Magnetic property zonation in a thick lava flow

Abstract

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.

Mössbauer spectroscopy of magnetic minerals in basalt on Earth and Mars

“…The Roza samples were obtained from H. Audunsson from a detailed study of thick lava flows from the Roza flow field of the Columbia River Basalt [16]. In all cases the magnetic phase was found to be titanomagnetite, with gradual changes in composition from the bottom of the flow (x ∼ 0.7) to the top of the flow (x ∼ 0.1).…”

“…Still, this reduction in the number of fitting variables leads to stable solutions, where the We have applied this approach to samples where the composition has been determined independently, either with X-ray diffraction and Curie temperatures or microprobe studies. Figure 3 shows spectra of titanomagnetite from the Roza flow field (Columbia River Basalt) [16] and titanomaghemite from the Kjalarnes magnetic anomaly [17], where it has been possible to determine the composition of the spinel phase with other methods.…”

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.…”

Geomagnetic field inclinations for the past 400 kyr from the 1‐km core of the Hawaii Scientific Drilling Project

“…The top of the flow, which cools more quickly than the middle, is typically finer grained and can result in magnetic domains of higher coercive force. A detailed study of a 30 to 60-m-thick basaltic lava flow in the northwestern United States [Audunsson and Levi, 1992] also showed that there can be extensive zonation of magnetic properties in a lava flow with the central section being less magnetically stable than the upper.…”