Stannern

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Eucrite
Monomict, noncumulate
(Stannern trend)
standby for stannern photo
Fell May 22, 1808
49° 17′ N., 15° 34′ E.

Following detonations, about 300 stones fell in Jihomoravsky, Czechoslavakia at approximately 6:00 A.M.. Sixty-six of these stones were subsequently recovered, having a combined total weight of ~52 kg, with the largest weighing 6 kg.

Stannern is a moderately equilibrated monomict breccia with a degree of metamorphism consistent with type 4 in the metamorphic sequence of Takeda and Graham (1991). As with the other Stannern Trend eucrites, which are all included in the lower metamorphic types of 1–4, Stannern’s relatively low degree of metamorphism is considered to be indicative of a late lava eruption which was not deeply buried thereafter.

Stannern has a composition that defines a separate trend among eucrites, one that is enriched in incompatible elements and exhibits a high Mg# (defined as molar 100×MgO/[MgO+FeO]). This is reflected in its plot on a TiO vs. FeO/MgO diagram, as well as in its major and trace element ratios. Although the incompatible element abundances for Stannern Trend eucrites are the highest found, they are not linked to the Mg# in the way they are in the Nuevo Laredo Trend eucrites. Instead, the incompatible trace elements have been decoupled from the major elements during in situ crystallization from a residual melt. The composition is analogous to lunar KREEP basalts produced during late-stage fractional magmatism.

An alternate petrogenesis of the Stannern Trend eucrites has been proposed by Barrat et al. (2007). They envisage a region of 10% partial melting at depth within the equilibrated eucritic crust, and the subsequent assimilation of a portion of this crustal partial melt by an ascending magma plume with Main Group composition in a ratio of approximately 15:85, respectively. They demonstrate that this would result in the enrichment of the incompatible trace elements, including REE, similar to that observed in Stannern and other members of this trend, along with the corresponding negative anomalies observed for Eu, Sr, and Be. This model is also consistent with other elemental abundances; the increased W content in Stannern Trend eucrites is consistent with the behavior of W as an incompatible element when it is associated with a metal-free, crustal partial melt. The degree of variation observed among non-cumulate eucrites is commensurate with the degree of crustal melt contamination they experienced during ascent. Significant complications with the relationships among established chemical trends were elucidated by Castle et al. (2012), and it was concluded that each geochemical trend may have originated on separate but similar parent bodies.

Stannern has an unusually young crystallization age for eucrites of 4.434 b.y., which is consistent with a late-stage initiation caused by an impact event. Another impact event occurred ~3.7 b.y. ago, which reset some isotopic clocks such as Ar–Ar. This event corresponds to the Late Heavy Bombardment period on the Moon ~3.8–4.1 b.y. ago. Both lunar and Vesta chronometer resetting events likely represent the same population of impactors, with impacts on Vesta continuing for a longer time. Stannern has a cosmic-ray exposure age of 35.1 (±0.7) m.y.

The Stannern Trend comprises a small number of eucrites including the falls of Stannern, Bouvante, and Pomozdino, together with the Saharan find NWA 4523 and several Antarctic finds; the newly found eucrite Bluewing 001 also shares close similarities with this trend. The specimen of Stannern shown above is a 2.58 g partial slice with fusion crust along the upper edge.

Thanks to Shawn Alan for sharing the following historical account of Stannern: Encyclopaedia Metropolitana; or, Universal Dictionary of Knowledge, Volume XXV, edited by Rev. Edward Smedley, Rev. Hugh James Rose, and Rev. Henry John Rose (London, 1845)

STONES, METEORIC. by William Hallows Miller (p. 82)

The next event of the kind which we shall describe deserves notice on account of the care with which the circumstances attending it were investigated by von Schreibers and von Widmanstatten on the spot, a week after it occurred. At Stannern, a small town in Moravia about ninety-two miles from Vienna on the post road to Prague, between half-past five and six A.M. on the 22d of May, 1808, the air, which had previously been clear, was suddenly obscured by a thick mist. A very loud explosion was then heard, followed by fainter reports and a noise like that of carriages drawn over a rough pavement. These sounds appeared to proceed from a point moving from north-west to south-east, and lasted about eight minutes. In the mean time a number of stones fell on the ground, scattering themselves over an oval surface about eight English miles and a half long, from north north-west to south south-east, and three miles and a quarter wide, having Stannern for its middle point. They formed three principal groups: one in and about Stannern; another, containing two of the largest stones, two of which weighed sixteen and fourteen pounds respectively, at the north end of the oval; the third, composed of the smallest, at the south end. Hence, as at L’Aigle, the largest stones appear to have fallen first. They were found to be hot a short time after their fall. One of them weighing a little more than four pounds made a hole two feet deep in a newly ploughed field. The number of stones actually gathered amounted to sixty-six, and their joint weight to one hundred and eighteen pounds avoirdupois. After the first explosion the mist became so dense that objects could not be discerned at the distance of twelve paces. It extended thirty-seven miles to the south, and nearly half that distance in other directions, and did not wholly disappear for four hours. About the time the explosion was heard, a fire-ball emitting sparks and leaving a train of fire behind it was seen from Triesch, four miles to the west of Stannern, and also from the Bohemian frontier, twenty miles to the north. The stones have a very uneven surface coated with a pitch-black crust. Their interior resembles a fine-grained porous white sandstone traversed by veins of a greyish substance. They contain small quantities of sulphuret of iron and oxide of iron, but no iron in a metallic state, and do not affect the magnetic needle. Their specific gravity varies from 2.95 to 3.16, which is less than that of most other meteoric stones. According to the analysis of Moser, a portion of one of them contained, in 100 parts, silica 46.25, lime 12.12, alumina 7.62, magnesia 2.50, oxide of iron 27, oxide of manganese 0.75, with traces of chrome, water, sulphur, and neutral hydrochlorates.

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