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 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 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 56-67 Andy Nowacki, J.-Michael Kendall, James Wookey Observations of seismic anisotropy at oceanic spreading centres offer insights into mid-ocean ridge processes and the formation of new plates. Here, remote observations of seismic anisotropy beneath mid-ocean ridges are made using measurements of source-side shear wave splitting. Over 100 high-quality measurements are made using earthquakes that occur near mid-ocean ridges and transform faults, but are observed at teleseismic distances. In general, for off-axis ridge events, the polarisation of fast shear waves,ϕ″, is approximately parallel to the spreading direction. Nearer the ridge (50 km),ϕ″ becomes more scattered and is often ridge-parallel. Delay times,δt, tend to increase from 〈 1 s near the ridge axis to ∼ 3 s further away. Slow-spreading regions (Gakkel and Southwest Indian Ridges) show smaller amounts of splitting than faster spreading centres. At transform zones, the pattern is more complex. Coverage beneath the East Pacific Rise is especially good, and we observe a systematic increase in delay times in S wave splitting measurements compared to previous SKS splitting observations made at ocean-bottom seismometers. One compatible explanation is the presence of horizontally-aligned, connected layers of melt at depth; this is also compatible with other observations of the ‘LAB’ discontinuity and surface-wave derived measurements of radial anisotropy. Highlights ► Mid-ocean ridge processes can be revealed by the seismic anisotropy they produce. ► We make ~ 120 shear wave splitting measurements beneath mid-ocean ridges globally. ► At ridge segments, the fast wave is usually parallel to the spreading direction. ► Delay times increase with distance from the ridge axis from 〈 1 up to 3 s. ► Splitting at the EPR is consistent with a layer of radial anisotropy at depth.

Description: Publication year: 2012 Source: Earth and Planetary Science Letters, Volumes 317-318, 1 February 2012, Pages 68-75 Samuel C. Schon, James W. Head Global climate models predict that ice will be deposited in tropical regions during obliquity excursions from the current mean obliquity of ~ 25° to ~ 35°, but no geological evidence for such deposits has been reported. We document the presence of very small (decameter scale) pedestal craters in the tropics of Mars (the Daedalia Planum–Tharsis region) that are superposed on an impact crater dated to ~ 12.5 Ma ago. The characteristics, abundance, and distribution of these small pedestal craters provide geological evidence that meters-thick ice accumulations existed in the tropical Tharsis region of Mars in the last few million years when mean obliquity was ~ 35° (~ 5–15 Ma) before it transitioned to a mean of ~ 25° (~ 0–3 Ma). A reconnaissance survey reveals similar small pedestal crater examples superposed on the older Amazonian Arsia Mons tropical mountain glacier deposit, suggesting that ice can accumulate in these tropical regions without initiating large-scale glacial conditions. These results support the predictions of general circulation models that ice can migrate to the equatorial regions during periods of moderate obliquity and then serve as a source for mid-latitude deposits. Highlights ► Observations of decameter-scale pedestal craters. ► Evidence of recent equatorial ice on Mars. ► Geological support for predictions of GCMs. ► Equatorial volatile transport pathway for mid-latitude glaciations.

Description: Publication year: 2012 Source: Earth and Planetary Science Letters, Volumes 317-318, 1 February 2012, Pages 111-119 Tomohiro Ohuchi, Takaaki Kawazoe, Yu Nishihara, Tetsuo Irifune The effects of water on the crystallographic preferred orientation (CPO) of olivine aggregates were investigated through simple-shear deformation experiments under asthenospheric upper mantle conditions (pressure = 2.1–5.2 GPa, temperature = 1490–1830 K) using a deformation-DIA apparatus. Formation of the A-type olivine fabric (developed by the (010)[100] slip system) was observed under water-depleted conditions (COH 〈 650 ppm H/Si in olivine), while B-type fabric (by the (010)[001] slip system) or a B-type-like fabric (by the (010)[h0l] slip system) were predominantly formed under water-rich conditions (〉 1000 ppm H/Si). In comparison with fabrics of anhydrous olivine (≤ 111 ppm H/Si), those of olivine having higher water contents (≥ 625 ppm H/Si in olivine) showed stronger anisotropic properties (e.g., P-wave anisotropy, S-wave polarization anisotropy, and the ratio of horizontally and vertically polarized shear waves). The water-induced olivine CPO transition from A-type to B-type(-like) fabric accompanies a change in the alignment of the seismic fast a-axes, resulting in flow-parallel and flow-perpendicular shear wave splitting under water-depleted and water-rich conditions, respectively. The rotation of the fast direction of shear-wave splitting across an arc, which is observed in many subduction zones, is well explained by the likely bimodal water distribution along the mantle wedge (i.e., water-rich in fore-arc and water-depleted in back-arc regions) and the developments of two different types of olivine fabrics (i.e., B-type(-like) fabric in fore-arc and A-type fabric in back-arc regions). Highlights ► The effects of water on developments of olivine fabrics were investigated. ► Formation of the A-type olivine fabric was observed under water-depleted conditions. ► B-type-like fabrics were predominantly formed under water-rich conditions. ► The water-induced fabric transition accompanies a change of the a-axis alignment. ► Flow-parallel and flow-perpendicular shear wave splitting are expected across an arc.

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 145-156 Olivier Galland Active volcanoes experience ground deformation as a response to the dynamics of underground magmatic systems. The analysis of ground deformation patterns may provide important constraints on the dynamics and shape of the underlying volcanic plumbing systems. Nevertheless, these analyses usually take into account simplistic shapes (sphere, dykes, sills) and the results cannot be verified as the modelled systems are buried. In this paper, I present new results from experimental models of magma intrusion, in which both the evolution of ground deformation during intrusion and the shape of the underlying intrusion are monitored. The models consisted of a molten vegetable oil, simulating low viscosity magma, injected into cohesive fine-grained silica flour, simulating the brittle upper crust; oil injection resulted is sheet intrusions (dykes, sills and cone sheets). The initial topography in the models was flat. While the oil was intruding, the surface of the models slightly lifted up to form a smooth relief, which was mapped through time. After an initial symmetrical development, the uplifted area developed asymmetrically; at the end of the experiments, the oil always erupted at the steepest edge of the uplifted area. After the experiment, the oil solidified, the intrusion was excavated and the shape of its top surface mapped. The comparison between the uplifted zone and the underlying intrusions showed that (1) the complex shapes of the uplifted areas reflected the complex shapes of the underlying intrusions, (2) the time evolution of the uplifted zone was correlated with the evolution of the underlying intrusion, and (3) the early asymmetrical evolution of the uplifted areas can be used to predict the location of the eruption of the oil. The experimental results also suggest that complex intrusion shapes (inclined sheet, cone sheet, complex sill) may have to be considered more systematically in the analyses of ground deformation patterns on volcanoes. Highlights ► I present quantitative experiments of ground deformation in volcanic systems. ► Setup allows 3D measurements of both intrusion and resulting ground deformation. ► Measured ground deformation pattern reflects the shape of the underlying intrusion. ► Stronger surface uplift gradients located the shallower parts of the intrusions. ► The asymmetrical ground deformation can be used to locate preparing eruptions.