Magnetic minerals in the Martian crust

Abstract

[1] Using rock magnetism and thermal modeling, we evaluate the candidate minerals responsible for strong magnetic anomalies in the Terra Sirenum and Terra Cimmeria regions of Mars' southern highlands. We assume an early global dynamo field similar in strength to the present Earth's field, enduring about 500 Myr after accretion and core formation, and a basaltic crust containing no more than 4-7 weight% of magnetic minerals. Thermal evolution models with a wide variety of initial crustal thicknesses, distributions of radioactive elements, and thermal expansion coefficients all yield similar thermal histories for the crust: warming in the first $1000 Myr (due mainly to radioactive heating) followed by monotonic cooling for the remainder of Mars' history. Primary thermoremanent magnetization (TRM) acquired by intrusive and extrusive bodies during the first 500 Myr was in part thermally demagnetized by general crustal warming after the dynamo field disappeared, from 500 to 1000 Myr. The Curie point isotherms around 1000 Myr established the maximum depth of TRM-bearing crust. Shock and heating due to impacts demagnetized the uppermost $10 km of the crust around the same time, resulting in potential magnetic layer thicknesses of 15-20 km for pyrrhotite, 40-50 km for magnetite, and 50-60 km for hematite. Other magnetic phases, such as iron and finely exsolved low-Ti titanohematite, are possible but less likely in a basaltic crust under oxidizing conditions. The prime candidates, in order of likelihood, are single-domain magnetite (0.2-0.4 volume% or 0.4-0.8 weight% required), single-domain pyrrhotite (1-2 volume% or 2-4 weight%), and either multidomain (>15 mm) or 5-15 mm single-domain hematite or a mixture of both (1.5-3 volume% or 3-6 weight%). A composite source with different combinations of these minerals at different depths is entirely possible. Viscous decay of TRM is difficult to assess without detailed knowledge of the distribution of minerals and blocking temperatures with depth but would increase the amounts of magnetic material required.

Crustal Magnetization of Mars: Terra Meridiani and Terra Sirenum

“…Table 1 lists possible magnetic carriers, their Curie temperatures [46], and magnetized depth estimates for various heat fluxes ranging from 3.7 -4.5 Ga. The magnetized depth is calculated using (Tc − Ts)k/F where k is thermal conductivity (3 W/(m·K)), Tc is Curie temperature (K), Ts is surface temperature (assumed to be 230 K [33]) and F is heat flux (W/m …”

“…Only iron oxide and iron sulfide minerals can retain considerable remanent magnetization over geologically long periods of time. A recent study [33] selected terrestrial magnetic minerals (i.e., magnetite, titanomagnetite, hematite, titanohematite, and pyrrhotite) as potential Martian magnetic minerals were selected for thermal evolution models. Martian meteorites have been found to contain such minerals, like magnetite, titanomagnetite, and pyrrhotite [8] [35].…”

“…Best-fit models of mul- one or more uniformly magnetized circular disks was used to determine the magnetic source [34]. However, based on assumptions made such as Martian core dynamo strength and Curie point isotherms for hematite, magnetite, and pyrrhotite [33], and an assumed maximum source thickness of 3 km, a minimum magnetization of 50 A/m was deduced [34].…”

“…By assuming the magnetization was confined to a 40 km thick layer (consistent with previous studies), the magnetization ranges between ±12 A/m. Additionally, a model made for the thermal evolution of Mars [33] used parameterized convection calculations to determine magnetic layer thickness for each potential mineral source. These models predicted thicknesses at ~4 Ga that range from ~30 -130 km for hematite, ~25 -120 km for magnetite, and ~20 -70 km for pyrrhotite.…”

New method for separation of magnetite from rock samples for oxygen isotope analysis

“…However, this technique does not permit the separation of magnetic minerals of similar magnetic susceptibility such as pyrrhotite (Fe 1-x S). This is important because magnetite and pyrrhotite are two minerals that are found associated in many geological environments, including: 1) sediments (Kao et al, 2004;Horng & Roberts, 2006;Raposo et al, 2006;Wakabayashi et al, 2006), 2) metallic ore deposits (Faure & Brathwaite, 2006), 3) hydrothermally altered igneous rocks (Alt et al, 1989), 4) carbonaceous chondrites (Hsu et al, 2006), and even 5) Martian crust (Dunlop & Arkani-Hamed, 2005) and Martian meteorites (McKay et al1996, Rochette et al, 2001. They are also disseminated in metamorphic and magmatic rocks, where they may constitute a continuous record of Earth´s magnetic field variations which can be used for paleomagnetic studies (Wehland et al, 2004).…”