El Taco Research Articles

A multi‐analytical approach to understand El Alto‐Ancasti's painted pottery (north‐western Argentina): Non‐invasive and micro‐invasive analyses of vessel's paintings from El Taco 19

AbstractArchaeometric analysis applied to the study of pre‐Hispanic pottery is a growing field in archaeological research in north‐western Argentina. Most of these investigations have focused on intra‐style variability, comparing samples from the same style but obtained in different sites throughout large regions. In this study, we propose to shift the approach and adopt a local micro‐scale perspective to deepen knowledge regarding pottery manufacturing. As a case study, pottery sherds from different styles discovered in the archaeological site El Taco 19 (El Alto‐Ancasti Mountain, Catamarca) were selected. To this end, we proposed a multi‐analytical approach employing non‐invasive and micro‐invasive analytical techniques.

Neutron capture 128Xe and 129Xe in the San Juan mass of the Campo del Cielo IAB iron meteorite: Evidence for a high fluence of thermalized neutrons

AbstractThe petrographic study of the San Juan A2 polished section demonstrated textural and compositional similarities with the Campo del Cielo IAB iron meteorite, with trace element abundances in metal following the pattern of bulk Campo del Cielo. Xenon, neon, and helium isotopic compositions have been measured in radial graphite rims, massive graphite inclusions, fine‐grained graphite aggregates, cliftonite, and platy graphite. Two silicate inclusions and two areas of metal were also analyzed. 3He/4He versus 4He/21Ne data for San Juan metal plot next to the values reported for the El Taco fragment of Campo del Cielo, supporting San Juan being a part of the Campo del Cielo meteorite shower. Based on the Ne isotopic composition of its components, and the observed correlation between 128Xe and 129Xe, the San Juan A fragment of Campo del Cielo was well shielded from the primary galactic cosmic ray high‐energy irradiation. Its size allowed the secondary neutrons to be fairly well thermalized, receiving an equivalent (normalized to the research reactor with highly thermalized neutron spectrum) fluence of thermal and epithermal neutrons of 6.6 × 1017 n cm−2. Considering 1.8 × 108 years single‐stage and constant exposure geometry irradiation history for Campo del Cielo, and assuming the identical neutron flux spectra for the research reactor and Campo del Cielo, the average thermal equivalent neutron flux for San Juan is about 1.2 × 102 n cm−2 s−1. Xe isotopic composition in the radial graphite rims and platy graphite shows evidence of live 129I in San Juan and is consistent with a mixture of iodine‐derived and tellurium‐derived Xe.

Animales y humanos en Las Cumbres de Ancasti (Siglos VIII y IX DC). Paisajes campesinos y recursos locales

La sierra de El Alto-Ancasti en el oriente de la provincia de Catamarca (Argentina) presenta una importante variabilidad paisajística que se traduce en una variada presencia de recursos faunísticos. Aunque el paradigma imperante definió a las poblaciones indígenas como pastores de camélidos, recientes investigaciones han evidenciado ocupaciones campesinas de larga duración y con un fuerte compromiso con el entorno local y sus recursos. En este trabajo presentaremos la información obtenida en el análisis faunístico del sitio El Taco 19 (ET19), entre los siglos VIII y IX d.C., que posee una representación importante de camélidos domesti- cados, pero también un rico registro de especies silvestres, implicando diferentes estrategias de aprovechamiento de la fauna. El estudio permitió reconstruir una historia donde los animales jugaron un rol importante como recursos económicos y estructuradores de las relaciones sociales. De esta suerte, como valoraremos en este trabajo, el pastoreo y la caza, constituyen prácticas centrales para comprender los vínculos establecidos entre animales y personas en las serranías del este catamarqueño.

