¹J.A. Barrat, ²R.C. Greenwood, ²A.B. Verchovsky, ³Ph. Gillet, ⁴C. Bollinger, ⁴J.A. Langlade, ¹C. Liorzou, ²I.A. Franchi
¹Université de Bretagne Occidentale, Institut Universitaire Européen de la Mer, CNRS UMR 6538, Place Nicolas Copernic, 29280 Plouzané, France
²Planetary and Space Sciences, Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA,United Kingdom
³EPSL, Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Station 3, CH-1015 Lausanne, Switzerland
⁴CNRS UMS 3113, I.U.E.M., Place Nicolas Copernic, 29280 Plouzané Cedex, France

The unique achondrite NWA 7325 is an unusual olivine gabbro composed chiefly of calcic plagioclase (An85-93), diopsidic pyroxene (En50.1-54.0 Wo44.8-49.3 Fs0.6-1.3), and forsteritic olivine (Fo97). It is Al and Mg-rich and Fe and Na-poor and displays very low concentrations of incompatible trace elements, much below 0.3 x CI abundances for many of them. It is also characterized by huge Eu and Sr anomalies (Eu/Eu∗=65, Srn/Cen=240). Although the O isotopic composition of NWA 7325 and some ureilites (those with olivine cores in the range Fo75 to Fo88) are similar, a genetic relationship between them is unlikely due to the Fe-poor composition of NWA 7325. It is almost certainly derived from a distinct planetesimal, not previously sampled by other achondrites. The low Na/Al, Ga/Al, Zn/Al ratios as well as the low K, Rb and Cs shown by NWA 7325, suggest a volatile-depleted parent body. This unique gabbro is demonstrably a cumulate, but the composition of its parental melt cannot be precisely assessed. However, the liquid from which NWA 7325 crystallized would have been very poor in incompatible trace elements (Yb in the range of 0.25 to 1.5 x CI abundance) with a very large positive Eu anomaly. Such a melt cannot be the product of the early magmatic activity on a small parent body. Instead, we propose that the parental melt to NWA 7325 formed as a consequence of the total melting of an ancient gabbroic lithology, possibly upon impact, in agreement with the systematics of 26Al-26Mg. Based on recent dating, the crustal material that was parental to NWA 7325 must have been older than 4562.8 Ma, and formed possibly ≈4566 Ma ago. If this scenario is correct, NWA 7325 provides evidence of one of the earliest crusts on a differentiated body so far studied.

Reference
Barrat JA, Greenwood RC, Verchovsky AB, Gillet P, Bollinger C, Langlade JA, Liorzou C, Franchi IA (2015)
Crustal differentiation in the early solar system: clues from the unique achondrite Northwest Africa 7325 (NWA 7325). Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.07.020]

Copyright Elsevier

^1,2Bashar Baghdadi, ²Albert Jambon, ³Jean-Alix Barrat
¹Damascus University, Faculty of Sciences, Department of Geology, Damascus, Syria
²Sorbonne Universités, UPMC Univ Paris 06, CNRS, Institut des Sciences de la Terre de Paris (iSTeP), 4 place Jussieu 75005 Paris, France
³Université de Brest, CNRS UMR 6538 (Domaines Océaniques), I.U.E.M., Place Nicolas Copernic, 29280 Plouzané Cedex, France

Northwest Africa (NWA) 3164 and 5167 are two angrites with a granulitic texture unlike that of other angrites, with a variable, up to millimeter grain size. Besides mineralogical and chemical similarities to other angrites, NWA 3164 and 5167 exhibit unique characteristics. Ca-rich olivine dominates (in NWA 3164: ∼49 vol%, Fo57; NWA 5167: 40 vol%, Fo59). Fassaitic clinopyroxene is the second major phase (in NWA 3164: 29 vol%; in NWA 5167: 36 vol%) with a significant Tschermak component. In addition, two Al-rich phases are present: plagioclase An99 and hercynitic spinel (∼6 and ∼7 vol% respectively for NWA 3164; 17 and 4 vol% respectively for NWA 5167). Heavily weathered iron sulfide and kamacite, (9 wt% in NWA 3164; 4 wt% in NWA 5167) are the remaining minor phases, a unique feature among angrites. All mineral phases are homogeneous. Like other angrites, NWA 3164 and 5167 exhibit a superchondritic Ca/Al ratio, with negligible amounts of alkalis and very low silica content. The presence of metal results from the incorporation of exogenous iron following impact. Subsequent annealing resulted in the observed granulitic texture. Major element composition indicates that both NWA 3164 and 5167 are derived from a picritic angrite precursor after incorporation of metal and annealing.

