Iron, IVA-an
(silica-rich orthopyroxenite, possibly LL chondrite related)
standby for steinbach photo
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 in Steinbach (~66%, quantifiably a stony-iron) is composed of a glomerocrystic assemblage of low-Ca orthopyroxene (37.3–42.9 vol%) and clinopyroxene (1.6–4.5 vol%), with a relatively large component of the silica mineral 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 both orthobronzite and clinobronzite, a ‘type-1 pyroxene’ (cloudy, troilite inclusion-rich) which is associated with clinobronzite, and tridymite are present. Of these silicates, the clinobronzite and type-1 pyroxene formed early and cooled rapidly, while the major component represented by incompatible element-depleted orthobronzite crystallized later (Ruzicka, 2014). The tridymite likely co-crystallized with pyroxenes as cumulate phases, but some 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 which 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 was evidently injected throughout the earlier-crystallized silicates during an impact event (Ruzicka, 2014).

Results of studies of Steinbach and São João Nepomuceno by Ruzicka and Hutson (2006) and Ruzicka (2014) indicate that their compositional trends are most consistent with formation over an ~60–70% fractional crystallization interval within a core or molten pod. The parental melt was most likely an ordinary chondrite protolith, probably similar to the L or LL chondrite parent asteroid (see isotopic diagram below), with Steinbach melts being derived from batch melting at a stage of >50% partial melting. Chemical variability among 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 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. Chromium vs. Oxygen-isotope Plot
standby for o-cr diagram
click on image for a magnified view

Diagram credit: Sanborn et al., 49th LPSC, #1780 (2018) 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 latest of the silica-bearing meteorites to form on the IVA parent body, which occurred after ~77% core crystallation (the others formed at or before the 30% fractional crystallization stage). Wasson et al. (2006) posited that crystallization might have occurred from the center of the core outward, with both stony-iron lithologies forming near the core–mantle boundary.

In light of all the available data, multiple plausible models for the formation of Steinbach and the other silica-bearing IVA iron meteorites have been proposed. A scenario based on the estimated cooling rate data of metal and orthopyroxene, and another based on O-isotope thermometry (Wang et al., 2004), suggest that Steinbach and two other paired silica-bearing IVA iron meteorites, São João Nepomuceno and Descoberto (Zucolotto and Monteiro, 2012), initially cooled from peak temperatures at a slow rate following accretion and heating by radiogenic nuclide decay. A molten metallic core would have been quickly formed through gravitational drainage. An in-depth analysis of Steinbach by Ruzicka and Hutson (2006) led them to suggest that this precursor body was disrupted during a catastrophic collision, with some portion of the original body being re-accreted to form a random mixture of molten metal and silicates. From the small number of silicated IVA meteorites known, it is probable that this re-accreted body was mostly composed of the metallic core component.

This impact 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 down 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. In paleomagnetic studies of Steinbach and São João Nepomuceno, Bryson et al. (2017) found evidence for a strong natural remanent magnetization in matrix metal in Steinbach. This is consistent with the hypothesis and models of an unmantled, inwardly solidifying core that generated a dynamo (probably through non-concentric solidification) with a directionally varying field of ≥100 µT.

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 the size of cloudy taenite islands. 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 for 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 possibly 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. standby for sjn photo
Photo courtesy of the J. Piatek Collection

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