An apatite for progress
Boulder, Colo., USA – Apatite has recently gained considerable attention as a mineral with many uses within the Earth and planetary sciences. Apatite chemistry has recently given new insight into a wide range of geological processes and tools, such as magmatism, metasomatism, planetary geochemistry, and geochronology. In their open-access Geology article, Emilie Bruand and colleagues expand the utility of apatite by presenting a novel way to fingerprint magma chemistry and petrogenesis using apatite inclusions within robust titanite and zircon.
Bruand and colleagues present trace element data from apatite mineral inclusions shielded within magmatic zircon and titanite. Importantly, apatite inclusion and host titanite chemistries detailed in this study allow estimation of the whole-rock Sr and SiO2. They show how these data can be used to assess the degree of fractionation of the host magma and to calculate key trace element abundances and ratios. They also demonstrate that the inclusions can be linked to discrete periods in the crystallization history of the host phases, thus providing insight into petrogenesis.
These results highlight the fact that apatite compositions might discriminate modern granitoids (younger than 2.5 Ga) from Archean-Proterozoic transitional granitoid compositions (sanukitoid signatures). Development of such a petrological tool has important potential for interpretation of provenance and a better understanding of the secular evolution of the continental crust, including that of early Earth.
An apatite for progress: Inclusions in zircon and titanite constrain petrogenesis and provenance
Emilie Bruand et al., School of Earth and Environmental Sciences, University of Portsmouth, Burnaby Building, Burnaby Road, Portsmouth PO1 3QL, UK. This paper is OPEN ACCESS online at http://geology.gsapubs.org/content/early/2016/01/04/G37301.1.abstract.
Other recently posted GEOLOGY articles are highlighted below:
Spatiotemporal variability of typhoon impacts and relaxation intervals on Jaluit Atoll, Marshall Islands
Murray R. Ford and Paul S. Kench, School of Environment, University of Auckland, 10 Symonds Street, Auckland 1010, New Zealand. This paper is online at http://dx.doi.org/10.1130/G37402.1.
Low-lying reef islands such as those in Tuvalu, Kiribati, and Marshall Islands are considered highly vulnerable to a range of climatic hazards including tropical storms. Within the Marshall Islands storms have had devastating impacts on atoll populations. However, the impacts of storms on the islands themselves are not well understood. Jaluit Atoll in the southern Marshall Islands was severely impacted by Typhoon Ophelia in 1958. Using a collection of aerial photos from before and after the typhoon, we were able to track the impacts of the storm around the atoll. Modern, high-resolution satellite imagery was then used to assess the current state of the islands over 50 years after the event. Our results show massive impacts of the typhoon, with widespread loss of land, including in some cases the complete destruction of islands. However, we also observed the significant ongoing reformation and recovery of the islands, to the extent that the land area on Jaluit Atoll now exceeds the pre-storm size and continues to increase. It appears that the storm generated vast quantities of sediment which has driven the growth of islands. Our observations occur in the context of local sea level rise and highlight the inherent dynamism of reef islands.
Paleogene laterites bearing the highest insect ichnodiversity in paleosols
Eduardo Bellosi et al., Museo Argentino de Ciencias Naturales, C1405DJR Buenos Aires, Argentina. This article is online at http://dx.doi.org/10.1130/G37250.1.
Laterites are red-colored residual deposits formed by intense and long-lasting subaerial weathering (i.e., soil/paleosol) of any rock or deposit under present or ancient warm and humid conditions. Eocene stacked laterites from Uruguay are unique in preserving in detail highly diverse and abundant assemblages of insect trace fossils. This paleopedological and ichnological study allows knowing the activity (nesting and pupating) of the invertebrate soil fauna from an ancient tropical-seasonal savanna. Trace fossils also helped for the reconstruction of complex and iterative sedimentologic and bio-geochemical processes that originated different facies in these laterites. Such processes were governed by cyclic changes in tectonics and climate, particularly iterative desiccation and rehydration stages.
Nanoscale constraints on porosity generation and fluid flow during serpentinization
Benjamin M. Tutolo et al., Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK. This paper is online at http://dx.doi.org/10.1130/G37349.1.