Hf–W chronometry of the IAB iron meteorite parent body

The IAB iron meteorites are unusual in having abundant silicate inclusions, but the relationship of metal and silicates is not well understood. In order to study the sequence and the timing of events that led to the observed structure of IAB iron meteorites, the Hf–W isotope system was applied to metal and silicate separates from seven IAB iron meteorites. A major metal segregation event on the IAB parent body is dated by the average 182W/ 184W ratio of the IAB metal phases (− 3.1 ± 0.2 ε 182W-units), corresponding to a time of 1.8 + 2.3 / − 2.0 Ma after the formation of Ca,Al-rich inclusions (CAIs). The 182Hf– 182W data for three bulk inclusions from Landes, Copiapo and El Taco define a linear trend interpreted as an isochron, corresponding to an age of 2.5 + 2.3 / − 2.0 Ma after CAI formation. This age most likely dates silicate differentiation on the IAB parent body and is, within analytical uncertainty, indistinguishable from the major metal segregation event. Several silicate inclusions are displaced from the IAB silicate isochron towards less radiogenic W compositions, most likely reflecting thermal events on the parent body. Metal segregation and silicate differentiation occurred early enough, so that these processes could have at least partially been triggered by an internal heat-source (e.g., 26Al). Basaltic inclusions of the Caddo County specimen define an age of 4.6 + 0.7 / − 0.6 Ma, most likely dating a partial melting event on the IAB parent body. Three silicate-separates from another El Taco inclusion yield an age that postdates CAI formation by 10.8 + 2.4 / − 2.0 Ma and the IAB silicate isochron by ~ 8.3 Ma. This prolonged interval of local thermal events on the IAB parent body are best explained by collisional heat sources (i.e. impacts). The Hf–W ages for thermal events on the IAB parent body are within the range of previously reported 129I– 129Xe ages but are significantly older than published 87Rb– 87Sr, 147Sm– 143Nd and most 40 K– 40Ar ages. Most likely, these chronometers were reset during later thermal events that had only a minor effect on the 182Hf– 182W and 129I– 129Xe systems.

Noble gas content and isotope abundances in phases of the Saint‐Aubin (UNGR) iron meteorite

Abstract— We analyzed the noble gas isotopes in the Fe‐Ni metal and inclusions of the Saint‐Aubin iron meteorite, utilizing the stepwise heating technique to separate the various components of noble gases.The light noble gases in all samples are mostly cosmogenic, with some admixture from the terrestrial atmosphere. Total abundances of noble gases in metal are one of the lowest found so far in iron meteorites and the 4He/21Ne ratio is as high as 503, suggesting that the Saint‐Aubin iron meteorite was derived from a very large meteoroid in space. The exposure ages obtained from cosmogenic 3He were 9–16 Ma.Saint‐Aubin is very peculiar because it contains very large chromite crystals, which—like the metal—contain only cosmogenic and atmospheric noble gases. The noble gases in all the samples do not reveal any primordial components. The only exception is the 1000 °C fraction of schreibersite which contained about 5% of the Xe‐HL component. The Xe‐Q and the El Taco Xe components were not found and only the Xe‐HL is present in this fraction. Some presolar diamond, the only carrier for the HL component known today, must have been available during growth of the schreibersite. However, it is also possible that this excess is due to the addition of cosmogenic and fission components. In this case, all the primordial components are masked (or lost) by the later events such as cosmic‐ray irradiation, heating, and radioactive decay.

Primordial noble gases in a graphite‐metal inclusion from the Canyon Diablo IAB iron meteorite and their implications

Abstract— We have carried out noble gas measurements on graphite from a large graphite‐metal inclusion in Canyon Diablo. The Ne data of the low‐temperature fractions lie on the mixing line between air and the spallogenic component, but those of high temperatures seem to lie on the mixing line between Ne‐HL and the spallogenic component. The Ar isotope data indicate the presence of Q in addition to air, spallogenic component and Ar‐HL. As the elemental concentration of Ne in Q is low, we could not detect the Ne‐Q from the Ne data. On the other hand, we could not observe Xe‐HL in our Xe data. As the Xe concentration and the Xe/Ne ratio in Q is much higher than that in the HL component, it is likely that only the contribution of Q is observed in the Xe data. Xenon isotopic data can be explained as a mixture of Q, air, and “El Taco Xe.” The Canyon Diablo graphite contains both HL and Q, very much like carbonaceous chondrites, retaining the signatures of various primordial noble gas components. This indicates that the graphite was formed in a primitive nebular environment and was not heated to high, igneous temperatures. Furthermore, a large excess of 129Xe was observed, which indicates that the graphite was formed at a very early stage of the solar system when 129I was still present. The HL/Q ratios in the graphite in Canyon Diablo are lower than those in carbonaceous chondrites, indicating that some thermal metamorphism occurred on the former. We estimated the temperature of the thermal metamorphism to about 500–600 °C from the difference of thermal retentivities of HL and Q. It is also noted that “El Taco Xe” is commonly observed in many IAB iron meteorites, but its presence in carbonaceous chondrites has not yet been established.

