Iron and Stony-Iron Meteorites

Benedix, Gretchen; Haack, H.; McCoy, T. J.

doi:10.1016/b978-0-08-095975-7.00109-1

Cited by 41 publications

(32 citation statements)

References 187 publications

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“…; and Benedix et al. ). For a more comprehensive plotting of Camel Donga metal compositions, including groundmass metals in proximity to the CD 2 and CD 3 nodules, cf.…”

Section: Results: Heterogeneous Camel Dongamentioning

confidence: 94%

Secondary‐volatiles linked metallic iron in eucrites: The dual‐origin metals of Camel Donga

Warren

Isa

Ebihara

et al. 2017

Meteorit & Planetary Scien

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The unique occurrence of abundant (~1 vol%) near‐pure‐Fe metal in the Camel Donga eucrite is more complicated than previously believed. In addition to that component of groundmass metal, scattered within the meteorite are discrete nodules of much higher kamacite abundance. We have studied the petrology and composition of two of these nodules in the form of samples we call CD2 and CD3. The nodules are ovoids 11 (CD2) to 15 (CD3) mm across, with metal, or inferred preweathering metal, abundances of 12–17 vol% (CD2 is unfortunately quite weathered). The CD3 nodule also includes at its center a 5 mm ovoid clumping (6 vol%) of F‐apatite. Both nodules are fine‐grained, so the high Fe metal and apatite contents are clearly not flukes of inadequate sampling. The metals within the nodules are distinctly Ni‐rich (0.3–0.6 wt%) compared to the pure‐Fe (Ni generally 0.01 wt%) groundmass metals. Bulk analyses of three pieces of the CD2 nodule show that trace siderophile elements Ir, Os, and Co are commensurately enriched; Au is enriched to a lesser degree. The siderophile evidence shows the nodules did not form by in situ reduction of pyroxene FeO. Moreover, the nodules do not show features such as silica‐phase enrichment or pyroxene with reduced FeO (as constrained by FeO/MgO and especially FeO/MnO) predicted by the in situ reduction model. The oxide minerals, even in groundmass samples well away from the nodules, also show little evidence of reduction. Although the nodule boundaries are generally sharp, groundmass‐metal Ni content is anti‐correlated with distance from the CD3 nodule. We infer that the nodules represent materials that originated within impactors into the Camel Donga portion of the eucrite crust, but probably were profoundly altered during later metamorphism/metasomatism. Origin of the pure‐Fe groundmass metal remains enigmatic. In situ reduction probably played an important role, and association in the same meteorite of the Fe‐nodules is probably significant. But the fluid during alteration was probably not (as previously modeled) purely S and O, of simple heat‐driven internal derivation. We conjecture a two‐stage metasomatism, as fluids passed through Camel Donga after impact heating of volatile‐rich chondritic masses (survivors of gentle accretionary impacts) within the nearby crust. First, reduction to form troilite may have been triggered by fluids rich in S2 and CO (derived from the protonodules?), and then in a distinct later stage, fluids were (comparatively) H2O‐rich, and thus reacted with troilite to form pure‐Fe metal along with H2S and SO2. The early eucrite crust was in places a dynamic fluid‐bearing environment that hosted complex chemical processes, including some that engendered significant diversity among metal+sulfide alterations.

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“…; and Benedix et al. ). For a more comprehensive plotting of Camel Donga metal compositions, including groundmass metals in proximity to the CD 2 and CD 3 nodules, cf.…”

Section: Results: Heterogeneous Camel Dongamentioning

confidence: 94%

Secondary‐volatiles linked metallic iron in eucrites: The dual‐origin metals of Camel Donga

Warren

Isa

Ebihara

et al. 2017

Meteorit & Planetary Scien

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“…These ataxites were obviously deposited at temperatures that lie between the temperature ranges for the formation of octahedrites and ataxites. According to the current hypothesis regarding the creation of irons by cooling of the melt, the ataxite Arltunga is the iron meteorite with the highest absolute cooling rate of 500 K/Myr (Buchwald, 1975), while the lowest absolute cooling rates are associated with the pallasites (2-18 K/Myr according to Benedix et al, 2014). Ataxites clearly play a special role in the characterization of the creation of irons.…”

Section: Argument 12: Formation Of Nickel-iron Meteorites With and Wimentioning

confidence: 99%

“…The role played by the trace elements germanium and gallium in the classification of irons has received considerable attention (Benedix et al, 2014;Buchwald, 1975;Goldstein et al, 2009;Lovering, Nichiporuk, Chodos, & Brown, 1957;Mason, 1979;Scott, 1972;Wasson, 1974). As Figure 4 shows, there is excellent correlation between the concentrations of germanium and gallium in octahedrites, hexahedrites, and ataxites (r = 0.91).…”

