Magnetic Properties from the East Rift Zone of Kilauea: Implications for the Sources of Aeromagnetic Anomalies over Hawaiian Volcanoes
Abstract
Aeromagnetic studies of the Island of Hawai‘i provide insights into geologic structure. High-amplitude short-wavelength anomalies occur along the southwest and east rift zones (ERZ) of Kilauea, the youngest volcano on the island. These anomalies have been attributed to contrast between highly magnetic intrusions at depth and less magnetic altered rocks. Anomalies along rift zones of the older volcanoes on the island have lower amplitude or are lacking. To better understand the origin of the high-amplitude anomalies, magnetic properties were obtained for samples from existing 1.7 - 2.0 km deep bore holes located on the ERZ 30 - 40 km east of the summit of Kilauea but not over associated aeromagnetic maxima. The bore holes penetrate subaerial flows, submarine flows, and intrusions. Average values of total magnetization (MT) based on measurements of magnetic susceptibility (κ) and NRM range from ~5.5 A/m for terrestrial flows to ~10 A/m for pillow basalts. MT of intrusions varys with depth. In shallow intrusions (< ~850 m depth), MT averages ~12 A/m, whereas in deep intrusions MT averages ~9 A/m. In contrast, to flows and shallow intrusions, deep intrusions have unstable NRMs that commonly diminish >80% during AF demagnetization at a peak field of 10 mT. The NRMs of deep intrusions were probably affected by drilling, and consequently their laboratory MT values may be much larger than in situ values. Therefore, the deep intrusions are more likely to have relatively low magnetizations rather than the high magnetizations that were used in previous aeromagnetic models.¶ The contrast in NRM stability for shallow and deep intrusions reflects differences in magnetic grain size. The average ARM/κ for shallow intrusions is ~4 times that of deep intrusions. Also, deep intrusions have high Curie temperatures (TC>550 °C) whereas shallow intrusions commonly have low TC, averaging ~165 °C. The fine magnetic grain size and low TC of shallow intrusions are interpreted as the result of rapid crystallization after degassing. Limited oxygen in the subsurface environment would inhibit formation of ilmenite and thereby preserve high Ti-magnetite.¶ After heating in air to ~300 °C and above, TC and room-temperature saturation magnetization (MS) of shallow intrusions increase dramatically. On average, MS at ~25 °C of shallow intrusions increases by a factor of 2.4 after heating to 600 °C. Susceptibility increases similarly after heating in air but does not increase after heating in argon. In the presence of oxygen, Ti apparently separates even at moderate temperature, raising the TC and thereby MS and κ. If NRM increases in a similar manner (as is reasonable if the fine magnetic grain size is preserved), these rocks could attain MT in excess of 20 - 25 A/m. We speculate that this process occurs naturally in proximity to vents where repeated intrusions reheated (or maintained) these rocks to moderate temperatures. If such rocks are the source of anomalies along the Kilauea rift zones then destruction of the fine-grained titanomagnetite over time could explain the lack of prominent anomalies along older rift zones.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMGP43A0836R
- Keywords:
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- 1517 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic anomalies: modeling and interpretation;
- 1519 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic mineralogy and petrology;
- 1540 GEOMAGNETISM AND PALEOMAGNETISM / Rock and mineral magnetism