Day: September 24, 2013

Geochemical studies of the SUBO 18 (Enkingen) drill core and other impact breccias from the Ries crater, Germany

Posted by

0

Wolf Uwe Reimold1,2*, Iain McDonald3, Ralf-Thomas Schmitt1, Birgit Hansen1, Juliane Jacob1 and Christian Koeberl4,5

1Museum für Naturkunde—Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstrasse 43, 10115 Berlin, Germany
2Humboldt Universitüt zu Berlin, Unter den Linden 6, 10099 Berlin, Germany
3School of Earth & Ocean Sciences, Cardiff University, Park Place, Cardiff CF10 3YE, UK
4Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria
5Natural History Museum, Burgring 7, 1010 Vienna, Austria

Suevite and melt breccia compositions in the boreholes Enkingen and Polsingen are compared with compositions of suevites from other Ries boreholes and surface locations and discussed in terms of implications for impact breccia genesis. No significant differences in average chemical compositions for the various drill cores or surface samples are noted. Compositions of suevite and melt breccia from southern and northeastern sectors of the Ries crater do not significantly differ. This is in stark contrast to the published variations between within-crater and out-of-crater suevites from northern and southern sectors of the Bosumtwi impact structure, Ghana. Locally occurring alteration overprint on drill cores—especially strong on the carbonate-impregnated suevite specimens of the Enkingen borehole—does affect the average compositions. Overall, the composition of the analyzed impact breccias from Ries are characterized by very little macroscopically or microscopically recognized sediment-clast component; the clast populations of suevite and impact melt breccia are dominated consistently by granitic and intermediate granitoid components. The Polsingen breccia is significantly enriched in a dioritic clast component. Overall, chemical compositions are of intermediate composition as well, with dioritic-granodioritic silica contents, and relatively small contributions from mafic target components. Selected suevite samples from the Enkingen core have elevated Ni, Co, Cr, and Ir contents compared with previously analyzed suevites from the Ries crater, which suggest a small meteoritic component. Platinum-group element (PGE) concentrations for some of the enriched samples indicate somewhat elevated concentrations and near-chondritic ratios of the most immobile PGE, consistent with an extraterrestrial contribution of 0.1–0.2% chondrite-equivalent.

Reference
Reimold WU, McDonald I, Ralf-Thomas Schmitt R-T, Hansen B, Jacob J and Koeberl C (2013) Geochemical studies of the SUBO 18 (Enkingen) drill core and other impact breccias from the Ries crater, Germany. Meteoritics & Planetary Science (in press)
[doi:10.1111/maps.12175]
Published by arrangement with John Wiley & Sons

Link to Article

Precise Determination of Sm and Nd Concentrations and Nd Isotopic Compositions in Highly Depleted Ultramafic Reference Materials

Posted by

0

Zhuyin Chu1,*, Jinghui Guo1, Yueheng Yang1, Liang Qi2 and Chaofeng Li1

1State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China
2State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550002, China

In this study, a high-precision method for the determination of Sm and Nd concentrations and Nd isotopic composition in highly depleted ultramafic rocks without a preconcentration step is presented. The samples were first digested using the conventional HF + HNO3 + HClO4 method, followed by the complete digestion of chromite in the samples using HClO4 at 190–200 °C and then complete dissolution of fluoride formed during the HF decomposition step using H3BO3. These steps ensured the complete digestion of the ultramafic rocks. The rare earth elements (REEs) were separated from the sample matrix using conventional cation-exchange chromatography; subsequently, Sm and Nd were separated using the LN columns. Neodymium isotopes were determined as NdO+, whereas Sm isotopes were measured as Sm+, both with very high sensitivity using single W filaments with TaF5 as an ion emitter. Several highly depleted ultramafic rock reference materials including USGS DTS-1, DTS-2, DTS-2b, PCC-1 and GSJ JP-1, which contain extremely low amounts of Sm and Nd (down to sub ng g-1 level), were analysed, and high-precision Sm and Nd concentration and Nd isotope data were obtained. This is the first report of the Sm-Nd isotopic compositions of these ultramafic rock reference materials except for PCC-1.

