Dhofar 081: A new lunar highland meteorite

Abstract— The lunar meteorite Dhofar 081, found as a single fragment of 174 g in the Dhofar region of Oman, is a shocked feldspathic fragmental highland breccia dominated by anorthosite‐rich lithic and mineral clasts embedded into a fine‐grained mostly shock melted clastic matrix. Major mineral phases in the bulk rock are Ca‐rich plagioclase (An_96.5–99.5), pyroxene (FS_21.9–46.2Wo_3.0–41.4), and olivine (Fa_29.3–47.8); accessory phases include Fe‐Ni metal, ilmenite, and Ti‐Cr‐rich spinel. Dhofar 081 contains subordinate crystalline fragments of large anorthosites, intersertal impact‐melt rocks, microporphyritic impact‐melt breccias, dark fine‐grained impact‐melt breccias, large cataclastic feldspars, and irregularly shaped brown glass clasts. Mafic components are rare and no genuine regolith components were found in the sections studied. Minerals in Dhofar 081 show homogeneously distributed shock features: intergranular recrystallization, strong fracturing and mosaicism in feldspar as well as a high density of mostly irregular fractures in pyroxene and olivine. Localized impact melting caused by one or several impacts led to a strong lithification. Based on these effects an equilibration shock pressure of about 15–20 GPa is estimated for the strongest shock event in Dhofar 081. Devitrification of the “glassy” material in the rock indicates thermal annealing after shock melting suggesting that the 15–20 GPa shock event predated the ejection event. According to the concentrations of implanted solar noble gases Dhofar 081 represents a polymict clastic breccia deposit with possibly a minor regolith component. A similar noble gas record of Dhofar 081 and MacAlpine Hills 88104/05 suggests the possibility of a source crater pairing of both meteorites. As indicated by noble gas measurements pairing of Dhofar 081 with the other lunar meteorites found in Oman, Dhofar 025 and Dhofar 026, is unlikely.     

Magmatic Evolution of the La Pacana Caldera System, Central Andes, Chile: Compositional Variation of Two Cogenetic, Large-Volume Felsic Ignimbrites

La Pacana is one of the largest known calderas on Earth, andis the source of at least two major ignimbrite eruptions witha combined volume of some 2700 km³. These ignimbrites have stronglycontrasting compositions, raising the question of whether theyare genetically related. The Toconao ignimbrite is crystal poor,and contains rhyolitic (76–77 wt % SiO₂) tube pumices.The overlying Atana ignimbrite is a homogeneous tuff whose pumiceis dacitic (66–70 wt % SiO₂), dense (40–60% vesicularity)and crystal rich (30–40 % crystals). Phase equilibriaindicate that the Atana magma equilibrated at temperatures of770–790°C with melt water contents of 3·1–4·4wt %. The pre-eruptive Toconao magma was cooler (730–750°C)and its melt more water rich (6·3–6·8 wt% H₂O). A pressure of 200 MPa is inferred from mineral barometryfor the Atana magma chamber. Isotope compositions are variablebut overlapping for both units (⁸⁷Sr/⁸⁶Sr_i 0·7094–0·7131;¹⁴³Nd/¹⁴⁴Nd 0·51222–0·51230) and are consistentwith a dominantly crustal origin. Glass analyses from Atanapumices are similar in composition to those in Toconao tubepumices, demonstrating that the Toconao magma could representa differentiated melt of the Atana magma. Fractional crystallizationmodelling suggests that the Toconao magma can be produced by30% crystallization of the observed Atana mineral phases. Toconaomelt characteristics and intensive parameters are consistentwith a volatile oversaturation-driven eruption. However, thelow H₂O content, high viscosity and high crystal content ofthe Atana magma imply an external eruption trigger. KEY WORDS: Central Andes; crystal-rich dacite; eruption trigger; high-silica rhyolite; zoned magma chamber     

Late Miocene–Pliocene eclogite facies metamorphism, D'Entrecasteaux Islands, SE Papua New Guinea

The D'Entrecasteaux Islands of south‐eastern Papua New Guinea are active metamorphic core complexes that formed within a region where the plate tectonic regime has transitioned from subduction to rifting. While rapid, post 4 Myr exhumation and cooling of amphibolite and greenschist facies rocks that constitute the footwall of the crustal scale detachment fault system have been previously documented on Fergusson and Goodenough Islands of the D'Entrecasteaux chain, the timing of eclogite facies metamorphism in rocks of the footwall was unknown. Recent work revealed that at least one of the eclogite bodies formed during the Pliocene. We present combined in situ ion microprobe U–Pb age analyses of zircon from five variably retrogressed eclogite samples from Fergusson and Goodenough Islands that document Late Miocene–Pliocene (8–2 Ma) eclogite formation on these islands. Textural relationships and zircon–garnet rare earth element partition coefficients indicate that U–Pb ages constrain zircon crystallization under eclogite facies conditions in all samples. Results suggest westward younging of eclogite facies metamorphism from Fergusson to Goodenough Island. Present‐day exposure of Late Miocene–Pliocene eclogites requires exhumation rates > 2.5 cm yr^?1.     

