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A new approach to thermal history modelling with detrital low temperature thermochronological data

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Kerry Gallagher, Mauricio Parra

Abstract

We present an inverse modelling strategy to infer thermal history information from detrital low temperature thermochronological data from modern sediment sampling the outlet of a single catchment. As presented, the method relies on the assumption that the geological timescale thermal history was the same across the catchment. The detrital sample is assumed to represent a mixture of grains originating from a potentially unknown sampling of the present elevation range in the catchment. The approach also implements a method to infer a function describing the topographic sampling represented in the detrital sample. In practice, this may reflect variations in erosion with elevation but also lithological differences in the catchment (fertility) and the nature of erosion/transport processes in the catchment. A combination of detrital and in-situ bedrock data are recommended to improve the resolution of the topographic sampling function. We demonstrate the application of the approach to a set of fission track data from the Fundación catchment in the Sierra Nevada de Santa Marta in northern Colombia. The inferred thermal history suggest a period of rapid cooling initiated around 50-30 Ma, followed by slower cooling to the present day, consistent with the regional geological history. The topographic sampling function estimates suggest that the hypsometric distribution is not appropriate in terms of the contributions from different elevations to the detrital sample. Rather, the data imply a higher proportion of sampling from lower elevations close to the location of the outlet where the detrital sample was collected.

A lower to middle Eocene astrochronology for the Mentelle Basin (Australia) and its implications for the geologic time scale

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Maximilian Vahlenkamp, David De Vleeschouwer, Sietske J. Batenburg, Kirsty M. Edgar, Emma Hanson, Mathieu Martinez, Heiko Pälike, Kenneth G. MacLeod, Yong-Xiang Li, Carl Richter, Kara Bogus, Richard W. Hobbs, Brian T. Huber, Expedition 369 Scientific Participants

Abstract

The geologic time scale for the Cenozoic Era has been notably improved over the last decades by virtue of integrated stratigraphy, combining high-resolution astrochronologies, biostratigraphy and magnetostratigraphy with high-precision radioisotopic dates. However, the middle Eocene remains a weak link. The so-called “Eocene time scale gap” reflects the scarcity of suitable study sections with clear astronomically-forced variations in carbonate content, primarily because large parts of the oceans were starved of carbonate during the Eocene greenhouse. International Ocean Discovery Program (IODP) Expedition 369 cored a carbonate-rich sedimentary sequence of Eocene age in the Mentelle Basin (Site U1514, offshore southwest Australia). The sequence consists of nannofossil chalk and exhibits rhythmic clay content variability. Here, we show that IODP Site U1514 allows for the extraction of an astronomical signal and the construction of an Eocene astrochronology, using 3-cm resolution X-Ray fluorescence (XRF) core scans. The XRF-derived ratio between calcium and iron content (Ca/Fe) tracks the lithologic variability and serves as the basis for our U1514 astrochronology. We present a 16 million-year-long (40-56 Ma) nearly continuous history of Eocene sedimentation with variations paced by eccentricity and obliquity. We supplement the high-resolution XRF data with low-resolution bulk carbon and oxygen isotopes, recording the long-term cooling trend from the Paleocene-Eocene Thermal Maximum (PETM – ca. 56 Ma) into the middle Eocene (ca. 40 Ma). Our early Eocene astrochronology corroborates existing chronologies based on deep-sea sites and Italian land sections. For the middle Eocene, the sedimentological record at U1514 provides a single-site geochemical backbone and thus offers a further step towards a fully integrated Cenozoic geologic time scale at orbital resolution.

