Planetary Science Letters Research Articles

Correlation of a 410-km Discontinuity Low Velocity Layer with Velocity Tomograms Beneath the Colorado Plateau Using the RISTRA Array

The transition zone water filter model (Nature 425(6953), 39–44; 2003) predicts that a hydrous partial melt layer is only actively produced in a region of upwelling mantle. We test the transition zone water filter model via stacking of P-to-S converted receiver functions by using the IRIS-PASSCAL RISTRA (Colorado Plateau/Rio Grande Rift Seismic Transect Experiment) array. Assuming the high velocity regions found at the northwest and southeast ends of the array at 350–440 km by teleseismic velocity tomograms e.g. Schmandt and Humphreys (Earth and Planetary Science Letters 297(3–4): 435–445; 2010) are cold and sinking vertically, the 410-km low velocity layer should be absent in these regions. The receiver function stacking profiles find the mean depths of the two primary discontinuities at 417 ± 7.1 km for the 410-km discontinuity and 667 ± 8.2 km for the 660-km discontinuity. The average arrival amplitudes with respect to Z component are 3.0% for the 410-km discontinuity, 2.8% for the 660-km discontinuity, and − 1.8% for the 410-km low velocity layer. The stacked Pds image show the 410-km low cabsent at ~ 350 to 390 km in the high velocity regions, but present in low velocity region. A correlation plot of sum of the 410-km low velocity arrival amplitudes and P-wave perturbation finds a positive linear relationship. Therefore, our findings provide seismic evidence for the transition zone water filter model at a small scale.

Introducing article numbering to Earth and Planetary Science Letters

Introducing article numbering to Earth and Planetary Science Letters

Geometry, kinematics and regional significance of faulting and related lamprophyric intrusion in the mineralised zone at the Pu Sam Cap complex, Northwest Vietnam

Geometry, kinematics and regional significance of faulting and related lamprophyric intrusion in the mineralised zone at the Pu Sam Cap complex, Northwest Vietnam

Petrology, geochemistry, and Sr, Nd isotopes of mantle xenolith in Nghia Dan alkaline basalt (West Nghe An): implications for lithospheric mantle characteristics beneath the region

Petrology, geochemistry, and Sr, Nd isotopes of mantle xenolith in Nghia Dan alkaline basalt (West Nghe An): implications for lithospheric mantle characteristics beneath the region

Reply to comment by Z. Yi et al. on “Remagnetization of the Paleogene Tibetan Himalayan carbonate rocks in the Gamba area: Implications for reconstructing the lower plate in the India‐Asia collision”

AbstractIn their comment on our publications on pervasive remagnetization of Jurassic‐Paleogene carbonate rocks of the Tibetan Himalaya (Huang et al., 2017, Journal of Geophysical Research: Solid Earth, 122, doi: 10.1002/2016JB013662 and 122, doi: 10.1002/2017JB013987), Yi et al. (2017) questioned our fold tests applied to their published paleomagenetic results from the Paleogene Zongpu and latest Cretaceous Zongshan carbonate rocks (Patzelt et al., 1996, Tectonophysics, 259(4), 259–284; Yi et al., 2011, Earth and Planetary Science Letters, 309(1), 153–165). They argued that authigenic magnetite pseudomorphic after pyrite, which is the dominant magnetic carrier within these carbonate rocks as indicated by our thorough rock magnetic and petrographic investigations, was formed during early diagenesis and that the primary natural remanent magnetization (NRM) is retained by these carbonate rocks. However, their statement for the invalidity of our fold tests is based on unrealistic assumptions that these carbonate rocks carry primary NRM and that the upper Zongpu Formation was deposited on a 10° primary dip. Their argument for immediate oxidization of pyrite to authigenic magnetite after carbonate deposition onto the continental passive margin ignores that sulfate‐reducing conditions were prevailing during early diagenesis, it is also inconsistent with the timing of the secondary remanence acquisition in remagnetized carbonate rocks elsewhere. As previously demonstrated, and agreed upon by Yi et al. (2017), the Zongpu and Zongshan carbonate rocks in Gamba are remagnetized; here we argue that the timing of remagnetization cannot be demonstrated to shortly postdate sedimentation. These data should therefore not be used for tectonic reconstructions.

