TIESCHITZ


H/L3.6
standby for tieschitz photo
Fell July 15, 1878
49° 36' N., 17° 7' E.

A single 27.4 kg stone was seen and heard to fall at 1:45 in the afternoon in Prostejov, Jihomoravsky, Czechoslovakia. Analysis was conducted at the Museum of the Technical High School of Brünn, and Tieschitz was classified as an unbrecciated, unequilibrated ordinary chondrite with a shock stage of S1/S2. This meteorite has preserved the early record of large-grained, pristine chondrites.

Tieschitz does not follow the ordinary chondrite metal–silicate trends in that it has an anomalous Fe content intermediate between the H and L groups, and it has a lower K content than is typical for both of those groups. In addition, Fe is more highly depleted compared to Ni. The Sm–Nd age of ~2 b.y. is evidence that a partial resetting event took place on the parent body at that time, possibly occurring during an aqueous alteration phase. The chemical composition of Tieschitz can support two petrogenetic scenarios—one in which formation occurred on a parent body unique from that of the H- and L-group ordinary chondrites, and another in which Fe was lost from an H-type chondritic body without disrupting the balance of other elemental systematics.

Tieschitz is texturally unique in that it contains both an opaque (black) matrix component and a transparent (white) matrix component. The white matrix material fills the interstitial space between chondrules and clasts, and is comprised of an amorphous phase primarily composed of albitic plagioclase (Al-, Na-, and Ca-rich) containing nanometer scale inclusions of Ca-rich pyroxene (Dobrică and Brearley, 2011, 2014, 2016). The white matrix is theorized to have precipitated from a leachate of chondrule feldspathic mesostasis glass that was dissolved by an aqueous, halogenated, metasomatic fluid (Dobrică and Brearley, 2014 and references therein). This scenario is consistent with the numerous voids found in ~30% of the chondrules and the observation that the white matrix shares a similar mineralogy with the altered "bleached" chondrules.

The low-alkali black matrix component of Tieschitz also shows the effects of metasomatism (Dobrică and Brearley, 2011). The black matrix contains micron-scale voids and veins, sometimes incorporating a polycrystalline fibrous mineral lining the walls which was determined to be sodic-calcic amphibole, a secondary mineral never before reported in an ordinary chondrite (Dobrică and Brearley, 2014). In addition, elongated ferroan olivine crystals are present within the voids, which were demonstrated to have formed under conditions of low pressure and low water:rock ratios. Both the amphibole and the olivine were precipitated from an aqueous fluid, probably during the same hydrothermal event that produced the voids in the chondrule mesostasis.

Although a Sm–Nd study (Smoliar et al., 2004) suggests that a late alteration event occurred ~2.0 b.y. ago, necessarily involving an impact heating event, the Ar–Ar chronometer has not been disturbed (4.45 [±0.05] b.y.; Turner et al., 1978). Upon consideration of all the evidence, Dobrică and Brearley (2014) argue that the metasomatic process that produced the amphibole-filled voids in the black matrix, and which led to the precipitation of the albitic white matrix, most likely occurred during primary metamorphism on the parent body of Tieschitz through radiogenic heating.

A native Cu assemblage has been identified by Komorowski et al. (2009, 2010, 2012) consisting of nm-sized metallic Hg spherules and HgS (cinnabar), associated with CuS (covellite) and native Cu. This first occurrence of native Hg in a meteorite likely reflects equilibration with subsequent sulfidation processes during accretion of fine-grained dust at low temperatures (<< 300°C) within the nebula. It has been demonstrated that these volatile-rich assemblages are not associated with shock-generated remobilization–condensation scenarios on the parent asteroid.

A wide variety of chondrule types are present in Tieschitz including BO, RP, and POP. Unlike other H3 chondrules, the porphyritic chondrules in Tieschitz have accretionary, fine-grained, dark rims, possibly formed by fine dust from impacts prior to planetary accumulation and lithification. Metal–troilite assemblages also occur in chondrule rims. Flat trace element abundance patterns of refractory lithophiles in nonporphyritic chondrules suggest that they originated by direct nebular condensation, which was followed by metasomatic processes (Engler et al., 2003). A cooling rate of 18°C/m.y. was calculated for Tieschitz based upon cloudy taenite particle size (Scott et al., 2013). This cooling rate, along with other literature cooling rate data for a broad spectrum of meteorite groups having a wide range of metamorphic grades, led to the conclusion that an onion shell model was not appropriate for the ordinary chondrites; instead, thorough impact-generated mixing of all the metamorphic layers after cooling is considered a more scenario.