Reference
Chu Z, Guo J, Yang Y, Qi L and Li C (2013) Precise Determination of Sm and Nd Concentrations and Nd Isotopic Compositions in Highly Depleted Ultramafic Reference Materials. Geostandards and Geoanalytical Research 121:652–666.
[doi:10.1111/j.1751-908X.2013.00224.x]
Published by arrangement with John Wiley & Sons

Link to Article

Nucleosynthetic Nd isotope anomalies in primitive enstatite chondrites

Posted by

0

M. Boyet*, A. Gannoun

Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, BP 10448, F-63000 Clermont-Ferrand, France CNRS, UMR 6524, LMV, F-63038 Clermont-Ferrand, France
IRD, R 163, LMV, F-63038 Clermont-Ferrand, France

We carried out stepwise dissolutions of four primitive enstatite chondrites (EC) belonging to the EH subgroup. Large Nd isotope anomalies are found in the most refractory phases, dissolved using strong acids. Residues are characterized by excesses in 142Nd and deficits in 145Nd, 148Nd and 150Nd isotopes. The Nd anomalies measured in the ALHA77295 residue are even greater than those measured in the Murchison carbonaceous chondrite (CC) using a similar analytical technique (Qin et al., 2011). Once corrected for a common Sm/Nd evolution, the 142Nd excess in the ALHA77295 residue is equal to 700 ppm relative to the terrestrial standard value. The Nd isotope patterns measured in EC and CC residues can be adjusted to coincide by adding a small amount of an s-process-rich carrier phase such as SiC and 0.075% is required to fit the ALHA7795 residue. Small isotope differences still persist between these residues even if they can be considered similar within error. In enstatite chondrites, residues have a deficit in 150Nd similar to or smaller than that measured in 148Nd, whereas in SiC extracted from carbonaceous chondrites or in whole rock, the deficit in 150Nd is always greater than that in 148Nd. Moreover in a binary 142Nd–148Nd diagram, the best-fit lines obtained for leachates and residues from carbonaceous chondrites and enstatite chondrites have slightly different slopes. For the same 148Nd/144Nd ratio, the anomalous component in an enstatite chondrite has a higher 142Nd/144Nd ratio compared to carbonaceous chondrites, a feature already observed at the whole rock scale. Our results suggest that different chondrite groups sample different reservoirs of presolar grains formed in different environments. Assuming that the carrier of this anomalous component measured in residues of enstatite chondrites are SiC, our results may suggest that different meteorite parent bodies sample reservoirs of presolar SiC formed in different stellar environments. This could explain why ALHA77295, the sample which is the most enriched in presolar grains, has a bulk 142Nd isotope composition similar to the terrestrial value. Further investigation of enstatite chondrites is needed to test whether the isotope composition of the most refractory phases is similar to that measured in carbonaceous chondrites and in particular the 144Sm that is a p-process isotope only. Finally this study highlights the difficulty of interpreting the 142Nd excess in terrestrial samples relative to chondrites since incomplete mixing of nucleosynthetic material in the solar nebula creates significant 142Nd variation, as shown by ALHA77295.

Reference
Boyet M and Gannoun A (2013) Nucleosynthetic Nd isotope anomalies in primitive enstatite chondrites. Geochimica et Cosmochimica Acta 121:652–666.
[doi:/dx.doi.org/10.1016/j.gca.2013.07.036]
Copyright Elsevier

Link to Article

Thermal history of nakhlites: A comparison between MIL 03346 and its terrestrial analogue Theo’s flow

Posted by

0

M.C. Domeneghettia,*, A.M. Fiorettib, F. Cámarac,d, C. McCammone, M. Alvarof,1

aDipartimento di Scienze della Terra e dell’Ambiente, Università degli Studi di Pavia, Italy
bIGG-CNR Unità Operativa di Padova, via Gradenigo, 6, Padova, Italy
cDipartimento di Scienze della Terra, Università di Torino, Italy
dCrisDi, Interdepartmental Centre for the Research and Development of Crystallography, Torino, Italy e Bayerisches Geoinstitut, Bayreuth, Germany
fInt’l Research School of Planetary Sciences, Università G. d’Annunzio, Chieti, Italy
1Present address: Dipartimento di Geoscienze, Università degli Studi di Padova, Italy.

