Keshav S.,University of Bayreuth |
Keshav S.,Montpellier University |
Gudfinnsson G.H.,Iceland GeoSurvey
Journal of Geophysical Research: Solid Earth | Year: 2013
Solidus phase relations of carbon dioxide-saturated (CO2 vapor) model peridotite in the system CaO-MgO-Al2O3-SiO 2-CO2 in the 1.1-2.1 GPa pressure range are reported. The solidus has a positive slope in pressure-temperature (PT) space from 1.1 to 2 GPa. Between 2 and 2.1 GPa, the melting curve changes to a negative slope. From 1.1 to 1.9 GPa, the liquid, best described as CO2-bearing silicate liquid, is in equilibrium with forsterite, orthopyroxene, clinopyroxene, spinel, and vapor. At 2 GPa, the same crystalline phase assemblage plus vapor is in equilibrium with two liquids, which are silicate and carbonatitic in composition, making the solidus at 2 GPa PT invariant. The presence of two liquids is interpreted as being due to liquid immiscibility. Melting reactions written over 1.1-1.9 GPa are peritectic, with forsterite being produced upon melting, and the liquid is silicate in composition. Upon melting at 2.1 GPa, orthopyroxene is produced, and the liquid is carbonatitic in composition. Hence, the invariance between 1.9 and 2.1 GPa is not only the reason for the dramatic change in the liquid composition over an interval of 0.2 GPa, but the carbonated peridotite solidus ledge itself most likely appears because of this PT invariance. It is suggested that because carbonatitic liquid is produced at the highest solidus temperature at 2 GPa in PT space in the system studied, such liquids, in principle, can erupt through liquid immiscibility, as near-primary magmas from depths of approximately 60 km. ©2013. American Geophysical Union. All Rights Reserved.
Rice M.S.,California Institute of Technology |
Cloutis E.A.,University of Winnipeg |
Bell J.F.,Arizona State University |
Bish D.L.,Indiana University |
And 6 more authors.
Icarus | Year: 2013
Hydrated silica-rich materials have recently been discovered on the surface of Mars by the Mars Exploration Rover (MER) Spirit, the Mars Reconnaissance Orbiter (MRO) Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), and the Mars Express Observatoire pour la Minéralogie, l'Eau, les Glaces, et l'Activité (OMEGA) in several locations. Having been interpreted as hydrothermal deposits and aqueous alteration products, these materials have important implications for the history of water on the martian surface. Spectral detections of these materials in visible to near infrared (Vis-NIR) wavelengths have been based on a H2O absorption feature in the 934-1009nm region seen with Spirit's Pancam instrument, and on SiOH absorption features in the 2.21-2.26μm range seen with CRISM. Our work aims to determine how the spectral reflectance properties of silica-rich materials in Vis-NIR wavelengths vary as a function of environmental conditions and formation. Here we present laboratory reflectance spectra of a diverse suite of silica-rich materials (chert, opal, quartz, natural sinters and synthetic silica) under a range of grain sizes and temperature, pressure, and humidity conditions. We find that the H2O content and form of H2O/OH present in silica-rich materials can have significant effects on their Vis-NIR spectra. Our main findings are that the position of the ~1.4μm OH feature and the symmetry of the ~1.9μm feature can be used to discern between various forms of silica-rich materials, and that the ratio of the ~2.2μm (SiOH) and ~1.9μm (H2O) band depths can aid in distinguishing between silica phases (opal-A vs. opal-CT) and formation conditions (low vs. high temperature). In a case study of hydrated silica outcrops in Valles Marineris, we show that careful application of a modified version of these spectral parameters to orbital near-infrared spectra (e.g., from CRISM and OMEGA) can aid in characterizing the compositional diversity of silica-bearing deposits on Mars. We also discuss how these results can aid in the interpretation of silica detections on Mars made by the MER Panoramic Camera (Pancam) and Mars Science Laboratory (MSL) Mast-mounted Camera (Mastcam) instruments. © 2012 Elsevier Inc.