Geochemistry of late miocene-pleistocene basalts in the Phu Quy island area (East Vietnam Sea): Implication for mantle source feature and melt generation

Geochemistry of late miocene-pleistocene basalts in the Phu Quy island area (East Vietnam Sea): Implication for mantle source feature and melt generation

Magma source feature and eruption age of volcanic rocks in the Tram Tau district, Tu Le Basin

Magma source feature and eruption age of volcanic rocks in the Tram Tau district, Tu Le Basin

Comment on “Cenozoic tectonic deformation and uplift of the South Tian Shan: Implications from magnetostratigraphy and balanced cross-section restoration of the Kuqa depression” by Tao Zhang, Xiaomin Fang, Chunhui Song, Erwin Appel, and Yadong Wang [Tectonophysics, 2014, doi:10.1016/j.tecto.2014.04.044

The recent paper entitled: “Cenozoic tectonic deformation and uplift of the South Tian Shan: Implications from magnetostratigraphy and balanced cross-section restoration of the Kuqa depression” by Zhang et al. [Tectonophysics, 2014, doi:10.1016/j.tecto.2014.04.044] discusses the Cenozoic tectonic deformation and uplift of the South Tianshan Mountains by integrating tectonic investigations, seismostratigraphic analysis and paleomagnetic dating of the thrust–fold belt in the Kuqa Depression at the southern border of the Tianshan. To support their conclusions the authors have reinterpreted a high resolution magnetostratigraphic study of the Kezilenuer Section in a paper entitled “Magnetostratigraphic study of the Kuche Depression, Tarim Basin, and Cenozoic uplift of the Tian Shan Range, Western China” by Huang et al. [Earth and Planetary Science Letters Volume 251, pages 346–364 (2006)]. We note here (i) that apparent conflicts in definition of the Xiyu Formation in the Kuche Depression as proposed by Zhang et al. (2014) require amplification and (ii) argue that their age assignment for the Kezilenuer Section is incorrect and yields an anomalously low sedimentation rate for the infilling of this foreland basin.

Geochemical consequences of intense pulse-like degassing during the onset of the Central Atlantic Magmatic Province

The Triassic–Jurassic boundary (TJB) is marked by one of the five largest mass extinctions of the Phanerozoic and the eruption of a large igneous province: the Central Atlantic Magmatic Province (CAMP). The TJB is characterized by a carbonate production crisis, by negative excursions in the carbon isotopic ratio of buried organic matter, and by successive peaks in atmospheric CO2. Here we use the numerical model GEOCLIM to explore the possible connections between the CAMP emplacement and the observed carbon cycle perturbations. Different degassing scenarios linked to the CAMP eruption are explored. We show that the emission of realistic amounts of CO2 that follow a geologically constrained degassing scenario as short-term peaks (less than 10ka) leads to successive decrease in carbonate production, as observed in the geological record. We also calculate the evolution of carbon isotopes and show that our model reproduces the amplitude of the isotopic excursion with a volcanic degassing of CO2 characterized by a carbon isotopic composition of −20‰. Such low values could be associated to carbon pools of light isotopic composition located at the transition zone [Cartigny, P. 2010, Earth and Planetary Science Letters, v. 296, p. 329–339] and not necessarily to biogenic methane release. Finally, the model predicts a succession of short-term CO2 rises, with an amplitude in close agreement with available proxy-based reconstructions.

