New Geology Articles Published Online Ahead of Print in July
Boulder, Colo., USA: Article topics include the central nervous system of a
310-million-year-old horseshoe crab; the Line Islands volcanic chain;
“Jurassic Arc”; the case of the Cali glass; the nature of magmatism; and
the supergiant Olympic Dam uranium deposit. These Geology articles
are online at
https://geology.geoscienceworld.org/content/early/recent
.
Linking rivers to the rock record: Channel patterns and paleocurrent
circular variance
C.P. Galeazzi; R.P. Almeida; A.H. do Prado
Abstract:
Alluvial rivers are the most important agents of sediment transport in
continental basins, whose fluvial deposits enclose information related to
the time when rivers were active. In order to extract the most information
from fluvial deposits in the sedimentary record, it is imperative to
quantify the natural variability of channel patterns at the global scale,
explore what controls may influence their development, and investigate
whether channel pattern information is preserved in the alluvial plains in
order to develop tools for recognizing them in the sedimentary record. By
surveying 361 reaches of modern alluvial rivers with available water
discharge data at a global scale, we present a quantitative channel pattern
classification based on sinuosity and channel count index applicable to the
recognition in the rock record. A continuum of channel patterns ranging
from high-sinuosity single channel to lowsinuosity multichannels is
documented, along with the proportion of depositional elements in their
alluvial plains and their conditions of occurrence. Preserved barforms in
the alluvial plains of these rivers are used to infer and quantify
paleoflow directions at the channel-belt scale and result in ranges of
paleocurrent circular variance that may lead to channel pattern
identification in the rock record. Data from this work indicate that the
recognition of channel patterns may be used to predict paleogeographic
features such as the scale of drainage basin area and discharge, slope, and
annual discharge regimes.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49121.1/606468/Linking-rivers-to-the-rock-record-Channel-patterns
Efficient release of bromine by super-eruptions
Clara M. Waelkens; John Stix; Brian Monteleone; Pierre Burckel
Abstract:
Bromine is a key halogen element in the quantification of volcanic
volatiles, but analytical difficulties in measuring its very low abundances
have prevented progress in understanding its behavior and its role in
volcanic emissions. We present a new data set of bromine, chlorine, and
fluorine concentrations in melt inclusions and matrix glasses for two
rhyolitic super-eruptions from the Toledo and Valles calderas, New Mexico,
USA. We show that before eruption, Br and Cl were efficiently partitioned
from the gas-saturated magma into a separate fluid phase, and we calculate
the mass of halogens in the fluid phase. We further demonstrate that
syn-eruptive magma degassing was negligible during the super-eruptions, so
that the main source of halogen emissions must have been the fluid phase.
If the fluid phase were erupted, the large mass of Br and Cl could have
severely impacted the atmospheric chemistry upon eruption.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49114.1/606469/Efficient-release-of-bromine-by-super-eruptions
The unevenness of the north Iberian crustal root, a snapshot of an
elusive stage in margin reactivation
Gabriela Fernández-Viejo; Patricia Cadenas; Jorge Acevedo; Sergio
Llana-Fúnez
Abstract:
Crustal roots are identified in collision chains worldwide. Frequently
mirroring the summits of mountain systems, they elegantly encapsulate the
concept of isostasy. The rugged topography of northern Iberia results from
convergence with the European plate during the Alpine orogeny that formed
the Pyrenean-Cantabrian mountain range. From east to west, the range
comprises three distinct parts: the Pyrenees, the Basque Cantabrian region,
and the Cantabrian Mountains. The identification of the Pyrenean root in
the 1980s and the observation of a similar geometry beneath the Cantabrian
range in the 1990s gave place to the current view of crustal thickening as
a continuous feature, resulting from the northward subduction of Iberian
crust. Recent developments in rift architecture have delivered a complex
rifting template for the area prior to convergence, and contrasting views
based on two-dimensional restorations have led to a debate over its
evolution. A crucial geophysical constraint is Moho topography. Using two
different data sets and techniques, we present the most accurate Moho
surface to date, evidencing abrupt changes throughout the orogen. The
complexity of hyperextended margins underlies the current Moho topography,
and this is ultimately transferred to the nonuniform orogenic pattern found
in northern Iberia.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49341.1/606470/The-unevenness-of-the-north-Iberian-crustal-root-a
Distinguishing volcanic from impact glasses—The case of the Cali glass
(Colombia)
Ludovic Ferrière; Alvaro P. Crósta; Wencke Wegner; Eugen Libowitzky; Fabio
Iwashita ...