Anomalous grains including presolar Al-rich oxide grains have been identified in Tieschitz. Most of these anomalous grains are known to originate in red giant stars located ~100 AU from the solar nebula; one particular 17O-rich grain has a composition consistent with an origin from a supernova. Another 17O-depleted grain has a composition more representative of a low mass star like the Sun. Also present are circumstellar grains of graphite, corundum, and spinel, and an abundance of SiC grains; these grains have anomalous isotopic ratios and are considered to have condensed around AGB or J-type stars. The few SiC X-grains present in Tieschitz were probably formed in type II supernovae. From their study of O-isotopic anomalies of the Sun, Lee et al. (2008) inferred that the Sun must have formed within a stellar cluster coeval with a massive star.

It was demonstrated by Szurgot (2016) that the mean atomic weight (Amean) of meteorites can be used to resolve the OC groups, including the intermediate groups L/LL and H/L. Amean values can also be predicted through various equations based on other parameters such as atomic Fe/Si ratio and grain density, and these Amean values, as well as the magnetic susceptibility values derived from X-ray fluorescence (XRF) scanning, all consistently resolve these groups into the ordered sequence LL < L/LL < L < H/L < H. Tieschitz has Amean values of 24.32 (chemical composition), 24.14 (Fe/Si atomic ratio), and 24.30 (grain density). The magnetic susceptibility value for Tieschitz (logχ = 4.91) corresponds to an Amean value of 23.92 utilizing the equation [Amean = 1.49 × logχ + 16.6]. The magnetic susceptibility values determined for both the historical and transitional OC groups are consistent with the ordered sequence above. Furthermore, it was demonstrated that Amean values are lower for unequilibrated type 3 samples than for equilibrated samples within each OC group due to the presence of water; Amean values for petrologic types 4–6 are indistinguishable within each group.

standby for amean diagram
Diagram credit: M. Szurgot, 47th LPSC, #2180 (2016)
Amean based on chemical composition (Eq. 1), Fe/Si atomic ratio (Eq. 2), and grain density (Eq. 3)

A hypothesis was presented by Trigo-Rodríguez and Williams (2016) to explain the notable coincidence in the timing of the four known H/L chondrite falls—all occurring within a three month period: Bremervorde on May 13, 1855; Famenin on June 27, 2015; Cali on July 6, 2007; and Tieschitz on July 15, 1878. The probability that all of the H/L meteorite falls would occur within this specific timeframe completely by chance was calculated to be only 6%. Trigo-Rodríguez and Williams (2016) consider that these H/L chondrites could be associated with the Bejar bolide that was tracked by the Spanish Meteor Network above Salamanca, Spain on July 11, 2008, and also fortuously photographed from Madrid by Javier Pérez Vallejo. Based on the available data, an orbital solution was constructed for this bolide which is consistent with a high-inclination orbit, and it is considered that it could represent material from the disruption of comet C/1919 Q2 Metcalf (see diagram below). They also propose that the Bejar bolide along with the H/L chondrites could be associated with the Omicron Draconids meteor stream which was shown by A. Cook to follow a similar orbit as comet C/1919 Q2 Metcalf. As demonstrated by Martínez-Jiménez et al. (2016) in their study of the Cali meteorite, not all H/L chondrites show such obvious features of aqueous alteration as those present in Tieschitz. Therefore, the H/L parent asteroid could be heterogeneous with respect to aqueous alteration, or alternatively, it could be a rubble pile composed of a broad diversity of material with variable densities and metamorphic histories.

standby for bejar orbit diagram
Diagram credit: Josep M. Trigo-Rodríguez/SPMN

Other meteorites assigned to this intermediate chondrite group include Famenin [3.8–3.9], Bremervörde [3.9], NWA 1955 [3–4], Haxtun [4], Yamato 74645 [4], Cali [4], and Yamato 8424; initial studies of Dhofar 008 indicate that it might also belong to this group. The specimen of Tieschitz shown above is a 4.8 g interior cut fragment, and the bottom image is an excellent petrographic thin section micrograph of Tieschitz, shown courtesy of Peter Marmet.

standby for tieschitz ts photo
click on photo for a magnified view
Photo courtesy of Peter Marmet