Testing the occurrence of Late Jurassic true polar wander using the La Negra volcanics of northern Chile

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Roger R. Fu, Dennis V. Kent, Sidney R. Hemming, Pablo Gutiérrez, Jessica R. Creveling

Abstract

True polar wander (TPW) is the reorientation of the crust-mantle system driven by the redistribution of masses in the mantle and on the Earth's surface. In the ideal case, characterization of TPW requires paleomagnetic constraints on the motion of all major plates and independent reconstructions of relative plate positions. While such complete datasets are absent for pre-Mesozoic TPW inferences due to the absence of oceanic plates, they are available for the Late Jurassic (165-145 Ma) “monster shift”, a ∼30° amplitude proposed TPW event. Here we perform paleomagnetic sampling and Ar-Ar geochronology on the La Negra volcanics of Northern Chile, producing two new paleomagnetic poles with ages 165.8 ± 1.8 Ma (1σ; 84.3°N 0.9°E; α95=7.6°; N=28) and 152.8 ± 0.8 Ma (84.5°N 256.4°E; α95=10.8°; N=18). By combining these data with other recently published results, we compute a net lithospheric rotation of 25.3° ± 7.3° (1σ) at a mean rate of 1.21° ± 0.35° My−1 between 170 and 145 Ma with a peak rate of 1.46° ± 0.65° My−1 between 160 and 145 Ma. These rates are consistent with inferences from the Pacific Plate, implying true whole lithosphere rotation. Given coherent motion involving the entire lithosphere, we conclude that the Earth underwent rapid TPW between approximately 165 and 145 Ma, potentially driven by the cessation of subduction along the western North American margin.

Dy, Er, and Yb isotope compositions of meteorites and their components: Constraints on presolar carriers of the rare earth elements

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Quinn R. Shollenberger, Gregory A. Brennecka

Abstract

One way to study the original building blocks of the Solar System is to investigate primitive meteorites and their components. Specifically, isolating these meteorites' individual components via sequential acid leaching can reveal isotopically diverse material present in the early Solar System, which can provide new insights into the mixing and transport processes that eventually led to planet formation. Such isotopic differences in the components are likely to be found in heavy rare earth elements, such as dysprosium (Dy), erbium (Er), and ytterbium (Yb), because their isotopes have different nucleosynthetic production pathways and the elements have significant differences in volatility; however, these specific elements have yet to be thoroughly investigated in the field of cosmochemistry. As such, we present the first combined Dy, Er, and Yb isotope compositions of sequential acid leachates from the Murchison meteorite, along with multiple bulk meteorites from different taxonomic classes. This work also presents a new method to separate, purify, and accurately measure Dy isotopes. Here we show that resolved Dy, Er, and Yb isotope variations in most bulk meteorites are due to neutron capture processes. However, Dy and Er isotopic compositions of bulk Murchison and Murchison leachates stem from the additions or depletions of a nucleosynthetic component formed by the s-process, most likely mainstream silicon carbide (SiC) grains. In contrast, the Yb isotope compositions of bulk Murchison and Murchison leachates display either unresolved or relatively small isotope anomalies. The disparate isotopic behavior between Dy-Er and Yb likely reflects their differing volatilities, with Dy and Er condensing/incorporating into the mainstream SiC grains, whereas the less refractory Yb remains in the gas phase during SiC formation. This work suggests that Yb is hosted in a non-SiC presolar carrier phase and, furthermore, that mainstream SiC grains may be the primary source of isotopic variation in bulk meteorites.

Kinetic carbon isotope fractionation links graphite and diamond precipitation to reduced fluid sources

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Nico Kueter, Max W. Schmidt, Marvin D. Lilley, Stefano M. Bernasconi

Abstract

At high temperatures, isotope partitioning is often assumed to proceed under equilibrium and trends in the carbon isotope composition within graphite and diamond are used to deduce the redox state of their fluid source. However, kinetic isotope fractionation modifies fluid- or melt-precipitated mineral compositions when growth rates exceed rates of diffusive mixing. As carbon self-diffusion in graphite and diamond is exceptionally slow, this fractionation should be preserved. We have hence performed time series experiments that precipitate graphitic carbon through progressive oxidization of an initially CH4-dominated fluid. Stearic acid was thermally decomposed at 800 °C and 2 kbar, yielding a reduced COH-fluid together with elemental carbon. Progressive hydrogen loss from the capsule caused CH4 to dissociate with time and elemental carbon to continuously precipitate. The newly formed C0, aggregating in globules, is constantly depleted by −6.2±0.3‰ in 13C relative to the methane, which defines a temperature dependent kinetic graphite-methane 13C/12C fractionation factor. Equilibrium fractionation would instead yield graphite heavier than the methane. In dynamic environments, kinetic isotope fractionation may control the carbon isotope composition of graphite or diamond, and, extended to nitrogen, could explain the positive correlation of δ13C and δ15N sometimes observed in coherent diamond growth zones. 13C enrichment trends in diamonds are then consistent with reduced deep fluids oxidizing upon their rise into the subcontinental lithosphere, methane constituting the main source of carbon.

