Description: Publication date: 15 February 2014 Source: Earth and Planetary Science Letters, Volume 388 Author(s): Matthew J. Kohn Mineral inclusions may develop residual pressures ( P incl ) during cooling and exhumation because of differential expansion and compression relative to the host entrapping mineral. Shifts to Raman spectra, as determined using laser Raman microspectroscopy, permit inference of P incl . Knowing the pressure–temperature ( P – T ) dependence of molar volumes from thermodynamic databases and the shear modulus of the host phase, entrapment P at assumed T or entrapment T at assumed P can be calculated for a measured P incl . This concept is developed theoretically to fit simplified empirical expressions of entrapment P ( T , P incl ) or T ( P , P incl ) for a wide range of common mineral host–inclusion pairs. Typical residuals are 〈200 bar for barometers and 〈5 °C for thermometers. Barometric slopes are flat (0–15 bar/°C), and even large measurement errors of ±0.5 cm −1 propagate to barometric precisions that are small (c. ±300 bar) for quartz inclusions in numerous silicates, apatite, oxides and pyrite, and acceptable for graphite in garnet (±2 kbar) and for garnet, olivine, and coesite in diamond (±3 kbar). Thermometric slopes are steep (40–400 bar/°C), but highly resolved measurements (at least ±0.1 cm −1 ) are needed for thermometric precisions better than ±30 °C for zircon and rutile inclusions in numerous silicates and for spinel in olivine. Applications to published data recover peak entrapment P ʼs in some instances, but imply higher or lower P ʼs than originally assumed in others. Disparities may arise from sample preparation, differences in conditions of mineral entrapment vs. final equilibration, or reequilibration of the host mineral via chemical diffusion, viscous flow or fracturing. Polymorphic transitions (e.g. coesite quartz, diamond graphite) can preclude recovery of entrapment P ʼs except for very high P ʼs or rapid exhumation. Graphical abstract

Description: Publication date: 15 February 2014 Source: Earth and Planetary Science Letters, Volume 388 Author(s): Arno Rohrbach , Sujoy Ghosh , Max W. Schmidt , Clazina H. Wijbrans , Stephan Klemme The Earthʼs mantle contains significant amounts of carbon and is at depths greater than ∼250–300 km potentially so reducing that the Fe–C redox couple determines the nature of the reduced phase(s), which may be diamond, metal and carbides. Carbides will be Fe-rich but their stability also depends on the presence of Ni. We thus have experimentally investigated the Fe–Ni–C subsolidus ternary at 10 GPa, and secondly determined eutectic melting temperatures in this system. At subsolidus, the Fe-rich side of the ternary has two of the phases: diamond, Fe 7 C 3 (to a molar X Ni = Ni / ( Fe + Ni ) = 0.11 ), Fe 3 C (to X Ni = 0.24 ) and metal stable, depending on bulk C-contents. At higher Ni-contents, (Fe, Ni) 3 C coexists with diamond and metal while at X Ni ⩾ 0.53 , carbides are absent and diamond coexists with metal. Because Ni is more noble than Fe, it partitions strongly into the reduced phases such that at low metal fractions the metal phase reaches X Ni > 0.5 (at a bulk Ni-content of 1800 ppm for the mantle). Thermodynamic calculations at subsolidus conditions suggest that the mantle contains 50–700 ppm Fe, Ni metal at ∼300 km depth. Adopting bulk C contents of 50 to 500 ppm in the mantle would result in the phase association (Fe, Ni) 3 C + metal + diamond (at 10 GPa). An unexpected finding of this study is that eutectic temperatures in the Fe–Ni–C system are very low, 1210 °C at the Fe–C side, decreasing to 1125 °C at an X Ni of 0.5 in the reduced phase. Hence we postulate that most of the deep reducing mantle will contain a small Fe–Ni–C melt fraction. These melts should be ubiquitous in the mantle, only those mantle regions where C-contents are less than what can be dissolved in the solid metal (50 ppm at 400 km depth) would not contain such a melt phase. However, the presence of a metal–carbon melt phase is probably of little long term consequence to mantle geochemistry as this melt is expected to remain in isolated pockets.

