Timing of magnetite formation in basaltic glass: Insights from synthetic analogs and relevance for geomagnetic paleointensity analyses

Citation:
Bowles, JA, Gee JS, Burgess K, Cooper RF.  2011.  Timing of magnetite formation in basaltic glass: Insights from synthetic analogs and relevance for geomagnetic paleointensity analyses. Geochemistry Geophysics Geosystems. 12

Date Published:

Feb

Keywords:

cooling rate, East Pacific Rise, experiments, field, intensity, magnetite, nucleation, Ontong Java Plateau, paleointensity, ridge lava flows, rocks, submarine, submarine basaltic glass, thellier

Abstract:

Absolute paleointensity estimates from submarine basaltic glass (SBG) typically are of high technical quality and accurately reflect the ambient field when known. SBG contains fine-grained, low-Ti magnetite, in contrast to the high-Ti magnetite in crystalline basalt, which has lead to uncertainty over the origin of the magnetite and its remanence in SBG. Because a thermal remanence is required for accurate paleointensity estimates, the timing and temperature of magnetite formation is crucial. To assess these factors, we generated a suite of synthetic glasses with variable oxygen fugacity, cooling rate, and FeO* content. Magnetic properties varied most strongly with crystallinity; less crystalline specimens are similar to natural SBG and have weaker magnetization, a greater superparamagnetic contribution, and higher unblocking temperatures than more crystalline specimens. Thellier-type paleointensity results recovered the correct field within 1 sigma error with 2 (out of 10) exceptions that likely result from an undetected change in the laboratory field. Unblocking and ordering temperature data demonstrate that low-Ti magnetite is a primary phase, formed when the glass initially quenched. Although prolonged heating at high temperatures (during paleointensity experiments) may result in minor alteration at temperatures <580 degrees C, this does not appear to impact the accuracy of the paleointensity estimate. Young SBG is therefore a suitable material for paleointensity studies.

Notes:

n/a

Website

DOI:

10.1029/2010gc003404

Scripps Publication ID:

Q02001