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GEOLOGY returns to Naica Cave, Mexico, and extends its reach to Mercury
21 December 2012
Geological Society of America, The (GSA)
New Geology articles posted online ahead of print 15 Nov. through 13 Dec. 2012
GEOLOGY ends 2012 with 23 new articles spanning a variety of geoscience phenomena and locations, including Mercury; Naica Cave, Mexico; Diamantina, Brazil; the Galápagos hotspot; China; the Aleutian island arc; Disko Bay and Uummannaq Fjord, central West Greenland; the California arc; the Pacific Ring of Fire; Po Plain, Italy; Torfajökull, Iceland; the U.S. Sierra Nevada; Spain; New Zealand; Turkey; Connecticut, USA; and Texas, USA.
1. High-resolution images obtained by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in orbit about Mercury;
2. Further study into the giant gypsum crystals of Naica, Mexico;
3. The provenance of North Atlantic ice-rafted debris;
4. Direct dating of gold at Diamantina, Minas Gerais, Brazil;
5. Influence of the Galápagos hotspot on the East Pacific Rise;
5. Earliest chitinozoans, an enigmatic group of marine microorganisms, found in China;
6. Deep seismic reflection profiling over the eastern Aleutian arc;
7. Marine geophysical images of seafloor landforms and sub-seafloor sediments at Disko Bay and Uummannaq Fjord, central West Greenland;
8. The life and death of volcanos, with the California arc for example;
9. New spectral analysis of well-preserved ash plume deposits in marine sediments along the Pacific Ring of Fire;
10. Sequence stratigraphy and the resolution of fossil records from Po Plain;
11. New results showing that very small amounts of water are highly effective in facilitating mineral reactions where water acts as a catalyst;
12. New research showing that magmatic water alone can power the explosivity of Icelandic rhyolite eruptions;
13. Air parcel trajectory analysis showing that high Sierran topography is a dominant control on how precipitating air parcels travel through the Sierra Nevada region;
14. Climate, landscape, and human impact on small mammal diversity in Pleistocene to early Holocene Spain;
15. The impact of microbial processes on maintaining terrestrial chemical weathering rates in cooling climates during glacial advance;
16. The unexpected find of a large, coherent, chunk of early Proterozoic mantle (~1.9 billion years old) underlying New Zealand’s South Island;
17. Laboratory experiments that investigate processes occurring in magma as it rises from depth and is erupted;
18. No evidence for the Cimmerian continental ribbon in the Jurassic accretionary complex and ophiolite from northeast Turkey;
19. Laboratory experiments regarding degassing-induced crystallization in basalts;
20. A new discovery that will force a rethinking of how the deep rocks of the Appalachian Mountains were formed;
21. Evidence that shell-crushing predators readily attacked gastropods, while still attacking certain types of brachiopods, as seen in a series of mixed brachiopod-mollusc communities preserved in 300-million-year-old shales from Texas, USA.
22. The chemical diversity preserved in large crystals in erupted lava show that magma interactions during transport from beneath mid-oceanic ridges are important for modulating the types of lavas eventually erupted; and
23. The discovery by geophysicists of a 2-km-thick salt body in the Gulf of California that extends over an area about the size of the U.S. state of Rhode Island, the largest salt body ever found there.
Highlights are provided below. GEOLOGY articles published ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary GEOLOGY articles by contacting Kea Giles at the address above.
Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Kea Giles for additional information or assistance.
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Extension and contraction within volcanically buried impact craters and basins on Mercury
Thomas R. Watters et al., Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington DC 20560, USA. Posted online 15 Nov. 2012; http://dx.doi.org/10.1130/G33725.1.