Editorial: The Depositional Record

Many of you will have already heard about the launch of a major new journal ‘The Depositional Record’ to be published alongside the flagship journal ‘Sedimentology’. The new journal, owned by the International Association of Sedimentologists (IAS) and published by Wiley, will be Open Access and digital only. The journal was launched at the 19th International Sedimentological Congress in Geneva and the web site has been ‘live’ since 19 August 2014. Our aim is to publish the first issue by May 2015 and obtain an Impact Factor by July 2017. Over this period, the authors’ publication charges will be met by the IAS. The purpose of this editorial is to define how we, the Editors, envisage that The Depositional Record will differ from Sedimentology. Historically, the journal Sedimentology has emphasized classic sedimentology, such as the description of sedimentary sequences and processes. While Sedimentology will continue to publish such studies and others, new analytical methods and approaches have been developed over the past 50 years. New geochemical and geobiological techniques, in particular, can provide information on climate, carbon dynamics, evolution of the oceans, formation of sediments and diagenesis within the depositional record. In addition, there are many types of deposits often not considered within the remit of classic sedimentology, yet they utilize the geological principles on which our science is based. Through environmental reconstruction using such proxies, the depositional record at all scales can be used to better understand anthropological, archaeological, palaeobotanical and palaeofaunal histories, including extinction events and environmental modelling more generally. Although papers on new methods and different types of geo-records have been published in Sedimentology, many of the fundamental papers in the areas listed above are published in journals such as Earth and Planetary Science Letters, Geobiology, Geochimica et Cosmochimica Acta, Paleoceanography, Biogeosciences, Quaternary Science Reviews and many others. The Depositional Record is aimed at such papers. We want to attract high-impact research papers that address processes which affect, or which are recorded in, the depositional record over all time and physical scales. We would be equally happy with studies addressing the impacts of ocean acidification upon the growth rate and chemical composition of a modern coral, the application of next-generation gene sequencing data to understand the role of microbes in carbonate mineral precipitation, investigations into the Neoproterozoic carbon cycle, the geological records of the Mg/Ca ratio of the oceans, the sedimentary record of ancient Roman harbour utilization or the reconstruction of past and future ice-sheet dynamics using glacial sediments. We encourage you to submit your exciting papers in these areas to The Depositional Record. A more detailed description of the scope and goals of the journal can be found at http://onlinelibrary. wiley.com/journal/10.1002/(ISSN)2055-4877.

Corrigendum to “The influence of magnetic fields in planetary dynamo models” [Earth Planet. Sci. Lett. 333–334 (2012) 9–20

Earth and Planetary Science Letters 392 (2014) 121–123 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Corrigendum Corrigendum to “The influence of magnetic fields in planetary dynamo models” [Earth Planet. Sci. Lett. 333–334 (2012) 9–20] K.M. Soderlund a ,∗ , E.M. King b , J.M. Aurnou c a b c Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78758, USA Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095, USA a r t i c l e i n f o Article history: Available online xxxx We have investigated the influence of magnetic fields on the convective properties of planetary dynamo models and the transi- tion between dynamos with dipolar and multipolar magnetic fields in Soderlund et al. (2012). We thank Uli Christensen for pointing out an error solely in our algorithm used to calculate the volu- metric average of relative axial helicity, H rel z . However, it should be noted that the main results of the paper remain intact, as dis- cussed below. Relative axial helicity is defined as axial helicity normalized by its maximum possible value, H rel z = u z ω z h ( u z u z h ω z ω z h ) 1 / 2 where u z is axial velocity, ω z = ∇ × u · z ˆ is axial vorticity, and h is the volumetric average in each hemisphere excluding bound- ary layers (e.g., Olson et al., 1999; Schmitz and Tilgner, 2010). We average the helicity magnitude over each hemisphere since axial helicity tends to be anti-symmetric across the equator and assume Ekman boundary layer thicknesses of δ E / D = 3E 1 / 2 following King et al. (2012). In Soderlund et al. (2012), the | H rel z | calculations are missing a factor of sin θ in the volume integral. The erroneously calculated helicity data are shown in Fig. 1a. The effect of this missing fac- tor is to weight the contributions within the tangent cylinder too heavily. Corrected | H rel z | are shown in Fig. 1b. While instantaneous val- ues were given in Soderlund et al. (2012), here we report the average of three random snapshots in time for each case. Temporal standard deviations ( σ ) are typically near 0 . 02. The corrected val- ues differ on average from the original values by [0.02 (0.02), 0.04 (0.04), 0.07 (0.02)] for cases with E = [ 10 − 3 , 10 − 4 , 10 − 5 ] where parentheses indicate non-magnetic cases. These differences only exceed 2 σ in the lowest Ekman number dynamo cases. Tables 1 and 2 provide corrected values of | H rel z | . In addition, we have also included time averaged values of axial vorticity colum- narity C ω z , in situ Lorentz to Coriolis force ratio F L / F C , and in situ inertial to viscous force ratio F I / F V . The two main conclusions of the paper are not modified sig- nificantly. They are: (i) The magnetic fields do not strongly affect relative axial helicity in cases with E 10 − 4 . This weak influence can be understood theoretically by estimating the strength of the Lorentz forces in the high magnetic Reynolds number limit with a dynamic Elsasser number, Λ d = B 2 /( 2 ρμ o Ω U B ) . Calculations of the dynamic Elsasser number correctly capture the secondary influence of Lorentz forces on convection. (ii) The breakdown of dipolar magnetic field generation still coincides with the degra- dation of helicity in the flow, even though the helicity decreases are markedly less abrupt than originally reported (see Fig. 1b). Im- portantly, the dipolarity breakdown still occurs when the inertial and viscous forces become comparable, F I / F V ∼ 1, for all Ekman numbers considered here (10 − 3 E 10 − 5 ). Because viscosity is expected to be negligible in planetary settings, the dipolarity breakdown in present day dynamo models with moderate Ekman numbers may not extrapolate to planets. Acknowledgements DOI of original article: http://dx.doi.org/10.1016/j.epsl.2012.03.038. Corresponding author. Tel.: +1 (218) 349 3006; fax: +1 (512) 471 8844. E-mail addresses: krista@ig.utexas.edu (K.M. Soderlund), eric.king@berkeley.edu (E.M. King), aurnou@ucla.edu (J.M. Aurnou). http://dx.doi.org/10.1016/j.epsl.2014.01.052 0012-821X/ © 2014 Elsevier B.V. All rights reserved. We thank Uli Christensen and Johannes Wicht for their assis- tance to verify our calculations.

