Multilayer bimineralic models for Southern Highlands magnetic anomalies on Mars:Insights from peridotitic laterites in New Caledonia
Résumé
Previous models built to explain the large amplitude Southern Highlands magnetic anomalies on Mars involve uniformly magnetized bodies and a single mineral origin. Common sources proposed to account for such large anomalies include mafic dikes, hematite deposits, serpentinized peridotites or layered mafic intrusions. In these models, the main magnetization component arises from the remanence of phases such as single domain magnetite, multi-domain hematite, or hemo-ilmenite.
Previous numerical simulations overlooked important aspects of the Mars early history. First, the Martian anomalies occur primarily in the Southern Highlands, where the magnetized crust is Noachian. A lot of information for previous models was derived SNC meteorites extracted from a much younger crust and is therefore probably not relevant. Second, many models consider chiefly anhydrous rock types as potential sources, yet growing evidence suggest not only the presence of liquid water in Noachian times but furthermore indicate abundance of water. Third, while the presence of unaltered olivine at the surface has been interpreted as evidence for lack of water, this inference is incorrect because fresh olivine exists at the surface of Earth. The presence of olivine rather indicates the existence of peridotitic source rocks at the exposure level. Fourth, the Southern Highlands magnetic anomalies are linear, broad, extensive and of large amplitude, although their linear shapes may arise from coalescence of stringed non-linear anomalies.
We propose a series of magnetic models built on an upper regolith layer of laterites produced by alteration of serpentinized harzburgites. These models are dominated by hematite in the upper layer and magnetite in the lower one. The magnetic characteristics of the materials involved are derived from those of thick terrestrial laterite deposits in New Caledonia.
An alternative hypothesis involves belts of ultramafic volcanic rocks, similar with komatiites (greenstone belts) that are quite common in Archean terrains on Earth. Yet, this model does not seem viable because ultramafic lavas typically have a low viscosity, resulting in the formation of high aspect ratio (thin) flows, hence this type of source would not form thick peridotitic bodies unlike terrestrial ophiolites.