Shock experiments with the H6 chondrite Kernouvé: Pressure calibration of microscopic shock effects

Abstract— Shock‐recovery experiments were carried out on samples of the H6 chondrite Kernouvé at shock pressures of 10, 15, 20, 25, 30, 35, 45, and 60 GPa and preheating temperatures of 293 K (low‐temperature experiments) and 920 K (high‐temperature experiments). Using a calculated equation of state of Kernouvé, pressure‐pulse durations of 0.3 to 1.2 μs were estimated. The shocked samples were investigated by optical microscopy to calibrate the various shock effects in olivine, orthopyroxene, oligoclase, and troilite. The following pressure calibration is proposed for silicates: (1) undulatory extinction of olivine <GPa; (2) weak mosaicism of olivine from 10–15 GPa to 20–25 GPa; (3) onset of strong mosaicism of olivine at 20–25 GPa; (4) transformation of oligoclase to diaplectic glass completed at 25–30 GPa (low‐temperature experiments) and at 20–25 GPa (high‐temperature experiments); (5) onset of weak mosaicism in orthopyroxene at 30–35 GPa (low‐temperature experiments) and at 25–30 GPa (high‐temperature experiments); and (6) recrystallization or melting of olivine starting at 45–60 GPa (low‐temperature experiments) and at 35–45 GPa (high‐temperature experiments), and completed above 45–60 GPa in the high‐temperature experiments. Troilite displays distinct differences between the samples shocked at low and high temperatures. In the low‐temperature experiments, the following effects can be observed in troilite: (1) undulatory extinction up to 25 GPa, (2) twinning up to 45 GPa, (3) partial recrystallization from 30 to 60 GPa, and (4) complete recrystallization >35 GPa; whereas in the high‐temperature experiments, troilite shows (1) complete recrystallization from 10 up to 45 GPa and (2) melting and crystallization above 45 GPa. Localized shock‐induced melting is observed in samples shocked to pressures >15 GPa in the high‐temperature experiments and >30 GPa for the low‐temperature experiments in the form of FeNi metal and troilite melt injections and intergrowths and as pockets and veins of whole‐rock melt. Obviously, the onset and abundance of shock‐induced localized melting strongly depends on the initial temperature of the sample.     

The Merzbacher & Eggler (1984) Geohygrometer: a Cautionary Note on its Suitability for High-K Suites

The use of the popular Merzbacher & Eggler (1984, Geology12, 587–590) experimental geohygrometer for calcalkalineandesites and dacites is critically evaluated and two pitfallsare found. First, calculation of the correct projection parametersis problematic because two endmember calculation schemes arefound in the literature; Baker & Eggler (1983, Journal ofVolcanology and Geothermal Research 18, 387–404) and Walkeret al. (1979, Contributions to Mineralogy and Petrology 70,111–125). Although related, these two schemes have crucialdifferences that can result in very different projection parametersfor the plagioclase component. This is most crucial for high-Kcompositions; H₂O contents estimated using the Walker et al.(1979) scheme can be as much as 100% higher than those estimatedusing the Baker & Eggler (1983) projection. Incorrect projectionparameter calculation has led to overestimation of water contentsin high-K andesites and dacites from the Central Andes. Second,for medium-K and high-K andesitic–dacitic compositionswater contents derived using the Merzbacher & Eggler (1984)geohygrometer deviate considerably from water contents estimatedusing other methods. Experimental data from the literature,and our studies of water contents inferred from melt inclusionsand plagioclase–melt equilibrium for dacites from theAltiplano–Puna Volcanic Complex of the Central Andes indicatethat the Merzbacher & Eggler (1984) geohygrometer shouldnot be applied to compositions with K₂O >1·9 wt %,as originally calibrated. KEY WORDS: magmatic volatiles; pre-eruptive water; experimental geohygrometer; high-K suites; melt inclusion     

Differentiation and emplacement of the Worthington Offset Dike of the Sudbury impact structure,Ontario