The innermost planet of our Solar System is generally crisscrossed by contractional landforms that formed as the interior cooled and surface area contracted. Contraction has been so dominant on Mercury that landforms produced by extension or horizontal stretching of crustal material had not been previously documented outside the interiors of large impact basins. High-resolution images obtained by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft in orbit about Mercury reveal extensional troughs or graben encircled by contractional wrinkle ridges on the largest expanses of volcanic plains. These systems of tectonic landforms, unlike any seen elsewhere, are associated with ghost craters -- impact craters that were flooded and buried by lava flows. The special arrangement of wrinkle ridges and graben in ghost craters on Mercury likely resulted from a combination of extension generated by cooling and thermal contraction of thick lava flow units and global contraction from cooling of the planet's interior. The rapid accumulation of highly fluid lava flows into thick cooling units on a planet undergoing a high rate of global contraction may be why these systems of tectonic landforms in ghost craters on Mercury have not been seen elsewhere in the Solar System.
Determining gypsum growth temperatures using monophase fluid inclusions—Application to the giant gypsum crystals of Naica, Mexico
Yves Krüger et al., Institute of Applied Physics, University of Bern, Bern, Switzerland; corresponding author: Juan Manuel García-Ruiz, Laboratorio de Estudios Cristalográficos, IACT, CSIC, Universidad de Granada, Granada, Spain. Posted online 28 Nov. 2012; http://dx.doi.org/10.1130/G33581.1.
Determining the formation temperature of minerals using fluid inclusions is a crucial step in understanding rock-forming scenarios. Unfortunately, fluid inclusions in minerals formed at low temperature, such as gypsum, are commonly in a metastable monophase liquid state. To overcome this problem, ultra-short laser pulses can be used to induce vapor bubble nucleation, thus creating a stable two-phase fluid inclusion appropriate for subsequent measurements of the liquid-vapor homogenization temperature, Th. In this study, Kruger and colleagues evaluate the applicability of Th data to accurately determine gypsum formation temperatures. Their results support the earlier hypothesis that the population and the size of the Naica crystals were controlled by temperature. In addition, this experimental method opens a door to determining the growth temperature of minerals forming in low-temperature environments.
Provenance of North Atlantic ice-rafted debris during the last deglaciation—A new application of U-Pb rutile and zircon geochronology
David Small et al., School of Geography and Geosciences, University of St Andrews, North Street, St Andrews, Fife KY16 9AL, Scotland, UK. Posted online 28 Nov. 2012; http://dx.doi.org/10.1130/G33594.1.
North Atlantic Ocean sediments, dated to the end of the last glacial period, contain material deposited from icebergs (Ice Rafted Detritus, IRD) that came from surrounding ice sheets. Establishing which ice sheets were responsible for depositing this material at any particular time allows inferences to be made regarding the behavior of said ice sheets. This paper presents the first study to use the well-established uranium-lead dating technique to investigate the sources of IRD by establishing the age of individual minerals and comparing these to the ages of potential source rocks that would have been eroded by ice sheets. The results of this study show that IRD found within an ocean core near to the former ice sheet that covered Scotland cannot have come from Scotland alone and that other, distant, ice sheets (Greenland or North America) must have contributed. This conclusion provides insights into how quickly IRD is deposited around the ocean and supports previous suggestions that such dispersal is rapid.
Direct dating of gold by radiogenic helium: Testing the method on gold from Diamantina, Minas Gerais, Brazil
Alexandre Raphael Cabral et al., Mineral Deposits, Technische Universität Clausthal, Adolph-Roemer-Strasse 2A, D-38678 Clausthal-Zellerfeld, Germany. Posted online 28 Nov. 2012; http://dx.doi.org/10.1130/G33751.1.
Natural gold, which formed in the geological past (at a million-year scale), retains the noble gas helium produced by radiogenic decay of uranium and thorium, elements that are incorporated in trace amounts in gold. New analytical advances make it feasible to measure within the same gold sample of as little as two milligrams (0.002 g) the contents of uranium, thorium and radiogenic helium. The amounts of these elements are then used to calculate the age of gold formation. This method of direct dating of gold has been applied to a historically important gold deposit in Diamantina, Minas Gerais, Brazil, giving a (U–Th)/He age of 515 ± 55 Ma (millions of years). The independent and well established U–Pb dating method on rutile, a titanium dioxide, which coexists with the Diamantina gold, confirms the result of the new dating technique.
