Description: Publication year: 2012 Source: Earth and Planetary Science Letters, Volumes 317-318, 1 February 2012, Pages 331-342 Aleksandra M. Mloszewska, Ernesto Pecoits, Nicole L. Cates, Stephen J. Mojzsis, Jonathan O'Neil, ... The composition of iron formations in the ≥ 3.75 Ga yr old Nuvvuagittuq Supracrustal Belt in northern Québec provides a proxy for seawater composition of the Eoarchean, and perhaps Hadean oceans, as well as constraints on the types of nutrients available to Earth's earliest life forms. Integrated petrologic and geochemical relationships, mapped between mineral phases in thin section and whole-rock chemistry, provide a framework for interpreting bulk and micro-scale variations in these chemical sedimentary precipitates. Results show that there are two distinct chemical sedimentary units in the Nuvvuagittuq belt: i) a banded iron formation (BIF) consisting of alternating micro-bands of magnetite, Ca–Mg–Fe-silicates and quartz, and ii) a more silicate-rich (Fe-poor) unit, the banded silicate-formation (BSF), of alternating micro-bands of quartz and Ca–Mg–Fe silicates. Precursor BIF and BSF deposits were likely layered amorphous silica and ferric-oxyhydroxides, fine-grained carbonate oozes and/or Ca–Mg–Fe rich silicate gels deposited in a marine setting. Low Al2O3, TiO2and HFSE concentrations show that they are relatively detritus-free, with distinctively seawater-like REE + Y profiles and consistently positive Eu anomalies. These features suggest that the rocks preserved their seawater-like compositions despite metamorphic overprinting. The most significant trace elements in the sediments are Ni and Zn. Experimentally-derived partitioning coefficients show that Ni was enriched in Eoarchean seawater as compared to today (up to 300 nM), while Zn was fairly similar (up to 20 nM). Compositional resemblances between the Nuvvuagittuq sediments and those documented in the ca. 3.8 Ga Isua supracrustals (West Greenland) provide a plausible case that global ocean processes – in terms of trace metal abundances – had reached steady-state by the Eoarchean. Highlights ► We examine the composition of chemical sediments of the ≥ 3.75 Ga Nuvvuagittuq belt. ► These chemical sediments are relatively pure, with seawater-like REE + Y signatures. ► Thus, they can be considered as potential proxies for Eoarchean seawater composition. ► The most significant trace metals in these sediments are Ni and Zn. ► Our results imply that compared to modern seawater Ni was high and Zn fairly similar.

Description: Publication year: 2012 Source: Earth and Planetary Science Letters, Volumes 317-318, 1 February 2012, Pages 305-318 Adriano Mazzini, Giuseppe Etiope, Henrik Svensen The 29th of May 2006 gas and mud eruptions suddenly appeared along the Watukosek fault in the north east of Java, Indonesia. Within a few weeks several villages were submerged by boiling mud. The most prominent eruption site was named Lusi. To date (November 2011) Lusi is still active and a ~ 7 kmarea is covered by the burst mud breccia.The mechanisms responsible for this devastating eruption remain elusive. While there is consensus about the origin of the erupted mud, the source of water is uncertain, the origin of the gas is unknown and the trigger of the eruption is still debated. In order to shed light on these unknowns, we acquired a wide set of data of molecular and isotopic composition of gas sampled in several Lusi vents, in the surrounding mud volcanoes, in the closest natural gas field (Wunut), and in the hydrothermal vents at the neighbouring volcanic complex in the period 2006–2011.The boiling fluids erupted in the crater zone are apparently CO2-dominated, while colder CH4-dominated and C2–C3bearing fluids are identified at several sites around the crater zone. Gas genetic diagrams, maturity plots and gas generation modelling suggest that the hydrocarbons are thermogenic (δC1up to − 35‰; δC2up to − 20‰), deriving from marine kerogen with maturity of at least 1.5%Ro, for instance in the ~ 4400 m deep Ngimbang source rocks. CO2released from the crater and surrounding seeps is also thermogenic (δC from − 15 to − 24‰) related to kerogen decarboxylation or thermal CH4oxidation in deep rocks, although three vents just outside the crater showed an apparent inorganic signature (− 7.5‰ 〈 δC = − 0.5‰) associated to mantle helium (R/Ra up to 6.5). High CO2–CH4equilibrium temperatures (200–400 °C) are typical of thermally altered hydrocarbons or organic matter. The data suggest mainly thermally altered organic sources for the erupted gases, deeper sourced than the mud and water (Upper Kalibeng shales). These results are consistent with a scenario of deep seated (〉 4000 m) magmatic intrusions and hydrothermal fluids responsible for the enhanced heat that altered source rocks and/or gas reservoirs.The neighbouring magmatic Arjuno complex and its fluid–pressure system combined with high seismic activity could have played a key role in the Lusi genesis and evolution. Within this new model framework, Lusi is better understood as a sediment-hosted hydrothermal system rather than a mud volcano. Highlights ► Gas from Lusi eruption shows that CO 2 and CH 4 have a deep thermogenic origin. ► Thermally altered Ngimbang source rocks (〉 4400 m depth) could generate the erupted gas. ► Lusi hydrocarbons derive from the Ngimbang–Kujung petroleum system. ► Mantle He from Lusi suggests deep magmatic intrusions from Arjuno–Welirang volcano. ► Lusi is not a mud volcano but rather a sediment-hosted hydrothermal system.

