STANNERN


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; i.e., the increased W content in Stannern Trend eucrites corresponds to the way in which W behaves as an incompatible element when 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 between 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., 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, as well as 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.