CRUSTAL MAGNETIC SIGNATURE OF PLANETS: TRACKING THE SOLID SOLUTION OF FE AND EXSOLUTION OF MAGNETITE IN PLAGIOCLASE OF 2.05 GA THE BUSHVELD COMPLEX, SOUTH AFRICA BY MAGNETIC, CHEMICAL AND MÖSSBAUER PROPERTIES

Magnetic mapping is a primary tool for planetary exploration. Understanding the nature of oxide exsolutions (or lack of) in silicate minerals gives constraints to interpret the magnetic signatures of planetary crusts, especially combined with chemical mapping techniques. Exsolution of magnetite (mgt) from silicates provides a strong recorder of the magnetic field at the time of exsolution. Such exsolutions can contribute to the crustal magnetic signature of planets due their long stability of a magnetic memory. Here we explore relationships between plagioclase chemistry and mgt exsolution, by microprobe, XRF, Mössbauer and magnetic measurements on separates and chemical and magnetic measurements on rock samples. We examine causality between chemical evolution and magnetic signatures.

28 gabbroic samples of the Bushveld Complex, with similar total FeO in plagioclase (0.2-0.6 wt%) contain Fe in very different forms: as Fe in solid solution, or exsolved, as micron to submicroscopic needles or blades of mgt. Minor chemical substitution of Fe in plagioclase can be represented by three logical but structurally implausible end members: Fe²⁺[Fe²⁺Si₃]0₈ with maximal Fe²⁺; Fe³⁺[Fe³⁺₃ Si]0₈ with maximal Fe³⁺; and an intermediate component Fe²⁺[Fe³⁺₂Si₂]0₈ equivalent to mgt + quartz. Mössbauer measurements of Fe³⁺/FeT on 28 feldspar separates indicate that the 1^st and 3^rd end members describe 20 compositions, so that exsolution of mgt and volumetrically equivalent quartz, though depleting total Fe, would increase the proportion of Fe²⁺. 5 sample compositions are colinear with the 3rd and only 3 samples appear to contain a significant proportion of the 2nd. Volume of exsolved mgt, 0 to 0.18%, was determined from magnetic saturation measurements.

Plagioclase (+rock) results are in three groups, where An indicates similar magma evolution: A) An_74-58, FeO_wt% 0.21-.39; B) An_69-58, FeO 0.35-.63; C) An_66-57; FeO 0.23-.42. A) with low Fe is dominantly paramagnetic with Fe in solid solution. B) with highest Fe lacks primary mgt, but has abundant exsolved mgt. Paradoxically Group C with compositions like A, has both primary mgt and up to 0.18% exsolved mgt. Presence of exsolved mgt must be controlled by varied cooling environments and a mgt + quartz saturation surface in the volume An-Ab-Fe²⁺[Fe²⁺Si₃]0₈-Fe²⁺[Fe³⁺₂Si₂]0₈.