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Retrograde clay mineral reactions (reverse weathering), including glauconite formation, are first-order controls on element sequestration in marine sediments. Here, we report substantial element sequestration by glauconite formation in shallow marine settings from the Triassic to the Holocene, averaging 3 ± 2 mmol·cm − ²·kyr −1 for K, Mg and Al, 16 ± 9 mmol·cm − ²·kyr −1 for Si and 6 ± 3 mmol·cm − ²·kyr −1 for Fe, which is ~2 orders of magnitude higher than estimates for deep-sea settings. Upscaling of glauconite abundances in shallow-water (0–200 m) environments predicts a present-day global uptake of ~≤ 0.1 Tmol·yr −1 of K, Mg and Al, and ~0.1–0.4 Tmol·yr −1 of Fe and Si, which is ~half of the estimated Mesozoic elemental flux. Clay mineral authigenesis had a large impact on the global marine element cycles throughout Earth’s history, in particular during ‘greenhouse’ periods with sea level highstand, and is key for better understanding past and present geochemical cycling in marine sediments. Here the authors show that reverse weathering reactions, such as the formation of glauconite minerals, are first-order controls on element sequestration in shallow marine sediments throughout Earth history, in particular during greenhouse periods with sea level highstand.

Serpentinization-fueled systems in the cool, hydrated forearc mantle of subduction zones may provide an environment that supports deep chemolithoautotrophic life. Here, we examine serpentinite clasts expelled from mud volcanoes above the Izu–Bonin–Mariana subduction zone forearc (Pacific Ocean) that contain complex organic matter and nanosized Ni–Fe alloys. Using time-of-flight secondary ion mass spectrometry and Raman spectroscopy, we determined that the organic matter consists of a mixture of aliphatic and aromatic compounds and functional groups such as amides. Although an abiotic or subduction slab-derived fluid origin cannot be excluded, the similarities between the molecular signatures identified in the clasts and those of bacteria-derived biopolymers from other serpentinizing systems hint at the possibility of deep microbial life within the forearc. To test this hypothesis, we coupled the currently known temperature limit for life, 122 °C, with a heat conduction model that predicts a potential depth limit for life within the forearc at ∼10,000 m below the seafloor. This is deeper than the 122 °C isotherm in known oceanic serpentinizing regions and an order of magnitude deeper than the downhole temperature at the serpentinized Atlantis Massif oceanic core complex, Mid-Atlantic Ridge. We suggest that the organic-rich serpentinites may be indicators for microbial life deep within or below the mud volcano. Thus, the hydrated forearc mantle may represent one of Earth’s largest hidden microbial ecosystems. These types of protected ecosystems may have allowed the deep biosphere to thrive, despite violent phases during Earth’s history such as the late heavy bombardment and global mass extinctions.

Metamorphic textures in medium-grade (~500–550°C) metasedimentary rocks from the Erzgebirge give evidence of prograde rutile crystallization from ilmenite. Newly-crystallized grains occur as rutile-rich polycrystalline aggregates that pseudomorph the shape of the ilmenites. In-situ trace element data (EMP and SIMS) show that rutiles from the higher-grade samples record large scatter in Nb content and have Nb/Ti ratios higher than coexisting ilmenite. This behavior can be predicted using prograde rutile crystallization from ilmenite and indicates that rutiles are reequilibrating their chemistry with remaining ilmenites. On the contrary, rutiles from the lowest grade samples (~480°C) have Nb/Ti ratios that are similar to the ones in ilmenite. Hence, rutiles from these samples did not equilibrate their chemistry with remaining ilmenites. Our data suggest that temperature may be one of the main factors determining whether or not the elements are able to diffuse between the phases and, therefore, reequilibrate. Newly-crystallized rutiles yield temperatures (from ~500 to 630°C, Zr-in-rutile thermometry) that are in agreement with the metamorphic conditions previously determined for the studied rocks. In quartzites from the medium-grade domain (~530°C), inherited detrital rutile grains are detected. They are identified by their distinct chemical composition (high Zr and Nb contents) and textures (single grains surrounded by fine grained ilmenites). Preliminary calculation, based on grain size distribution of rutile in medium-grade metapelites and quartzites that occur in the studied area, show that rutiles derived from quartzites can be anticipated to dominate the detrital rutile population, even if quartzites are a minor component of the exposure.