Influence of the Galápagos hotspot on the East Pacific Rise during Miocene superfast spreading
J. Geldmacher et al., GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstrasse 1-3, 24148 Kiel, Germany. Posted online 28 Nov. 2012; http://dx.doi.org/10.1130/G33533.1.
This study investigates the geochemical composition of lava rocks that formed between 7 and 23 million years ago at the equatorial East Pacific Rise, one of the major seafloor spreading centers where oceanic crust is produced by submarine volcanism. Spreading center magmas generally originate at relatively shallow depths. It is found that lavas formed between 11 and ca. 22 m.y. show the influence of a geochemically enriched magma source, which is commonly ascribed to the deep Earth's mantle. This material is believed to rise to the surface at local melting anomalies, the so-called hotspots. Although the Galápagos hotspot (forming the famous volcanic Galápagos Islands) is located near the investigated segment of the East Pacific Rise, no evidence for any influence of the hotspot on the current spreader center and its lava compositions can be seen. However, the observed geochemical influence of hotspot material in the 11-22 m.y. lavas correlates with an interval of extreme ("superfast") seafloor spreading (up to 200 mm/y in contrast to today's 130 mm/y). It is suggested that increased spreading and magma production could have facilitated shallow flow of Galápagos hotspot material to the East Pacific Rise were it has contaminated the local upper mantle spreading center source region.
Earliest chitinozoans discovered in the Cambrian Duyun fauna of China
Cen Shen et al. (Xi-guang Zhang, corresponding), Key Laboratory for Paleobiology, Yunnan University, Kunming, Yunnan 650091, China. Posted online 28 Nov. 2012; http://dx.doi.org/10.1130/G33763.1.
Chitinozoans, an enigmatic group of marine microorganisms, have long been discovered from the early Ordovician to the latest Devonian strata with a worldwide distribution. Usually their organic-walled tests may be preserved as individual vesicles, in aggregate masses, in chains, or associated with organic envelopes (cocoons), exhibiting a strikingly morphological diversification. These advantages have enabled this microfossil group to be successfully applied for biostratigraphic subdivision of the Paleozoic. However information about their origin and early evolution is sparse. With efficient collaboration, a team of paleontologists coming from three countries has reported the oldest hitherto known chitinozoan Eisenackitina? sp. from the Cambrian Stage 5 (~510 million years old) of China, extending the record of this group back by at least 20 million years. The exceptional occurrence of the Cambrian chitinozoan along with many other phosphatized fossils, including embryos in Orsten-type preservation, might imply a benthic mode of life for itself. This discovery not only supplies critical data for evaluation of the basic morphology of the Cambrian chitinozoans, but also sheds new light on assessment of their early evolutionary process.
Seismic reflection imaging of ultradeep roots beneath the eastern Aleutian island arc
Andrew J. Calvert (corresponding) and Susan E. McGeary, Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada. Posted online 28 Nov. 2012; http://dx.doi.org/10.1130/G33683.1.
Volcanic island arcs such as the present day Aleutian and Izu-Bonin-Mariana island chains form where an oceanic tectonic plate subducts into the mantle beneath another piece of oceanic lithosphere. Ancient island arcs are believed to have been the fundamental building blocks that were assembled by plate tectonics to create the continents over the last approx. three billion years. However, the process through which the relatively buoyant continental crust is derived from the denser rocks that form much of island arc crust is unclear. Seismic refraction surveys, which estimate a proxy for crustal composition, namely the seismic wave speed, indicate that arc crust is commonly too thin (12-35 km) to contain the dense mafic/ultramafic root that should be produced when continental-like andesitic rocks that form the middle crust of many island arcs are generated. Deep seismic reflection profiling over the eastern Aleutian arc shows a 25 km-thick package of reflectors, which extends from the lower crust to depths as great as 50 km. However, reflective roots are not apparent further west in the central Aleutian arc. Deep arc roots can be denser than the uppermost mantle, detach from the base of the evolving island arc and sink back into the underlying mantle, and this process may explain the differences observed beneath the Aleutian arc.