High resolution single-crystal X-ray diffraction (HR-SCXRD) and Mössbauer spectroscopy of the intracrystalline cation distribution have been performed on augitic core-crystals from a Miller Range nakhlite (sample MIL 03346,13) with approximate composition of En36Fs24Wo40. The Mössbauer data on the single-crystal yielded a very low Fe3+ content [Fe3+/Fetotal – 0.033(23) a.p.f.u.] that, together with the Electron microprobe analysis (EMPA) and the X-ray structural data allowed us to obtain the accurate cation site distribution and the Fe2+–Mg degree of order. This leads to a closure temperature (Tc) of 500 with a standard deviation of ±100 °C that would correspond to a slow cooling rate, which is in disagreement with petrologic evidence that indicates that this sample originates from a fast cooled (~3–6 °C/h) lava flow.
In order to clarify this discrepancy we undertook (i) a SC-XRD study of an augite (~En49Fs9Wo42) from a pyroxenite (TS7) of Theo’s flow, a 120-m-thick lava flow regarded as a terrestrial analogue of MIL 03346; (ii) an annealing experiment at 600 °C on a crystal from exactly the same fragment of MIL 03346. SC-XRD data from TS7 augite yields a Tc = 600(20) °C, consistent with the cooling rate expected at 85 m below the surface. This Tc is higher, although similar within error, to the Tc = 500(100) °C obtained for MIL 03346; thus suggesting relatively slower cooling for MIL 03346 with respect to TS7. The annealing experiment on the MIL 03346 crystal clearly showed that the degree of order remained unchanged, further confirming that the actual Tc is close to 600 °C.
This result appears inconsistent with the shallow depth of origin (~<2 m) assumed for MIL 03346, further supporting the discrepancy between MIL 03346 textural and petrologic evidence of fast cooling and the above mentioned Tc results obtained for augite. Therefore, a tentative scenario is that, soon after eruption and initial quench and while still at relatively high-T (~600 °C), MIL 03346 was blanketed with subsequent lava flows that slowed down the cooling rate and allowed the augite Fe2+–Mg exchange reaction to proceed.

Reference
Domeneghetti MC, Fioretti AM, Cámara F, McCammon C and Alvaro M (2013) Thermal history of nakhlites: A comparison between MIL 03346 and its terrestrial analogue Theo’s flow. Geochimica et Cosmochimica Acta 121:571–581.
[doi:/dx.doi.org/10.1016/j.gca.2013.07.044]
Copyright Elsevier

Link to Article

The abundance, distribution, and isotopic composition of Hydrogen in the Moon as revealed by basaltic lunar samples: Implications for the volatile inventory of the Moon

Posted by

0

Romain Tartèsea,*, Mahesh Ananda,b, Jessica J. Barnesa,b, Natalie A. Starkeya, Ian A. Franchia, Yuji Sanoc

aPlanetary and Space Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, United Kingdom
bDepartment of Earth Sciences, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom
cAtmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8564, Japan

In this study we have measured the OH contents and D/H ratios in apatite grains in lunar basalts. These new data considerably expand the limited dataset published so far. The data presented in this study also show that there is a major difference between high- and low-Ti mare basalts in terms of their OH and D/H systematics. Apatites in high-Ti basaltic samples display a relatively restricted range in OH contents (~1500–3000 ppm) with large δD variations (~600–1000‰) whereas apatites in low-Ti Apollo basalts and lunar meteorites display a comparatively larger range in OH contents (~500–15000 ppm), each sample displaying relatively restricted variations in their D/H ratios. Analyses of apatites in basaltic meteorites Miller Range 05035 and LaPaz Icefield 04841 substantially expand the lower bound for δD values measured in apatites from Apollo mare basalts, down to δD values of ~100‰. In these meteorites, high resolution mapping of the distribution of secondary ions of H and C was used to avoid cracks and hotspots. Together with mixing calculations for terrestrial contamination, this analytical protocol ensured that most of the values reported for MIL 05035 and LAP 04841 correspond to their actual lunar signatures. We interpret the large variations of apatite δD values in mare basalts between ~200‰ and 1000‰ as a result of different amounts of degassing of H-bearing species initially dissolved in the basaltic parental melts. Indeed, the average δD values measured in different low-Ti basalts are consistent with ~85–99% degassing of H as H2, starting from a δD value of 100‰. Degassing of H-bearing species essentially as H2 was favoured by the reduced nature of lunar magmas. In low-Ti mare basalts, apatite crystallisation occurred after degassing of the H-bearing species and the OH variations reflect different degrees of fractional crystallisation. In high-Ti mare basalts, large δD variations with relatively restricted range in OH contents imply that apatite crystallisation and degassing of H-bearing species were mostly coeval. Geochemical modelling integrating corrections for degassing and fractional crystallisation suggests that the mantle source regions of the different low-Ti mare basalts could have contained ~5–50 ppm H (equivalent to ~45–450 ppm H2O), which are similar to the estimated range of ~60–350 ppm water for the Earth’s upper mantle. Finally, the H isotopic composition of pre-degassed lunar hydrogen in mare basalts is consistent with a CI-chondrite-type value of ~100‰, which is consistent with the increasing evidence suggesting that the Earth, Mars and the Moon might have accreted similar water of chondritic origin.

Reference
Tartèse R, Mahesh Anand M, Barnes JJ, Starkey NA, Franchi IA and Sano Y (2013) The abundance, distribution, and isotopic composition of Hydrogen in the Moon as revealed by basaltic lunar samples: Implications for the volatile inventory of the Moon. Geochimica et Cosmochimica Acta 122:58–74.
[doi:dx.doi.org/10.1016/j.gca.2013.08.014]
Copyright Elsevier

Link to Article