Monazite is a common accessory phosphate mineral that occurs under a wide range of pressure and temperature conditions in sedimentary, metamorphic and igneous rocks. Monazite contains high amounts of Th and U, rendering single monazite grains suitable for in-situ U-Th/Pb dating using laser ablation inductively-coupled mass spectrometry (LA-ICP-MS). Two key aspects of monazite dating that are critical for accurate age data with maximum precision are (i) optimized instrumental conditions to minimize analytical scatter and (ii) a well characterized reference material to ensure the accuracy of the obtained aged. Here, we analyzed five monazite reference materials (USGS 44069, 94-222, MAdel, Moacir and Thompson Mine Monazite) for their U-Th/Pb ages using LA-ICP-MS technique and applied a variety of laser spot diameters and repetition rates to find the best operational conditions to achieve accurate age data while maintaining maximum precision. We find that a spot diameter of 10 µm and a repetition rate of 10 Hz yield the most precise ages with a deviation of ±2.0% from their respective high-precision U/Pb literature age data. Ages were reproduced in three different LA-ICP-MS laboratories using these parameters. Each reference material was tested for its suitability as a matrix-matched age reference material. For this, a rotating, iterative approach was adopted in which one reference monazite was used as calibration reference material against all others, which were treated as unknowns. The results reveal that USGS 44069, 94-222, Thompson Mine Monazite and MAdel all agree with their respective calculated ages and ID-TIMS reference ages and thus are suggested as suitable calibration reference materials. Moacir, however, appears slightly older than previously suggested (up to 4%), thus, caution is advised here when using Moacir as reference material for U-Th/Pb LA-ICP-MS dating in the absence of further absolute age calibration.

An undated high-pressure low-temperature tectonic melange in the Elekdagbreve area (central Pontides, N Turkey) comprises blocks of MORB-derived lawsonite eclogite within a sheared serpentinite matrix. In their outer shells, some of the eclogite blocks contain large (up to 6 cm) tourmaline crystals. 'Prograde' inclusions in poikiloblastic garnet from a well-preserved eclogite block are lawsonite, epidote/clinozoisite, omphacite, rutile, glaucophane, chlorite, Ba-bearing phengite, minor actinolite, winchite and quartz. In addition, glaucophane, lawsonite and rutile occur as inclusions in omphacite. These inclusion assemblages document the transition from a garnet-lawsonite-epidote-bearing blueschist to a lawsonite eclogite with the peak assemblage garnet + omphacite I + lawsonite + rutile. Peak metamorphic conditions are not well-constrained but are estimated approximately 400-430 degrees C and >1.35 GPa, based on Fe-Mg exchange between garnet and omphacite and the coexistence of lawsonite + omphacite + rutile. During exhumation of the eclogite-serpentinite melange in the hanging wall of a subduction system, infiltration of B-rich aqueous fluids into the rims of eclogite blocks caused retrogressive formation of abundant chlorite, titanite and albite, followed by growth of tourmaline at the expense of chlorite. At the same time, omphacite I (XJd=0.24-0.44) became unstable and partially replaced by omphacite II characterized by higher XJd (0.35-0.48), suggesting a relatively low silica activity in the infiltrating fluid. Apart from Fe-rich rims developed at the contact to chlorite, tourmaline crystals are nearly homogeneous. Their compositions correspond to Na-rich dravite, perhaps with a small amount of excess (tetrahedral) boron (about 5.90 Si and 3.10 B cations per 31 anions). delta11 B values range from -2.2 to +1.7permil. The infiltrating fluids were most probably derived from subducting altered oceanic crust and sediments. [PUBLICATION ABSTRACT]