Olivine, the primary constituent of mantle rocks, is converted to hydrous phases when brought into contact with water at upper lithospheric temperatures. This reaction, known as serpentinization, results in some of the most extreme redox and pH environments on Earth and provides nutrients for unique, chemosynthetic microbial ecosystems, which have been inferred to represent possible modern analogues for the origin of life on Earth. Field samples of mantle rocks are nearly always serpentinized — commonly to completion –but, paradoxically, their intrinsic porosity and permeability are diminishingly low, and serpentinization is a solid volume-increasing reaction. Here, we utilize neutron scattering techniques to present the first measurements of the evolution of pore size and specific surface area distribution in partially serpentinized rocks. Our measurements and analyses demonstrate that serpentine and accessory phases form with their own, inherent porosity, which accommodates the bulk of diffusive fluid flow during serpentinization and thereby permits continued serpentinization after voluminous serpentine minerals fill reaction-generated porosity.
Slow cooling of the lowermost oceanic crust at the fast-spreading East Pacific Rise
Kathrin Faak, Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Universitätsstrasse 150, D-44801 Bochum, Germany; and Kathryn M. Gillis School of Earth and Ocean Sciences, University of Victoria, P.O. Box 3055 STN CSC, Victoria, British Columbia V8W 3P6, Canada. This paper is online at http://dx.doi.org/10.1130/G37353.1.
The processes involved in the accretion of molten rock from Earth's mantle to form new ocean crust and how the new crust cools is one of the great questions in earth science. Recent recovery of deep gabbroic rocks by the IODP Expedition 345 allowed us to determine the cooling history of the lowermost ocean crust formed along a mid-ocean ridge in the Eastern Pacific for the first time. As these rocks cool, their minerals exchange certain elements in ways that were calibrated in the laboratory. By measuring the distribution of these elements in different minerals it was possible to calculate how fast this cooling happens. Our results show that the lower oceanic crust cools slowly, at about 0.001 degrees C per year. Cooling rates for the deep gabbros are key to testing predictions of the thermal structure of the lower crust based on remote imaging techniques and thermal modeling. The slow cooling rates are most consistent with models dominated by conductive heat transport within the lower oceanic crust, rather than heat transport by deep hydrothermal circulation. This implies that the lower crust formed at fast spreading mid-ocean ridges remains "hot" (above 900 degrees C) up to 10 km away from the ridge axis.
Early hydrothermal carbon uptake by the upper oceanic crust: Insight from in situ U-Pb dating
Laurence A. Coogan et al., School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8P 5C2, Canada. This paper is online at http://dx.doi.org/10.1130/G37212.1.
The long-term carbon cycle involves the degassing of carbon from the solid earth (e.g. through volcanoes) and the drawdown of this CO2 into rocks through fluid-rock reactions. This cycle requires a feedback mechanism to prevent atmospheric CO2 levels becoming either too high or too low for Earth to maintain a clement climate. It has generally been though these fluid-rock reactions are largely associated with continental chemical weathering with rivers carrying the solutes derived from these reactions to the ocean. An alternative model proposes that the fluid-rock reactions occur in the oceanic crust with seawater reacting with the lavas that make up the upper oceanic crust forming carbonate minerals. Here we use a new dating approach to show that these seafloor carbonates are formed very early in the lifetime of a piece of oceanic crust. In turn this suggests that fluid-rock reactions within the oceanic crust rapidly sequester carbon from the ocean and that this process may provide an effective feedback on the long-term carbon cycle.
Calcite/aragonite ratio fluctuations in Aptian rudist bivalves: Correlation with changing temperatures
Enric Pascual-Cebrian et al., GeoScience Limited, Falmouth Business Park, Bickland Water Road, Falmouth TR11 4SZ, UK. This paper is online at http://dx.doi.org/10.1130/G37389.1.
