WOODBINE


Iron, IAB complex, Pitts grouplet
standby for woodbine photo
Found Spring 1953
42° 20' 48" N., 90° 10' 6" W.

A mass of 48.2 kg was plowed up on Henry Albrecht's farm ~2 km west of Woodbine, Illinois. The weathered angular mass measuring 30 × 27 × 17 cm was without its fusion crust and heat-affected zone, indicating a very long terrestrial residence.

The IAB iron-meteorite complex, recently proposed by Wasson and Kallemeyn (2002), comprises iron meteorites from the former IAB-IIICD group, as well as numerous related irons. Many of the members contain silicate inclusions with chondritic compositions. Woodbine is a low-Au member of the IAB complex that is closely related to the main group. On a Ni–Au diagram, a grouplet of three members—Woodbine, Colfax, and Pitts—has been resolved in an area intermediate between the sLL and sLM subgroups, but with higher Ni contents, and they were designated the Pitts grouplet. A new study conducted by Worsham et al. (2016) coupling Pd vs. other HSEs found that the Pitts grouplet and the sLM subgroup both show enrichments in Pd abundances, but otherwise they are considered to have experienced distinct petrogenetic histories.

The metal in Woodbine has a small-grained polycrystalline texture, exhibiting independently oriented, fine-textured Thomson (Widmanstätten) structures. Bulk analysis reveals that 20.4 wt% of Woodbine consists of non-metal phases composed of 7.6% enstatite, 4.2% Mg-rich olivine, 2.6% Na-rich anorthite, and 1.1% diopside which form clusters up to 6 mm wide. Schreibersite forms rims on silicate grains, and troilite has been melted by shock and injected into the cracks of the silicates, cementing individual grains into large aggregates. Minor graphite occurs between troilite and kamacite and as rims around silicate grains. Carbon is also present as cohenite and haxonite.

Dey et al. (2019) employed 17O and ε54Cr values for several irons and their associated silicates/oxides to investigate i) if each iron and its associated phases originated on a common parent body (i.e., an endogenous mixture of core and mantle vs. an exogenous mixture through impact), and ii) if any genetic connection exists between the irons and other meteorite groups (e.g., IAB with winonaites, IIE with H chondrites, and Eagle Station pallasites with CK chondrites). Three IAB irons were employed in the study, and it was demonstrated on a coupled diagram that although the ε54Cr values for the iron component plot in the winonaite field, values for the silicate component plot in a distinct region on an O–Cr coupled diagram (see diagram below). From these results they ascertained that the IAB silicated irons formed through an impact-generated mixture comprising iron from a winonaite-related parent body and silicate from an unrelated and otherwise unsampled parent body. Incorporation of the silicates into the FeNi-metal host took place at a depth greater than 2 km, allowing time for a Thomson (Widmanstätten) structure to develop during a long cooling phase. Fractional crystallization occurred in some large molten metal pools, followed by very slow cooling, which produced the broad range of features found in certain IAB meteorites (e.g., silicate-poor, graphite–troilite-rich inclusions and extremely high Ni contents). Other results from their study can be found on the Miles and Eagle Station pages.

17O vs. ε54Cr for Irons and Pallasites
standby for o-cr isotope diagram
click on diagram for a magnified view

Diagrams credit: Dey et al., 50th LPSC, #2977 (2019)

The composition of Woodbine is consistent with mixing, compression, and annealing of incompletely segregated metal and silicate on a small chondritic asteroid which never experienced temperatures high enough for complete melting to occur. Upon cooling, the graphite and schreibersite precipitated on the silicate grains, followed by kamacite nucleation, which gradually developed into a Thomson (Widmanstätten) structure. Neumann bands were formed by the impact shock that dislodged the mass from the asteroid. The absolute I–Xe retention age for Woodbine, relative to the Shallowater standard, was calculated to be 4.5659 (±0.0003) b.y (Niemeyer, 1979). A Pb–Pb age of 4.563 (±0.029) b.y. (isochron for all samples, see diagram below) was calculated by Litasov et al. (2019) for apatite grains in Woodbine and some other group IAB irons; a somewhat younger Pb–Pb age was obtained when excluding several incongruent data points.

standby for udei pb-pb diagram
Diagram credit: Litasov et al., 82nd MetSoc, #6125 (2019)

In a study of iron meteorite exposure histories, Welten et al. (2008) found that Pitts exhibits a complex exposure history comprising two stages. During the first stage of irradiation, which involved high shielding at a depth of 63–77 cm within an object >2 m in diameter, cosmogenic noble gas data indicate a CRE age of 600 (+190/–150) m.y. A second stage irradiation lasting only ~0.7 (±0.2) m.y. occurred on a <40 cm diameter body. The investigators argue that the cosmogenic radionuclide and noble gas data for Pitts are consistent with a scenario in which a fragment was ejected during a minor impact on a km-sized near-Earth object (NEO), followed by its rapid delivery to Earth. A possible source object for the Pitts meteorite is the Earth-crossing M-type asteroid 1986 DA, itself possibly derived from a larger main belt object. Since 1986 DA has a semi-major axis that is close to the 5:2 mean motion resonance with Jupiter, one of the M-type asteroids similarly located such as 16 Psyche and 216 Kleopatra might be the source parent body of the Pitts iron, and by association, the Woodbine and Colfax irons.

Woodbine is a fine octahedrite of the IAB iron-meteorite complex. To learn more about the relationship within the IAB complex and among other iron chemical groups, see the Appendix Part III. The specimen of Woodbine pictured above is a 9.7 g partial slice containing coarse silcate inclusions.