Helium isotope systematics in the vicinity of the Azores triple junction: Constraints on the Azores geodynamics

We present new He isotopic data from subaerial and submarine samples collected along the Terceira Rift (Azores Plateau) in order to constrain the sources involved in the Azores magmatism in the unique geodynamic context of the Azores triple junction. Lower than MORB 4He/3He isotopic ratios were determined on olivines from the Terceira Island and the adjacent Serreta submarine ridge (down to ≈69,000; ≈10.5 R/Ra), which in addition to the Ne isotopes previously published in Madureira et al. (2005) [Primitive helium and neon isotopes in Terceira Island: Constraints on the origin of the Azores archipelago. Earth and Planetary Science Letters 233, 429–440], support the geochemical models that consider the involvement of the lower mantle in the Azores magmatism. The analysis of He data published for the Mid-Atlantic Ridge (MAR) points to the existence of chemically distinct mantle domains underneath Eurasia and Nubia lithospheric plates, which can result from an heterogeneous distribution of asthenosphere metasomatism during the initial stages of plume activity in the Azores region. At the Terceira Rift, lavas from Graciosa Island, as well as from the western end of São Miguel Island and D. João de Castro Bank, display 4He/3He ratios similar to those observed along the MAR segments located to the north of the Azores triple junction area. Conversely, samples from the south Hirondelle Basin and also Faial display a He isotopic signature similar to that of MORB erupted along MAR segments located to the south of the Azores Plateau. The Terceira Rift and the Azores triple junction area are thus characterized by the mingling of two different asthenospheric domains referred as “Eurasia” and “Nubia” type and by the presence of mantle plume derived material having primitive helium signature.