After correction for iron and Fo90 olivine incorporation, bulk rock REE abundances of both NWA 3164 and NWA 5167 appear lower than those of quenched angrites, showing the lowest absolute abundances among angrites. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) analyses of clinopyroxene, olivine and anorthite indicate that difference of Rare Earth Element (REE) abundances between NWA 3164 and NWA 5167 can be explained by adding traces of phosphate to NWA 3164. NWA 3164/5167 trace elements abundances are similar, showing depletion in volatile elements and enrichment in refractory lithophile elements such as Ca, Ti, and Al. The most incompatible elements are depleted as well, unlike other angrites. This indicates that the source of these younger angrites was more depleted in incompatible elements when compared to the older magmatic angrites. The low Hf/W is understood as the result of exogenous iron incorporation and therefore the Hf/W and W isotopic heterogeneity of the Angrite Parent Body (APB) mantle is secondary. Comparison with other angrites suggests that iron incorporation may be necessary to explain their low Hf/W and W isotopic compositions.

Reflectance
Baghdadi B, Jambon A, Barrat J-A (2015) Metamorphic Angrite Northwest Africa 3164/5167 Compared to Magmatic Angrites. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.07.022]

Copyright Elsevier

¹Richard C. Greenwood, ²Jean-Alix Barrat, ³Edward R.D. Scott, ⁴Henning Haack, ⁵Paul C. Buchanan, ¹Ian.A. Franchi, ^6,7Akira Yamaguchi, ¹Diane Johnson, ⁸Alex W.R. Bevan, ⁹Thomas H. Burbine
¹Planetary and Space Sciences, Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom
²CNRS UMR 6538 (Domaines Océaniques), U.B.O.-I.U.E.M., Place Nicolas Copernic, 29280 Plouzané Cedex, France
³Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI 96822, USA
⁴Centre for Star and Planet Formation, Natural History Museum of Denmark, Øster Voldgade 5-7, DK-1350 Copenhagen K, Denmark
⁵Department of Chemistry and Geology, Kilgore College, 1100 Broadway, Kilgore, TX 75662, USA
⁶National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan
⁷Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tachikawa, Tokyo 190-8518, Japan
⁸Department of Earth and Planetary Sciences, Western Australian Museum, Locked Bag 49 Welshpool DC, WA 6986, Australia
⁹Astronomy Department, Mount Holyoke College, South Hadley, MA 01075, USA

Evidence from iron meteorites indicates that a large number of differentiated planetesimals formed early in Solar System history. These bodies should have had well-developed olivine-rich mantles and consequentially such materials ought to be abundant both as asteroids and meteorites, which they are not. To investigate this “Great Dunite Shortage” we have undertaken a geochemical and oxygen isotope study of main-group pallasites and dunitic rocks from mesosiderites.