An extensive and dynamic ice sheet on the West Greenland shelf during the last glacial cycle
C. Ó Cofaigh et al., Department of Geography, Durham University, Durham DH1 3LE, UK. Posted online 30 Nov. 2012; http://dx.doi.org/10.1130/G33759.1.
The extent and timing of the advance and retreat of the Greenland Ice Sheet on the West Greenland continental shelf bordering Baffin Bay during the last glaciation remains uncertain. This paper resolves this issue for the continental shelf offshore of Disko Bay and Uummannaq Fjord, central West Greenland. Marine geophysical images of seafloor landforms and sub-seafloor sediments, combined with dated sediment cores, show that the Greenland Ice Sheet extended to the shelf edge as fast-flowing ice streams, including the ancestral Jakobshavn Isbræ, during the last glaciation about 20,000 years ago. The ice sheet had commenced retreat from the shelf edge by 14,800 years ago but in "Disko Trough," offshore of Disko Bay, sediment core evidence shows that it underwent a subsequent re-advance onto the outer shelf during the Younger Dryas cold period (12,800-11,700 years ago). This is the first evidence of a major Younger Dryas advance of the Greenland Ice Sheet on the West Greenland shelf, although the short lived duration suggests that it may have been out of phase with Younger Dryas temperatures.
Detrital zircon as a proxy for tracking the magmatic arc system: The California arc example
A.P. Barth et al., Department of Earth Sciences, Indiana University–Purdue University Indianapolis, 723 West Michigan Street, Indianapolis, Indiana 46202, USA. Posted online 30 Nov. 2012; http://dx.doi.org/10.1130/G33619.1.
The life and death of volcanos record the tempo and pulses of magma flow at the Earth's surface, but much of the plumbing and the early history of these volcanoes are hidden from view. This study by Andrew Barth and colleagues suggests that combining signals recorded in intrusive igneous rocks and in volcanic sediment can illuminate the waxing and waning of these magma systems over time. Geochemical signals of magma generation and intrusion are recorded in the plumbing of deeply-eroded volcanoes, and these same signals can be preserved in volcanic material washed into adjacent sedimentary basins. The combined geochemical records reveal the timing and nature of pulses and lulls in magmatism where subduction has occurred over very long periods of time. This novel set of observations demonstrates a new method for reconstructing the very long-term pulse of magma generation processes above subduction zones.
A detection of Milankovitch frequencies in global volcanic activity
Steffen Kutterolf et al., Collaborative Research Center (SFB) 574, GEOMAR, Wischhofstrasse 1-3, 24148 Kiel, Germany. Posted online 30 Nov. 2012; http://dx.doi.org/10.1130/G33419.1.
A rigorous detection of Milankovitch periodicities in volcanic output across the Pleistocene-Holocene ice age has remained elusive. Steffen Kutterolf and colleagues report on a new spectral analysis of a large number of well-preserved ash plume deposits recorded in marine sediments along the Pacific Ring of Fire. Our analysis yields a statistically significant detection of spectral peaks at the obliquity and 100,000-year periods. We propose that the variability in volcanic activity results from crustal stress changes associated with ice age mass redistribution. In particular, increased volcanism lags behind the highest rate of increasing sea level by 4.0 plus or minus 3.6 thousand years and correlates well with numerical predictions of stress changes at volcanically active sites. These results strongly support the presence of a feedback amongst ice age climate, volcanism and continental stress field.