Abstract:
Natural glass occurs on Earth in different geological contexts, mainly as
volcanic glass, fulgurites, and impact glass. All these different types of
glasses are predominantly composed of silica with variable amounts of
impurities, especially the alkalis, and differ in their water content due
to their mode of formation. Distinguishing between different types of
glasses, on Earth and also on the Moon and on other planetary bodies, can
be challenging. This is particularly true for glasses of impact and
volcanic origin. Because glass is often used for the determination of the
age of geological events, even if out of geological context, as well as to
derive pressure and temperature constraints, or to evaluate the volatile
contents of magmas and their source regions, we rely on methods that can
unambiguously distinguish between the different types of glasses. We used
the case of the Cali glass, found in an extended area close to the city of
Cali in western Colombia, which was previously suggested to be of impact or
volcanic origin, to show that, using a multimethod approach (i.e.,
combining macroscopic observations, chemical and isotopic data, and H 2O content), it is possible to distinguish between different
formation modes. A suite of Cali glass samples was analyzed using electron
microprobe, instrumental neutron activation analysis, thermal ionization
mass spectrometry, and Fourier-transform infrared spectroscopy, allowing us
to definitively exclude an impact origin and instead classify these glasses
as a rhyolitic volcanic glass (obsidian). Our results suggest that other
“unusual glass occurrences” that are claimed, but not convincingly proven,
to be of impact origin should be reexamined using the same methodology as
that applied here.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G48925.1/606471/Distinguishing-volcanic-from-impact-glasses-The
Current and sea level control the demise of shallow carbonate
production on a tropical bank (Saya de Malha Bank, Indian Ocean)
Christian Betzler; Sebastian Lindhorst; Thomas Lüdmann; John J. Reijmer;
Juan-Carlos Braga ...
Abstract:
Carbonate platforms are built mainly by corals living in shallow
light-saturated tropical waters. The Saya de Malha Bank (Indian Ocean), one
of the world’s largest carbonate platforms, lies in the path of the South
Equatorial Current. Its reefs do not reach sea level, and all carbonate
production is mesophotic to oligophotic. New geological and oceanographic
data unravel the evolution and environment of the bank, elucidating the
factors determining this exceptional state. There are no nutrient-related
limitations for coral growth. A switch from a rimmed atoll to a
current-exposed system with only mesophotic coral growth is proposed to
have followed the South Equatorial Current development during the late
Neogene. Combined current activity and sea-level fluctuations are likely
controlling factors of modern platform configuration.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49090.1/606472/Current-and-sea-level-control-the-demise-of
Central nervous system of a 310-m.y.-old horseshoe crab
: Expanding the taphonomic window for nervous system preservation
Russell D.C. Bicknell; Javier Ortega-Hernández; Gregory D. Edgecombe;
Robert R. Gaines; John R. Paterson
Abstract:
The central nervous system (CNS) presents unique insight into the behaviors
and ecology of extant and extinct animal groups. However, neurological
tissues are delicate and prone to rapid decay, and thus their occurrence as
fossils is mostly confined to Cambrian Burgess Shale–type deposits and
Cenozoic amber inclusions. We describe an exceptionally preserved CNS in
the horseshoe crab Euproops danae from the late Carboniferous
(Moscovian) Mazon Creek Konservat-Lagerstätte in Illinois, USA.
The E. danae CNS demonstrates that the general prosomal
synganglion organization has remained essentially unchanged in horseshoe
crabs for >300 m.y., despite substantial morphological and ecological
diversification in that time. Furthermore, it reveals that the euarthropod
CNS can be preserved by molding in siderite and suggests that further
examples may be present in the Mazon Creek fauna. This discovery fills a
significant temporal gap in the fossil record of euarthropod CNSs and
expands the taphonomic scope for preservation of detailed
paleoneuroanatomical data in the Paleozoic to siderite concretion Lagerstätten of marginal marine deposits.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49193.1/606398/Central-nervous-system-of-a-310-m-y-old-horseshoe
Late Paleoproterozoic mafic magmatism and the Kalahari craton during
Columbia assembly
Cedric Djeutchou; Michiel O. de Kock; Hervé Wabo; Camilo E. Gaitán; Ulf
Söderlund ...