Water controls the seasonal rhythm of rock glacier flow

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): A. Cicoira, J. Beutel, J. Faillettaz, A. Vieli

Abstract

Rock glaciers are creeping periglacial landforms experiencing strong acceleration during recent atmospheric warming and raising concerns with regard to their future behaviour and stability. High resolution kinematic observations show strong seasonal and multi-annual variations in rock glacier creep, but the linking mechanisms to environmental forcing remain poorly understood and lack quantitative models. Here we investigate the interaction between rock glacier creep and climatic forcing - temperature and precipitation - by developing a novel conceptual and numerical modelling approach. The model is constrained and the results are compared with data from the Dirru Rock Glacier (Vallis - CH). We are able to reproduce the observed velocity variations both in magnitude and phase on seasonal and inter-annual time scales. We find that water from liquid precipitation and snow melt, rather than air temperature, is the main driver of variations in rock glacier creep. Our results imply that the influence of water on rock glacier creep is fundamental and must be considered when investigating the historic and future evolution of rock glaciers.

Triple oxygen isotope investigation of fine-grained sediments from major world's rivers: Insights into weathering processes and global fluxes into the hydrosphere

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 15 December 2019

Source: Earth and Planetary Science Letters, Volume 528

Author(s): Ilya N. Bindeman, Germain Bayon, James Palandri

Abstract

Continental weathering is accompanied by formation of clays and other secondary minerals and their δ18O and Δ17O values should hence reflect to some extent signatures of meteoric water (δ18OMW) and mean annual temperatures (MAT). Our ability to extract climate information from weathered products across the geologic history relies on analytical methods tested and calibrated against modern climate conditions. We here present triple-oxygen isotope analyses of clay-size sediments from 45 rivers worldwide, as well as δ18O analyses of corresponding silt- and sand-rich detrital fractions, which altogether cover about 25% of the continental area that drained into the oceans, extending from the tropics to polar regions. The majority of studied clays closely approximate weathering products, always having high-δ18O signatures regardless of the bedrock type, and in equilibrium with local meteoric waters. Silts are only ∼1.9‰ lighter on average due to greater detrital dilution. Overall, bulk clays from across different climatic regions do not vary much isotopically; an observation which we attribute to opposing effects of temperature on clay-water fractionation and hydrologic relationship between temperature and δ18OMW. Mathematical inversion of measured clay δ18O and Δ17O values (corrected for detrital contribution) into MAT and δ18OMW, compiled for each studied watershed, returns satisfactory estimates. Globally, triple O isotopes in clays appear to be water-dominated, being controlled almost exclusively by δ18OMW at respective temperature of weathering, with minor effects related to evaporation. Using sand from rivers, correlation of δ18O silts with detrital proportions, and estimated surface outcrop of different rock types, we additionally arrive at a +11.5‰ estimate for the exposed silicate crust undergoing weathering.

Globally-averaged, sediment-flux weighted clay δ18O and Δ17O values are +14.80‰ and −0.164‰, respectively. These values are significantly skewed toward O isotope signatures for the southeast Asia and western Pacific regions, characterized by very high sediment fluxes to the ocean. Using both clay- and silt-size fractions, the total weighted silicate weathering δ18O signature exported to the world's ocean is −2.59‰, almost 50% higher the previous estimate, yielding an ice-free world hydrosphere estimate of −0.78‰.