Sequence stratigraphy and the resolution of the fossil record
Daniele Scarponi et al., Dipartimento di Scienze Biologiche Geologiche e Ambientali, University of Bologna, via Zamboni 67, 40126 Bologna, Italy. Posted online 30 Nov. 2012; http://dx.doi.org/10.1130/G33849.1.
Sequence stratigraphy, a key geoscientific achievement of the last decades, offers geologists a process-based framework for understanding how sediments assemble into sedimentary successions as the sea level and other geological processes change through time. Yet, the basic model's key predictions have never been tested rigorously using numerical dating methods that allow for quantification of changes in the pace at which sedimentary strata accumulate. This report, focused on the recent geological past (latest Quaternary), shows that direct dating of fossil shells provides direct quantitative estimates on the temporal resolution, frequency of bed-forming events and sedimentation rates at the level of individual strata. In addition, this dating effort confirms that the quality of fossil record is strongly influenced by glacio-eustatic sea-level changes. The extensive dating of shells from Po Plain sediments highlights the fact that the changes in sea level not only may produce distinct sequence stratigraphic architectures, but also affect the quality, quantity, and the temporal nature of paleontological and sedimentological records.
Very little water is necessary to make a dry solid silicate system wet
Ralf Milke et al., Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstrasse 74-100, 12249 Berlin, Germany. Posted online 30 Nov. 2012; http://dx.doi.org/10.1130/G33674.1.
The standard rim-forming mineral reaction olivine + quartz = orthopyroxene has been experimentally performed at very dry conditions, i.e., only 20 wt ppm of water present in the sample container as a fluid, and most "water" was hydrogen dissolved within the solid crystals. Reaction rates and resulting fabrics at olivine-quartz interfaces mimic reaction features previously known from water-rich conditions (tens of thousands ppm). This experiment by Ralf Milke and colleagues indicates that very small amounts of water (tens of ppm of the entire sample) are highly effective in facilitating mineral reactions where water acts as a catalyst and creates porosity that might start to migrate from the initial centers of self-propagating mineral reaction, if rock deformation allows. The threshold between dry and wet in granulite and eclogite facies rocks is crossed once the nominally anhydrous minerals are saturated with hydrogen.
Explosive subglacial rhyolitic eruptions in Iceland are fueled by high magmatic H2O and closed-system degassing
Jacqueline Owen et al., Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK. Posted online 30 Nov. 2012; http://dx.doi.org/10.1130/G33647.1.
New research has discovered that magmatic water alone can power the explosivity of Icelandic rhyolite eruptions. Previously, scientists were aware of explosive rhyolitic eruptions from Iceland, but firm evidence of what might be powering the explosions and how much magmatic water the magma could contain were both lacking. The study by Jacqui Owen, Hugh Tuffen (both Lancaster University) and Dave McGarvie (The Open University) investigated samples collected from numerous subglacial rhyolitic edifices in Torfajökull (Iceland) that showed contrasting eruptive behavior. This included material from one of Iceland's most powerful subglacial rhyolitic eruptions which occurred 70,000 years ago and distributed thick ash layers across Europe. This new research shows a strong correlation between the explosivity of the eruptions and both the initial water content and "openness" of degassing; suggesting that magmatic volatile content alone is sufficient to drive powerful Icelandic eruptions, even those that initiate beneath thick ice sheets. Furthermore, Icelandic magma may contain much more water than previously thought, with values twice as high as expected being measured. This research helps explain the power behind some of Iceland's most explosive ash producing eruptions and may assist scientists working on the potential impact of future explosive rhyolitic eruptions on air travel.
Refining paleoaltimetry reconstructions of the Sierra Nevada, California, using air parcel trajectories
Alex R. Lechler and Joseph Galewsky, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA. Posted online 30 Nov. 2012; http://dx.doi.org/10.1130/G33553.1.
