"Rock of the Month # 121, posted July 2011" ---

Putorana is a place-name in Siberia. Meteorites are traditionally named for their location where they were seen to fall, or were found later, often many years later. The lustrous, rounded blebs of silvery-white metal in a fine-grained groundmass are reminiscent of a mesosiderite, the less-flashy cousins of pallasites, the better-known class of stony iron meteorites. This material was analysed soon after its discovery, and deemed to be an elegant "pseudometeorite", a fine facsimile of a class of meteorite, but nonetheless of terrestrial origin.

The possible meteoritic provenance was studied by Treiman et al. (2001, 2002). and their negative finding was later cited in the valuable handbook by Norton and Chitwood (2008) in a list of "meteorwrongs" (ibid., pp.175-180). The purported meteorite(s) are from the Putorana plateau of the Noril'sk district of Siberia, a region known to economic geologists for the huge nickel-copper-platinum-palladium deposits of the Noril'sk-Talnakh "ore junction", associated with rift basalts and related magmas of Permian age. The region, east and southeast of Noril'sk, includes a nature reserve (Montaigne and Olson, 2000), near which the sample was apparently recovered.

The samples were first identified not as meteorites but as fine-grained basaltic breccia with basalt, fine-grained "anorthosite" and feldspathic dunite clasts, lacking the cosmogenic radionuclide ²⁶Al. Plagioclase, olivine and rare ilmenite co-crystallized, followed by pigeonite. There is a rusted exterior, no obvious fusion crust, and a fresh interior. The lack of a crust obviously speaks against a meteoritic origin for such fresh material, although the mineralogy is certainly unusual for terrestrial lavas, including cohenite and rare native copper (Treiman et al., 2001). The rock contains Ni-bearing native iron (essentially, but not quite, kamacite, generally found only in meteorites) but despite this unusual metal, it is not a meteorite - in addition to the evidence above, oxygen isotope ratios appear terrestrial, and some of the silicate mineral-chemical details are not appropriate to meteorites. This "Putorana" rock is presumed to be related to Siberian Trap basalt magmatism. The metallic copper can be seen in hand specimen, or within kamacite. There is no taenite, the typical nickel-rich alloy complement of kamacite in meteorites. Iron carbide (Fe₃C, cohenite) encloses kamacite. Comparison with native iron on Earth (e.g., Disko Island, west Greenland) and with planetary basalt suites and mesosiderites identifies Putorana as terrestrial (Treiman et al., 2002). Electron microprobe analysis of Putorana iron reveals that the iron contains minor amounts of Ni (2.31%), Co (0.54%) and Cu (0.12%), very comparable to published data on iron from Disko Island (see below).

An extra note on native iron: as recorded in the MINLIB bibliography (March 2017) the relatively rarity of iron in its unalloyed (reduced, metallic) state is indicated by the relative scarcity of reports, given that iron is one of the most abundant elements in the Earth's crust. Indeed, iron is orders of magnitude more abundant than the metals in the following table, of which only copper, besides iron, forms oxidized phases to a significant degree. The estimated average bulk abundance for each element (in all its varied chemical states and mineral forms) is given below, in parts per billion, parts per million, or (iron) PERCENT. Estimated average abundances in the Earth's crust from Rudnick and Gao (2005) except Te (Zemann and Lautwein, 1974).

The Cu-Ni-PGE sulphide ores of the region include a diversity of Cu-Fe-(Ni) sulphides such as the familiar chalcopyrite and rarer or easily-overlooked minerals such as putoranite, talnakhite and mooihoekite, often with unusual discrete platinum-group mineral species of palladium, copper and tin, such as cabriite (Evstigneeva and Genkin, 1983; Torgashin, 1992).

The Putorana area is part of the extensive Siberian Traps, a "large igneous province" with basalt flows and interflow ash fall deposits, and suites of intercalated mafic-ultramafic intrusions. The Permo-Triassic flood basalts cover an estimated area of 337,000 km², average thickness 1 km. There are two main subprovinces (Noril'sk, 5-10% of the total volume, up to 3 km thick) and Putorana (90-95% of volume, >2 km thick, Sharma et al., 1991), plus a third smaller component, the Maimecha-Kotui suite. The lavas of the Putorana plateau are highly homogeneous (Nesterenko et al., 1992), and isotopic data suggest a homogeneous mantle source (Sharma et al., 1992). In oxygen isotopes, the Putorana lavas are also very consistent, whereas the Noril'sk suite are variable (Das Sharma et al., 1994).

Geochronology indicates that the voluminous flood basalts were emplaced rapidly, over an interval of the order of 900,000 years, commencing about 248 Ma, with a mean eruption rate >1.3 km³/year (Renne and Basu, 1991). These authors postulated a mantle plume, as opposed to a simple rift event, and noted that the available data could not preclude the magmatism being responsible for the Permian-Triassic mass extinction (249±4 Ma). The Siberian traps are the largest known flood basalt province, and the end-Permian mass extinction is the largest event of its type on record (Saunders and Reichow, 2006).

The large Putorana sample displays an emulsion of mm-scale metal blebs in a silicate matrix. This resembles an oil-water "mixture". Within this presumed mixture of two at least temporarily immiscible liquids (metal and silicate) there are cm-scale xenoliths of silicate rock with <1% metal. The metal blebs appear to have locally coalesced to form metal veinlets several mm wide and several cm long. It is not immediately clear why the iron precipitated in the native form: a likely explanation would be interaction of the rising magma with a strong reductant, such as a graphitic schist or coal seam.

The magnetic susceptibility of the material is, not surprisingly, higher than just about any stony meteorite: only stony irons, irons and other meteorites with substantially more than 50% metal will be more magnetic. The minimum mean of 5 measurements on the slab, which weighs 1,362 grams, is 902x10^-3 SI units, cf. an average estimate of 746 for a comparable, 1,555-gram slab of the EH4 Abee (sample ROM 53274), which is one the most magnetic "stony" meteorites I've seen. The lowest value on the Putorana slab, over a silicate xenolith, was 694, followed by 862, 958, and two values >999 (upper limit in normal operating mode, "cut" to 999 for the purposes of a minimum estimate). Eight subsequent measurements averaged 903, the lowest value 671. For comparison, a sample with magnetic susceptibility ~10x10^-3 SI units will but weakly attract a suspended pen magnet, and so in low-tech field terms can be said to be "weakly magnetic".

Appearance in thin section: The PUT-1 sample is a roughly 40:60 metal-silicate mixture, the metal hosted within a holocrystalline, intergranular matrix dominated by fine-grained rounded olivine and tabular, twinned plagioclase feldspar, plus feldspar- and olivine-crowded poikilitic overgrowths of coarser, later clinopyroxene and scattered phenocrysts of olivine. There is a trace of tabular ilmenite, and local patches of beautiful goethite, iron oxyhydroxide secondary after iron. The metal appears single-phase, isotropic, and browner than the kamacite-taenite intergrowths that are so familiar in most meteorites. The variable mineral proportions across a single section can vary from olivine-bearing anorthosite to troctolite to olivine gabbro.

Above left: rounded olivine and tabular plagioclase crystals, in part enclosed by overgrowth of later poikilitic / ophitic clinopyroxene.
Above right: Small native iron grains in the granular silicate host rock, with a partially resorbed tabular grey plate of ilmenite (ideal formula FeTiO₃). Note the absence of an obvious second phase in the metal, cf. the taenite often found in meteoritic kamacite. The Putorana iron may display one or two sets of ragged fractures.
Images at nominal 50X magnification, long-axis field of view 1.7 mm, in (left) crossed-polarized transmitted and (right) plane-polarized reflected light.