Oxygen isotope analysis of 24 main-group pallasites (103 replicates) yielded a mean Δ17O value of -0.187±0.016‰ (2σ), which is fully resolved from the HED Δ17O value of -0.246 ± 0.014 (2σ) obtained in our earlier study and demonstrates that both groups represent distinct populations and were derived from separate parent bodies. Our results show no evidence for Δ17O bimodality within the main-group pallasites, as suggested by a number of previous studies.
Olivine-rich materials from the Vaca Muerta, Mount Padbury and Lamont mesosiderites, and from two related dunites (NWA 2968 and NWA 3329), have Δ17O values within error of the mesosiderite average. This indicates that these olivine-rich materials are co-genetic with other mesosiderite clasts and are not fragments from an isotopically distinct pallasite-like impactor. Despite its extreme lithologic diversity the mesosiderite parent body was essentially homogeneous with respect to Δ17O, a feature best explained by an early phase of large-scale melting (magma ocean), followed by prolonged igneous differentiation.
Based on the results of magma ocean modeling studies, we infer that Mg-rich olivines in mesosiderites formed as cumulates in high-level chambers and do not represent samples of the underlying mantle. By analogy, recently documented Mg-rich olivines in howardites may have a similar origin.
Although the Dawn mission did not detect mesosiderite-like material on Vesta, evidence linking the mesosiderites and HEDs includes: i) their nearly identical oxygen isotope compositions; ii) the presence in both of coarse-grained Mg-rich olivines; iii) both have synchronous Lu-Hf and Mn-Cr ages; iv) there are compositional similarities between the metal in both; and v) mesosiderite-like material has been identified in a howardite breccia. The source of the mesosiderites remains an outstanding question in meteorite science.
The underrepresentation of olivine-rich materials amongst both asteroids and meteorites results from a range of factors. However, evidence from pallasites and mesosiderites indicates that the most important reason for this olivine shortage lies in the early, catastrophic destruction of planetesimals in the terrestrial planet-forming region and the subsequent preferential loss of their olivine-rich mantles.

Reference
Greenwood RC, Barrat J-A, Scott ERD, Haack H, Buchanan PC, Franchi IA, Yamaguchi A, Johnson D, Bevan AWR, Burbine TH (2015) Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites: Implications for the “Great Dunite Shortage” and HED-mesosiderite connection. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.07.023]

Copyright Elsevier

¹Alan E. Rubin
¹Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA

Paris is the least aqueously altered CM chondrite identified to date, classified as subtype 2.7; however, literature data indicate that some regions of this apparently brecciated meteorite may be subtype 2.9. The suite of CAIs in Paris includes 19% spinel–pyroxene inclusions, 19% spinel inclusions, 8% spinel–pyroxene–olivine inclusions, 43% pyroxene inclusions, 8% pyroxene–olivine inclusions, and 3% hibonite-bearing inclusions. Both simple and complex inclusions are present; some have nodular, banded, or distended structures. No melilite was identified in any of the inclusions in the present suite, but other recent studies have found a few rare occurrences of melilite in Paris CAIs. Because melilite is highly susceptible to aqueous alteration, it is likely that it was mostly destroyed during early-stage parent-body alteration. Two of the CAIs in this study are part of compound CAI–chondrule objects. Their presence suggests that there were transient heating events (probably associated with chondrule formation) in the nebula after chondrules and CAIs were admixed. Also present in Paris are a few amoeboid olivine inclusions (AOI) consisting of relatively coarse forsterite rims surrounding fine-grained, porous zones containing diopside and anorthite. The interior regions of the AOIs may represent fine-grained rimless CAIs that were incorporated into highly porous forsterite-rich dustballs. These assemblages were heated by an energy pulse that collapsed and coarsened their rims, but failed to melt their interiors.

Reference
Rubin AE (2015) An American on Paris: Extent of aqueous alteration of a CM chondrite and the petrography of its refractory and amoeboid olivine inclusions. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12482]
Published by arrangement with John Wiley&Sons

¹Charles K. Shearer, ¹Paul V. Burger, ¹Aaron S. Bell, ²Yunbin Guan, ³Clive R. Neal
¹Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA
²Division of Earth and Planetary Sciences, California Institute of Technology, Pasadena, California, USA
³Department of Civil & Environmental Engineering & Earth Sciences, University of Notre Dame, Notre Dame, Indiana, USA