Thermobarometry, Sm/Nd Ages and Geophysical Evidence for the Location of the Suture Zone Between Cuyania and the Western Proto-Andean Margin of Gondwana

Basement rocks comprising ortho- and paragneisses and schists whose tectono-metamorphic evolution is poorly known, are exposed in the Sierras de Umango, Maz-Espinal and Las Ramaditas, in the northwest of the La Rioja Province, Argentina. These units were included in the Maz, El Taco, El Zaino complexes, as well as the Tambillos Metamorphics that would be part of the northern end of the Cuyania terrane, a microcontinent derived from Laurentia that collided with Western Gondwana during the early Paleozoic, or belong to the active margin of the continent. To recognize rocks belonging to each of these tectonic units and to understand the history and physical conditions of accretion were some of the main goals of the multidisciplinary investigation whose preliminary results are presented here. Geochemical studies, trace and REE elements and Sm-Nd model ages allowed the recognition of several episodes of crustal accretion in these rocks. The oldest one occurred at ca. 2.2 Ga in an arc/back-arc environment along the eastern segment of the Sierra de Maz, and was possibly coeval with development of a early Proterozoic continental crust that acted as source to sediments of Maz Complex. The following episode of crustal accretion that formed rocks in this region was at ca. 1.4 Ga and is registered by tonalites emplaced in an extensional environment cropping out in the western flank of the Sierra del Espinal. In the Sierra de Umango, an arc/back-arc sequence registered an episode of crustal accretion during the Grenvillian Cycle (ca. 1.3 Ga). The last episode of crustal accretion detected in this area (800 Ma) is represented by an old alkaline volcanism in the Sierra de Umango. This episode could be representing the first stage of break-up of the Rodinia supercontinent during the Neoproterozoic. The metamorphic grade reached by these rocks is mostly represented by fabrics with mineral assemblages of intermediate to high pressures and high temperatures, typical of collisional environments. The oldest rock-forming fabrics tectono-metamorphic episode recognized is of middle Proterozoic age (ca. 1.04 to 0.969 Ga, garnet-whole-rock Sm/Nd age) being registered by metapelites from Maz Complex that attained temperatures of 650°C-6.3 kbar. A younger metamorphic event (463 Ma, garnet - whole-rock Sm/Nd age) is verified in metatonalites intrusive in these metapelites. Another metamorphic event at ca. 301 Ma (garnet-WR Sm/Nd age) was recognized in metasediments from El Taco Complex. Peak metamorphic conditions of this event, probably registering the last major tectonic episode that affected rocks of this area was established in 868°C-9.8 kbar. It is impossible to distinguish fabrics belonging to totally different tectonic episodes based on structural or metamorphic data. Therefore, distinction between major tectono-thermal events of totally different ages such as the high-T middle Proterozoic deformation and with N-NWestwards tectonic transport direction registered in the Sierras de Maz-Espinal and Umango from the youngest one (ca. 301 Ma) that attained the highest-P/T conditions, recognized in the Sierra de Las Ramaditas, had to be done on the basis of Sm/Nd ages. Geophysical evidence indicates the presence of extensive WNW-oriented lineaments that separate basements blocks of different magnetic and gravimetric signatures that are thought to represent ancient Grenvillian age suture zones. On the other hand, the northern segment of the Valle Fértil lineament that runs between Sierras de Umango and Maz-Espinal is at present interpreted as marking the eastern boundary of the Cuyania terrane. This is supported by isotopic data as well as the contrasting history of tectono-metamorphic events as determined for both of these segments of the NW Sierras Pampeanas.