The elevation history of the Sierra Nevada Mountains of California remains enigmatic. Debate is generally centered on whether the Sierra has been a high-standing topographic feature, similar to modern, for at least the last 30-40 million years or if the Sierra experienced 1-2 km of elevation increase within the last 10 million years. University of New Mexico researchers Alex Lechler and Joseph Galewsky present a modern (A.D. 1979-2010) air parcel trajectory analysis that shows that high Sierran topography is a dominant control on how precipitating air parcels travel through the Sierra Nevada region. Specifically, precipitation at sites on the leeward (east) side of the Sierra is commonly delivered by parcels that travel around, rather than over, the Sierran crest. This trajectory analysis indicates that stable isotope-based estimates for Sierran paleo-elevations are likely limited by incorrect assumptions that precipitating air parcels surmount Sierra Nevada topography relatively unimpeded. As a result, stable isotope paleoaltimetry approaches alone cannot fully resolve past Sierra Nevada topography and are incapable of ruling out 1-2 km of elevation gain in the last 10 million years.
Small-mammal diversity in Spain during the late Pleistocene to early Holocene: Climate, landscape, and human impact
Juan Manuel López-García et al., IPHES (Institut Català de Paleoecologia Humana i Evolució Social), C/Escorxador s/n, E- 43003 Tarragona, Spain. Posted online 30 Nov. 2012; http://dx.doi.org/10.1130/G33744.1.
This article shows, by the application of quantitative ("Simpson Diversity Index" and "Habitat Weighting Method" estimating the diversity of past ecosystems and the landscape evolution) and qualitative methods ("Mutual Climatic Range Method or MCR" -- estimating past temperatures and precipitations) on the small mammal assemblages, new data about the human influence on small mammal species This influence is reflected in the Iberian archaeological sites since the Neolithic period with the establishment of the agriculture. However, in previous periods (late Pleistocene and early Holocene) environmental and climatic factors, such as the vegetation coverage or the temperatures, were the determinant causes in the specific composition of the small mammal species.
A microbial driver of chemical weathering in glaciated systems
Scott N. Montross et al., Department of Geography, Queen's University, Kingston, Ontario K7L 3N6, Canada. Posted online 13 Dec. 2012; http://dx.doi.org/10.1130/G33572.1.
Glaciological processes under ice sheets provide sustainable ecosystems for microbes, forming an aquatic environment through basal melting, and providing nutrients and energy from bedrock. Microbes facilitate solute production in most Earth-surface environments, but the balance of biotic and abiotic weathering in subglacial environment is presently unknown. This study by Scott N. Montross and colleagues demonstrates an up to eightfold increase in dissolved major cations in biotic relative to abiotic weathering experiments using glacial sediments and meltwater. This conclusion greatly expands our view of Earth's biogeochemically active weathering zone by incorporating the large wet-based portions of glaciated continents, both at present and during Earth's history. The profound environmental significance is that microbial processes have the ability to maintain terrestrial chemical weathering rates in cooling climates during glacial advance.
Extreme persistence of cratonic lithosphere in the southwest Pacific: Paleoproterozoic Os isotopic signatures in Zealandia
Alex J. McCoy-West et al., Research School of Earth Sciences, Australian National University, Canberra, NSW 0200, Australia. Posted online 13 Dec. 2012; http://dx.doi.org/10.1130/G33626.1.
Pieces of the mantle in the form of xenoliths entrained in basaltic magmas provide a rare opportunity to directly study the composition and age structure of the largely inaccessible deep Earth. Osmium isotopic data from mantle xenoliths from around Zealandia, a largely submerged, continental fragment, provide new constraints on continent formation in the southwest Pacific. Alex McCoy-West and colleagues have discovered a large, coherent, chunk of early Proterozoic mantle (~1.9 billion years old) underlying New Zealand's South Island. This is a highly unexpected find because the basement crustal rocks in this region are only 200 million years old. This >1.7 billion years age difference is the largest temporal decoupling between the mantle lithosphere and overlying crust yet observed, suggesting that old cratonic mantle lithosphere may be responsible for the stabilization of new continental crust, even on the modern Earth. Additionally, the discovery of extensive ancient lithosphere within Zealandia provides new information on its origins and assembly history, with tectonic implications for the present-day development of the Australia-Pacific plate boundary cutting through New Zealand.