Remotely sensed observations from recent missions (e.g., GRAIL, Kaguya, Chandrayaan-1) have been interpreted as indicating that the deep crust and upper mantle are close to or at the lunar surface in many large impact basins (e.g., Crisium, Apollo, Moscoviense). If this is correct, the capability of either impact or volcanic processes to transport mantle lithologies to the lunar surface should be enhanced in these regions. Somewhat problematic to these observations and interpretations is that examples of mantle lithologies in the lunar sample collection (Apollo Program, Luna Program, lunar meteorites) are at best ambiguous. Dunite xenoliths in high-Ti mare basalt 74275 are one of these ambiguous examples. In this high-Ti mare basalt, olivine occurs in three generations: olivine associated with dunite xenoliths, olivine megacrysts, and olivine microphenocrysts. The dunite xenoliths are anhedral in shape and are generally greater than 800 μm in diameter. The interior of the xenoliths are fairly homogeneous with regard to many divalent cations. For example, the Mg# (Mg/Mg + Fe × 100) ranges from 82 to 83 in their interiors and decreases from 82 to 68 over the 10–30 μm wide outer rim. Titanium and phosphorus X-ray maps of the xenolith illustrate that these slow diffusing elements preserve primary cumulate zoning textures. These textures indicate that the xenoliths consist of many individual olivine grains approximately 150–200 μm in diameter with low Ti, Al, and P cores. These highly incompatible elements are enriched in the outer Fe-rich rims of the xenoliths and slightly enriched in the rims of the individual olivine grains. Highly compatible elements in olivine such as Ni exhibit a decrease in the rim surrounding the xenolith, an increase in the incompatible element depleted cores of the individual olivine grains, and a slight decrease in the “interior rims” of the individual olivine grains. Inferred melt composition, liquid lines of descent, and zoning profiles enable the reconstruction of the petrogenesis of the dunite xenoliths. Preservation of primary magmatic zoning (Ti, P, Al) and lack of textures similar to high-pressure mineral assemblages exhibited by the Mg-suite (Shearer et al. 2015) indicate that these xenoliths do not represent deep crustal or shallow mantle lithologies. Further, they are chemically and mineralogically distinct from Mg-suite dunites identified from the Apollo 17 site. More likely, they represent olivine cumulates that crystallized from a low-Ti mare basalt at intermediate to shallow crustal levels. The parent basalt to the dunite xenolith lithology was more primitive than low-Ti basalts thus far returned from the Moon. Furthermore, this parental magma and its more evolved daughter magmas are not represented in the basalt sample suite returned from the Taurus-Littrow Valley by the Apollo 17 mission. The dunite xenolith records several episodes of crystallization and re-equilibration. During the last episode of re-equilibration, the dunite cumulate was sampled by the 74275 high-Ti basalt and transported over a period of 30–70 days to the lunar surface.

Reference
Shearer CK, Burger PV, Bell AS, Guan Y, Neal CR (2015) Exploring the Moon’s surface for remnants of the lunar mantle 1. Dunite xenoliths in mare basalts. A crustal or mantle origin? Meteoritics&Planetary Science
Link to Article [DOI: 10.1111/maps.12480]

Published by arrangement with John Wiley&Sons

^1,2Wei Jia Ong, ¹Christine Floss
¹Laboratory for Space Sciences and Physics Department, Washington University, St. Louis, Missouri, USA
²National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, Michigan, USA