Overall, the modern river clays represent a snapshot of modern weathering conditions on continents, and associated first-order climatic signatures related to MAT and δ18O of the hydrosphere. This implies that measured increase in δ18O and stepwise decrease in Δ17O in shales in the geologic record capture: evolving global hydrologic cycle upon continental emergence, decrease in global MAT or diagenetic conditions, and decreasing ocean mass via rehydrating of the mantle by subduction of hydrated low-δ18O, high-Δ17O slabs.

Corsica: A 20-Yr Multi-Mission Absolute Altimeter Calibration Site

Publication date: Available online 9 October 2019

Source: Advances in Space Research

Author(s): P. Bonnefond, P. Exertier, O. Laurain, T. Guinle, P. Féménias

Abstract

Initially developed for monitoring the performance of TOPEX/Poseidon and follow-on Jason legacy satellite altimeters, the Corsica geodetic facilities that are located both at Senetosa Cape and near Ajaccio have been developed to calibrate successive satellite altimeters in an absolute sense. Since 1998, the successful calibration process used to calibrate most of the oceanographic satellite altimeter missions has been regularly updated in terms of in situ instruments, geodetic measurements and methodologies. In this study, we present an assessment of the long-term stability of the in situ instruments in terms of sea level monitoring that include a careful monitoring of the geodetic datum. Based on this 20-yr series of sea level measurements, we present a review of the derived absolute Sea Surface Height (SSH) biases for the following altimetric missions based on the most recent reprocessing of their data set: TOPEX/Poseidon and Jason-1/2/3, Envisat and ERS-2, CryoSat-2, SARAL/AltiKa and Sentinel-3A&B. For the longest time series the standard error of the absolute SSH biases is now at a few millimeters level which is fundamental to maintain the high level of confidence that scientists have in the global mean sea level rise.

On the uncertainty associated with validating the global mean sea level climate record

Publication date: Available online 9 October 2019

Source: Advances in Space Research

Author(s): Christopher S. Watson, Benoit Legresy, Matt A. King

Abstract

Satellite altimetry provides the ongoing sea level climate data record that provides evidence for one of the most significant manifestations of climate change on Earth. External and independent validation of satellite altimetry is a core component of mission design, providing confidence in such a seminal climate record. The global tide gauge network, corrected for the effects of vertical land motion, forms one of a suite of approaches used to validate satellite altimetry. Used as a tool to identify potential systematic error, the altimeter minus tide gauge approach ultimately leads to an improved geophysical data record through iterative diagnosis, correction and reprocessing of the mission record. A recent example includes the detection of a small but significant drift in the early part of the record associated with the TOPEX record. Here, we return to the approach that quantified the apparent drift in the TOPEX record and focus on further elucidating the uncertainty of that technique as a function of mission duration. We show that approximately 2.9 years is required to reach a validation uncertainty of ±1 mm/yr (1σ). This result appears optimistic by a factor of 1.5 to 2.2 in comparison to an error budget approach reported in the literature. Our results highlight the challenge of validating the altimeter record using a sparse and irregularly distributed network of tide gauges and points towards possible areas of future improvement in the validation approach.

Juno/JIRAM: Planning and Commanding activities

Publication date: Available online 8 October 2019

Source: Advances in Space Research

Author(s): R. Noschese, A. Cicchetti, R. Sordini, M. Cartacci, A. Mura, S. Brooks, M. Lastri, G. Filacchione, A. Migliorini, M.L. Moriconi, A. Adriani, H.N. Becker, A. Bini, C. Pasqui, D. Grassi, F. Altieri, B.M. Dinelli, G. Piccioni, S. Stefani, F. Tosi

Abstract

In the context of space missions, where science is the most important goal, careful planning and detailed commanding are fundamental. The planning and commanding phases are activities whose complexity depends on the instrument characteristics, environmental constraints and scientific goals. The purpose of this work is to describe in detail these activities for the Jovian Infrared Auroral Mapper (JIRAM) on board the Juno spacecraft, a NASA mission to Jupiter.