Description: Publication date: 15 February 2014 Source: Earth and Planetary Science Letters, Volume 388 Author(s): Rong Ren , Bao-Fu Han , Zhao Xu , Yin-Zhang Zhou , Bo Liu , Lei Zhang , Jia-Fu Chen , Li Su , Jiao Li , Xian-Hua Li , Qiu-Li Li We report here a Cambrian southward-subducting intra-oceanic arc system in the southern West Junggar, NW China, where an immature arc occurred initially on SSZ-type ophiolites and finally evolved into a more mature one. The immature arc is dominantly represented by Early–Middle Cambrian (∼510 Ma) low-K tholeiitic felsic rocks, whereas the mature arc is characterized by Late Cambrian (∼495 Ma) medium- and high-K calc-alkaline felsic and mafic rocks. The SSZ-type ophiolites show remarkable depletion of Nb and Ta and contain high-Cr spinel (Cr# > 0.6), resembling these formed in the forearc. Altogether, they record the initiation of subduction and transformation of crust during early subduction of the Paleo-Asian Ocean in the southern part of the Central Asian Orogenic Belt (CAOB). The subduction initiation might occur in the Early Cambrian (>515 Ma), as constrained by both the SSZ-type ophiolites (516 Ma) and the oldest arc plutons (515–509 Ma) that crosscut the ophiolites. The immature felsic plutons have high SiO 2 (>72 wt%) contents and variable MgO (0.42–1.49 wt%) and Mg# values (22–62). Crustal anatexis may be responsible for the genesis of those plutons and thus the transformation from oceanic to continental crust. These results, combined with regional data, convincingly indicate that it is one of the oldest intra-oceanic arc systems in the southern CAOB, which may mark the initial subduction of the Paleo-Asian Ocean in its southern part, much later than those reported in the north. An archipelago-type model is proposed for the evolution of the southern West Junggar and has implications for the development of the southern part of the CAOB.

Description: Publication date: 15 February 2014 Source: Earth and Planetary Science Letters, Volume 388 Author(s): X.M. Pellicer , R. Linares , F. Gutiérrez , X. Comas , C. Roqué , D. Carbonel , M. Zarroca , J.A.P. Rodríguez The Isona tufa mound complex (ITMC), associated with artesian springs of the Areny-Montsec aquifer, Spanish Pyrenees, is a potential analog for water constructed landforms on Mars. We used Ground Penetrating Radar (GPR), trenching, sedimentological description of exposures, and radiocarbon and U-series dating methods for the geological characterization of the ITMC. Preliminary geomorphological mapping combined with sedimentological analyses permitted the recognition of the different facies and their spatial distribution. GPR surveys conducted next to an outcrop and a trench provided electromagnetic wave velocity in tufas (0.09 and 0.11 m ns −1 ) and determined the correspondence of the radar signatures with facies types. This was used to reconstruct the tufas internal structure and the depositional stages for two different contexts: (1) an upper unit representing the morpho-stratigraphic record of paleosprings – Tufa 1 – composed of relict tufa mounds older than 350 ka BP; and (2) a lower unit – Tufa 3 – associated with groundwater aquifer outlets (Basturs Lakes). The GPR data allowed depicting the signatures for the vent, pool, rimstone, palustrine, dam, cascade and slope facies. A relationship was inferred between the age of the tufas and the radar signature, in terms of relative amplitude and signal attenuation. Older dry tufas with advanced diagenesis and karstification are characterized by well-defined GPR reflectors and lower attenuation than younger tufas, associated with aquifer discharge and shallower water tables. U-series and radiocarbon ages obtained from the Basturs Lakes tufas indicate that these have been active since 106 ka BP during both cold and mild Marine Isotopic Stages (MIS). We hypothesize that tufas related to the deep-seated Areny-Montsec confined karst aquifer were insensitive to climate variations. Landforms reminiscent of the ITMC have been detected during the last decade on Mars. Since GPR will be part of the ExoMars Rover of the European Space Agency (ESA) mission projected for 2018, we anticipate that our results may be able to constrain the interpretation of landforms possibly related to water on Mars.

Description: Publication date: 15 February 2014 Source: Earth and Planetary Science Letters, Volume 388 Author(s): James F.J. Bryson , Nathan S. Church , Takeshi Kasama , Richard J. Harrison Nanoscale intergrowths unique to the cloudy zones (CZs) of meteoritic metal display novel magnetic behaviour with the potential to reveal new insight into the early development of magnetic fields on protoplanetary bodies. The nanomagnetic state of the CZ within the Tazewell IIICD iron meteorite has been imaged using off-axis electron holography. The CZ is revealed to be a natural nanocomposite of magnetically hard islands of tetrataenite (ordered FeNi) embedded in a magnetically soft matrix of ordered Fe 3 Ni. In the remanent state, each tetrataenite island acts as a uniaxial single domain particle with its [001] magnetic easy axis oriented along one of three 〈100〉 crystallographic directions of the parent taenite phase. Micromagnetic simulations demonstrate that switching occurs via the nucleation and propagation of domain walls through individual tetrataenite particles. The switching field (H s ) varies with the length scale of the matrix phase (L m ), with H s > 1 T for L m ∼10 nm (approaching the intrinsic switching field for isolated single domain tetrataenite) and 0.2 〈 H s 〈 0.6 T for L m ∼30 nm. The reduction in H s with increasing L c is caused by exchange coupling between the hard tetrataenite islands and the soft magnetic matrix, which lowers the critical field for domain wall nucleation, providing an explanation for previously observed coercivity variations throughout the CZ. Non-random distributions of the tetrataenite easy axes are observed locally throughout the CZ, suggesting a magnetic field could have been present during nanostructure formation. This observation demonstrates the potential for stable chemical transformation remanent magnetisation to be encoded by the nanostructure, with variations in the proportions of the six possible magnetisation states reflecting the intensity and relative direction of the magnetic fields present during cooling. According to recent cooling models, the cooling rate of meteoritic metal originating near the surface of differentiated planetesimals was such that the magnetic signal across the CZ could potentially record dynamo field intensity and direction variations over time (10–100 Ma), which would enable events such as magnetic reversals and the decay of an asteroid dynamo to be observed.