Description: Publication year: 2012 Source: Earth and Planetary Science Letters, Volumes 317-318, 1 February 2012, Pages 319-330 Gary R. Eppich, Kari M. Cooper, Adam J.R. Kent, Alison Koleszar Uranium-series crystal ages, interpreted within a textural and geochemical framework, can provide insight into crystal storage timescales, especially in cases where crystals may derive from multiple sources. We report hereTh–Ra model ages of two distinct populations of plagioclase from low silica dacites from Mount Hood, Oregon, a volcano where previous studies show that the compositions of erupted magmas are controlled by magma recharge, mixing, and incorporation of plagioclase derived from mafic and silicic end-member magmas. We have measured trace element concentrations andU–Th–Ra disequilibria in four plagioclase size fractions from the Timberline (1500 a) and Old Maid (215 a) eruptive sequences. After correction for groundmass and apatite contamination, averageTh–Ra model ages of large (〉 500 μm) plagioclase are 〉 4.5 ka (Timberline) and 〉 5.5 ka (Old Maid), with ages of cores that are 〉 10 ka in each case, indicating that plagioclase derived from silicic magmas crystallized thousands of years before eruptions. These model ages are longer than timescales of repose between eruptions, indicating that these crystals resided in the sub-surface over multiple eruptions, likely stored in a silicic crystal mush zone that periodically interacts with mafic recharge magmas, remobilizing a fraction of the large plagioclase crystals during each eruptive event. After correction for large plagioclase contamination, small (〈 500 μm) plagioclase, derived from mafic magmas, have high (Ra)/Ba relative to equilibrium with liquid proxies (groundmass and mafic inclusion), leading toTh–Ra model ages that are 〈~3 ka for Old Maid and undefined for Timberline separates. However, the preservation of significantTh–Ra disequilibria require that the majority of crystals in the separate are young (〈〈10 ka). The high (Ra)/[Ba] could potentially be explained by rapid crystallization immediately prior to and/or during mixing events, consistent with evidence of rapid crystallization of rims. Rapid crystallization of mafic intrusions may trigger eruption at Mount Hood by producing a partially-crystalline mafic magma capable of mixing with a reheated silicic crystal mush. Highlights ► NewTh–Ra model ages for two populations of plagioclase from Mt Hood magmas.► Large crystals have average ages 〉 4.5 ka, and core ages 〉 10 ka.► Small crystals have average ages 〈 3 ka and may have crystallized rapidly.► Plagioclase stored for 〉 10 ka is remobilized during mixing events.► Mixing likely triggered eruption which may be characteristic of andesitic magmas.