We carried out Fe isotopic analyses on 21 O-rich presolar grains from the Acfer 094 ungrouped carbonaceous chondrite. Presolar grains were identified on the basis of oxygen isotopic ratios, and elemental compositions were measured by Auger spectroscopy. The Fe isotopic measurements were carried out by analyzing the Fe isotopes as negative secondary oxides with the NanoSIMS to take advantage of the higher spatial resolution of the Cs+ primary ion beam. Our results demonstrate the effectiveness of this approach for measuring both 54Fe/56Fe and 57Fe/56Fe. The ion yield for FeO– is significantly lower than for Fe+, but this is not a serious limitation for presolar silicate grains with Fe as a major element. Most of the grains analyzed are ferromagnesian silicates, but we also measured four oxide grains. Iron contents are high in all of the grains, ranging from 10 to 40 atom%. Three of the grains belong to oxygen isotope Group 4. All of them have 54Fe/56Fe and 57Fe/56Fe ratios that are solar within errors, consistent with an origin in the outer zones of a Type II supernova, as indicated by their oxygen isotopic compositions. The remaining grains belong to oxygen isotope Group 1, with origins in low-mass AGB stars. The majority of these also have solar 54Fe/56Fe and 57Fe/56Fe ratios. However, four grains are depleted in 57Fe; one is also slightly depleted in 54Fe. Current AGB models predict excesses in 57Fe with 54Fe/56Fe ratios that largely reflect the metallicity of the parent star. While the solar 57Fe/56Fe ratios are consistent with formation of the grains in early third dredge-up episodes, these models cannot account for the grains with 57Fe depletions. Comparison with galactic evolution models suggests formation of these grains from stars with significantly subsolar metallicity; however, these models also predict large depletions in 54Fe, which are not observed in the grains. Thus, the isotopic compositions of these grains remain unexplained.

Reference
Ong WJ, Floss C (2015) Iron isotopic measurements in presolar silicate and oxide grains from the Acfer 094 ungrouped carbonaceous chondrite. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12475]

Publsihed by arrangement with John Wiley&Sons

⁵D. Schwander, ^2,3,4L. Kööp, ¹T. Berg, ⁵G. Schönhense, ^2,3,4P.R. Heck, ^2,3,4,5A.M. Davis, ^1,6,7U. Ott
¹Institut für Physik, Johannes Gutenberg-Universität, Staudingerweg 7, D-55128 Mainz, Germany
²Department of the Geophysical Sciences, The University of Chicago, Chicago, IL, United States
³Chicago Center for Cosmochemistry, The University of Chicago, Chicago, IL, United States
⁴Robert A. Pritzker Center for Meteoritics and Polar Studies, Field Museum of Natural History, Chicago, IL, United States
⁵Enrico Fermi Institute, The University of Chicago, Chicago, IL, United States
⁶University of West Hungary, H-9700 Szombathely, Hungary
⁷Max-Planck-Institut für Chemie, Hahn-Meitner-Weg 1, D-55128 Mainz, Germany

Ca, Al-rich inclusions (CAIs) often contain numerous refractory metal nuggets (RMNs), consisting of elements like Os, Ir, Mo, Pt and Ru. The nuggets are usually thought to have formed by equilibrium condensation from a gas of solar composition, simultaneously with or prior to oxide and silicate minerals. However, the exact mechanisms responsible for their extremely variable compositions, small sizes and associations with CAI minerals remain puzzling. Expanding on previous work on chemically separated RMNs, we have studied a large number of RMNs within their host CAIs from three different meteorite types, i.e., the highly primitive chondrite Acfer 094, Allende (CV3ox) and Murchison (CM2). Our results show several inconsistencies between the observed features and a direct condensation origin, including a lack of correlated abundance variations in the refractory metals that is expected from variations in condensation temperature. Instead, we show that most RMN features are consistent with RMN formation by precipitation from a CAI liquid enriched in refractory metals. This scenario is additionally supported by the common occurrence of RMNs in CAIs with clear melt crystallization textures as well as the occurrence of synthetic RMNs with highly variable compositions in run products from Schwander et al. (2015). In some cases, the sizes of meteoritic RMNs correlate with the sizes of their host minerals in CAIs, which indicates common cooling rates.