To maximize the scientific return, we fully employ the flexibility offered by the JIRAM operational modes to efficiently plan observations of various Jovian targets, in spite of the harsh Jovian radiation environment and the spinning state of the Juno spacecraft. Moreover, the JIRAM observations are limited by the challenging pointing and timing scheme of the mission, which impose constraints on both the observation planning and instrumental commanding.

On a century of extragalactic novae and the rise of the rapid recurrent novae

Publication date: Available online 8 October 2019

Source: Advances in Space Research

Author(s): Matthew J. Darnley, Martin Henze

Abstract

Novae are the observable outcome of a transient thermonuclear runaway on the surface of an accreting white dwarf in a close binary system. Their high peak luminosity renders them visible in galaxies out beyond the distance of the Virgo Cluster. Over the past century, surveys of extragalactic novae, particularly within the nearby Andromeda Galaxy, have yielded substantial insights regarding the properties of their populations and sub-populations. The recent decade has seen the first detailed panchromatic studies of individual extragalactic novae and the discovery of two probably related sub-groups: the ‘faint–fast’ and the ‘rapid recurrent’ novae. In this review we summarise the past 100 years of extragalactic efforts, introduce the rapid recurrent sub-group, and look in detail at the remarkable faint–fast, and rapid recurrent, nova M31N 2008-12a. We end with a brief look forward, not to the next 100 years, but the next few decades, and the study of novae in the upcoming era of wide-field and multi-messenger time-domain surveys.

Spread-F prediction model for the equatorial Chumphon station, Thailand

Publication date: Available online 8 October 2019

Source: Advances in Space Research

Author(s): P. Thammavongsy, P. Supnithi, W. Phakphisut, K. Hozumi, T. Tsugawa

Abstract

This work proposes a range spread-F (RSF) prediction model using the neural network (NN) over the equatorial Chumphon (CPN) region in Thailand. The RSF model is constructed by using five input spaces including the diurnal variation, seasonal variation, geographic latitude, solar flux index (F10.7), and magnetic index (Ap). The RSF NN model is trained with three years of RSF data during 2013 to 2015 from Chumphon (CPN) station (Latitude = 10.7°N, Longitude = 99.4°E) and the performance of the proposed RSF NN model is validated using the dataset of 2016. As a result, the RSF NN model achieves 98.3% accuracy of all correct predictions even with the limited available data. The results show that the proposed NN model yields a lower RSF probability than the actual observation by about 7.3%, but the overestimation of the proposed NN model is 2.5% in both the equinoxes and solstices. In addition, we discover that the IRI-2016 model mostly overestimates the RSF probability when compared with the actual observation for all seasons in 2016, particularly, in equinoctial months over Chumphon station.

PDE Model-Based Boundary Control of a Spacecraft with Double Flexible Appendages under Prescribed Performance

Publication date: Available online 8 October 2019

Source: Advances in Space Research

Author(s): Junteng Ma, Hao Wen, Dongping Jin

Abstract

This paper investigates a boundary control scheme of a spacecraft with double flexible appendages under prescribed performance. The flexible spacecraft system comprises a rigid central hub and two flexible appendages regarded as continuum models, so that the motion of the system can be portrayed by using partial differential equations (PDEs). In this paper, only one control torque and two control forces are applied to guarantee the desired attitude angle of the spacecraft and simultaneously suppress the vibration of the two flexible appendages. Moreover, the angle tracking error of the spacecraft can be restricted in a small residual set under a minimum convergence rate by adopting the prescribed performance technique (PPT). The stability of the boundary control is analyzed by employing LaSalle’s invariance principle. Finally, the feasibility of the proposed controller is verified through numerical results.

Nonthermal effects on the cylindrical dusty ion shocks in nonthermal viscous space plasma

Publication date: Available online 8 October 2019

Source: Advances in Space Research

Author(s): Abdelwahed, E.K. El-Shewy, A.A. El-Rahman, N.F. Abdo

Abstract

The propagating cylindrical shock dust ion wave (CDISW) in dusty four component plasma with three viscous component (ion and two polarity charged grains) has been introduced. The three dimensional (3D) Cylindrical Burgers (CB) equation is derived. The propagating cylindrical shock characteristics are established to becomes a very significantly improved by the supports of electron nonthermality, ion and negative (positive) kinematics viscosity coefficients. Furthermore, the shock strength depends on cylindrical directions. The obtained results may be profitable in understanding both the laboratory and space applications of plasmas.

