1) it is characterized by high intensity and low stability; (2) the D I M intensity increases by at least a factor of 5 from the centre of the drill core to the drill string's cutting surface, where it appears to have been produced; (3) it is directed down and radially inward towards the centre of the drill core; and (4) it is relatively more dominant and more intense in magnetically less stable MD grains.The observed DIRM can be modelled as a pure IRM acquired in a field of the order of 10mT. Therefore, the DIRM in this drill core is most easily explained as having been produced during the initial drilling by a strong non-uniform field concentrated near the cutting rim of the drill string. Other processes which might contribute to DIRM production include tearing of grains and possible changes in strain, mechanical shocks and piezo remanent magnetization (PRM).In this drill core, DIRM in the magnetically less stable grains was more effectively cleaned by alternating fields (AF) than by thermal demagnetization, and judicious AF demagnetization was usually successful at defining the primary remanence, especially for specimens from the centre of the drill core, which are less affected by DIRM overprinting. The use of a non-magnetic drill string would further reduce, and might possibly eliminate, DIRM production.
In this study, grain size and composition‐dependent magnetic properties of titanomagnetite minerals are used as indicators of intraflow structures and magmatic evolution in an extensive and thick (30–60 m) basaltic lava flow. Similar zonation occurs in this flow at three localities separated by tens of kilometers. The magnetic properties subdivide the flow to three zones. The upper layer, representing the top 1/3 of the lava (≤ 20 m), has higher magnetic stability due to smaller and more deuterically oxidized titanomagnetite grains, approaching pure magnetite. The central layer in the underlying 2/3 of the flow (≤ 35 m) has larger, magnetically less stable, and less oxidized grains with relatively uniform magnetic properties. The basal layer, the bottom 1/10 of the flow (≤ 5 m), has near primary, least oxidized titanomagnetites (Ulv68Mag32). The upper intraflow boundary of the magnetic properties appears to coincide with the transition from entablature (above) to colonnade (below), distinguishing between regions of faster and slower cooling. Microprobe data indicate that the intraflow oxidation state (Fe3+/Fe2+) of the initially precipitated primary titanomagnetites increases with falling equilibrium temperature from the flow margins to a maximum near the center, the position of lowest equilibrium temperature. In contrast, Curie temperature measurements indicate that titanomagnetite oxidation increases with height in the flow. Modification of the initially symmetric equilibrium titanomagnetite compositions was caused by subsolidus high‐temperature oxidation possibly due to hydrogen loss produced by dissociation of magmatic water, as well as unknown contributions of circulating air and percolating water from above. The titanomagnetites of the basal layer of the flow remain essentially unaltered.
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 ...
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