Abstract:
The 1.87–1.84 Ga Black Hills dike swarm of the Kalahari craton (South
Africa) is coeval with several regional magmatic provinces used here to
resolve the craton’s position during Columbia assembly. We report a new
1850 ± 4 Ma (U-Pb isotope dilution–thermal ionization mass spectrometry
[ID-TIMS] on baddeleyite) crystallization age for one dike and new
paleomagnetic data for 34 dikes of which 8 have precise U-Pb ages. Results
are constrained by positive baked-contact and reversal tests, which
combined with existing data produce a 1.87–1.84 Ga mean pole from 63
individual dikes. By integrating paleomagnetic and geochronological data
sets, we calculate poles for three magmatic episodes and produce a
magnetostratigraphic record. At 1.88 Ga, the Kalahari craton is
reconstructed next to the Superior craton so that their ca. 2.0 Ga poles
align. As such, magmatism forms part of a radiating pattern with the coeval
ca. 1.88 Ga Circum-Superior large igneous province.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48811.1/606399/Late-Paleoproterozoic-mafic-magmatism-and-the
The products of primary magma fragmentation finally revealed by pumice
agglomerates
Thomas Giachetti; Kathleen R. Trafton; Joshua Wiejaczka; James E. Gardner;
James M. Watkins ...
Abstract:
Following rapid decompression in the conduit of a volcano, magma breaks
into ash- to block-sized fragments, powering explosive sub-Plinian and
Plinian eruptions that may generate destructive pyroclastic falls and
flows. It is thus crucial to assess how magma breaks up into fragments.
This task is difficult, however, because of the subterranean nature of the
entire process and because the original size of pristine fragments is
modified by secondary fragmentation and expansion. New textural
observations of sub-Plinian and Plinian pumice lapilli reveal that some
primary products of magma fragmentation survive by sintering together
within seconds of magma break-up. Their size distributions reflect the
energetics of fragmentation, consistent with products of rapid
decompression experiments. Pumice aggregates thus offer a unique window
into the previously inaccessible primary fragmentation process and could be
used to determine the potential energy of fragmentation.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48902.1/606362/The-products-of-primary-magma-fragmentation
Translithospheric magma plumbing system of intraplate volcanoes as
revealed by electrical resistivity imaging
Yabin Li; Aihua Weng; Wenliang Xu; Zonglin Zou; Yu Tang ...
Abstract:
The magma plumbing systems of volcanoes in subduction and divergent
tectonic settings are relatively well known, whereas those of intraplate
volcanoes remain elusive; robust geophysical information on the magma
pathways and storage zones is lacking. We inverted magnetotelluric data to
image the magma plumbing system of an intraplate monogenetic volcanic field
located above the stagnant Pacific slab in northeast China. We identified a
complex, vertically aligned, low-resistivity anomaly system extending from
the asthenosphere to the surface consisting of reservoirs with finger- to
lens-like geometries. We show that magma forms as CO2-rich melts
in a 150-km-deep asthenospheric plume crossing the whole lithosphere as
hydrated melt, inducing underplating at 50 km depth, evolving in crustal
reservoirs, and erupting along dikes. Intraplate volcanoes are
characterized by low degrees of melting and low magma supply rates. Their
plumbing systems have a geometry not so different from that of volcanoes in
subduction settings.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49032.1/606363/Translithospheric-magma-plumbing-system-of
Multiple melt source origin of the Line Islands (Pacific Ocean)
Robert Pockalny; Ginger Barth; Barry Eakins; Katherine A. Kelley; Christina
Wertman
Abstract:
The Line Islands volcanic chain in the central Pacific Ocean exhibits many
characteristics of a hotspot-generated seamount chain; however, the lack of
a predictable age progression has stymied previous models for the origin of
this feature. We combined plate-tectonic reconstructions with seamount age
dates and available geochemistry to develop a new model that involves
multiple melt regions and multiple melt delivery styles to explain the
spatial and temporal history of the Line Islands system. Our model
identifies a new melt source region (Larson melt region at ~17°S, ~125°W)
that contributed to the formation of the Line Islands, as well as the
Mid-Pacific Mountains and possibly the Pukapuka Ridge.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49306.1/606364/Multiple-melt-source-origin-of-the-Line-Islands
South African speleothems reveal influence of high- and low-latitude
forcing over the past 113.5 k.y.
Brian Chase; Chris Harris; Maarten J. de Wit; Jan Kramers; Sean Doel ...
