Magnetic properties and oxidation of iron-titanium oxide minerals in Alae and Makaopuhi Lava Lakes, Hawaii

Abstract

Measurements of strong‐field magnetization over the temperature range −196° to 700°C have been made on forty‐eight drill core samples of tholeiitic basalt from Alae and Makaopuhi lava lakes, Kilauea volcano, Hawaii. These samples were originally obtained at temperatures ranging from 50° to 1020°C. Nearly all samples contain abundant hemoilmenite with Curie temperatures in the range −100° to −160°C. Samples quenched from high temperatures (800° to 1000°C) have second Curie temperatures ranging from 150° to 290°C, due to unoxidized titanomagnetite, and samples obtained at lower temperatures (50° to between 400° and 700°C) have second Curie temperatures ranging from 500° to 580°C. This transition from medium to high Curie temperatures occurs between 850° and 300°C, varying from one drill hole to another, and is accompanied by a marked increase in the strong‐field magnetization at room temperature. Oxidation of original titanomagnetite to Ti‐poor titanomagnetite containing ilmenite lamellas is the cause of the increase in Curie temperature. Comparison of the compositions of the oxide minerals with the oxygen fugacity data of Sato and Wright and the equilibrium reaction data of Buddington and Lindsley shows that oxygen fugacity was controlled largely by the buffering action of the oxide minerals; hence titanomagnetite was oxidized, whereas the more abundant hemoilmenite was little changed as the lava cooled. This oxidation occurred at temperatures well below equilibrium, the difference being generally of the order of 100°C but as much as 400°C. We conclude that in some basaltic lavas the magnetic minerals may form through subsolidus reactions at temperatures well below their final Curie temperatures. In such lavas the natural remanent magnetization is a mixture of thermoremanent magnetization and high‐temperature chemical remanent magnetization.

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

“…The Curie temperature values one obtains from an I S (T) curve for a particular titanomaghemite may vary considerably (>100°C) depending on the method of T c determination (Moskowitz, 1981). We chose a graphic construction that uses the intersection of two tangents to the I S (T) curve above and below T c (Grommé et al, 1969). This graphic method tends to underestimate T c in comparison with the two other methods discussed by Moskowitz (1981).…”

“…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).…”

Magnetic and Mineralogical Investigations of Opaque Minerals: Preliminary Results

“…The comparison suggests that agreement between the two instruments is very good (Table 1, Sample 51-1, 24 cm). The instrument at Menlo Park is an automated recording magnetic balance (Doell and Cox, 1965;Grommé et al, 1969). All samples were measured in vacuum (less than 10 ~4 torr) with heating to 610 to 615°C.…”

“…Small chips (totaling 150-200 mg) were heated and cooled at rates of 25 to 30°C/min. All Curie temperature values were determined using the graphic method of Grommé et al (1969).…”

Thermomagnetic Behavior of Magnetic Susceptibility—Heating Rate and Sample Size Effects

“…Cooling curve is indicative for the occurrence of mineralogical changes caused by laboratory heating. Considerable effort has been put on theoretical and experimental studies, defining physical basis of the observed susceptibility changes (Egli, 2009;Fabian et al, 2013;Dunlop, 2014) and methodologies for estimation of the Curie temperature (Gromme et al, 1969;Petrovsky and Kapicka, 2006). The shape and the observed peaks on the heating and cooling curves of k-T cycles are interpreted as a reflection of different environmental processes [e.g., degree of soil formation and synthesis of pedogenic strongly magnetic fraction; presence of substitutions; reactions of Fe minerals with organic matter; destruction of thermally unstable minerals such as carbonates, organic matter, clay minerals (Liu et al, 2005;Hanesch et al, 2006;Jiang et al, 2015)].…”