Abstract— The Offset Dikes of the 1.85 Ga Sudbury Igneous Complex (SIC) constitute a key topic in understanding the chemical evolution of the impact melt, its mineralization, and the interplay between melt migration and impact‐induced deformation. The origin of the melt rocks in Offset Dikes as well as mode and timing of their emplacement are still a matter of debate. Like many other offset dikes, the Worthington is composed of an early emplaced texturally rather homogeneous quartz‐diorite (QD) phase at the dike margin, and an inclusion‐ and sulfide‐rich quartz‐diorite (IQD) phase emplaced later and mostly in the centre of the dike. The chemical heterogeneity within and between QD and IQD is mainly attributed to variable assimilation of host rocks at the base of the SIC, prior to emplacement of the melt into the dike. Petrological data suggest that the parental magma of the Worthington Dike mainly developed during the pre‐liquidus temperature interval of the thermal evolution of the impact melt sheet (>1200 °C). Based on thermal models of the cooling history of the SIC, the two‐stage emplacement of the Worthington Dike occurred likely thousands to about ten thousand years after impact. Structural analysis indicates that an alignment of minerals and host rock fragments within the Worthington Dike was caused by ductile deformation under greenschist‐facies metamorphic conditions rather than flow during melt emplacement. It is concluded that the Worthington Offset Dike resulted from crater floor fracturing, possibly driven by late‐stage isostatic readjustment of crust underlying the impact structure.     

New impact‐melt rock from the Roter Kamm impact structure,Namibia: Further constraints on impact age,melt rock chemistry,and projectile composition

Abstract— A new locality of in situ massive impact‐melt rock was discovered on the south‐southwestern rim of the Roter Kamm impact structure. While the sub‐samples from this new locality are relatively homogeneous at the hand specimen scale, and despite being from a nearby location, they do not have the same composition of the only previously analyzed impact‐melt rock sample from Roter Kamm. Both Roter Kamm impact‐melt rock samples analyzed to date, as well as several suevite samples, exhibit a granitic‐granodioritic precursor composition. Micro‐chemical analyses of glassy matrix and Al‐rich orthopyroxene microphenocrysts demonstrate rapid cooling and chemical disequilibrium at small scales. Platinum‐group element abundances and ratios indicate an ordinary chondritic composition for the Roter Kamm impactor. Laser argon dating of two sub‐samples did not reproduce the previously obtained age of 3.7 ± 0.3 (1s?) for this impact event, based on ⁴⁰Ar/³⁹Ar dating of a single vesicular impact‐melt rock. Instead, we obtained ages between 3.9 and 6.3 Ma, with an inverse isochron age of 4.7 ± 0.3 Ma for one analyzed sub‐sample and 5.1 ± 0.4 Ma for the other. Clearly a post‐5 Ma impact at Roter Kamm remains indicated, but further analytical work is required to better constrain the currently best estimate of 4–5 Ma. Both impactor and age constraints are clearly obstructed by the inherent microscopic heterogeneity and disequilibrium melting and cooling processes demonstrated in the present study.     

Maribo—A new CM fall from Denmark

Abstract– Maribo is a new Danish CM chondrite, which fell on January 17, 2009, at 19:08:28 CET. The fall was observed by many eye witnesses and recorded by a surveillance camera, an all sky camera, a few seismic stations, and by meteor radar observatories in Germany. A single fragment of Maribo with a dry weight of 25.8 g was found on March 4, 2009. The coarse‐grained components in Maribo include chondrules, fine‐grained olivine aggregates, large isolated lithic clasts, metals, and mineral fragments (often olivine), and rare Ca,Al‐rich inclusions. The components are typically rimmed by fine‐grained dust mantles. The matrix includes abundant dust rimmed fragments of tochilinite with a layered, fishbone‐like texture, tochilinite–cronstedtite intergrowths, sulfides, metals, and carbonates often intergrown with tochilinite. The oxygen isotopic composition: (δ¹⁷O = ?1.27‰; δ¹⁸O = 4.96‰; Δ¹⁷O = ?3.85‰) plots at the edge of the CM field, close to the CCAM line. The very low Δ¹⁷O and the presence of unaltered components suggest that Maribo is among the least altered CM chondrites. The bulk chemistry of Maribo is typical of CM chondrites. Trapped noble gases are similar in abundance and isotopic composition to other CM chondrites, stepwise heating data indicating the presence of gas components hosted by presolar diamond and silicon carbide. The organics in Maribo include components also seen in Murchison as well as nitrogen‐rich components unique to Maribo.