Description: Publication date: 15 February 2014 Source: Earth and Planetary Science Letters, Volume 388 Author(s): Catherine A. Rychert , Nicholas Harmon , Cynthia Ebinger The Galápagos Archipelago represents an opportunity to investigate the properties of young oceanic lithosphere, the effects of a hotspot anomaly on lithospheric thickness, and melting dynamics in a hotspot-ridge interaction. Here we use data recorded by the SIGNET array and permanent station PAYG on the Islands Santa Cruz and Isabela, respectively. We used P-to-S (Ps) and S-to-P (Sp) receiver functions to constrain crust and mantle structure. A simultaneous deconvolution method was used to constrain 1-D structure and also for the modeling of robust features. A migrated extended multitaper method was used to investigate 3-D structural variations. Ps images a velocity increase with depth at 11 ± 7   km , probably the base of the pre-plume crust, or old Moho. Sp imaging and modeling images a second, deeper velocity increase at 37 ± 7   km depth. A velocity decrease with depth is imaged on average at 75 ± 12   km likely associated with the lithosphere–asthenosphere boundary. This discontinuity is imaged deeper, 82 km, in the southwest and shallower, 66 km, in the northeast near the spreading ridge. Although the trend is consistent with lithospheric thickening with age, the thickness is much larger than predicted by conductive cooling models of 5–10 My oceanic lithosphere. We infer a compositional contribution to velocity variations. Finally, a velocity increase with depth is imaged at ∼125 to 145 ± 15   km depth that is likely associated with the onset of melting. The discontinuity is imaged deeper in 3 sectors of the Galápagos platform-ridge region, all coincident with the slowest surface wave shear velocity anomalies in the upper 100 km. One is located in the southwest in a hypothesized plume location. The other two are to the northwest and northeast, possibly illuminating multiple plume diversions related to complex plume–ridge interactions.

Description: Publication date: 1 February 2014 Source: Earth and Planetary Science Letters, Volume 387 Author(s): L.A. Berbesi , R. di Primio , Z. Anka , B. Horsfield , H. Wilkes The immense mass of organic carbon contained in sedimentary systems, currently estimated at 1.56 × 10 10 Tg ( Des Marais et al., 1992 ), bears the potential of affecting global climate through the release of thermally or biologically generated methane to the atmosphere. Here we investigate the potential of naturally-occurring gas leakage, controlled by petroleum generation and degradation as a forcing mechanism for climate at geologic time scales. We addressed the potential methane contributions to the atmosphere during the evolution of petroleum systems in two different, petroliferous geological settings: the Western Canada Sedimentary Basin (WCSB) and the Central Graben area of the North Sea. Besides 3D numerical simulation, different types of mass balance and theoretical approaches were applied depending on the data available and the processes taking place in each basin. In the case of the WCSB, we estimate maximum thermogenic methane leakage rates in the order of 10 − 2 – 10 − 3 Tg / yr , and maximum biogenic methane generation rates of 10 − 2 Tg / yr . In the case of the Central Graben, maximum estimates for thermogenic methane leakage are in the order in 10 − 3 Tg / yr . Extrapolation of our results to a global scale suggests that, at least as a single process, thermal gas generation in hydrocarbon kitchen areas would not be able to influence climate, although it may contribute to a positive feedback. Conversely, only the sudden release of subsurface methane accumulations, formed over geologic timescales, can possibly allow for petroleum systems to exert an effect on climate.