Rosenkjaer G.K.,University of British Columbia |
Gasperikova E.,Lawrence Berkeley National Laboratory |
Newman G.A.,Lawrence Berkeley National Laboratory |
Arnason K.,Iceland GeoSurvey |
Lindsey N.J.,Lawrence Berkeley National Laboratory
Geothermics | Year: 2015
The magnetotelluric (MT) method is important for exploration of geothermal systems. The information on the Earth's resistivity obtained with MT methods has been valuable in imaging the hydrothermal alteration of such systems. Given its ability to recover complex resistivity models for the Earth, three-dimensional (3D) MT inversion has become a common practice in geothermal exploration. However, 3D inversion is a time-consuming and complicated procedure that relies on computer algorithms to search for a model that can explain the measured data to a sufficient level. Furthermore, many elements of inversion require input from the practitioner, which can easily bias the results. Consequently, final 3D MT results depend on various factors, including the inversion code, the model mesh used to represent the Earth, data quality and processing, and constraints imposed during the inversion procedure.In this paper, to explore how this variability in 3D MT modeling impacts the final model, we invert MT data sets from the Krafla and Hengill geothermal areas in Iceland, using three different inversion codes. In each case, the modelers had the freedom to select a subset of the data and implement the inversion for the respective code in an optimized way. We compare the results from all the inversion codes, as well as consider the setup and assumptions made during the inversion process, all of which helps enhance the robustness and quality of the results. The comparison is done in multiple ways, using visual comparison of the recovered resistivity models, as well as comparing the structural similarities of the models by employing a structural correlation metric based on cross-gradients and other types of metrics for structural correlation. This approach highlights structures that are common in all three models, and implies that these structures are independent of the inversion code and necessary to fit the data.All modeling results from both Krafla and Hengill are consistent to first order, recovering a conductive layer on top of a resistive core typical of high temperature geothermal systems. For Hengill, the models show strong structural agreement, with all inversions recovering a moderately layered resistivity model but adding detail to previous work done in the area. Major differences are found in areas with coarse data coverage and hence questionable model resolution. Where the recovered structures in different models coincide, our confidence that these structures are well-constrained by the data is elevated, in spite of the different setup and assumptions in the codes these structures are required; so they can be interpreted in terms of geology with more certainty. Results from Krafla are not as consistent as results for Hengill, related in part to the Krafla data being nosier than the Hengill data. The models from Krafla have coinciding larger structures, but small-scale structures there are less coherent. One of the consistent structures in all the models is a conductive zone reaching from a depth of 5. km to shallower depths in the northern part of the area. © 2015 Elsevier Ltd.
Gernigon L.,Geological Survey of Norway |
Blischke A.,Iceland GeoSurvey |
Nasuti A.,Geological Survey of Norway |
Sand M.,Norwegian Petroleum Directorate
Tectonics | Year: 2015
We have acquired and processed new aeromagnetic data that cover the entire oceanic Norway Basin located between the Møre volcanic rifted margin and the Jan Mayen microcontinent (JMMC). The new compilation allows us to revisit the structure of the conjugate volcanic (rifted) margins and the spreading evolution of the Norway Basin from the Early Eocene breakup time to the Late Oligocene when the Aegir Ridge became extinct. The volcanic margins (in a strict sense) that formed before the opening of the Norway Basin have been disconnected with the previous Jurassic-Mid-Cretaceous episode of crustal thinning. We also show evidence of relationships between the margin architecture, the breakup magmatism distribution along the continent-oceanic transition, and the subsequent oceanic segmentation. The Norway Basin shows a complex system of asymmetric oceanic segments locally affected by episodic ridge jumps. The new aeromagnetic compilation also confirms that a fan-shaped spreading evolution of the Norway Basin was clearly active before the cessation of seafloor spreading and extinction of the Aegir Ridge. An important Mid-Eocene kinematic event at around magnetic chron C21r can be recognized in the Norway Basin. This event coincides with the onset of diking and increasing rifting activity (and possible oceanic accretion?) between the proto-JMMC and the East Greenland margin. It led to a second phase of breakup and microcontinent formation in the Norwegian-Greenland Sea ~26 Myrs later in the Oligocene. Key Points 88.000 line km of new aeromagnetic data in the Norwegian-Greenland Sea Complete aeromagnetic coverage of the Norway Basin spreading system Prebreakup and postbreakup evolution of the rift/margin system/microcontinent ©2015. American Geophysical Union. All Rights Reserved.
Hey R.,University of Hawaii at Manoa |
Martinez F.,University of Iceland |
Hoskuldsson A.,University of Iceland |
Eason D.E.,University of Hawaii at Manoa |
And 4 more authors.
Earth and Planetary Science Letters | Year: 2016
The previous orthogonal ridge/transform staircase geometry south of Iceland is being progressively changed to the present continuous oblique Reykjanes Ridge spreading geometry as North America-Eurasia transform faults are successively eliminated from north to south. This reorganization is commonly interpreted as a thermal phenomenon, caused by warmer Iceland plume mantle progressively interacting with the ridge, although other diachronous seafloor spreading reorganizations are thought to result from tectonic rift propagation. New marine geophysical data covering our reinterpretation of the reorganization tip near 57°N show successive transform eliminations at a propagation velocity of ~110 km/Myr, ten times the spreading half rate, followed by abrupt reorganization slowing at the Modred transform as it was converted to a migrating non-transform offset. Neither the simple thermal model nor the simple propagating rift model appears adequate to explain the complicated plate boundary reorganization process. © 2015 Elsevier B.V..