Laser ablation ICP-MS study of IIIAB irons and pallasites: constraints on the behaviour of highly siderophile elements during and after planetesimal core formation
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Abstract We have measured the abundances of seven precious metals in NiFe phases (kamacite and plessite) in six iron meteorites. These in-situ analyses, obtained by accelerator mass spectrometry (AMS) on small polished samples previously characterized by electron microprobe techniques, constrain the distribution of the rare siderophile elements. Within our set of irons, a small but varied suite, concentrations measured by AMS vary by factors of 9–13 for Au, Pd, Pt, Rh and Ru, and by factors of 90 and 250 for Ir and Os respectively. Data are presented for all six platinum group elements (PGE) plus gold. The AMS data suggest a variation in overall precious-metal abundances of a factor of 16 between the most-enriched (Negrillos, Σ PGE + Au = 270 ppm) and the least-enriched (Welland, 16–19 ppm). A clear illustration of the use of AMS data for provenance studies of meteoritic iron is presented for the Welland IIIA iron, an 1888 find from Ontario. Few published data are available for Welland: comparison of a type sample with a smaller piece of unknown metal, with respect to chondrite-normalized PGE patterns, major-element chemistry and textures of the metals, strongly support a suggestion that the latter is a fragment of the same iron.
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Abstract— We studied 26 IAB iron meteorites containing silicate‐bearing inclusions to better constrain the many diverse hypotheses for the formation of this complex group. These meteorites contain inclusions that fall broadly into five types: (1) sulfide‐rich, composed primarily of troilite and containing abundant embedded silicates; (2) nonchondritic, silicate‐rich, comprised of basaltic, troctolitic, and peridotitic mineralogies; (3) angular, chondritic silicate‐rich, the most common type, with approximately chondritic mineralogy and most closely resembling the winonaites in composition and texture; (4) rounded, often graphite‐rich assemblages that sometimes contain silicates; and (5) phosphate‐bearing inclusions with phosphates generally found in contact with the metallic host. Similarities in mineralogy and mineral and O‐isotopic compositions suggest that IAB iron and winonaite meteorites are from the same parent body. We propose a hypothesis for the origin of IAB iron meteorites that combines some aspects of previous formation models for these meteorites. We suggest that the precursor parent body was chondritic, although unlike any known chondrite group. Metamorphism, partial melting, and incomplete differentiation ( i.e. , incomplete separation of melt from residue) produced metallic, sulfide‐rich and silicate partial melts (portions of which may have crystallized prior to the mixing event), as well as metamorphosed chondritic materials and residues. Catastrophic impact breakup and reassembly of the debris while near the peak temperature mixed materials from various depths into the re‐accreted parent body. Thus, molten metal from depth was mixed with near‐surface silicate rock, resulting in the formation of silicate‐rich IAB iron and winonaite meteorites. Results of smoothed particle hydrodynamic model calculations support the feasibility of such a mixing mechanism. Not all of the metal melt bodies were mixed with silicate materials during this impact and reaccretion event, and these are now represented by silicate‐free IAB iron meteorites. Ages of silicate inclusions and winonaites of 4.40‐4.54 Ga indicate this entire process occurred early in solar system history.
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The IABs represent one of only two groups of iron meteorites that did not form by fractional crystallization of liquid Fe-Ni in the core of a differentiated planetesimal. Instead, they are believed to originate from a partially differentiated body that was severely disrupted by one or more impacts during its early history. We present a detailed microstructural and paleomagnetic study of the Odessa and Toluca IAB meteorites, with a view to further constraining the complex history of the IAB parent body. X-ray photoemission electron microscopy and energy dispersive spectroscopy were used to generate high-resolution Ni/Fe maps. The crystallographic architecture of Odessa was analysed using electron backscatter diffraction. Paleomagnetic signals and the magnetic properties of several microstructures were also assessed using X-ray magnetic circular dichroism. Odessa exhibits a complex series of microstructures, requiring an unusual evolution during slow cooling. A conventional Widmanstätten microstructure, consisting of multiple generations of kamacite lamellae surrounded by M-shaped diffusion profiles, developed via continuous precipitation to temperatures below ∼400 °C. Multiple generations of pearlitic plessite nucleated from kamacite/taenite (T > 400 °C) and tetrataenite rim/taenite interfaces (T < 400 °C), via a process of discontinuous precipitation. Rounded rafts of Ni-rich taenite, observed within some regions of pearlitic plessite, are shown to have the same crystallographic orientation as the parental taenite, and a non-standard orientation relationship with the enclosing kamacite. Contrary to current theories, these rafts cannot have formed by coarsening of pre-existing pearlitic plessite. A new bowing mechanism is proposed, whereby rafts of Ni-enriched taenite form between advancing lobes of an irregular reaction front during discontinuous precipitation. Subsequent coarsening leads to the growth of the taenite rafts, and the partial or complete removal of pearlite lamellae, resulting in spheroidised plessite with a crystallographic architecture matching the experimental observations. We find no evidence for a strong magnetic field on the IAB parent body, suggesting it did not have an active core dynamo at the time of cloudy zone formation. This supports the prediction that the IAB parent body was unable to form a significant core due to the redistribution of metal during an earlier impact event.
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Abstract The Krymka chondrite contains an exotic graphite-bearing fragment that appears to be of a new type of material added to unequilibrated LL-chondrite during agglomeration on the surface of the parent body. The fine-granular texture without chondrules, two morphological groups of graphite crystals which differ in size and occurence, high content of troilite (11.3 vol.%), the high Ni (55.5–66.6 wt.%) and Co (1.59–2.87 wt.%) contents of the taenite and absence of kamacite, the presence of F-apatite, which is rare for meteorites but common for lunar and terrestrial igneous rocks, are the main features of the fragment. The mineralogy and texture indicate: (1) the fragment probably formed by crystallization from a highly reduced silicate melt, which had been enriched in carbon; 2) the subsequent metal sulphidization lowered its abundance and resulted in the formation of troilite and the compositional features of the residual metal; (3) terrestrial weathering of an exotic fragment and the host part of the chondrite produced iron hydroxides, pentlandite and quite possibly magnetite.
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