Understanding how bivalves responded to past temperature fluctuations may help us to predict specific responses of complex calcifiers to future climate change. During the Aptian cold episode, aragonite-rich rudist bivalves decreased in abundance in northern Tethyan carbonate platforms, while rudists with a thickened calcitic outer shell layer came to dominate those of Iberia. Seawater cooling and variations in calcium carbonate saturation states may have controlled this faunal turnover. However, our understanding of how rudist lineages responded to changing environmental conditions is constrained by a lack of quantitative data on the evolution of thickness, size, and mineralogy of the shell. This study is based on volumetric measurements of the shell and shows the transition of a lineage of rudist bivalves from aragonite-rich mineralogy to calcite-dominated mineralogy, and returning to aragonite-dominated mineralogy. Calcite-dominated shells occurred during the Aptian cold episode, and more aragonitic shells were observed during the warmer intervals. These results imply a correlation between shell composition and temperature and suggest that some bivalves adapted to less favorable calcification conditions by changing calcite and aragonite proportions of their bimineralic shells and decreasing skeletal thickness, thereby reducing the cost of shell growth.
Age of the Laschamp excursion determined by U-Th dating of a speleothem geomagnetic record from North America
Ioan Lascu et al., Institute for Rock Magnetism, University of Minnesota, 100 Union Street SE, Minneapolis, Minnesota 55455, USA. This paper is online at http://dx.doi.org/10.1130/G37490.1.
The Laschamp excursion is a short-lived geomagnetic event during which the polarity of Earth's magnetic field reversed for a brief period, and its intensity decreased by an order of magnitude. Geomagnetic field decay rates during the Laschamp excursion are often cited as analogs for modern field decay rates. In addition to its geophysical significance, the Laschamp excursion is an important global geochronologic marker, occurring around the time of the demise of Homo neanderthalensis, in conjunction with rapid climatic oscillations leading into the Last Glacial Maximum, and coeval with a major supervolcano eruption. Precise determination of the timing and duration of the Laschamp would elucidate major scientific questions situated at the intersection of the physical and natural sciences. This study presents a North American speleothem geomagnetic record of the Laschamp excursion that is dated using a combination radioisotopic (U/Th) and incremental dating. This allows for the excursion timing and duration to be determined directly from a geological archive for the first time. In this record, the Laschamp excursion spans the interval 42,250 to 39,700 yrs before present (BP), with an age of 41,100 +/- 350 yrs BP for the period in which the virtual geomagnetic pole was situated in the southern hemisphere.
Water-fluxed crustal melting produces Cordilleran batholiths
William J. Collins et al., NSW Institute for Frontiers Geoscience, The University of Newcastle, NSW 2308, Australia. This paper is online at http://dx.doi.org/10.1130/G37398.1.
Most granites and related calc-alkaline silicic volcanic rocks from the United States and New Zealand Cordillera are saturated with zircon between 65 and 70 wt% SiO2. For this silica interval, zircon saturation temperatures (Tzr) are universally lower (900 degrees C). The values contrast with Tzr from alkaline rocks from the Cenozoic U.S. Cordillera, which are typically >800 degrees C for 65-70 wt% SiO2. Case studies of titanium-in-zircon thermometry from the U.S. Cordillera also suggest that alkaline magma injections into granitic magma chambers are hot, but calc-alkaline magma injections are usually cooler. A model is presented suggesting that silicic Cordilleran magmas form in magmatic arcs where hydrous basaltic magmas solidify in the arc root, producing mafic underplates that exsolve aqueous fluids, which transfer to the crust and promote water-fluxed partial melting at ambient pressure-temperature (~750-800 degrees C at 8 kbar) conditions. Subsequent rock-buffered melting reactions modulate the water content of arc magmas. The granitic partial melts are water undersaturated, rise adiabatically as increments, but stall in the middle to upper crust, building cool and hydrous, crystal-rich magma chambers (batholiths). However, injections of hotter magmas are required to drive volcanic eruption. In the backarc, granitic magma chambers are intermittently recharged with hotter, drier alkaline magmas, which are produced mostly by decompression melting during lithospheric extension, not hydrous fluxing. This highlights the control of subduction dynamics on water content and consequently magmatic temperatures in silicic magma systems.
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