Abundance and isotopic composition of noble gases in metal and graphite of the Bohumilitz IAB iron meteorite

Abstract— Abundances and isotopic compositions of noble gases in metal and graphite of the Bohumilitz IAB iron meteorite were measured. The abundance ratios of spallogenic components in metal reveal a 3He deficiency which is due to the diffusive loss of parent isotopes, that is, tritium (Tilles, 1963; Schultz, 1967). The diffusive loss likely has been induced by thermal heating by the Sun during cosmic‐ray exposure (∼160 Ma; Lavielle et al, 1999). Thermal process such as impact‐induced partial loss may have affected the isotopic composition of spallogenic Ne. The 129Xe/131Xe ratio of cosmogenic components in the metal indicates an enhanced production of epi‐thermal neutrons.The abundance ratios of spallogenic components in the graphite reveal that it contained small amounts of metal and silicates. The isotopic composition of heavy noble gases in graphite itself was obtained from graphite treated with HF/HCl. The isotopic composition of the etched graphite shows that it contains two types of primordial Xe (i.e., Q‐Xe and El Taco Xe). The isotopic heterogeneity preserved in the Bohumilitz graphite indicates that the Bohumilitz graphite did not experience any high‐temperature event and, consequently, must have been emplaced into the metal at subsolidus temperatures. This situation is incompatible with an igneous model as well as the impact melting models for the IAB‐IIICD iron meteorites as proposed by Choi et al. (1995) and Wasson et al (1980).

Re-definition of "El Taco Xe" based on 132Xe-normalized data. Multiple primordial components in IAB irons.

The “El Taco Xe” trapped in graphite fractions of the El Taco IAB iron meteorite proposed by Mathew and Begemann (1995) was reexamined on the basis of the data normalized to 132Xe. On the Xe three-isotope diagrams, their data did not form a clear linear trend between atmospheric Xe and “El Taco Xe” due to the existence of Xe-HL in the samples. The Xe-HL released at high-temperature demands a slight modification of “El Taco Xe” by subtracting Xe-HL. The modified El Taco Xe proposed in this study can be explained by a mass-fractionation from solar Xe or U-Xe. High temperature data could be explained by the addition of a Q-HL mixture with a constant Q/HL ratio of ∼5 to the modified El Taco Xe.

Isotopic composition of xenon and krypton in silicate-graphite inclusions of the El Taco, Campo del Cielo, IAB iron meteorite

Noble gases, extracted by temperature-stepped pyrolysis from acid-resistant silicate, schreibersite, and graphite residues of inclusions from the IAB iron meteorite El Taco (Campo del Cielo), have been analyzed for their isotopic composition. The concentrations of spallogenic (light) gases agree with previous data on adjacent specimens. Nominal feldspar and pyroxene fractions have lost up to 80% of their 3He during, or towards the end of, the exposure of El Taco to the cosmic radiation. In olivine the 3 Ne 21 Ne and NePNe ratios are as low as 2.18 ± 0.01 and 1.09 ± 0.03, respectively, presumably because the silicates were irradiated within a FeNi matrix under extremely heavy shielding. Argon, Kr, and Xe in the silicates are dominated by a trapped “planetary” component with 100 ≤ 36 Ar 132 Xe ≤ 700 and 1.7≤ 84 Kr 132 Xe ≤ 2.2 . Xenon released from the silicates at extraction temperatures > 1000°C is isotopically close to ureilite Xe, while the high- T Xe from graphite and schreibersite is unlike any of the presently well-established types of Xe. The isotopic composition of this Xe can be generated as a mixture of mass-fractionated UXe, with light isotopes depleted by (2.2 ± 0.1) % per atomic mass unit, and 244PuXe. The fission component, 10% in the case of 136Xe, is not due to in situ-production; it must have been added prior to incorporation of the Xe into the El Taco parent body and presumably underwent the mass fractionation process together with UXe. From the silicates 127I( n, γ)-produced 128Xe ∗ and radiogenic 129Xe ∗ are released together, with the same 128 Xe ∗ 129 Xe ∗ ratio in nominal olivine, pyroxene, and feldspar although the maximum release from pyroxene occurs about 300°C higher than from feldspar. From this the cooling rate of silicates for the temperature range 1500-1200°C is deduced to be ≥100°C per million years. The 80 Kr 82 Kr ratio of 79,81Br( n, γ)-produced 80,82Kr is 2.86 ± 0.04, in agreement with the value found in inclusions of the IAB Linwood but higher than observed in almost all stone meteorites.

