Haraldur Audunsson

Late Pleistocene geomagnetic excursion in Icelandic lavas : Confirmation of the Laschamp excursion

In 1980 Kristjansson and Gudmundsson [1] reported a late glacial geomagnetic excursion in three hills in the Reykjanes peninsula, Iceland, with shallow negative inclinations and westerly declinations. They named it the Skalamaelifell excursion. More extensive field work has identified the same excursional paleomagnetic direction (declination= 258°,inclination= −15°) at four additional outcrops in a 10 × 10 km area in the Reykjanes peninsula. The excursion lavas are olivine tholeiites with similar petrography and chemical compositions. Paleointensity determinations by the Thellier method average 4.2 ± 0.2 μT for 8 samples, more than an order of magnitude weaker than the present geomagnetic field in Iceland. Together, these results suggest extrusion of the excursion lavas in a very brief span of time, probably less than a few hundred years. KAr dating of the excursion lavas gives a mean age for 19 determinations of 42.9 ± 7.8ka(2σ). Compilation of thirty KAr ages of the Laschamp and Olby flows by three laboratories yield a new age for the Laschamp excursion in France of 46.6 ± 2.4ka(2σ). The age of the excursion in southwestern Iceland is statistically indistinguishable from the Laschamp excursion at the 95% confidence level, and both have very low paleointensities. Therefore, we suggest that the Laschamp and Olby flows in France and the Skalamaelifell units of Iceland recorded essentially the same geomagnetic excursion. Differences in the virtual paleomagnetic poles (VGPs) of these excursions may be due to (1) the probable non-dipole character of the geomagnetic field during the excursion, (2) rapid geomagnetic secular variation and possible small age differences of the extrusive rocks in France and Iceland, and/or (3) crustal magnetic anomalies which might dominate the local geomagnetic field during the excursion at either or both locations.

Magnetic property zonation in a thick lava flow

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.

Geomagnetic fluctuations during a polarity transition

The extensive Roza Member of the Columbia River Basalt Group (Washington State) has intermediate paleomagnetic directions, bracketed by underlying normal and overlying reverse polarity flows. A consistent paleomagnetic direction was measured at 11 widely distributed outcrops; the average direction has a declination of 189° and an inclination of −5°, with greater variation in the inclination [Rietman, 1966]. In this study the Roza Member was sampled in two Pasco Basin drillcores, where it is a single cooling unit and its thickness exceeds 50 m. Excellent core recovery allowed uniform and dense sampling of the drillcores. During its protracted cooling, the Roza flow in the drillcores recorded part of a 15.5 Ma geomagnetic polarity transition. The inclination has symmetric, quasicyclic intraflow variation, while the declination is nearly constant, consistent with the results from the outcrops. Thermal models of the cooling flow provide the timing for remanence acquisition. The inclination is inferred to have progressed from 0° to −15° and back to −3°over a period of 15 to 60 years, at rates of 1.6° to 0.5°/yr. Because the geomagnetic intensity was probably weak during the transition, these apparently high rates of change are not significantly different from present-day secular variation. These results agree with the hypothesis that normal secular variation persists through geomagnetic transitions. The Iow-amplitude quasicyclical fluctuations of the field over tens of years, recorded by Roza, suggest that the geomagnetic field reverses in discrete steps, and that more than 15–60 years were required to complete this reversal.