Paleomagnetism of the Notch Peak contact metamorphic aureole, revisited: Pyrrhotite from magnetite+pyrite under submetamorphic conditions

Abstract

[1] On the basis of new demagnetization and rock magnetic experiments, a scattered, twopolarity remagnetization extending throughout the contact metamorphic aureole surrounding the Jurassic Notch Peak granitic stock in western Utah is now thought to reside in metamorphic pyrrhotite rather than in fine-grained hematite. In part, the pyrrhotite formed by in situ reduction of magnetite by pyrite and organic compounds; it could not have been formed by the large-scale introduction of reducing fluids, as isotopic evidence indicates that these metalimestones remained impermeable. At higher grades, some pyrrhotite probably also formed by the reaction of pyrite and Fe-bearing silicates. As magnetite destruction and pyrrhotite formation extends into rocks that are not obviously metamorphosed, the reactions responsible could proceed at very low (less than $200°C) temperatures, and the feasibility of such low-temperature reactions in a closed system is shown by detailed thermodynamic calculations. This study demonstrates that under reducing conditions in the presence of sufficient pyrite, pyrrhotite can form at the expense of magnetite under very mild conditions; indeed, magnetite may be destroyed merely by deep burial. Such a mechanism may help explain the paucity of primary magnetite in ancient sedimentary rocks, particularly since the single-domain and pseudo-single-domain grains capable of retaining a stable paleomagnetic signature over geologic time are extremely fine-grained and hence particularly vulnerable to destruction.

Magnetic properties related to thermal treatment of pyrite

“…It plays an important role in the sulfur and iron cycles of marine sediments [2] . Because of its importance and prevalence, a number of researches were carried out on pyrite by many workers from the aspects of mineralogy, mineral phase transformation and thermal decomposition [3][4][5][6][7][8][9][10] . Hong and Fegley [5] conducted the experiment on pyrite decomposition in 100 μL/L O 2 -CO 2 gas mixture and noted that at lower temperatures (392-460℃), hematite and pyrrhotite were the oxidation and decomposition products of pyrite, while at higher temperatures (484-538℃), no detectable hematite was observed in the products.…”

The effects of metamorphism on iron mineralogy and the iron speciation redox proxy

“…On the other hand, pyrrhotite can form from the decomposition of iron oxides and iron silicates even at low-metamorphic grade (<200°C) upon interaction with H 2 S-bearing fluids or codecomposition of pyrite (Gillett, 2003;Hall, 1986;Nesbitt and Kelly, 1980;Tracy and Robinson, 1988). In these circumstances, placing pyrrhotite in the Fe py pool will drive iron speciation ratios toward the euxinic zone.…”

“…Additional isotopic study could help determine the importance of S-rich fluids (e.g. Gillett, 2003), although determining "background" or "normal" can be difficult in ancient, relatively metamorphosed samples. Mineral assemblages could give clues as well, but often clear-cut metamorphic grades cannot be or are not sampled.…”

Burial Diagenesis of Magnetic Minerals: New Insights from the Grès d’Annot Transect (SE France)

“…It has been proposed that pyrrhotite could form by the reaction of pyrite with magnetite and organic matter in metamorphic conditions [1], even at a low temperature (<200 °C) [15]. Framboidal grains, mainly composed of pyrite, were examined, because they are often associated with organic matter as a result of the bacterial activity [60].…”

“…where "FeS" is pyrrhotite [15]. The formation of pyrrhotite (Fe 1−x S with 0 < x < 0.13) is hence of importance, as it may inform on low-grade metamorphic conditions (>200 °C) (e.g., [1][2][3][4]).…”