Reference
Schwander D, Kööp L, Berg T, Schönhense G, Heck PR, Davis AM, Ott U (2015) Formation of refractory metal nuggets and their link to the history of CAIs. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.07.014]

Copyright Elsevier

^1,2,3Kasia Biron, ^1,2Sylvie Derenne, ³François Robert, ⁴Jean-Noël Rouzaud
¹CNRS, UMR 7619 METIS, Paris, France
²Sorbonne Universités, UPMC Univ Paris 06, CNRS, EPHE, UMR 7619 METIS, Paris, France
³IMPMC, CNRS/MNHN UMR 7202, Paris, France
⁴Laboratoire de Géologie, ENS/CNRS UMR 8538, Paris, France

Based on the statistical model proposed for the molecular structure of the insoluble organic matter (IOM) isolated from the Murchison meteorite, it was recently proposed that, in the solar T-Tauri disk regions where (photo)dissociation of gaseous molecules takes place, aromatics result from the cyclization/aromatization of short aliphatics. This hypothesis is tested in this study, with n-alkanes being submitted to high-frequency discharge at low pressure. The contamination issue was eliminated using deuterated precursor. IOM was formed and studied using solid-state nuclear magnetic resonance, pyrolysis coupled to gas chromatography and mass spectrometry, RuO4 oxidation, and high-resolution transmission electron microscopy. It exhibits numerous similarities at the molecular level with the hydrocarbon backbone of the natural IOM, reinforcing the idea that the initial precursors of the IOM were originally chains in the gas. Moreover, a fine comparison between the chemical structure of several meteorite IOM suggests either that (i) the meteorite IOMs share a common precursor standing for the synthetic IOM or that (ii) the slight differences between the meteorite IOMs reflect differences in their environment at the time of their formation i.e., related to plasma temperature that, in turn, dictates the dissociation–recombination rates of organic fragments.

Reference
Biron K, Derenne S, Robert F, Rouzaud J-N (2015) Toward an experimental synthesis of the chondritic insoluble organic matter. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12477]

Published by arrangement with John Wiley & Sons

¹Charles A Schambeau, ¹Yanga R. Fernández, ²Carey M. Lisse, ³Nalin Samarasinha, ⁴Laura M. Woodney
¹Department of Physics, University of Central Florida, Orlando, FL 32816, USA
²Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723
³Planetary Science Institute, Tucson, AZ 85719, USA
⁴Department of Physics, California State University San Bernardino, San Bernardino, CA 92407

We present a new analysis of Spitzer observations of comet 29P/Schwassmann-Wachmann 1 taken on UT 2003 November 21, 23, and 24, similar to a previous investigation of the observations (Stansberry et al., 2004), but using the most recent Spitzer data pipeline products and intensive image processing techniques. Analysis of images from the IRAC 5.8 & 8.0 μμm bands and the MIPS 24.0 & 70.0 μμm bands resulted in photometry measurements of the nucleus after a suite of coma modeling and removal processes were implemented. SW1 was not identified in the 5.8 μμm image from the previous work so its incorporation into this analysis is entirely new. Using the Near Earth Asteroid Thermal Model ( Harris, 1998) resulted in a nucleus radius measurement of R = View the MathML source30.2-2.9+3.7 km and an infrared beaming parameter value of View the MathML sourceη=0.99-0.19+0.26. We also measured an infrared geometric albedo, p5.8p5.8 = 0.5 ±± 0.5. Extrapolating a 0.04 V-band albedo and using a normalized reflectivity gradient S′=14.94±1.09S′=14.94±1.09 [% (1000 Å)−1] ( Duffard et al., 2014) we recover an infrared albedo of p5.8p5.8 = 0.31 in the near infrared consistent with the value recovered from thermal modeling. The dust composition extracted from IRS spectra are very comet-like, containing mainly amorphous ferromagnesian silicates (but with a minority of crystalline silicates as well), water ice, and metal sulfides.

Reference
Schambeau CA, Fernández YR, Lisse CM, Samarasinha N, Woodney LM (2015) A New Analysis of Spitzer Observations of Comet 29P/Schwassmann-Wachmann 1. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.06.038]
Copyright Elsevier

¹V. Sautter et al. (>10)*
¹IMPMC, Muséum d’Histoire Naturelle, 75005 Paris, France

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

Reference
Sautter V et al. (2015) In situ evidence for continental crust on early Mars. Nature Geoscience (in Press)
Link to Article [doi:10.1038/ngeo2474]