Description: Publication year: 2012 Source: Earth and Planetary Science Letters, Volumes 317-318, 1 February 2012, Pages 282-294 Torben Stichel, Martin Frank, Jörg Rickli, Brian A. Haley We present the first combined dissolved hafnium (Hf) and neodymium (Nd) concentrations and isotope compositions of deep water masses from the Atlantic sector of the Southern Ocean. Eight full depth profiles were analyzed for Hf and twelve for Nd. Hafnium concentrations are generally depleted in the upper few hundred meters ranging between 0.2 pmol/kg and 0.4 pmol/kg and increase to relatively constant values of around 0.6 pmol/kg in the deeper water column. At the stations north of the Polar Front (PF), Nd concentrations increase linearly from about 10 pmol/kg at depths of ~ 200 m to up to 31 pmol/kg close to the bottom indicating particle scavenging and release. Within the Weddell Gyre (WG), however, Nd concentrations are essentially constant at 25 pmol/kg at depths greater than ~ 1000 m. The distributions of both elements show a positive correlation with dissolved silicon implying a close linkage to diatom biogeochemistry.Hafnium essentially shows invariant isotope compositions with values averaging at εHf = + 4.6, whereas Nd isotopes mark distinct differences between water masses, such as modified North Atlantic Deep Water (NADW, εNd = − 11 to − 10) and Antarctic Bottom Water (AABW, εNd = − 8.6 to − 9.6), but also waters locally advected via the Agulhas Current can be identified by their unradiogenic Nd isotope compositions. Mixing calculations suggest that a small fraction of Nd is removed by particle scavenging during mixing of water masses north of the PF. Nevertheless, the Nd isotope composition has apparently not been significantly affected by uptake and release of Nd from particles, as indicated by mixing calculations. A mixing envelope of an approximated North Pacific and a North Atlantic end-member shows that Nd isotope and concentration patterns in the Lower Circumpolar Deep Water (LCDW) can be fully explained by ~ 30:70 percentage contributions of these respective end-members. Highlights ► We present the first combined Hf and Nd isotope data from the Southern Ocean. ► Concentration patterns suggest that Hf and Nd are governed by remineralization. ► Hf isotopes are invariant, whereas Nd isotopes label distinct water masses. ► Mixing calculations show that Nd isotopes are largely governed by water mass mixing. ► Nd isotope composition of AABW suggests influences by the Antarctic continent.

Description: Publication year: 2012 Source: Earth and Planetary Science Letters, Volumes 317-318, 1 February 2012, Pages 295-304 A. Bekker, H.D. Holland During the Lomagundi Event, ca. 2.22 to 2.06 Ga, marine carbonates recorded the largest and longest uninterrupted positive carbon isotope excursion, the earliest extensive marine sulfate evaporites were deposited, and the average ferric iron to total iron (expressed as Fe2O3/∑Fe|Fe2O3|) ratio of shales increased dramatically. At the end of the Lomagundi Event, the first economic sedimentary phosphorites were deposited, and the carbon isotope values of marine carbonates returned to ~ 0‰ VPDB. Thereafter marine sulfate evaporites and phosphorites again became scarce, while the average Fe2O3/∑Fe|Fe2O3|ratio of shales decreased to values intermediate between those of the Archean and Lomagundi-age shales. We propose that the large isotopic and chemical excursions during the Lomagundi Event were caused by a positive feedback between the rise of atmospheric O2, the weathering of sulfides in the pre-2.3 Ga continental crust, and the flux of phosphate to the oceans (cf. Holland, 2002). The rise in the terrestrial phosphate flux led to an increase in the burial rate of organic carbon and a major transfer of oxygen from the carbon to the sulfur cycle.The end of the Lomagundi Event was probably caused by a decrease in the terrestrial phosphate flux related to the weathering of low-pyrite sediments that were deposited during the Lomagundi Event. The rate of deposition of organic matter and the precipitation of sulfate evaporites decreased, the isotopic and chemical excesses of the Lomagundi Event were eliminated, and the ocean-atmosphere system entered the period frequently called the Boring Billion. Highlights ► The Lomagundi Event (LE) is linked to the atmospheric oxygenation. ► Continental weathering of sulfides increased acidity and P flux to the oceans. ► High P flux increased C org burial and led to oxygen transfer to the sulfur cycle. ► Terrestrial P flux later decreased due to weathering of low-pyrite LE sediments. ► Organic matter and sulfate evaporite burial decreased after the LE.