Abstract:
Variation in δ18O and δ13C values in a speleothem
from the Cango Caves in southernmost South Africa enable the construction
of coherent regional composite records spanning the past 113,500 yr. Novel
for the region in terms of both their length and detail, these records
indicate environmental and climatic changes that both are consistent with
records from the wider region and show a clear evolution from low- to
high-latitude forcing dominance across the last glacial period. Prior to
ca. 70 ka, the influence of direct low-latitude insolation forcing is
expressed through increases in summer rainfall during austral summer
insolation maxima. With the onset of Marine Isotope Stage 4, cooler global
conditions and the development of high-latitude ice sheets appear to have
supplanted direct insolation forcing as the dominant driver pacing patterns
of environmental change, with records from the Southern and Northern
Hemisphere tropics exhibiting a positive relationship until after the Last
Glacial Maximum. These results highlight the complexity of South African
climate change dynamics as a response to changing global boundary
conditions and provide a critical reference for regional and global
comparisons.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49323.1/606365/South-African-speleothems-reveal-influence-of-high
How carbonate dissolution facilitates sediment-hosted Zn-Pb
mineralization
Weihua Liu; Sam C. Spinks; Matthew Glenn; Colin MacRae; Mark A. Pearce
Abstract:
Most of the world’s Zn and Pb is extracted from sediment-hosted Zn-Pb
deposits. The Zn-Pb deposits hosted in carbonate rocks are hypothesized to
form by mixing of acidic metal-bearing brines with reduced sulfur-bearing
fluids while dissolving sedimentary carbonate. To test the role of
carbonate in this process, we conducted hydrothermal experiments simulating
ore formation by reacting Zn ± Pb ± Ba–bearing brines with H2S
and SO42– produced by native sulfur, with and without
carbonate minerals (calcite or dolomite crystals), at 200 °C and
water-saturated pressure. Sphalerite, galena, and barite (or anhydrite)
crystals formed only when carbonate was present in the experiment,
accompanied by carbonate dissolution. The textures of sphalerite clusters
are similar to those observed in ancient and modern hydrothermal deposits.
Thermodynamic modeling at 150 °C and 250 °C demonstrates that mixing of
metal-rich brines and H2S causes most of the Zn in solution to
precipitate as sphalerite only when carbonate dissolution occurs to buffer
the pH, consistent with the experimental observations. The need for a pH
buffer increases with increasing temperature, and different pH buffers may
play a role for different deposit types. We propose that carbonate-buffered
fluid mixing is a critical process for forming post-sedimentary Zn ± Pb ±
Ba deposits in sedimentary carbonate rocks.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49056.1/606366/How-carbonate-dissolution-facilitates-sediment
Jurassic Arc: Reconstructing the Lost World of eastern Gondwana
Elliot K. Foley; R.A. Henderson; E.M. Roberts; A.I.S. Kemp; C.N. Todd ...
Abstract:
The tectonic setting of the Australian sector of the eastern Gondwanan
margin during the Jurassic and Cretaceous is enigmatic. Whether this
involved convergent tectonism and a long-lived continental magmatic arc or
rift-related extension unrelated to subduction is debated. The paucity of
Australian Jurassic–Cretaceous igneous outcrops makes resolving these
competing models difficult. We used the detrital zircon record of the
Jurassic–Cretaceous Great Australian Superbasin (GAS) as a proxy for
igneous activity. We attribute the persistent magmatism recorded in GAS
sedimentary fill throughout the Mesozoic to ca. 95 Ma to continuation of
the established Paleozoic continental arc system. The detrital zircon
record signals short (~10 m.y.) pulses of elevated Jurassic and Cretaceous
magmatic activity and strongly positive εHf values, indicating juvenile
crust or mantle-derived magmatism. Margin reconstruction indicates
sustained continental growth at rates of at least ~55 km3 km –1 m.y.–1, mainly to the tract now represented by
submerged northern Zealandia, due to the retreat of this arc system. We
posit that arc retreat was a key factor in rapid crust generation and
preservation, and that continental sedimentary systems globally may host
cryptic records of juvenile crustal addition that must be considered in
estimating crustal growth rates along convergent plate margins.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49328.1/606368/Jurassic-Arc-Reconstructing-the-Lost-World-of
Extreme fractionation of selenium isotopes and possible deep biospheric
origin of platinum nuggets from Minas Gerais, Brazil
Alexandre Raphael Cabral; Stephan König; Benjamin Eickmann; Michael Brauns;
Miguel Tupinambá ...