GPS and BDS combined PPP model with inter-system differenced observations

Publication date: Available online 8 October 2019

Source: Advances in Space Research

Author(s): Rui Tu, Ju Hong, Rui Zhang, Junqiang Han, Lihong Fan, Pengfei Zhang, Jinhai Liu, Xiaochun Lu

Abstract

This study proposes a combined precise point positioning (PPP) model, called the inter-system differenced PPP model, in which it formed the satellite-differenced observation between different satellite systems. Compared with the traditional combined PPP model, the inter-system differenced PPP model has the following characteristics: (1) The satellite difference between various systems can eliminate the receiver clock bias parameter, it enhances the strength of the PPP model. (2) Inter-satellite differences can eliminate some common model errors and reduce the impact of observation errors. (3) The system time difference can be calculated directly to provide the basis for time offset monitoring. In order to verify the positioning results of the inter-system differenced model, static PPP and dynamic PPP experiments were carried out to test the positioning accuracy and convergence time, and the results were compared with those of the traditional combination PPP model. In addition, the offset characteristics introduced by the inter-system differenced model were analysed, and the optimal estimation strategy was determined by dynamic PPP tests. Preliminary results are as follows: (1) For inter-system differenced static PPP, the convergence time of stations is less than 20 min. The standard deviation (STD) of the position bias for the East (E), North (N), and Up (U) components are better than 2.5 cm, 1.0 cm and 3.0 cm, respectively, and the corresponding root mean square deviation (RMS) are better than 3.0 cm, 1.5 cm and 6.0 cm, respectively. Compared with the traditional combined PPP model, the average convergence time of the inter-system differenced model is nearly the same, but its overall positioning accuracy is better. (2) For the inter-system differenced dynamic PPP model, the convergence time of stations is better than 30 min. The STD of the position bias for the E, N, and U components is better than 3.0 cm, 2.0 cm and 4.5 cm respectively, and the RMS is better than 4.0 cm, 2.5 cm and 7.0 cm respectively. Compared with the traditional combined PPP model, the convergence of the inter-system differenced model is slightly better, and the positioning accuracy does not differ significantly. Moreover, the inter-system differenced model is much better than the traditional model when fewer satellites are available. (3) The offset between GPS and BDS corresponding with the precise products of GFZ is related to the type of receiver, and the daily standard of offset is less than 0.5 ns. In addition, we determined optimum process noise of the offset parameter to be a 10-3/3(m2/s) random walk after comparing several options.

Simulation of Cosmic Rays in the Earth’s Atmosphere and Interpretation of Observed Counts in an X-ray Detector at Balloon Altitude Near Tropical Region

Publication date: Available online 8 October 2019

Source: Advances in Space Research

Author(s): Ritabrata Sarkar, Abhijit Roy, Sandip K. Chakrabarti

Abstract

The study of secondary particles produced by the cosmic-ray interaction in the Earth’s atmosphere is very crucial as these particles mainly constitute the background counts produced in the high-energy detectors at balloon and satellite altitudes. In the present work, we calculate the abundance of cosmic-ray generated secondary particles at various heights of the atmosphere by means of a Monte Carlo simulation and use this result to understand the background counts in our X-ray observations using balloon-borne instruments operating near the tropical latitude (geomagnetic latitude: ∼14.50° N). For this purpose, we consider a 3D description of the atmospheric and geomagnetic field configurations surrounding the Earth, as well as the electromagnetic and nuclear interaction processes using Geant4 simulation toolkit. Subsequently, we use a realistic mass model description of the detector under consideration, to simulate the counts produced in the detector due to secondary cosmic-ray particles.