Description: Publication date: 15 February 2014 Source: Earth and Planetary Science Letters, Volume 388 Author(s): Kate Selway , Jun Yi , Shun-Ichiro Karato Hydrogen strongly influences mantle rheology and is therefore an important factor in the growth and stability of cratons. Hydrogen also strongly affects electrical conductivity so it is possible to infer the hydrogen content of the lithospheric mantle in-situ and test models of craton formation using magnetotelluric data. Tanzania is an ideal natural laboratory to test hypotheses on lithospheric hydrogen content since it contains regions with very different tectonic regimes including the stable Tanzanian Craton and the East Africa Rift that is reworking lithosphere previously deformed in the East African Orogeny. Additionally, the lithosphere is well sampled by voluminous xenoliths that constrain lithospheric composition and the geotherm, which also affect electrical conductivity. Hydrogen contents were calculated for two locations in Tanzania: the first in the stable central Tanzanian Craton and the second on the eastern margin of the craton where incipient rifting is occurring. The central Tanzanian Craton was found to have a high lithospheric mantle water content of ∼10 −2 wt% which is comparable to that of the oceanic asthenosphere and is hard to reconcile with the long-term survival of the craton. It is possible that the water was introduced into the lithosphere recently by kimberlite volcanism or that, if the lithosphere has had a high water content throughout its history, the central craton has been shielded from deformation by weaker orogens that surround it. The eastern margin of the craton has a water content of 10 −3 to 10 −4 wt% throughout much of the lithospheric mantle that decreases to 10 −4 to 10 −5 wt% at the base of the lithosphere and at depths corresponding to the uppermost plume head. Xenolith data show evidence for partial melting of the plume head and the base of the lithosphere in this dehydrated region. The partial melting and dehydration of a plume head beneath a craton is a present-day observation of the processes that may have formed cratonic roots.

Description: Publication date: 15 February 2014 Source: Earth and Planetary Science Letters, Volume 388 Author(s): Luca Caricchi , Juliet Biggs , Catherine Annen , Susanna Ebmeier Deformation of volcanic edifices is typically attributed to the movement of magma within the volcanic plumbing system, but a wide range of magmatic processes are capable of producing significant volume variations and may also produce deformation. In order to understand the evolution of magmatic systems prior to eruption and correctly interpret monitoring signals, it is necessary to quantify the patterns and timescales of surface deformation that processes such as crystallisation, degassing and expansion of the hydrothermal system can produce. We show how the combination of petrology and thermal modelling can be applied to geodetic observations to identify the processes occurring in a magmatic reservoir during volcanic unrest. Thermal modelling and petrology were used to determine the timescales and volumetric variations associated with cooling, crystallisation and gas exsolution. These calculations can be performed rapidly and highlight the most likely processes responsible for the variation of a set of monitoring parameters. We then consider the magnitude and timescales of deformation produced by other processes occurring within the vicinity of an active magma system. We apply these models to a time series of geodetic data spanning the period between the 1997 and 2008 eruptions of Okmok volcano, Aleutians, examining scenarios involving crystallisation, degassing and remelting of the crystallising shallow magmatic body and including a viscoelastic shell or hydrothermal system. The geodetic observations are consistent with the injection of a water-saturated basalt, followed by minor crystallisation and degassing. Other scenarios are not compatible either with the magnitude or rate of the deformation signals.

Description: Publication date: 1 February 2014 Source: Earth and Planetary Science Letters, Volume 387 Author(s): D. Chavrit , E. Humler , O. Grasset Quality criteria have been used to select ∼400 vesicularity measurements on zero-age mid-ocean ridge glasses from ∼600 data available in the literature published over the past ∼30 years. At face value, observations show that for a given depth of sampling, enriched basalts from slow spreading ridge segments are more vesicular than those from depleted and intermediate or fast spreading ridges. A shallower depth of eruption enhances these effects because lower hydrostatic pressure favours bubble expansion. In order to get an insight into these complex and intermingled processes, we used empirical and semi-quantitative approaches based on a limited number of inputs (segment depth D, spreading rate τ and K 2 O/TiO 2 ratios). Both models give equivalent results and predict vesicularities within ±50%. From these calculations, we compute the equivalent CO 2 concentration at the depth of eruption all along the oceanic ridge system. The total calculated CO 2 fluxes are low ranging from 6.5 ± 1.8 to 8.7 ± 2.8 × 10 11   mol / yr and the CO 2 mantle content displays large variabilities from 66 − 19 + 27 to 78 − 40 + 82   ppm . In order to test these results, the mantle 3 He fluxes have been evaluated using the calculated CO 2 fluxes and a CO 2 / 3 He ratio of 2.2 × 10 9 . These fluxes range from 295 ± 82 to 395 ± 127   mol / yr and are close to the values reported by Jean-Baptiste (1992) (267–534 mol/yr) and the most recent estimate ( Bianchi et al., 2010 , ∼ 527 ± 102   mol / yr ) using box-model of the three main ocean basins constrained by measurements of 3 He and radiocarbon data. As these independent methods give similar helium fluxes at regional and global scales, it provides strong support to a low and heterogeneous mantle carbon concentration and distribution. Finally, the calculated volcanic CO 2 emissions at oceanic ridges correspond to ∼30 seconds of anthropogenic emissions, at current rates.