The long alteration history of peridotites in the New Caledonia ophiolite:from ocean ridge to obduction and beyond
Abstract
We use the New Caledonia ophiolite to investigate the alteration of oceanic peridotites. This begins at the ridge through pervasive HT serpentinization along grain boundaries and along normal faults. As spreading continues seawater interacts with rocks through hydrothermal circulation. Later on, obduction takes place creating a hydraulic head with meteoric fluids, permeating through the nappe as shown by abundant karstic features. Finally, weathering forms laterites. We use paleomagnetism to define the timing of events. Serpentinization produces magnetite, followed by maghemite, hematite, and ultimately goethite.
Oriented samples from the Tontouta Valley preserve a record of a protracted alteration history. Stepwise thermal demagnetization experiments inform about the magnetic assemblages and chemical remanent magnetization (CRM) events. The magnetic susceptibility correlates positively with NRM intensity, suggesting that serpentinization boosts remanence. This would not occur if serpentinization produced mostly MD grains. NRM intensity and magnetic susceptibility do not correlate with density, which is in agreement with observations made elsewhere. The NRM directions show a dominance of normal polarities, with scattered directions and a few reverse polarities. This indicates that the serpentinization process took place before the Matuyama-Brunhes reversal. Goethite-dominated samples have an NRM close to today's field which confirms that the last increment of CRM was acquired recently. The variations of magnetic susceptibility with NRM intensity during stepwise thermal demagnetization to 600ºC indicate that magnetite and goethite together carry over 90% of the magnetic remanence while hematite, surprisingly accounts for only a moderate portion. Some specimens characterized by a drastic loss of remanence between 120 and 250ºC as well as a distinct gain in magnetic susceptibility above 200ºC may host Fe-Ni sulfides capable of carrying a remanence. Finally, in a few samples, the NRM intensity vs temperature and Zijderveld plots demonstrate the preservation of a reverse component of magnetization corresponding to an early phase of alteration.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMGP41A0758F
- Keywords:
-
- 1513 Geomagnetic excursions;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1521 Paleointensity;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1522 Paleomagnetic secular variation;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1535 Reversals: process;
- timescale;
- magnetostratigraphy;
- GEOMAGNETISM AND PALEOMAGNETISM