Taphonomic bias in exceptionally preserved biotas

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Farid Saleh, Jonathan B. Antcliffe, Bertrand Lefebvre, Bernard Pittet, Lukáš Laibl, Francesc Perez Peris, Lorenzo Lustri, Pierre Gueriau, Allison C. Daley

Abstract

Exceptionally preserved fossil biotas provide crucial data on early animal evolution. Fossil anatomy allows for reconstruction of the animal stem lineages, informing the stepwise process of crown group character acquisition. However, a confounding factor to these evolutionary analyses is information loss during fossil formation. Here we identify that the Ordovician Fezouata Shale has a clear taphonomic difference when compared to the Cambrian Burgess Shale and Chengjiang Biota. In the Fezouata Shale, soft cellular structures are most commonly associated with partially mineralized and sclerotized tissues, which may be protecting the soft tissue. Also, entirely soft non-cuticularized organisms are absent from the Fezouata Shale. Conversely, the Cambrian sites commonly preserve entirely soft cellular bodies and a higher diversity of tissue types per genus. The Burgess and Chengjiang biotas are remarkably similar, preserving near identical proportions of average tissue types per genus. However, the Burgess shale has almost double the proportion of genera that are entirely soft as compared to the Chengjiang Biota, indicating that the classic Burgess Shale was the acme for soft tissue preservation. Constraining these biases aids the differentiation of evolutionary and taphonomic absences, which is vital to incorporating anatomical data into a coherent framework of character acquisition during the earliest evolution of animals.

How to make porphyry copper deposits

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): Cin-Ty A. Lee, Ming Tang

Abstract

Much of the world's economic copper resources are hosted in porphyry copper deposits (PCDs), shallow level magmatic intrusions associated mostly with thick (>45km) magmatic arcs, such as mature island arcs and continental arcs. However, a well-known, but unresolved paradox, is that arc magmas traversing thick crust, particularly in continental arcs, are generally depleted in Cu whereas in island arcs, where PCDs are less common, magmas become enriched in Cu. Here, we show that the formation of PCDs requires a complex sequence of intra-crustal magmatic processes, from the lower crust to the upper crust. PCDs form when the crust becomes thick (>45km) enough to crystallize garnet. Garnet fractionation depletes Fe from the magma, which drives sulfide segregation and removal of most of the magma's Cu into the lower crust, leaving only small amounts of Cu in the residual magma to make PCDs. However, because garnet is depleted in ferric iron, the remaining Fe in the magma becomes progressively oxidized, which eventually oxidizes sulfide to sulfate, thereby releasing sulfide bound Cu from the magma into solution. This auto-oxidation of the magma, made possible by deep-seated garnet fractionation, increases the ability of endogenic magmatic fluids to self-scavenge Cu from large volumes of otherwise Cu-poor magmas and then transport and concentrate Cu to the tops of magmatic bodies. Examination of the occurrence of PCDs in the central Andes shows that ore formation occurs when continental arcs reach their maximum thickness (>60km), just before the termination of magmatism.

Graphical abstract

In continental arcs, where oceanic lithosphere subducts beneath continental lithosphere, intracrustal magmatic differentiation likely plays a dominant role in controlling the composition of magmas. Of particular interest are the occurrence of Cu-porphyry systems, which are typically found late in the “life-cycle” of an arc when arc crustal thickness is the greatest. During this time, deep-seated fractionation of garnet-rich cumulates, known as arclogites, results in oxidation of the residual magma, increasing the scavenging power of late stage magmatic fluids and the possibility of making Cu-porphyries.

