Iron, IVA-an
(silica-bearing)
Found 1724
50° 30' N., 12° 30' E. approx.
A unique meteorite 40 cm in diameter was found in Karl-Marx-Stadt, Germany, and classified as a IVA-anomalous stony-iron. Steinbach is possibly associated with a fall near Grimma, Germany around 1550. Three separate masses were found including a 1.0 kg mass found in Gotha in 1724, an 86.5 kg mass found in Rittersgrün in 1833, and a 10.5 kg mass found in Breitenbach in 1861, for a combined weight of 98 kg.
The silicate fraction of Steinbach (66%) is composed of a coarse, granular assemblage of low-Ca orthopyroxene (37.3–42.9 vol%) and clinopyroxene (1.6–4.5 vol%), with a relatively large component of tridymite (20.2–30.1 vol%). Approximately one-third of the volume consists of FeNi-metal, while various troilite assemblages and minor chromite are also present, but no olivine has been identified. In a more precise investigation of the silicate fraction it was determined that clinobronzite, type-1 pyroxene (inclusion-rich), type-2 pyroxene (incompatible-element enriched), type-2 pyroxene (incompatible-element depleted), and tridymite are present, each associated with early or late crystallization. Only enriched type-2 pyroxene crystallized late. The tridymite likely co-crystallized with pyroxenes as cumulate phases, but some was possibly formed from the oxidation of Si from the metal. Since no plagioclase is present, in can be inferred that all feldspathic melt was expelled from the cumulus pile. A Thomson (Widmanstätten) structure is revealed upon etching that exhibits a continuity throughout the meteorite. This indicates that the metal grains are connected in 3-D space, and represent a single taenite crystal, even though the metal appears to be cm-sized when viewed within the area of a slice. The metal and troilite evidently flowed through the earlier-crystallized silicates.
According to their study of Steinbach, Ruzicka and Hutson (2006) determined the compositional trends to be most consistent with ~65% fractional crystallization of pyroxene within a core or molten pod. The parental melt was basically an ordinary chondrite protolith, probably similar to the LL chondrite body, with Steinbach melts being derived from batch melting of >50% partial melting. Chemical variability between different grains in the low-Ca pyroxene favor a cumulate origin, and the variability of the grains is attributed to an extended formation period within an evolving melt. In light of this new data, Ruzicka and Hutson (2005) proposed that the Steinbach parental melt had previously lost an evolved melt fraction, either during an earlier, separate melt phase, or during a single, progressive heating event.
The low Ni content is thought to be a result of Fe dilution derived both from dissociation of FeS and by reduction of FeO (Wasson et al., 2006). A lower than normal Ge and Ga content for this iron suggests their loss through high temperatures in an open system. Melting and differentiation of the iron and its subsequent drainage to form a core was likely initiated by a large impact(s) onto a porous L–LL-type asteroid between ~20 and 160 km in diameter, rather than by radiogenic heating (26Al, 60Fe) alone (Wasson and Kunihiro, 2004; Kunihiro et al., 2004). It was calculated that Steinbach contains metal and silicates more evolved than in other silica-bearing meteorites, consistent with a greater depth of penetration as a melt phase into fractures in the metallic core. Steinbach was the youngest of the silica-bearing meteorites to form on the IVA parent body, which occurred after ~77% core crystallation (the others formed at or below 30% crystallization). Wasson et al. (2006) proposed that crystallization occurred from the bottom of the core upward.
In light of all the available data, more than one plausible model for the formation of Steinbach and the other silica-bearing IVA iron meteorites has been posited. A scenario based on the estimated cooling rate data of metal and orthopyroxene, and on O-isotope thermometry (Wang et al., 2004), suggests that Steinbach and two other probably paired silica-bearing IVA iron meteorites, São João Nepomuceno and Descoberto, initially cooled at a slow rate following accretion and heating by radiogenic nuclide decay from peak temperatures. A molten metallic core would have been quickly formed through gravitational drainage. An in-depth analysis of Steinbach by Ruzicka and Hutson (2006) has led them to suggest that this precursor body was disrupted during an impact collision, with some portion of the original body being re-accreted to form a random mixture of molten metal and silicates. By the small number of silicated IVA meteorites known, this re-accreted body mostly constituted the metallic component.
This disruption event disrupted most of the silicate mantle and caused rapid cooling (~100°C/hr) at higher temperatures, initiating the production of clinobronzite from protopyroxene, as well as hindering metal–silicate separation. After the remaining silicates were crystallized (by 1240°C), slow cooling persisted to temperatures as low as ~350°C, with cooling rate variations among different samples attributable to differences in burial depth. Cooling of this mantle-less metallic body is thought to have progressed from the surface inward, and metallographic cooling models for the IVA meteorites are consistent with such a scenario given an asteroidal diameter of ~300 km.
During the high temperature stage (1450–1490°C) of this proposed disruption/re-accretion event, the solid olivine-rich mantle was segregated from the remaining molten phases, silicate melt was injected into metallic melt, a significant degree of reduction induced the loss of substantial FeO, and some volatile losses occurred. Although a rapid cooling phase 207–375 m.y. ago attests to an additional disruption, the silicates in Steinbach show no shock effects, a factor which would need to be accounted for provided there were further impacts.
An alternative scenario was developed by Wasson et al. (2006) in which they dispute the possibility of an impact-scrambling model, and suggest instead that this silicated IVA group experienced a typical cooling scenario with no large cooling-rate deviations as evidenced by cloudy taenite island sizes. They propose that a late impact event converted some orthopyroxene into clinopyroxene. However, more exacting measurements of high-Ni particles in the cloudy zone by Goldstein et al. (2009) indicate a much different thermal history of the IVA irons (see further details and a likely formation scenario on the Gibeon page.
Two other IVA irons, Gibeon and Bishop Canyon, contain tridymite veins that were formed from vapor deposition (or melt injection) of a SiO-rich source. The specimen of Steinbach pictured above is a 5.8 g partial slice showing the FeNi-metal network that encloses granular aggregates of low-Ca pyroxene and tridymite. The porosity of Steinbach was determined by M. Strait (2010) to range from 0% for the metallic component to 3.56% for the silicate component. The photo below is a 631 g slice of São João Nepomuceno, shown courtesy of the Jay Piatek Collection.
