Origins of magnetite nanocrystals in Martian meteorite ALH84001

Abstract

The Martian meteorite ALH84001 preserves evidence of interaction with aqueous fluids while on Mars in the form of microscopic carbonate disks. These carbonate disks are believed to have precipitated 3.9 Ga ago at beginning of the Noachian epoch on Mars during which both the oldest extant Martian surfaces were formed, and perhaps the earliest global oceans. Intimately associated within and throughout these carbonate disks are nanocrystal magnetites (Fe 3 O 4 ) with unusual chemical and physical properties, whose origins have become the source of considerable debate. One group of hypotheses argues that these magnetites are the product of partial thermal decomposition of the host carbonate. Alternatively, the origins of magnetite and carbonate may be unrelated; that is, from the perspective of the carbonate the magnetite is allochthonous. For example, the magnetites might have already been present in the aqueous fluids from which the carbonates were believed to have been deposited. We have sought to resolve between these hypotheses through the detailed characterization of the compositional and structural relationships of the carbonate disks and associated magnetites with the orthopyroxene matrix in which they are embedded. Extensive use of focused ion beam milling techniques has been utilized for sample preparation. We then compared our observations with those from experimental thermal decomposition studies of sideritic carbonates under a range of plausible geological heating scenarios. We conclude that the vast majority of the nanocrystal magnetites present in the carbonate disks could not have formed by any of the currently proposed thermal decomposition scenarios. Instead, we find there is considerable evidence in support of an alternative allochthonous origin for the magnetite unrelated to any shock or thermal processing of the carbonates.

Chemical signature of magnetotactic bacteria

“…Although not without controversy, it was suggested that low concentrations in minor elements observed in magnetite from Martian meteorite ALH84001 could indicate a biological origin for magnetite (18). This interpretation was supported by abiotic formation of magnetite in the laboratory, leading to high levels of elements other than iron in the crystal products (19,20), except if initial materials highly depleted in doping elements were used (21). However, the degree of magnetite chemical purity in these previous studies was estimated by energy dispersive x-ray spectroscopy (EDXS) coupled with transmission electron microscopy, which is usually limited to the quantification of relatively elevated elemental concentrations, typically higher than 0.1-1% (22).…”

Magnetita biomimética mediada por proteínas del magnetosoma vs. magnetita del meteorito ALH84001: ¿Son ambas comparables?

“…The problem persists, however, since other inorganic techniques, as About magnetite crystals within ALH84001, they probably depict a complex history. Thermal decomposition, that has been claimed for many authors as a possible mechanism to explain the occurrence of magnetite crystals in ALH84001 (Golden et al, 2001;Barber & Scott, 2002;Brearly, 2003;Golden et al, 2004), has been discarded by Thomas-Keprta et al, (2009) andJiménez-López et al, (2012) who have proven that magnetite crystals in ALH84001 are allochthonous, meaning that those crystals were not originated within the carbonate globules that fills the meteorite fractures in which magnetite is embedded. Instead, those crystals might have been washed away from different external sources maybe by the action of water that is assumed to be present in huge quantities in early Mars (Carter et al, 2015) and deposited within the meteorite fractures.…”

“…Lastly, Vuong et al, (2015) have reported high quality magnetite with a high saturation magnetization for particles produced by thermal decomposition. Although along those experiments crystals that meet the biogenicity criteria have been produced, Jiménez-López et al, (2012) 52 from their carbonate precursor that has not been observed in ALH84001, backing the proposition of Thomas-Keprta et al, (2009), that the majority of magnetite crystals found in ALH84001 are allochthonous. Regarding the sets of magnetite crystals synthetised in different MPMS experiments with different proteins, SD size particles with a better trend to crystallographic perfection have been consistently obtained (Amemiya et al, 2007;Prozorov et al, 2007;Rawlings et al, 2014;Valverde-Tercedor et al, 2015).…”

Discovery of extra-terrestrial life: assessment by scales of its importance and associated risks

“…But the original group announcing the discovery continued to investigate all possibilities. Recently they have published a paper, which concluded that the 'magnetic bacteria' features found in the meteorite were not formed by some non-biological thermal event [11].…”