Abstract:
Platinum-rich nuggets offer an opportunity for understanding how precious
metals accumulate. We analyzed the selenium (Se) isotopic composition of
Se-rich (102–103 μg g–1)
platinum-palladium (Pt-Pd) nuggets from a recent placer deposit in Minas
Gerais, Brazil, for which a biogenic origin has been inferred. We obtained
Se isotopic values with a relatively narrow range (δ82/ 76SeNIST3149 = –17.4‰ to –15.4‰ ± 0.2‰, two standard
deviations [2 SD]). The Pt-Os age of the nuggets is 181 ± 6 Ma (2 SD). The
data indicate that the nuggets did not form in the recent placer deposit,
but by replacement of hydrothermal vein minerals at ~70 °C and at least 800
m below the surface. The high abundance and extreme isotopic composition of
Se as well as the presence of other biophilic elements like iodine, organic
carbon, and nitrogen within the nugget matrix are consistent with a
microbial origin. Although abiogenic reduction of Se oxyanions cannot be
ruled out, the nuggets plausibly record Se-supported microbial activity in
the deep biosphere.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49088.1/606369/Extreme-fractionation-of-selenium-isotopes-and
Skarn fluid sources as indicators of timing of Cordilleran arc
emergence and paleogeography in the southwestern United States
M. Gevedon; J.S. Lackey; J.D. Barnes
Abstract:
Oxygen isotope ratios of garnet provide well-established means to
investigate crustal fluid histories. Traditionally, δ18O values
from skarn garnets have been used to track the hydrothermal evolution of an
individual skarn body through time. We, however, use garnet from 14 skarns
from the Jurassic (ca. 175 to ca. 148 Ma) Cordilleran margin arc
(southwestern United States) to provide regional tectonic context to arc
magmatism and hydrothermal activity. We document arc-wide garnet δ 18O variability of ~19‰ (–8.9‰ to +10.3‰, n = 159), providing a
record of contrasting meteoric fluid ingress between northern (Sierra
Nevada) and southern (Mojave Desert) arc segments. Strongly negative garnet
δ18O values (≤–3‰) are limited to the Mojave Desert arc segment
and can only form in the presence of meteoric fluid, requiring shallow
formation in subaerial crust. When combined with U-Pb garnet ages, the δ 18O data provide a minimum radiometric age of local subaerial
arc emergence and temporal constraint on the migration of the Jurassic
paleoshoreline in the Mojave Desert section of the arc.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49005.1/606266/Skarn-fluid-sources-as-indicators-of-timing-of
Hydrothermal alteration can result in pore pressurization and volcano
instability
Michael J. Heap; Tobias Baumann; H. Albert Gilg; Stephan Kolzenburg; Amy G.
Ryan ...
Abstract:
The collapse of a volcanic flank can be destructive and deadly.
Hydrothermal alteration is common to volcanoes worldwide and is thought to
promote volcano instability by decreasing rock strength. However, some
laboratory studies have shown that not all alteration reduces rock
strength. Our new laboratory data for altered rhyodacites from Chaos Crags
(Lassen volcanic center, California, USA) show that pore- and crack-filling
mineral precipitation can reduce porosity and permeability and increase
strength, Young’s modulus, and cohesion. A significant reduction in
permeability, by as much as four orders of magnitude, will inhibit fluid
circulation and create zones of high pore fluid pressure. We explored the
consequences of pore fluid pressurization on volcano stability using
large-scale numerical modeling. Upscaled physical and mechanical properties
for hydrothermally altered rocks were used as input parameters in our
modeling. Results show that a high-pore-pressure zone within a volcano
increases volcano deformation and that increasing the size of this zone
increases the observed deformation. Hydrothermal alteration associated with
mineral precipitation, and increases to rock strength, can therefore
promote pore pressurization and volcano deformation, increasing the
likelihood of volcano spreading, flank collapses, and
phreatic/phreatomagmatic explosions. We conclude that porosity-decreasing
alteration, explored here, and porosity-increasing alteration can both
promote volcano instability and collapse, but by different mechanisms.
Hydrothermal alteration should therefore be monitored at volcanoes
worldwide and incorporated into hazard assessments.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49063.1/606268/Hydrothermal-alteration-can-result-in-pore
Ice-crystal traces imply ephemeral freezing in early Permian equatorial
Pangea
Lily S. Pfeifer; Brooke A. Birkett; Jean Van Den Driessche; Stéphane
Pochat; Gerilyn S. Soreghan
Abstract:
Delicate impressions in lacustrine strata of the lower Permian (lower
Cisuralian) Usclas Formation record ephemeral freezing in equatorial
Pangea. These sediments accumulated in the paleoequatorial and intramontane
Lodève Basin (southern Massif Central, France) during peak icehouse
conditions of the Late Paleozoic Ice Age. Experimental replication of these
features supports the interpretation that they are ice-crystal molds.