Description: Publication year: 2012 Source: Earth and Planetary Science Letters, Volumes 317-318, 1 February 2012, Pages 136-144 Jun Kameda, Shoko Hina, Kyoko Kobayashi, Asuka Yamaguchi, Yohei Hamada, ... The diagenesis and deformation processes of ribbon cherts embedded in a Jurassic accretionary complex, central Japan, were investigated in detail to gain a better understanding of the mechanical behavior of the plate boundary at depth in cold subduction zones. The analyzed cherts record two stages of deformation: (1) map- to outcrop-scale ductile folding, and (2) subsequent brittle faulting. The ductile deformation was facilitated by silica dehydration–precipitation, and is represented by multiple phases of vein networks. The folds are cut by brittle faults, indicating lithification and the concurrent mechanical transition from ductile to brittle behavior. Slip zones along the faults are typically filled with brecciated chert in a chlorite matrix. Geothermometry analysis of the matrix chlorite suggests that faulting occurred following the completion of opal-CT to quartz transition reaction. This is also confirmed by the kinetic simulation of silica conversion reactions. The results suggest that ductile deformation of thick pelagic deposits with abundant fluids results in an aseismic plate boundary, whereas chemical diagenesis of the deposits, producing crystalline cherts, results in interplate coupling in cold subduction zones such as the Japan Trench. Highlights ► Ribbon cherts record diagenesis and deformation processes in cold subduction zones. ► Opal-CT to quartz transition occurs at ~ 50–130 °C along the plate boundary. ► Chert lithification results in transition from brittle to ductile deformation. ► Chert diagenesis may define the up-dip limit of seismicity in cold subduction zones.

Description: Publication year: 2012 Source: Earth and Planetary Science Letters, Volumes 317-318, 1 February 2012, Pages 196-203 Andrea Bizzarri In this paper we consider a wide catalog of synthetic earthquakes, numerically modeled as spontaneous, fully dynamic, 3-D ruptures on extended faults, governed by different friction laws, including slip-dependent and rate- and state-dependent equations. We analyze the spatial correlations between the peak of fault slip velocity (vpeak) and the rupture speed (vr) at which the earthquake spreads over the fault. We found thatvpeakpositively correlates withvrand that the increase ofvpeakis roughly quadratic. We found that near the transition between sub- and supershear regimesvpeaksignificantly diminishes and then starts to increase again with the square ofvr. This holds for all the governing models we consider and for both homogeneous and heterogeneous configurations. Moreover, we found that, on average,vpeakincreases with the magnitude of the event (vpeak ~ M0). Our results can be incorporated as constraints in the inverse modeling of faults. Highlights ► Rupture speed and peak in fault slip velocity positively correlates. ► The governing law does not alter the correlation. ► Peak slip velocity correlates with seismic moment. ► The spatial can be implemented as constraints in kinematic modeling of faults.

Description: Publication year: 2012 Source: Earth and Planetary Science Letters, Volumes 317-318, 1 February 2012, Pages 76-84 Xiao Zhang, Matthias Prange, Silke Steph, Martin Butzin, Uta Krebs, ... The early Pliocene warm phase was characterized by high sea surface temperatures and a deep thermocline in the eastern equatorial Pacific. A new hypothesis suggests that the progressive closure of the Panamanian seaway contributed substantially to the termination of this zonally symmetric state in the equatorial Pacific. According to this hypothesis, intensification of the Atlantic meridional overturning circulation (AMOC) – induced by the closure of the gateway – was the principal cause of equatorial Pacific thermocline shoaling during the Pliocene. In this study, twelve Panama seaway sensitivity experiments from eight ocean/climate models of different complexity are analyzed to examine the effect of an open gateway on AMOC strength and thermocline depth. All models show an eastward Panamanian net throughflow, leading to a reduction in AMOC strength compared to the corresponding closed-Panama case. In those models that do not include a dynamic atmosphere, deepening of the equatorial Pacific thermocline appears to scale almost linearly with the throughflow-induced reduction in AMOC strength. Models with dynamic atmosphere do not follow this simple relation. There are indications that in four out of five models equatorial wind-stress anomalies amplify the tropical Pacific thermocline deepening. In summary, the models provide strong support for the hypothesized relationship between Panama closure and equatorial Pacific thermocline shoaling. Highlights ► We study the effect of the Panama seaway on Pacific equatorial thermocline depth. ► Results from twelve model experiments are examined. ► Eastward net throughflow leads to a reduction in Atlantic overturning. ► We find a relationship between Panama closure and Pacific thermocline depth.