Within the Tethyan belt, the NW-Iranian block is bound by two sutures, the Sevan-Akera suture in the north, and the Zagros suture in the south, where the pre-Cambrian geology is poorly constrained. Lower-Middle Cambrian sandstone in NW Iran is represented by quartz and feldspar dominated sub-arkose and sub-litharenite with well-developed cross bedding structures. They are interlayered with shale and limestone and overlain unconformably by Middle Cambrian dolomite. Here we describe and present U–Pb ages of the detrital zircon and rutile together with in-situ ε-Hf data from Lower to Middle Cambrian sandstones. Some zircons are characterized by oscillatory zoning and Th/U ratios of 0.3 to 2.0, pointing to an igneous provenance. Ages of the detrital zircons range from 481 to 2762 Ma, whereby the majority of zircons have ages between 524 and 1000 Ma (Neoproterozoic), and small peaks at 1800–2000 and 2400–2600 Ma. Late Paleoproterozoic to Mesoproterozoic zircons (1100–1750 Ma) are absent in the detrital record which is a distinctive feature of Gondwana-related detrital zircons. Detrital rutile has elevated Cr contents and variable Zr content (88 to 1087 ppm), suggesting derivation mainly from amphibolite-facies meta-basic rocks. U–Pb rutile ages range from 561 to 1253 Ma. Initial ε-Hf values of the Neo-Proterozoic zircons range from -10 to 20, suggesting an association of highly depleted mantle and crustal magmas. Thus, the Lower-Middle Cambrian sandstones in Iranian Azerbaijan were supplied from a Neoproterozoic igneous and metamorphic provenance. Sandstone whole-rock major and trace elements are in accordance with a detrital material provenance from continental granitoids and Precambrian recycled sediments. These results support the hypothesis that Iranian Azerbaijan Cambrian detritus was supplied from repetitive recycling of the crystalline basement and related sedimentary cover of the Saharides and the Arabian-Nubian Shield and deposited in a passive-margin setting in northern Gondwana land. Our results show a close geological tie between Iranian Azerbaijan, Sanandaj-Sirjan, Zagros, Alborz, and other Gondwana-Land-derived units in terms of Early Paleozoic detritus sources. Northwest Iranian Block was probably a passive continental margin in Late Ediacaran-Cambrian after the so-called Cadomian orogeny. Graphical Abstract

The U–Pb age and trace element composition of detrital rutile provide information about the metamorphic history of the source region that cannot be constrained by traditional U–Pb dating of detrital zircon. Previous provenance investigations focussed on only one of these methods. Based on a large LA-ICP-MS trace element and U–Pb isotopic dataset of detrital rutile and U–Pb isotopic data of detrital zircon from late Ordovician sedimentary rocks of Saxo-Thuringia, Germany, this paper discusses the application and significance of combining these methods in provenance investigations. U–Pb age spectra from the detrital zircons analysed show multiple age components (multimodal age spectra) in all samples. This is in contrast with the detrital rutile data, as only one sample yielded a multimodal U–Pb age distribution. Multimodal age spectra of detrital rutile are most likely preserved in sediments from (large) catchment areas with complex geological histories. They may also be related to specific sedimentation events, such as glacial washout during the retreat of large ice shields (e.g. the Hirnantian glaciation of Gondwana). Unimodal age spectra are however not restricted to small catchment areas, if the provenance region is characterized by a pervasive thermal overprint such as the Pan-African orogeny throughout Gondwana. Unimodal age distributions may further consist of overlapping age cluster detectable by different trace element composition of the detrital rutile grains. The combined U–Pb age and trace element data from detrital rutile grains demonstrate that rutile sourced from metapelitic rocks yield reliable and precise U–Pb ages. In contrast, detrital rutile classified to be of metamafic origin generally has too low uranium concentrations to be dated reliably by LA-ICP-MS. Detrital rutile records low- to medium-grade metamorphic events in the source region and therefore has the potential to better constrain the maximum depositional age of sedimentary rocks in comparison to U–Pb dating of detrital zircon.

