Title: Magnetic minerals in the Martian crust

Author(s): Dunlop DJ, Arkani-Hamed J

Source: JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS 110 (E12): 1-11 OCT 12 2005

Article No.: E12S04

Document Type: Article

Language: English

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 similar to 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 similar to 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 mu m) or 5 - 15 mu m 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.

KeyWords Plus: DRILLING PROGRAM HOLE-735B; MID-ATLANTIC-RIDGE; THERMOREMANENT MAGNETIZATION; REMANENT MAGNETIZATION; THERMAL EVOLUTION; IMPACT DEMAGNETIZATION; OCEANIC GABBROS; GUSEV CRATER; MARS; ANOMALIES

Addresses: Dunlop DJ (reprint author), Univ Toronto, Dept Phys, 3359 Mississauga Rd, Mississauga, ON L5L 1C6 Canada
Univ Toronto, Dept Phys, Mississauga, ON L5L 1C6 Canada
McGill Univ, Dept Earth & Planetary Sci, Montreal, PQ H3A 2A7 Canada

E-mail Addresses: jafar@eps.mcgill.ca, dunlop@physics.utoronto.ca

Publisher: AMER GEOPHYSICAL UNION, 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA

Subject Category: GEOCHEMISTRY & GEOPHYSICS

IDS Number: 975UL