Control of fluid pressures on the formation of listric normal faults

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): X.P. Yuan, Y.M. Leroy, B. Maillot

Abstract

Listric normal faults are widespread in the extension of the upper crust. Despite major advances in understanding the formation of listric faults through various experiments, the mechanical conditions that allow their formation are highly debated. In particular, Anderson's faulting theory predicts that newly formed normal faults are planar and are dipping at least at 45∘, and in practice, at 60–65∘ for most rock types. Here, we develop Limit Analysis to investigate the formation of a listric fault at the onset of slip linking a deep detachment to the topographic surface. We find that listric normal faults can occur in the brittle upper crust without appealing to viscous or ductile behaviours, nor to flexural stresses. The disequilibrium-compaction fluid pressures, typically observed in many sedimentary basins, are essential for the formation of listric faults. The fluid pressure is hydrostatic down to the fluid-retention depth ZFRD and sustains a higher gradient below this depth. Parametric studies show that the surface slope is also essential for the formation of listric faults even with a gently dipping surface slope (≤4∘), whereas a flat topographic surface leads to a simple Andersonian geometry at the onset of slip. The method is applied to two field examples in order to determine fluid overpressures that best match the fault shapes interpreted from seismic data. For the offshore Niger Delta, the simulated normal faults match the observed listric faults with a very shallow ZFRD = 0.5–0.75 km, and below ZFRD the fluid pressure has a lithostatic gradient, consistent with the observed fluid-pressure profiles. To reproduce a series of listric faults joining on one common low-angle detachment in the NW Gulf of Mexico, we demonstrate that a shallow ZFRD = 0.7–1.1 km is required, below which the fluid pressure increases to the lithostatic pressure on the detachment, in agreement with the fluid-pressure observations.

Late Cretaceous (99-69 Ma) basaltic intraplate volcanism on and around Zealandia: Tracing upper mantle geodynamics from Hikurangi Plateau collision to Gondwana breakup and beyond

Earth and Planetary Science Letters - Mon, 10/21/2019 - 19:10

Publication date: 1 January 2020

Source: Earth and Planetary Science Letters, Volume 529

Author(s): K. Hoernle, C. Timm, F. Hauff, V. Tappenden, R. Werner, E.M. Jolis, N. Mortimer, S. Weaver, F. Riefstahl, K. Gohl

Abstract

Margins resulting from continental breakup are generally classified as volcanic (related to flood basalt volcanism from a starting plume head) or non-volcanic (caused by tectonic processes), but many margins (breakups) may actually be hybrids caused by a combination of volcanic and tectonic processes. It has been postulated that the collision of the Hikurangi Plateau with the Gondwana margin ∼110 Ma ago caused subduction to cease, followed by large-scale extension and ultimately breakoff of the Zealandia micro-continent from West Antarctica through seafloor spreading which started at ∼85 Ma. Here we report new geochemical (major and trace element and Sr-Nd-Pb-Hf isotope) data for Late Cretaceous (99-69 Ma) volcanism from Zealandia, which include the calc-alkalic, subduction-related Mount Somers (99-96 Ma) and four intraplate igneous provinces: 1) Hikurangi Seamount Province (99-88 Ma), 2) Marlborough Igneous Province (98-94 Ma), 3) Westland Igneous Province (92-69 Ma), and 4) Eastern Chatham Igneous Province (86-79 Ma). Each of the intraplate provinces forms mixing arrays on incompatible-element and isotope ratio plots between HIMU (requiring long-term high μ=238U/204Pb) and either a depleted (MORB-source) upper mantle (DM) component or enriched continental (EM) type component (located in the crust and/or upper mantle) or a mixture of both. St. Helena end member HIMU could be the common component in all four provinces. Considering the uniformity in composition of the HIMU end member despite the type of lithosphere (continental, oceanic, oceanic plateau) beneath the igneous provinces, we attribute this component to a sublithospheric source, located beneath all volcanic provinces, and thus most likely a mantle plume. We propose that the plume material rose beneath the active Gondwana margin and flowed along the subducting lithosphere beneath the Hikurangi Plateau and neighboring seafloor and through slab tears/windows beneath the Gondwana (later to become Zealandia) continental lithosphere. We conclude that both plateau collision, resulting in subduction cessation, and the opening of slab tears/windows, allowing hot asthenosphere and/or plume material to upwell to shallow depths, were important in causing the breakup of Zealandia from West Antarctica. Combined tectonic-volcanic processes are also likely to be responsible for causing breakup and the formation of other hybrid type margins.

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