2013 Nier Prize for Audrey Bouvier

Dr. Audrey Bouvier It is my pleasure to introduce Dr. Audrey Bouvier, who has made significant contributions through her isotopic investigations of planetary materials, particularly the lead-lead systematics of the solar system's earliest solids as well as a variety of undifferentiated and differentiated meteorites. Audrey received her Bachelor's (2001) and Master's (2002) degrees in Earth Sciences from the Laboratoire Magmas et Volcans at Université Blaise Pascal in Clermont-Ferrand, followed by a PhD (2005) from Département des Sciences de la Terre at Ecole Normale Supérieure de Lyon. In Lyon, she was coadvised by Francis Albarède and Janne Blichert-Toft, and her dissertation work there focused on the Pb-Pb chronology of chondrites and the SNC (Martian) meteorites. The work on chondrites has provided important constraints on the accretionary and cooling histories of their parent bodies. While the interpretation and implications of her work on the Martian meteorites (discussed in a series of articles published between 2005 and 2009) have been hotly debated, it is clear that the data are of high quality and have stimulated dialog and further research into the differentiation history of Mars by other research groups. In addition to the Pb-Pb investigations, Audrey also determined the first ever Lu-Hf internal isochron for a Martian meteorite (Zagami), which yielded an age consistent with the internal isochron ages for this sample based on other radiochronometers. Following her PhD, Audrey worked as a Postdoctoral Research Associate (2005–2007) in the Department of Geosciences at the University of Arizona. There she began studies in collaboration with Jonathan Patchett and Jeff Vervoort that helped to define the Lu-Hf and Sm-Nd compositions of the chondritic uniform reservoir (CHUR), which have significant implications for the bulk composition and evolution of crustal and mantle reservoirs on Earth. The paper reporting this work has been the most cited one in Earth and Planetary Science Letters since its publication in 2008 in that journal. Audrey subsequently joined my research group in the Center for Meteorite Studies at Arizona State University as a Postdoctoral Research Associate for 3 years (2007–2010), and then as a Faculty Research Associate for another year. At ASU, Audrey investigated the high-resolution chronology of chondritic components and differentiated meteorites using the Pb-Pb and Al-Mg chronometers. This work has important implications for the age of the solar system, as well as the initial distribution of 26Al and the timing of accretion and differentiation of planetesimals in the early solar system. One of the publications resulting from her study of the Pb-Pb and Al-Mg systematics of a refractory inclusion from the Northwest Africa 2364 CV3 chondrite was highlighted this past January by the editors of Nature Geoscience as one of their ten favorite papers chosen across disciplines in the web focus celebrating the 5th anniversary of that journal. In October 2011, Audrey left ASU to begin an appointment as Research Associate in the Department of Earth Sciences at the University of Minnesota. It is for her substantial body of work focusing on the high-resolution chronology and evolution of the solar system and planetary bodies, and including such topics as the age of the earliest solar system solids to the differentiation history of Mars, that Audrey is deserving of the Nier Prize. The quality and productivity of her work have also been recognized by other successes in recent years. She was awarded grants from the NSF Geochemistry and Petrology program and the NASA Lunar Advanced Science and Exploration Research program. Finally, she was offered a position as Assistant Professor of Geochemistry in the Department of Earth Sciences at the University of Western Ontario and began her appointment there in July of this year. I have no doubt that she has a bright future ahead of her at UWO! Audrey has published numerous papers in the area of high-precision chronology and evolutionary history of the solar system that have had a tremendous impact in the field of meteoritics and cosmochemistry. On a more personal note, it has been a great pleasure to work with her over the years and to see her evolve and succeed in her personal and professional life. She is highly deserving of the Nier Prize of the Meteoritical Society for this body of work; I am very pleased to be able to present her as the recipient of this year's award.

Sr[sbnd]Nd[sbnd]Pb isotopic compositions of Early Cretaceous granitoids from the Dabie orogen: Constraints on the recycled lower continental crust

