Finn Ulff-Møller

Chemical Classification of Iron Meteorites: XII. New Members of the Magmatic Groups

Abstract Data are reported for thirty iron meteorites that are members of the magmatic groups, for three main group pallasites, one anomalous mesosiderite, and for three ungrouped irons and an ungrouped pallasite that are similar to IIIAB irons in their Ni, Ga, and Ge contents. The set includes four observed falls (11% of iron falls) Ban Rong Du, Chisenga, Nyaung and Sterlitamak, and Zaisho, one of two known pallasite falls. Two of the ungrouped irons (Ban Rong Du and Mount Howe 88403) and the ungrouped pallasite Yamato 8451, although having Ni, Ga, and Ge contents in the same general range as IIIAB, have very different contents of Co and exhibit significant differences for several other elements; they are clearly not related to IIIAB or to its little sister, group IIIE. A fourth ungrouped iron, Tres Castillos, chiefly differs from IIIAB in terms of its low Ga and high Ge contents; its Ga/Ge ratio is 35% higher than that of any other IIIAB iron. We report data on four new IIAB irons, all falling within established fields; the Bilibino iron is somewhat unusual, having a low Ir content (0.12 μg/g) and a structure altered by reheating. The IVA irons are also typical. One, Albion, may be a mislabeled specimen of Gibeon; another, Page City, exhibits large cracks (up to 3 cm). The Chaunskij anomalous mesosiderite has exceptionally high Ni and very low Ir concentrations. Two of three new main group pallasites are anomalous; Pecora Escarpment 91004 has an Ir content above the normal range, and Zaisho has an exceptionally high Fa content in the olivine.

The Portales Valley meteorite breccia: evidence for impact-induced melting and metamorphism of an ordinary chondrite

Abstract The Portales Valley H-chondrite fall is an annealed impact-melt breccia with coarse metal interstitial to angular and subrounded silicate clasts. The large metal-rich regions exhibit a Widmanstatten structure and contain very little troilite. We were able to examine a 16.5 kg metal-rich specimen of Portales Valley. Silicates contain numerous flecks of metallic Cu and curvilinear trails of tiny metallic Fe–Ni blebs, characteristic of shocked and annealed chondrites. One silicate clast appears to have experienced little (

The thermal evolution of IVA iron meteorites: evidence from metallographic cooling rates

Metallographic cooling rates of group IVA iron meteorites have been recalculated based on the most recent Ni diffusion coefficients and phase diagram. The cooling rates are revised upwards by a factor of ca. 15 relative to previous estimates. A large range in cooling rate is found in the low-Ni part of group IVA (Ni < 8.4 wt%), while the high-Ni part shows approximately constant cooling rates. Undercooling is observed only in the high-Ni IVA members. Some of the taenite lamellae in the high-Ni IVA irons, which were apparently affected by moderate undercooling, can, alternatively, be interpreted to have experienced a nonlinear cooling history. The variation in cooling rate of the entire group IVA spans two orders of magnitude (19–3400 K/My). This span is still so large that it constitutes severe problems for both a core origin model and a raisin-bread model but seemingly it does not contradict a model where the parent body is broken up and reassembled after core crystallization but prior to Widmanstatten pattern formation.

Massive chromite in the Brenham pallasite and the fractionation of Cr during the crystallization of asteroidal cores

Abstract Large (≥2 mm) chromite grains are present in IIIAB iron meteorites and in the main-group pallasites ( pmg ), closely related to high-Au IIIAB irons. Pallasites seem to have formed by the intrusion of a highly evolved metallic magma from a IIIAB-like core into fragmented olivine of the overlying dunite mantle. High Cr contents are commonly encountered during the analyses of metallic samples of high-Au IIIAB irons and main-group pallasites, an indication that Cr contents were high in the intruding liquid and that Cr behaved as an incompatible element during the crystallization of the IIIAB magma, contrary to expectations based on the negative IIIAB Cr-Ni and Cr-Au trends among low-Au IIIAB irons. In a region about 10 cm across in the Brenham main-group pallasite massive chromite fills the interstices between olivine grains, the site normally occupied by metal in Brenham and other pallasites. The massive chromite may have formed as a late cumulus phase; because Fe-Ni was also crystallizing, its absence in the chromite-rich region suggests a separation associated with differences in liquid buoyancy. The coexisting chromite and olivine are zoned; in the olivine FeO is highest in pallasitic (olivine-metal) regions, lowest in rims adjacent to chromite, and intermediate in the cores of these olivines. Chromite shows the opposite zoning, with the highest FeO contents at grain edges adjacent to olivine. The observed gradients are those expected to form by Fe-Mg exchange between olivine and chromite during slow cooling at subsolidus temperatures. Compared to normal Brenham, contents of phosphoran olivine and phosphates are higher in the chromitic pallasitic region. We also report data for large-to-massive chromites present in pmg Molong and in high-Au IIIAB Bear Creek that, like Brenham, formed from a highly evolved magma. The Bear Creek chromite has a much lower Mg content than that in the pallasites, implying that, in the pmg , the Mg was extracted from the olivine during high-temperature reaction with the precipitating chromite. There are other circumstantial arguments indicating that Cr was incompatible in the metal during the crystallization of the IIIAB magma, with the concentration in the residual magma rising from an initial value of about 300 μg/g to a value around 700 μg/g when Bear Creek and Brenham were formed. We consider possible explanations for these negative Cr-Au and Cr-Ni trends and find the most probable one to be that they reflect sampling artefacts resulting from analysts avoiding visible chromite (and the commonly associated phase FeS) when choosing metal samples.