Investigations of cosmic-ray-produced nuclides in iron meteorites, 4. Identification of noble gas abundance anomalies

The literature on cosmic-ray-produced nuclides in iron meteorites occasionally reports unusual (“anomalous”) abundance proportions for the associated noble gases. The anomalies are in some cases ascribed to excesses of 4He, caused by the presence of primordial or radiogenic components; in other cases to abundance deficiencies of 3He, caused by partial loss of cosmogenic tritium. The arguments and data used previously for the recognition, identification and determination of anomalies are, however, imperfect or incorrect. New procedures are here proposed. They are based on more reliable data on the abundance patterns of the cosmogenic component and on a novel system of test correlations which describes these patterns. In most cases in which anomalies are recognised, the system allows an unequivocal identification of the nuclide which is the cause of the anomaly. This is a prerequisite for the quantitative determination of excesses and deficiencies. The procedures are applied to evaluate anomalous noble gas data reported in the literature for about 15 samples of various iron meteorites. In some cases, previous identifications of 3He deficiencies and of 4He excesses prove to be correct. However, guesses that 4He excesses were present in certain specimens from Arispe, Cranbourne, El Taco, Hoba, Pin˜on, Pitts and Sandia Mountains are invalidated by the present investigation. 4He excesses are more exceptional than heretofore believed.

A Low-Blank, Metal System for Rare-Gas Analysis

A low-blank, metal system for extracting and purifying rare gases was constructed to connect with a low-background, metal mass spectrometer which had been used in analyses of xenon previously. In melting samples, different two kinds of inner crucibles are employed, which are put in a tantalum crucible to prevent it from destructive attack by sample materials. With the inner crucible of tantalum, blanks are typically 1.4×10-9cm3STP for4He, (1±0.3)×10-11cm3STP for 20Ne, (0.97±0.13)×10-8cm3STP for40Ar, <7×10-13cm3STP for84Kr, and<3×10-13cm3STP for132Xe. With the inner crucible of crystallized alumina, the blanks for 20Ne and40Ar are more by a factor of five on the average than those with the tantalum inner crucible, while the 4He blank is similar to that with the tantalum inner crucible. Most of hindrance ions could be separated from rare gases with a high resolution power(=500)of the spectrometer. To examine the system, rare gases from iron meteorite El Taco were measured. Data of the isotopic analyses of He, Ne and Ar from El Taco are given. The system is suitable for the isotopic analyses of extremely low quantities of rare gases

A contribution to the stable carbon isotope geochemistry of iron meteorites

For most iron meteorites studied, the carbon isotopic composition of nodular graphite falls in the range −4.8 to −8.2%. vs PDB and shows a mode between −5 and −6%.. Fourteen cohenite analyses from the Magura meteorite fall between −18.1 and −19.2%. with a pronounced clustering around −18.5%.. Carbon of a taenite separate from the same meteorite has an isotopic composition of −18.8%.; compositions between −19.7 and −22.1%. were found for taenite carbon in five other octahedrites. It is suggested that the 12C enrichment in cohenite and taenite relative to the nodular graphite is a general phenomenon in iron meteorites, and that the study of 13C abundances in iron meteorites may aid in the elucidation of their history. To this end an experimental study of carbon isotope fractionations in the system Fe-Ni-C is essential. The 13C content of carbon from several silicate inclusions in the Four Corners and ‘El Taco’ (Campo del Cielo) meteorites is generally similar to the nodular graphite, the 12C enrichment (−13%.) in one specimen may be interpreted in terms of a mixing model involving an original inclusion carbon and carbon exsolved from the taenite upon cooling.

