Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: Publication year: 2012 Source: Earth and Planetary Science Letters, Volumes 317-318, 1 February 2012, Pages 8-19 B. David A. Naafs, Jens Hefter, Gary Acton, Gerald H. Haug, Alfredo Martínez-Garcia, ... Here we present orbitally-resolved records of terrestrial higher plant leaf wax input to the North Atlantic over the last 3.5 Ma, based on the accumulation of long-chainn-alkanes andn-alkanl-1-ols at IODP Site U1313. These lipids are a major component of dust, even in remote ocean areas, and have a predominantly aeolian origin in distal marine sediments. Our results demonstrate that around 2.7 million years ago (Ma), coinciding with the intensification of the Northern Hemisphere glaciation (NHG), the aeolian input of terrestrial material to the North Atlantic increased drastically. Since then, during every glacial the aeolian input of higher plant material was up to 30 times higher than during interglacials. The close correspondence between aeolian input to the North Atlantic and other dust records indicates a globally uniform response of dust sources to Quaternary climate variability, although the amplitude of variation differs among areas. We argue that the increased aeolian input at Site U1313 during glacials is predominantly related to the episodic appearance of continental ice sheets in North America and the associated strengthening of glaciogenic dust sources. Evolutional spectral analyses of then-alkane records were therefore used to determine the dominant astronomical forcing in North American ice sheet advances. These results demonstrate that during the early Pleistocene North American ice sheet dynamics responded predominantly to variations in obliquity (41 ka), which argues against previous suggestions of precession-related variations in Northern Hemisphere ice sheets during the early Pleistocene. Highlights ► Biomarker record of aeolian input to the North Atlantic covering the last 3.5 Ma. ► Aeolian input significantly increased during glacials starting at 2.7 Ma. ► Increase related to episodic appearance of continental ice sheets in North America. ► Strengthening of glaciogenic dust sources at 2.7 Ma. ► North American ice sheet dynamics obliquity controlled during early Pleistocene.

Beschreibung: 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.

Beschreibung: Publication year: 2012 Source: Earth and Planetary Science Letters, Volumes 317-318, 1 February 2012, Pages 27-43 A. Khan, T.J. Shankland This paper applies electromagnetic sounding methods for Earth's mantle to constrain its thermal state, chemical composition, and “water” content. We consider long-period inductive response functions in the form ofC-responses from four stations distributed across the Earth (Europe, North America, Asia and Australia) covering a period range from 3.9 to 95.2 days and sensitivity to ~ 1200 km depth. We invertC-responses directly for thermo-chemical state using a self-consistent thermodynamic method that computes phase equilibria as functions of pressure, temperature, and composition (in the Na2O–CaO–FeO–MgO–Al2O3–SiO2model system). Computed mineral modes are combined with recent laboratory-based electrical conductivity models from independent experimental research groups (Yoshino (2010) and Karato (2011)) to compute bulk conductivity structure beneath each of the four stations from whichC-responses are estimated. To reliably allocate water between the various mineral phases we include laboratory-measured water partition coefficients for major upper mantle and transition zone minerals. This scheme is interfaced with a sampling-based algorithm to solve the resulting non-linear inverse problem. This approach has two advantages: (1) It anchors temperatures, composition, electrical conductivities, and discontinuities that are in laboratory-based forward models, and (2) At the same time it permits the use of geophysical inverse methods to optimize conductivity profiles to match geophysical data. The results show lateral variations in upper mantle temperatures beneath the four stations that appear to persist throughout the upper mantle and parts of the transition zone. Calculated mantle temperatures at 410 and 660 km depth lie in the range 1250–1650 °C and 1500–1750 °C, respectively, and generally agree with the experimentally-determined temperatures at which the measured phase reactions olivine → β-spinel and γ-spinel → ferropericlase + perovskite occur. The retrieved conductivity structures beneath the various stations tend to follow trends observed for temperature with the strongest lateral variations in the uppermost mantle; for depths 〉 300 km conductivities appear to depend less on the particular conductivity database. Conductivities at 410 km and at 660 km depth are found to agree overall with purely geophysically-derived global and semi-global one-dimensional conductivity models. Both electrical conductivity databases point to 〈 0.01 wt.% H2O in the upper mantle. For transition zone minerals results from the laboratory database of Yoshino (2010) suggest that a much higher water content (up to 2 wt.% H2O) is required than in the other database (Karato, 2011), which favors a relatively “dry” transition zone (〈 0.01 wt.% H2O). Incorporating laboratory measurements of hydrous silicate melting relations and available conductivity data allows us to consider the possibility of hydration melting and a high-conductivity melt layer above the 410-km discontinuity. The latter appears to be 1) regionally localized and 2) principally a feature from the Yoshino (2010) database. Further, there is evidence of lateral heterogeneity: The mantle beneath southwestern North America and central China appears “wetter” than that beneath central Europe or Australia. Highlights ► We invert electromagnetic sounding data for mantle water content and thermal state. ► Upper mantle temperatures are found to vary laterally by several hundred degrees. ► The upper mantle generally appears “dry” containing 〈 0.01 wt.%H 2 O. ► The transition zone is overall “wetter” with water contents reaching up to 2 wt.%. ► A highly conductive melt layer atop the 410-km transition is a localized feature.

Beschreibung: 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.

Beschreibung: 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.