Magmatic activity on the IVA parent body: Evidence from silicate-bearing iron meteorites

Abstract Four of the magmatic IVA iron meteorites contain tridymite or clinobronzite-orthobronzitetridymite which are quite unlike silicate assemblages in other iron meteorites. The textures, the bulk chemistry, and the zoning preserved in the pyroxenes strongly suggest that they are igneous cumulates. The pyroxenes have extremely low Fe/Mn-ratios (less than 20) and low contents of REEs and other incompatible elements. These cumulates crystallized from magmas of unusual composition, with some similarity to terrestrial boninites, at the protobronzite-tfdymite cotectic in the olivine-plagioclase-silica system. A liquidus temperature in the range 1400-1350°C was inferred for the Steinbach meteorite from the estimated distribution coefficient for Cr in pyroxene (DCrsolid/liquid ∼ 0.66). The low levels of incompatible elements show that less than 1% of the residual liquid was trapped in the cumulates. During cooling at subsolidus temperatures, most of the protobronzite transformed to orthobronzite and the rest inverted to a fine inter-growth of clino- and orthobronzite. In addition, the igneous zoning of Ca, Fe, Mg, and Mn was modified by diffusion, whereas Ti, Al, and Cr were not or only slightly affected. The silica-saturated magmas could not have evolved in an olivine-rich mantle. We assume that the magmas became incorporated in the metal core, possibly due to solidification shrinkage of the metal. We propose that the IVA parent magmas were formed by high degrees of partial melting (>40%) of a chondritic precursor along the olivine-pyroxene peritectic reaction curve in the olivine-plagioclase-silica system at low pressures. The precursor may have been depleted in incompatible elements by a preceding melting episode. The partial melts were then separated from the olivine residue and subsequently reduced to account for the low Fe Mn - ratios and the unusually high Si content.

Formation of native iron in sediment-contaminated magma: I. A case study of the Hanekammen Complex on Disko Island, West Greenland

Abstract For the first time a compositional range of native iron bodies is described in a cogenetic series of sediment-contaminated volcanic rocks from the Tertiary West Greenland Basalt Province. The iron-bearing rocks occur in a high-level composite intrusion, the Hanekammen Complex. Reaction between a tholeiitic parent magma with > 11% MgO and carbonaceous Al2O3-rich shale took place in a reservoir >3 km below the paleosurface and created magmatic layering with basaltic magma overlain by less dense andesitic magma. The contaminated rock series bears a strong imprint of assimilation but very little fractional crystallization, which implies that the two processes were not intimately coupled in the present case. Most of the iron was formed at depth in a range of fO2 from 10−13 atm 1200°C to 10−16 atm 1100°C as determined by the P distribution between solid iron and magma. This is far below the graphite-gas (CCO) buffer at the estimated pressure (>800 bars) and suggests a high hydrogen fugacity. Together, iron in basalt and andesite form a general trend, defined by Co versus Ni concentrations, that reflects the degree of assimilation, the amount of immiscible sulphide liquid, and the degree of reduction (in order of decreasing importance). The zoning of single iron grains reflects the dynamics of their growth and, to some extent, subsequent homogenization and reaction with the magma. Weakly zoned iron spherules in viscous andesite were formed and remained in situ, whereas iron grains in basalt settled through the layered magma and developed strong zoning. All iron types contain Co-rich domains (