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

Abstract

[1] 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 1s 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°C, this does not appear to impact the accuracy of the paleointensity estimate. Young SBG is therefore a suitable material for paleointensity studies.

New paleointensity results from rapidly cooled Icelandic lavas: Implications for Arctic geomagnetic field strength

“…Our samples are not pure glass and likely contain measurable amounts of very fine grained, crystalline volcanic material. In addition, the timescales referenced for relaxation of the glass structure for devitrification (75–120 Ma [ Bowles et al , ]) are 1–2 orders of magnitude longer than our oldest specimens. How these differences might affect interpretations of devitrification and mineral growth is uncertain.…”

“…Thirty three of our 44 successful sites are younger than 200 ka and therefore unlikely to be affected by any new mineral growth at standard laboratory temperatures [ Bowles et al , ]. Eleven sites are determined to be at least Brunhes age, with the oldest site estimated to be ∼3.35 Ma.…”

Accurate determination of Fe3+/∑Fe of andesitic glass by Mössbauer spectroscopy

“…Superparamagnetic behavior has been documented in otherwise fresh natural basaltic glasses (Pick and Tauxe 1994), and in Fe-rich basaltic glasses quenched in the laboratory (Bowles et al 2011) but is not generally considered to contribute to Möss-bauer spectra of laboratory-quenched glasses. Non-paramagnetic components of Mössbauer spectra, including broad absorptions approximately symmetric about CS = 0 mm/s as well as sextets, are a common feature in such glasses, in many cases comprising a larger fraction of the Fe than documented for VF3 or M544 .…”

Paleointensity results from the Jurassic: New constraints from submarine basaltic glasses of ODP Site 801C

“…While magnetite does grow at elevated temperatures [Pick and Tauxe, 1994;Smirnov and Tarduno, 2003], the rate of growth drops dramatically below the glass transition temperature [Bowles et al, 2011]; hence, the low-Ti magnetite that is present in even zero-age pillow margin glass is likely to have formed during quenching. Paleointensity experiments on basaltic glass from sites of recent eruptions recover the ambient magnetic field at those locations Tauxe, 1993a, 1993b;Kent and Gee, 1996;Carlut and Kent, 2000].…”