In order to characterize the recycled lower continental crust (LCC) in the Dabie orogen, 17 Early Cretaceous low-Mg adakitic (LMA) and 9 normal (non-adakitic) granitoids have been investigated for SrNdPb isotopes. Combined with literature data, LMA have low ƐNd(t) (−27.8 to −14.7) and 206Pb/204Pb(i) (15.69–17.16) and low to moderately high 87Sr/86Sr(i) (0.7066 to 0.7087) ratios. Normal granitoids yield isotope ratios similar to adakitic rocks, except a few with 87Sr/86Sr (i) up to 0.7105. Dabie LMA define a linear trend parallel to the North Hemisphere Reference Line (NHRL) in a 208Pb/204Pb(i)– 206Pb/204Pb(i) diagram. For a given 206Pb/204Pb(i), the 208Pb/204Pb(i) or ∆8/4 (152–217) of Dabie LMA are close to the majority of UHP gneisses and the Neoproterozoic mafic rocks from the northern margin of the South China Block (SCB), but significantly higher than adakitic rocks from the North China Block (∆8/4<150). Considering the commonly present Neoproterozoic inherited zircons, we suggest that the LMA in the Dabie orogen are derived from a thickened LCC that could be dominantly composed of ancient SCB lower crust.The SrNdPb isotopic composition of LMA are similar to those of Post-Collisional Mafic Igneous rocks (PCMI) from the Dabie orogen, but different to exhumed UHP rocks. The SrNdPb isotopic system of the mantle source of the PCMI could be enriched in components dominantly from delaminated LCC of the Dabie orogen instead of deeply subducted continental crust, which is in contrast to O- and C-isotope data by [Zhao, Z.F., Zheng, Y.F., Wei, C.S., Wu, Y.B., Chen, F.K., and Jahn, B.M., 2005. Zircon UPb age, element and CO isotope geochemistry of post-collisional mafic–ultramafic rocks from the Dabie orogen in east-central China. Lithos 83(1–2), 1–28; Dai, L.Q., Zhao, Z.F., Zheng, Y.F., Li, Q.L., Yang, Y.H., and Dai, M.N., 2011. Zircon HfO isotope evidence for crust–mantle interaction during continental deep subduction. Earth and Planetary Science Letters 308, 229–244]. Combining the SrNdPbHf and OC isotopic data, two types of commonly present magma components have been identified in the PCMI, reflecting crust–mantle interaction during the Triassic subduction and the Early Cretaceous orogen collapse, respectively.

Limits in pyrolysis–GC–MS analysis of kerogen isolated from Archean cherts

Pyrolysis–GC–MS was recently proposed as an emerging technique for establishing biogenicity of 3.5Gyr old insoluble organic matter in cherts (Derenne et al., 2008. Earth and Planetary Science Letters 272, 476–480). When applied to four kerogens isolated from Archean chert samples for biogenicity determination, this method highlights the importance of having a significant aliphatic component remaining in the kerogen. If not, fatty acid decarboxylation may bias the n-alkane distribution. However, in most cases, mature samples such as Archean cherts usually do not have such an aliphatic fraction. This shows the limits of the technique and the need for a new biogenicity marker.

Bio-magnetochronology for the upper Campanian - Maastrichtian from the Gubbio area, Italy: new results from the Contessa Highway and Bottaccione sections

A new bio-chronostratigraphic framework is presented for the upper Campanian – Maastrichtian pelagic sediments of the Gubbio area (Bottaccione and Contessa Highway sections, Italy). New planktonic foraminiferal (FO of P. hantkeninoides), calcareous nannofossil (FO of M. prinsii, base acme of M. murus) and magnetostratigraphic data are provided and integrated to construct an age-depth curve based on the recent astronomical calibration of the late Campanian – Maastrichtian magnetic polarity time scale (Husson, D., Galbrun, B., Laskar, J., Hinnov, L. A., Thibault, N., Gardin, S., Locklair, R. E., 2011. Astronomical calibration of the Maastrichtian, Late Cretaceous. Earth and Planetary Science Letters 305, 328–340). All bio-horizons recorded in both sections fall on or are very close to the Line of Correlation (LOC), which testifies to the reliability of biochronologic studies in the Umbria-Marche basin. The proposed age model allows to estimate the sedimentation rates and the ages of calcareous nannofossil and planktonic foraminiferal bio-horizons in both sections. Also, it provides an excellent late Campanian – Maastrichtian reference for sections in the Umbria-Marche area and the entire Tethys. Due to the presence of a ~ 425 kyr-long hiatus in Chron C31n of the Bottaccione section, the Contessa Highway section constitutes a more complete late Campanian – Maastrichtian reference for bio-magnetochronology in the Tethyan realm.

Melt inclusions in olivines from early Iceland plume picrites support high 3He/4He in both enriched and depleted mantle

