Paleomagnetic properties of hydrothermally altered rocks from IODP Expedition 376 at Brothers Volcano, New Zealand
Abstract
We present results of paleo and rock magnetic experiments on hydrothermally altered dacite rocks from sites U1528 on the Upper Cone and U1527 and U1530 on the NW Caldera, respectively, drilled during Expedition 376 of the International Ocean Discovery Program (IODP) at Brothers volcano, the most hydrothermally active submarine volcano in the Kermadec arc.
The altered samples show a significant reduction of Natural Remanent Magnetization (NRM) intensity (10-2-10-4 A/m) compared with unaltered dacite samples from the same sites (1-10 A/m), associated with replacement of titanomagnetite by significant amounts of pyrite. No overprint Chemical Remanent Magnetization (CRM) was observed in the altered samples, which is consistent with the formation of paramagnetic pyrite. Alternating Field (AF) and thermal demagnetization experiments showed that most samples possess a weak Characteristic Remanent Magnetization (ChRM) of stable direction carried by the residual primary titanomagnetite. ChRM inclinations of samples from sites U1527 and U1528 average ~ -60°, which is close to the inclination of a Geocentric Axial Dipole (GAD) of -55°. NRM intensities from hydrothermally altered samples at site U1527 are much higher (10-2-100 A/m) than those at site U1528 (10-4-10-2 A/m). U1527 samples also have lower coercivities. At site U1530, we generally found shallower inclinations. A significant number of U1530 samples record positive inclinations, indicating antipodal directions, the cause of which is still being investigated. Thermal demagnetization generally shows a gradual decrease of NRM above ~ 300° C. Many samples from sites U1528 and U1530 record a sharp increase in NRM intensity above ~ 400°C, caused by transformation of pyrite to pure magnetite and/or hematite. This is accompanied by a large increase in magnetic susceptibility (10-4-10-2 SI) after thermal demagnetization compared to values measured before (10-6-10-4 SI). Consistent results confirming this irreversible change in magnetic mineralogy were obtained from Isothermal Remanent Magnetization (IRM) acquisition experiments, where, after thermal demagnetization, some samples acquired IRM intensities > 100 A/m, comparable with IRM intensities achieved in unaltered dacite samples with relatively abundant primary titanomagnetite minerals.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V33E0222C
- Keywords:
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- 0450 Hydrothermal systems;
- BIOGEOSCIENCES;
- 3616 Hydrothermal systems;
- MINERALOGY AND PETROLOGY;
- 8135 Hydrothermal systems;
- TECTONOPHYSICS;
- 8424 Hydrothermal systems;
- VOLCANOLOGY