Variation of Palaeomagnetic Stability and Other Parameters in a Vertical Traverse of a Single Icelandic Lava

Abstract

An intensive study has been made of 30 samples from within, and two bakes samples from below a single Tertiary basalt in Eastern Iceland. The samples are spaced along a measured vertical traverse from bottom to top. The lava is 16.8 m thick, and is reversely magnetized.The quantities measured for each core were:sections of the iron-titanium oxide.analyses.(I) Oxidation state as revealed by microscope studies in polished(2) Oxidation state of the whole rock as determined by FeO and Fe,O,(3) Titanomagnetite and separate ilmenite grain sizcs.(4) Magnetic susceptibility.(5) Saturation magnetization. (6) Curie points and total heating curve in a strong magnetic field. (7) Natural magnetization after demagnetization in several alternating (8) Koenigsberger's Q factor, slightly modified.(9)A new measure of the resistance of the specimen to alternating field demagnetization, the 'stability factor'. (10) The position of the core above the base of the lava. The above quantities vary greatly with position in the lava. The results of these measurements show the relationships of these variables to each other; in particular how the palaeomagnetic stability depends on oxidation state of the iron-titanium oxides. Stability is high in the lower two-thirds of this lava, and not just at the chilled margins.Two anomalous results are (I) that the Curie point is virtualIy constant regardless of oxidation state (unlike some other lavas), and (2) that the whole rock oxidation state measured by Fe20,/(2Fe0 + Fe20,) does not correlate simply with the oxidation state of the iron-titanium oxides as seen in the microscope. fields.

Variation of Rock Magnetic Parameters and Paleointensities over a Single Holocene Lava Flow.

“…Yet individual flows may show considerable variability in their magnetic properties (Watkins and Haggerty, 1965;Ade-Hall et al, 1968a;Wilson et al, 1968; Lawley and Ade-Hall, 1971;Peterson, 1976;Herzog et al, 1988;Audunsson et al, 1992), and the extent to which this impacts on the paleomagnetic signal recorded in the flow will limit its potential resolution. Natural remanent magnetisation (NRM) intensity and stability have been shown to be far from uniform, NRM intensity in particular varying by more than a factor of 10 within individual flows.…”

“…In most cases this variation seems related to high temperature (HT) deuteric oxidation of the remanence-bearing titanomagnetites, the development of which can be highly variable within any single flow giving rise to complex intra-flow NRM distributions (e.g. Wilson et al, 1968;Peterson, 1976;Herzog et al, 1988).…”

“…Samples with low NRM stability often show greater scatter about the flow-mean direction (e.g. Watkins and Haggerty, 1965;Wilson et al, 1968;Herzog et al, 1988). This is associated with the development of a magnetically soft secondary overprint and on alternating field demagnetization to relatively low fields (<40 mT) the directions converge with the flowmean value.…”

The Opaque Mineralogy of Basalts from DSDP Leg 26

“…Typical features of deuteric oxidation are ilmenite exsolution in octahedral planes in magnetite and the formation of titanohematite, rutile, and pseudobrookite from magnetite and ilmenite. Various degrees of deuteric oxidation can be recognized and a scale from Class 1 (no oxidation) to Class 6 (maximum oxidation) has been widely used in classifying basalts (Wilson et al, 1968). Thus, most samples studied to date are from quenched basalts, pillows, or flow tops, and contain Class 1 titanomagnetite.…”

Rock-Magnetic Investigation of Basalts from the Southern Kerguelen Plateau (Leg 120)

“…Low-temperature oxidation of titanomagnetite to titanomaghemite has been shown to be a cause for the decrease in intensity of marine magnetic anomalies with increasing age of the ocean floor (Irving, 1970;Prévot and Grommé, 1975;Bleil and Petersen, 1983). Another form of alteration is the high-temperature oxidation of basalts that is common in subaerial flows (Lindsley, 1962;Wilson et al, 1968;Grommé et al, 1969;Ade-Hall et al, 1976;Haggerty, 1976). Although low-T oxidation of titanomagnetite is a single-phase reaction, deuteric oxidation leads to exsolution of different phases, essentially a rhombohedral Ti-rich phase (hemo-ilmenite) and a low-Ti titanomagnetite (spinel structure).…”