High 3He/4He ratios (up to 50 Ra) are present in near-primary picrite flows from Baffin Island and West Greenland (BIWG) that span a range of compositions from depleted to relatively enriched in incompatible trace elements [Starkey, N.A., Stuart, F.M., Ellam, R.M., Fitton, J.G., Basu, S., Larsen, L.M., 2009. Helium isotopes in early Iceland plume picrites: constraints on the composition of high 3He/4He mantle. Earth and Planetary Science Letters 277, 91–100]. This association has put into question the prevailing models in which high 3He/4He resides in either unprocessed, undegassed mantle or in depleted residues of melting. Whereas helium isotopes are measured in gas extracted from melt inclusions, radiogenic isotopes and incompatible trace elements are measured in whole-rock powders. The possibility exists, therefore, that some helium-rich melts could be sampled in melt volumes that are too small to affect the bulk lithophile element composition of an erupted magma but which are rich enough in helium to dominate its helium-isotope composition. In this case high 3He/4He would appear to reside in a range of mantle compositions.We have tested this possibility by analysis of major and trace elements in olivine-hosted melt inclusions in high-3He/4He BIWG picrites. We find no evidence for the existence of depleted melt inclusions in the enriched picrites, or enriched inclusions in the depleted picrites. REE profiles of melt inclusions in individual samples are parallel to each other and to that of their respective whole-rock. These data support the conclusion that high 3He/4He is present in a wide range of mantle source compositions. No model in which the BIWG picrites originate in a discrete, high-3He/4He mantle source can account for these new data. We propose that the primordial He in the early Iceland plume picrites must originate in a separate reservoir that has a sufficiently high helium concentration to allow it to impose its high 3He/4He equally on the enriched and depleted mantle sources of the BIWG picrites.

Turbulent mixing in the Amazon River: The isotopic memory of confluences. Earth and Planetary Science Letters, 290 (2010), pp. 37–43

Turbulent mixing in the Amazon River: The isotopic memory of confluences. Earth and Planetary Science Letters, 290 (2010), pp. 37–43

Revisiting the magnetostratigraphy of the Central Atlantic Magmatic Province (CAMP) in Morocco

The origin of the Triassic–Jurassic (Tr–J) mass extinction is still a matter of debate: proponents of the idea that continental flood basalts of the Central Atlantic Magmatic Province (CAMP) are responsible for the crisis are opposed by those who favor an extraterrestrial origin linked to the impact of meteorite. Principal limitations reside in the difficulty to date and correlate CAMP lavas with the marine realm turnover. One argument widely used to suggest that CAMP lavas pre-dated the Tr–J boundary in Morocco is based on the presence of two brief magnetic reversals in the intermediate units of the Tiourjdal and Oued Lahr sections (Morocco) that were correlated to the E23r chron from the Newark basin and to the SA5n.2r/3r and SA5r chrons of the Saint Audrie Bay [Knight, K.B., Nomade, S., Renne, P.R., Marzoli, A., Betrand, H., Youbi, N., 2004. The Central Atlantic Magmatic Province at the Triassic–Jurassic boundary: paleomagnetic and 40Ar/ 30Ar evidence from Morocco for brief, episodic volcanism. Earth and Planetary Science Letters 228, 143–160]. However the primary origin for these negative (reverse) magnetic components is questionable since no field or reversal test was provided to constrain the primary character of the remanence as well as because the small number of samples. Here we have conducted a detailed paleomagnetic and magnetic mineralogy study of the interbedded limestones of the Tiourjdal section and of other CAMP lavas sections where the intermediate unit is complete, namely the Tizi El Hajaj, Jbel Imzar and Aït Ourir sections, to better constrain the origin and stratigraphic location of these negative magnetic components. We show that the interbedded limestones of the Tiourjdal section were entirely remagnetized by chemical processes via acid and oxidizing hydrothermal fluids generated by eruptions of CAMP lavas. In addition, magnetostratigraphic data of the Tizi El Hajaj, Jbel Imzar and Aït Ourir sections show that the entire intermediate unit encompassed a positive (normal) magnetic interval. A good quality paleomagnetic pole for the CAMP lava in Morocco is then provided (Plat = 60.0°; Plong = 241.6°; A95 = 2.6; N = 99) that is now in better agreement with its trans-Atlantic counterpart.

Comment on ‘Conduit convection, magma mixing, and melt inclusion trends at persistent degassing volcanoes’ by Fred Witham, published in Earth Planetary Science Letters (2011) 301, 345–352

► We discuss the influence of magma mixing on the CO 2 -H 2 O behavior in magmas. ► Witham ‘s model does not fit our melt inclusion data set for Mt Etna and Stromboli. ► Magma fluxing by a deep derived CO 2 -rich gas better explains the CO 2 -H 2 O evolution.