Description: Publication year: 2012 Source: Earth and Planetary Science Letters, Volumes 317-318, 1 February 2012, Pages 177-184 Martin Butzin, Matthias Prange, Gerrit Lohmann A critical problem in radiocarbon dating is the spatial and temporal variability of marineC reservoir ages. This is particularly true for the time scale beyond the tree-ring calibration range. Here, we propose a method to assess the evolution of marine reservoir ages during the last deglaciation by numerical modeling. We apply a self-consistent iteration scheme in which existing radiocarbon chronologies can be readjusted by transient, three-dimensional simulations of marine and atmospheric ΔC. To estimate the uncertainties regarding the ocean ventilation during the last deglaciation, we consider various ocean overturning scenarios which are based on different climatic background states. An example readjustingC data from the Caribbean points to marine reservoir ages varying between 200 and 900 a during the last deglaciation. Correspondingly, the readjustment leads to enhanced variability of atmospheric ΔC by ± 30‰, and increases the mysterious drop of atmospheric ΔC between 17.5 and 14.5 cal ka BP by about 20‰. Highlights ► We propose to assess glacial marineC reservoir ages by numerical modeling ► Caribbean reservoir ages may have varied from 200–900 a during the last deglaciation ► This may have increased the drop of atmospheric ΔC between 17.5 and 14.5 cal ka BP

Description: Publication year: 2012 Source: Earth and Planetary Science Letters, Volumes 317-318, 1 February 2012, Pages 157-164 Peter I. Nabelek, Anne M. Hofmeister, Alan G. Whittington We explore the conductive cooling rates of plutons and temperature-time paths of their wall rocks using numerical methods that explicitly account for the temperature dependence of thermal diffusivity (α) and heat capacity (CP). We focus onαbecause it has the strongest influence on the temperature-dependence of thermal conductivity (k = ρ·CP·α) at high temperatures. Latent heats of crystallization are incorporated into the models as apparent CP's. Two sets of models are presented, one for a 50 m thick basalt sill emplaced into rocks with diffusivity of the average crust, and one for a 5 km wide and 2 km thick granite pluton emplaced into dolostones. The sill's liquidus is 1230 °C, the solidus 980 °C, and the sill is emplaced into wall rocks that are at 150 °C. The pluton's liquidus is 900 °C, the solidus 680 °C, and the pluton is emplaced into wall rocks with initial geothermal gradient of 30 °C/km. The top of the pluton is at 3 km depth. Incorporating appropriateα = f(T) into calculations can more than double the solidification times of intrusions in comparison with incorporating constantαof 1 mm·sthat is used in most petrologic heat transport models. The instantaneously emplaced basalt sill takes ~ 51 a to completely solidify, whereas the granite pluton takes ~ 52 ka to completely solidify. The solidification time is longer because of low thermal diffusivity of magma and because wall rocks become more insulating as temperature rises.In contact aureoles, maximum temperatures reached withα = f(T) are somewhat lower in comparison withα = 1 mm·s; however, temperatures in inner aureoles stay elevated significantly longer withα = f(T), and consequently promote approach to mineralogical and textural equilibrium. The elevated temperature regime in inner aureoles enhances temperature gradients that may promote greater fluid fluxes if fluid pore pressures are controlled by temperatures. The results demonstrate the need to incorporate temperature-dependent transport properties of magmas and wall rocks into models of metamorphism and fluid flow in contact aureoles. Highlights ► Temperature-dependence of thermal diffusivity influences heat flow in magmatic systems. ► Duration of model magmatic regimes of plutons can double compared to previous models. ► Elevated temperatures in contact aureoles remain longer than in previous models.