Section: Argument 5 Alteration Of Meteorites By Cftmentioning

confidence: 99%

“…Bottke, Nesvorný, Grimm, Morbidelli, & O'Brien, 2006;Yang, Goldstein, & Scott, 2007). Papers published by Goldstein et al (2009), Benedix et al (2014); and others do not provide a convincing explanation of the observed distributions of trace elements, such as the variation in the abundances of germanium found in iron meteorites. Some of the formation models described above, including the role of impact events, are applicable to the creation of stonyiron meteorites, although in this case there is no need to postulate extremely low cooling rates.…”

Section: Introductionmentioning

confidence: 99%

“…According to current opinion, iron meteorites and some stonyiron meteorites are fragments of asteroids. These asteroidsthe parent bodies of the meteorites-underwent a process of geological evolution during a very early phase of the solar system that included the melting of metals and silicates, and differentiation to create asteroids with a core-mantle structure (Chabot & Haack, 2006;Goldstein, Scott, & Chabot, 2009;McCoy, Mittlefehldt, & Wilson, 2006in Benedix, Haack, & McCoy, 2014Krot, Keil, Scott, Goodrich, & Weisberg, 2008;Weisberg, McCoy, & Krot, 2006in Goldstein et al, 2009). After cooling extremely slowly, these parent asteroids are assumed to have been broken up by impacts occurring in the asteroid belt (Goldstein et al, 2009), resulting in the release of iron meteorites.…”

Section: Introductionmentioning

confidence: 99%

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Formation of nickel–iron meteorites by chemical fluid transport

Schrön¹

2017

ScienceOpen Research

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The deposition of solid material from the gas phase via chemical vapor transport (CVT) is a well-known process of industrial and geochemical relevance. There is strong evidence that this type of thermodynamically driven chemical transport reaction plays a significant role in certain natural processes. This article presents detailed evidence that CVT is a highly plausible mechanism for the formation of iron meteorites. In this study, naturally occurring CVT is referred to as "chemical fluid transport" (CFT) and the end products deposited from the gas phase as "fluidites."Treating iron meteorites as cosmic fluidites enables simple solutions to be found to the problem of how they formed and to numerous related and in some cases unresolved questions. This study is based on a thermodynamic trend analysis of solid-gas equilibrium reactions involving chlorine-and fluorinecontaining compounds of 42 chemical elements that include a systematic examination of reaction dominance switching behavior. In order to assess the transport behavior of the individual elements, the reaction-conditioned pressures p MeX were calculated from the equilibrium constants. For a selected group of minerals, the relative propensity of these minerals to deposit from the gas phase was then derived from the equilibrium constants. The study shows that octahedrites, hexahedrites and ataxites formed as a result of the transport of metal chlorides and fluorides (CFT) during accretion within the solar nebula. Siderophile elements are characterized by the similarities in their chemical transport properties. These chemical properties of the elements, expressed in the form of the reaction-conditioned pressure, play a key role in determining the chemical composition of iron meteorites. The mobilization process that leads to the formation of the gaseous metal halides MeX includes the reduction of oxides. The deposition of nickel-iron bodies occurs via back reaction after the transport of the gaseous halides. The back reaction leads to the thermodynamically favored deposition of schreibersite before troilite and of troilite before kamacite/taenite. The deposition temperature of octahedrites and hexahedrites lies below the temperature at which Widmanstätten patterns would be destroyed, while that of ataxites lies slightly above. Similarly, the occurrence of thermally instable cohenite in meteorites provides further support for the fluidite character of irons.The variation in the trace element concentrations in iron meteorites is explained by enrichment and depletion mechanisms in the gas phase. The striking correlation between gallium and germanium abundances in iron meteorites is the result of similarities regarding the mobilization phase and the reaction dominance switching behavior of both elements, and crystal isomorphism.These findings are supported by numerous arguments that provide evidence for the CFT model. The occurrence of the mineral lawrencite FeCl 2 in meteorites is interpreted as an indication of the effectiveness of the chemical transp...

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Timing of Nebula Processes That Shaped the Precursors of the Terrestrial Planets

Chaussidon¹,

Liu²

2015

The Early Earth

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Iron and Stony-Iron Meteorites

Cited by 41 publications

References 187 publications

Secondary‐volatiles linked metallic iron in eucrites: The dual‐origin metals of Camel Donga

Secondary‐volatiles linked metallic iron in eucrites: The dual‐origin metals of Camel Donga

Formation of nickel–iron meteorites by chemical fluid transport

Timing of Nebula Processes That Shaped the Precursors of the Terrestrial Planets

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