Formation processes of framboidal pyrite

Magnetic mineral diagenesis

http://web.gps.caltech.edu/~jkirschvink/pdfs/magnetofossils.pdf

Elongated prismatic magnetite crystals in ALH84001 carbonate globules: Potential Martian magnetofossils

A paleomagnetic investigation at the Gran Dolina site excavation (Atapuerca, Spain) shows that the sediments containing the recently discovered human occupation were deposited more than 780,000 years ago, near the time of the Matuyama-Brunhes boundary. Forty-one oriented samples were obtained from 22 sites along an 18-meter section of the Gran Dolina karst filling. The lower 16 sites displayed reversed-polarity magnetizations whereas the upper six sites were normal. The reversal spans the hominid finds at stratigraphic level TD6 (the Aurora stratum), and these hominid fossils are therefore the oldest in southern Europe.

https://science.sciencemag.org/content/sci/269/5225/830.full.pdf

Paleomagnetic age for hominid fossils at Atapuerca archaeological site, Spain

[1] Magnetic hysteresis measurements of sediments have resulted in widespread reporting of “pseudo-single-domain”-like magnetic properties. In contrast, the ideal single domain (SD) properties that would be expected to be responsible for high quality paleomagnetic records are rare. Determining whether SD particles are rare or common in sediments requires application of techniques that enable discrimination among different magnetic components in a sediment. We apply a range of such techniques and find that SD particles are much more common than has been reported in the literature and that magnetite magnetofossils (the inorganic remains of magnetotactic bacteria) are widely preserved at depth in a range of sediment types, including biogenic pelagic carbonates, lacustrine and marine clays, and possibly even in glaci-marine sediments. Thus, instead of being rarely preserved in the geological record, we find that magnetofossils are widespread. This observation has important implications for our understanding of how sediments become magnetized and highlights the need to develop a more robust basis for understanding how biogenic magnetite contributes to the magnetization of sediments. Magnetofossils also have grain sizes that are substantially smaller than the 1–15μm size range for which there is reasonable empirical support for relative paleointensity studies. The different magnetic response of coexisting fine biogenic and coarser lithogenic particles is likely to complicate relative paleointensity studies. This issue needs much closer attention. Despite the fact that sediments have been subjected to paleomagnetic investigation for over 60 years, much remains to be understood about how they become magnetized.

/pdf/searching-for-single-domain-magnetite-in-the-pseudo-single-497718s53u.pdf

Searching for single domain magnetite in the “pseudo-single-domain” sedimentary haystack: Implications of biogenic magnetite preservation for sediment magnetism and relative paleointensity determinations

LIMESTONES are an important source of palaeomagnetic data, with magnetite the dominant carrier of the magnetization. Because the magnetite is present in low concentrations, however, it has been directly observed only rarely and in trace amounts in acid-treated separates. Such residues are composed primarily of spheres, the ultimate origin of which has been enigmatic7. Yet interpretation of the palaeomagnetic data depends on a knowledge of the origin of the magnetite, which has been inferred to carry either a chemical or a viscous remanent magnetization. Here we present electron microscope data obtained from a sequence of samples of Onondaga Limestone from an east–west traverse across northern New York, which unambiguously show that magnetite spheres are derived by alteration and replacement of framboidal pyrite. Remagnetization is thus caused by chemical, rather than viscous, processes. This chemical remanent magnetization is compatible with a fluid-mediated event occurring on a regional scale, induced by tectonic stress of Alleghenian (late Palaeozoic) age.

/pdf/replacement-of-pyrite-framboids-by-magnetite-in-limestone-4o1vn0wws9.pdf

R. Vand Der Voo

Papers

Replacement of pyrite framboids by magnetite in limestone and implications for palaeomagnetism