Direct observations of degassing-induced crystallization in basalts
L. Jane Applegarth et al. (Hugh Tuffen, corresponding), Lancaster Environment Centre, Lancaster University, Bailrigg, Lancaster LA1 4YQ, UK. Posted online 13 Dec. 2012; http://dx.doi.org/10.1130/G33641.1.
This article describes laboratory experiments that investigate processes occurring in magma as it rises from depth and is erupted. During magma ascent, the degassing of dissolved water disturbs the chemical equilibrium of the melt, and can trigger crystal growth without any drop in temperature. Isothermal crystal growth can dramatically increase magma viscosity, and therefore influence whether an eruption is explosive or effusive. These experiments by Jane Applegarth and colleagues use a microscope and heated stage to make the first direct observations of degassing-driven crystal growth in natural basaltic samples at magmatic temperatures. Their results therefore indicate that when melts are sufficiently water-rich, degassing can trigger rapid crystal growth. Direct observation also allows maximum crystal growth rates to be measured, which may be more important than average growth rates in determining the impact of degassing.
Jurassic accretionary complex and ophiolite from northeast Turkey: No evidence for the Cimmerian continental ribbon
Gültekin Topuz et al., Istanbul Teknik Üniversitesi, Avrasya Yer Bilimleri Enstitüsü, 34469 Maslak, Istanbul, Turkey. Posted online 13 Dec. 2012; http://dx.doi.org/10.1130/G33577.1.
Permian-Triassic and Late Cretaceous accretionary complexes, ascribed to the consumption of two distinct oceans, the Paleo- and Neo-Tethys, are exposed over extensive areas in the Eastern Mediterranean region. However, a separating continental ribbon, the so-called Cimmeride continent, between the Paleo- and Neo-Tethys during early Mesozoic time cannot be defined. Topuz and colleagues report a previously unknown Early Jurassic metamorphic oceanic accretionary complex and ophiolite from northeast Turkey, bounded by oceanic accretionary complexes of Permian-Triassic and Late Cretaceous age to the north and the south, respectively, without a continental domain in between. This special tectonic position and widespread coexistence of Permian-Triassic and Late Cretaceous accretionary complexes alongside the Izmir-Ankara-Erzincan suture imply that (1) the southern margin of Laurasia in the eastern Mediterranean region grew by episodic accretionary processes from late Paleozoic to end-Mesozoic time without involvement of a Cimmerian continental ribbon, and (2) the Paleo-Tethys and northern branch of the Neo-Tethys were not distinct oceans in the Eastern Mediterranean region.
Discovery of ultrahigh-temperature metamorphism in the Acadian orogen, Connecticut, USA
Jay J. Ague et al., Department of Geology and Geophysics, Yale University, P.O. Box 208109, New Haven, Connecticut 06520-8109, USA. Posted online 13 Dec. 2012; http://dx.doi.org/10.1130/G33752.1.
Over the past two decades it has become clear that the continental crust can be heated to extreme, "ultrahigh" temperatures (UHT) in excess of 900 degrees C (about 1650 degrees F) over large regions during metamorphism and mountain building. How metamorphic rocks of the crust reach such extreme temperatures remains as a fundamental unresolved problem in the geosciences, with major implications for rock strength, generation of magmas (which may ascend from depth to produce ore deposits and volcanic eruptions), heat transfer, and Earth's overall heat budget. This paper by Jay Ague and colleagues documents newly-discovered UHT rocks in northeastern Connecticut, the first regional UHT locality found in the United States. The rocks underwent extreme temperature conditions of roughly 1000 degrees C (more than 1800 degrees F) at depths of at least 35 km (about 22 miles) below the surface during Appalachian mountain building in the Devonian Period. This discovery will force a rethinking of how the deep rocks of the Appalachian Mountains were formed, and shed new light on how UHT conditions can be reached.