Translating burial and exhumation histories from the petrological and geochronological record of high-pressure assemblages in subduction channels is key to understanding subduction channel processes. Convective return flow, either serpentinite or sediment hosted, has been suggested as a potential mechanism to retrieve rocks from significant depths and exhume them. Numerical modelling predicts that during convective flow, subducted material can be cycled within a serpentinite-filled subduction channel. Geochronological and petrological evidences for such cycling during subduction are preserved in lawsonite eclogite from serpentinite melange in the Southern New England Orogen, eastern Australia. Ar–Ar, Rb–Sr phengite and U–Pb titanite geochronology, supported by phase equilibrium forward modelling and mineral zoning, suggest Cambro–Ordovician eclogite underwent two stages of burial separated by a stage of partial exhumation. The initial subduction of the eclogite at ca. 490 Ma formed porphyroblastic prograde-zoned garnet and lawsonite at approximate P–T conditions of at least 2.9 GPa and 600 °C. Partial exhumation to at least 2.0 GPa and 500 °C is recorded by garnet dissolution. Reburial of the eclogite resulted in growth of new Mg-rich garnet rims, growth of new prograde-zoned phengite and recrystallization of titanite at P–T conditions of approximately 2.7 GPa and 590 °C. U–Pb titanite, and Ar–Ar and Rb–Sr phengite ages constrain the timing of reburial to ca. 450 Ma. This was followed by a second exhumation event at approximately 1.9 GPa and 520 °C. These conditions fall along a cold approximate geotherm of 230 °C/GPa. The inferred changes in pressure suggest the lawsonite eclogite underwent depth cycling within the subduction channel. Geochronological data indicate that partial exhumation and reburial occurred over ca. 50 M y., providing some estimation on the timescales of material convective cycling in the subduction channel.

Conventional dating of molybdenite (187Re-187Os) provides one of few options for direct dating of sulfide mineralization. Unfortunately, in situ dating of molybdenite is considered unreliable due to intra-granular decoupling of 187Re-187Os. In this study, we developed a new analytical protocol for studies of micron- to grain scale 187Re-187Os systematics in molybdenite. Online chemical separation using ICP-MS/MS technology enables in situ dating by β-decay systems (e.g., Rb-Sr and K-Ca in micas) using laser ablation. Here, the methodology is extended to the 187Re-187Os system, another β-decay system that cannot be resolved by mass spectrometry. Several reaction gases were evaluated, and production of OsCH2 by reaction with CH4 was found to produce strong separation of Os from Re. However, in contrast to the e.g., 87Rb-87Sr system, 1–2% of the parent isotope Re also reacted to ReCH2, leaving a significant interference. A mathematical correction of this remaining interference is possible, and 187Re-187Os (mass-shifted) can be measured accurately even for fairly extreme ratios. For laser ablation, standards were developed by pressing particulate pellets of conventionally dated molybdenite (Moly Hill and Merlin), because there are no appropriate reference materials available. Six natural molybdenite samples from a range of geological settings, containing > 10 ppm Re, were analyzed by 70 μm laser ablation spots, and ages were calibrated by analysis of molybdenite pellets. Contrary to our expectation, weighted average ages obtained were in agreement (within 1%) with conventional age determinations, with fairly good precision (from ~ 1 to 5% 2σ depending on Re concentration), suggesting limited or essentially nonexistent decoupling within crystals. Two important implications of this result are that decoupling Re-Os is not universal, and that our new analytical protocol is useful both for dating and for studies of decoupling. The benefit of in situ dating compared to conventional dating is, apart from lower cost and time consumption, the possibility of targeting smaller molybdenite crystals (≥ 100 μm) in thin sections and epoxy mounts. The youngest sample in the study is 920 Ma, but we see potential of dating significantly younger Re-rich molybdenite.