Neutron-activation potassium-argon dating of meteorites

Argon analyses are presented for stepwise heating of nine specimens of seven neutron-irradiated meteorites, and potassium-argon ages calculated by the Ar 40-Ar 39 method. Within the limitations of the data, it is found that the chondrites Karoonda, St. Severin and Allegan, and the aubrites Peña Bianca Spring and Bishopville, all have identical potassium-argon gas retention ages, assumed to be 4.6 × 10 9 yr. In particular, Bishopville, for which conflicting age results have been reported, has the same K-Ar age as the chondrites and Peña Bianca Spring. For these five meteorites, there is no evidence for significant loss of radiogenic Ar 40, such as has been observed in hypersthene chondrites. Gas released at low extraction temperatures indicates that a silicate inclusion from the hexahedrite El Taco experienced a heating event 3.4 × 10 9 yr ago. Gas released from El Taco silicate at higher extraction temperatures yields a K-Ar age identical to that of the chondrites, large amounts of trapped Ar 38, and indicates a ratio ( Ar 40 Ar 36 ) $ ̆ 2 in the trapped argon. The results for separated chondrules and matrix of the unequilibrated chondrite Chainpur differ qualitively from those observed for other meteorites in a manner for which no tenable explanation is offered.

Dating of meteorites by the high-temperature release of iodine-correlated Xe 129

Correlations between the amounts of Xe 129 and Xe 128 released in stepwise heating of neutron-irradiated meteorites are used to determine the initial ratio I 129 I 127 and hence a relative formation time for the various samples. In such an experiment, we obtain the formation times in millions of years (relative to the L4 chondrite Bjurböle) of nine specimens: −3.9 ± 0.7 for Karoonda (C4 chondrite); 3.1 ± 0.6 and 10.5 ± 0.7 for the matrix and chondrules, respectively, of Chainpur (LL3); 7.5 ± 1.0 for St. Severin (LL6); 3.9 ± 2.9 and −2.3 ± 1.0 for the matrix and chondrules, respectively, of Allegan (H5); 3.6 ± 0.7 for Peña Blanca Spring and 20.8 ± 9.5 for Bishopville (aubrites); 3.8 ± 0.7 for a silicate inclusion of the iron meteorite El Taco (Campo del Cielo). No assumptions are made about the amount of Xe 129 in the trapped gas. The correlations for previously published data have also been recalculated in the same way, with no assumptions about trapped Xe 129. For a group of chondrites previously reported to be isochronous with a mean simultaneity of 2.5 million years, the recalculation confirms this mean simultaneity, but significant differences in formation times are resolved. An iodine-xenon age of 53 ± 9 million years (after Bjurböle) is obtained from unpublished data for the achondrite Lafayette. The reliability of iodine-xenon ages of individual meteorites is considered; in particular, the ages of Bishopville and Lafayette are less reliable than those of most other meteorites studied, especially in view of the anomalous ages reported. The relevance of iodine-xenon dating to theories of nucleosynthesis, early solar system chronology, and theories of meteorite parent-body formation is discussed.

Spallogenic rare gases in the El Taco meteorite

He, Ne and Ar concentrations were determined in three samples of the El Taco meteorite. They were found to vary by about a factor of 10, with the highest 21Ne concentration being 0.0074 ± 0.0002 × 10 −8 cm 3 STP/g. The 3He, 21Ne and 38Ar concentrations are compatible with the production model of Signer and Nier and lead to an exposure age of 14 ± 8 m.y. The 3He/ 4He ratios in two samples are abnormally low, which is attributed to 4He excess rather than 3H diffusion in space. The results are compared to rare gas data of other meteorites from the Campo del Cielo and North Chile strewnfields. We conclude that a relationship between the two groups appears unlikely.