Ecological effects of the Paleozoic-Modern faunal transition: Comparing predation on Paleozoic brachiopods and molluscs
Lindsey R. Leighton et al., Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada. Posted online 13 Dec. 2012; http://dx.doi.org/10.1130/G33750.1.
Marine life, like life on land, has changed considerably through time. Over 350 million years ago, brachiopods, a type of shellfish, were among the most common animals living on the sea-floor. In modern oceans, brachiopods are rare, whereas molluscs, such as bivalves (clams) and gastropods (snails) are very common. The transition from one group of animals to the other was gradual, but by approximately 300 million years ago, molluscs and brachiopods were living together in some environments. The appearance of molluscs in these settings meant that predators that previously had fed on brachiopods now had access to new, molluscan, prey. Molluscs are usually fleshier than brachiopods, and so, all other things being equal, we would predict that predators (animals that eat other animals) would prefer to eat molluscs because doing so would increase nutritional gain relative to the effort spent capturing the prey. Lindsey Leighton and colleagues tested this hypothesis by examining a series of these mixed brachiopod-mollusc communities, which were preserved in 300 million year old shales from Texas. They found that shell-crushing predators readily attacked gastropods, while still attacking certain types of brachiopods. Thus, the nature of predation also started becoming more modern at this early date.
Diverse Sr isotope signatures preserved in mid-oceanic-ridge basalt plagioclase
Amy E. Lange et al., College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA. Posted online 13 Dec. 2012; http://dx.doi.org/10.1130/G33739.1.
The mid-ocean ridge system encompasses 70,000 kilometers of volcanic activity caused by spreading of the oceanic plates. Beneath the ridges, magma must travel through several kilometers of crust before it erupts. Due to our inability to directly observe what happens beneath the volcanoes, erupted material is regularly studied to better understand subsurface processes. Large crystals in erupted lava form at depth and preserve a chemical fingerprint of the conditions in which they grew, allowing us to have a window into the behavior of magma below the ridges. Using these crystals, Amy Lange and colleagues determined that the crystals often interact with multiple magmas beneath the ridge prior to eruption. Furthermore, the range of chemical values observed in a limited number of crystals approaches the range observed for erupted lavas at an entire ridge segment. The chemical diversity preserved in the crystals suggests that a wide range of magmas are delivered below mid-ocean ridge volcanoes on a small enough spatial and temporal time scale to be sampled by individual crystals. They observed these phenomena at the multiple ridges examined in this global study. These results suggest that magma interactions during transport are important for modulating the types of lavas that are eventually erupted.
Thick evaporites and early rifting in the Guaymas Basin, Gulf of California
Nathaniel C. Miller, Marine Geology and Geophysics, Massachusetts Institute of Technology/Woods Hole Oceanographic Institution Joint Program, 266 Woods Hole Road, Woods Hole, Massachusetts 02543, USA; and Daniel Lizarralde, Department of Geology and Geophysics, Woods Hole Oceanographic Institution, 266 Woods Hole Road, Woods Hole, Massachusetts 02543, USA. Posted online 13 Dec. 2012; http://dx.doi.org/10.1130/G33747.1.
Geophysicists working in the Gulf of California have discovered a 2-km-thick salt body that extends over an area about the size of the U.S. state of Rhode Island. The Gulf of California is a young ocean basin that formed when relative movement between the Pacific and North American plates caused the Earth's crust and upper mantle to break apart there about 6 million years ago. In rift zones like the Gulf of California, thick salt bodies often form as seawater seeps into the newly forming basin and evaporates. Such salt bodies have not been found in the Gulf of California before, and details about the shape, size, and age of the salt provide geologists with new clues about how this young ocean basin formed.