Evidence for films of ice in marginal-lacustrine sediment at such low
latitudes and inferred low to moderate altitudes (1–2 km) calls for a
reevaluation of climate conditions in eastern equatorial Pangea. Ephemeral
freezing implies either cold tropical temperatures (~5 °C cooler than the
Last Glacial Maximum) and/or lapse rates that exceeded those of the Last
Glacial Maximum. Extreme continentality of the Lodève Basin would have
amplified seasonality, albeit the climatic forcing(s) necessary to have
promoted cold temperatures in equatorial Pangea remain enigmatic.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49011.1/606269/Ice-crystal-traces-imply-ephemeral-freezing-in
Superhydrous hematite and goethite: A potential water reservoir in the
red dust of Mars?
Si Athena Chen; Peter J. Heaney; Jeffrey E. Post; Timothy B. Fischer; Peter
J. Eng ...
Abstract:
Water can be stored in nominally anhydrous minerals as substitutional
hydroxyl, generating vast but commonly unrecognized H2O
reservoirs in ostensibly dry regimes. Researchers have long known that
hematite (α-Fe2O3) can accommodate small
concentrations of hydroxyl through the substitution of Fe3+ by
3H+. Our study of natural hematite has demonstrated the
occurrence of “hydrohematite” phases that are 10–20 mol% deficient in Fe
and accordingly contain 3.6–7.8 mol% structural water. Intergrown with
natural hydrohematite samples were superhydrous goethite-like phases
exhibiting an Fe deficiency of 10–20 mol% relative to endmember goethite
(α-FeOOH). We synthesized hydrohematite in alkaline solutions (pH 9–12) at
low temperatures (T < 200 °C) using fresh ferrihydrite as the
transient precursor, and we observed a nonclassical crystallization pathway
involving vacancy inoculation by Fe as nanocrystals evolved. The high level
of incorporation of H2O in iron (hydr)oxides dramatically alters
their behaviors as catalysts and pigments, and the presence of
hydrohematite in rocks may rule out high-T diagenesis. We propose
that hydrohematite is common in low-T occurrences of Fe oxide on
Earth, and by extension it may inventory large quantities of water in
apparently arid planetary environments, such as the surface of Mars.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48929.1/606270/Superhydrous-hematite-and-goethite-A-potential
Tiny, glassy, and rapidly trapped: The nano-sized planktic diatoms in
Messinian (late Miocene) gypsum
Luca Pellegrino; Marcello Natalicchio; Kenta Abe; Richard W. Jordan; Sergio
E. Favero Longo ...
Abstract:
Primary gypsum represents an excellent paleobiological archive due to its
early and fast growth, favoring the preservation of delicate biomineralized
structures. The Mediterranean region is renowned for evaporite deposits
that formed during the Messinian salinity crisis (MSC), an event that
supposedly annihilated most of the marine biota. However, the Messinian
evaporites have been scarcely studied for their fossil content. Abundant
nano-sized planktic diatoms and associated organic matter are observed for
the first time in bottom-grown gypsum crystals that formed during the early
stage of the MSC in different marginal basins of the western Mediterranean.
This discovery increases our knowledge of the Messinian biota and reveals
that nano-sized planktic diatoms played a prominent role in carbon and
silicon export during gypsum deposition. The co-occurrence of these diatoms
with larger diatoms, possibly associated with a deep chlorophyll maximum,
suggests that Messinian gypsum formed in stratified and relatively deep
basins (far below the photic zone), typified by marine conditions in the
upper water column. The nano-sized planktic diatoms may have taken
advantage of the hydrological reconfigurations experienced by the
Mediterranean since the onset of the MSC. This study confirms that primary
gypsum represents a promising archive of information for elucidating the
marine biotic response to an ancient environmental crisis.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49342.1/606271/Tiny-glassy-and-rapidly-trapped-The-nano-sized
First identification of a Cathaysian continental fragment beneath the
Gagua Ridge, Philippine Sea, and its tectonic implications
Shengping Qian; Xiaozhi Zhang; Jonny Wu; Serge Lallemand; Alexander R.L.
Nichols ...
Abstract:
The tectonic history of the Philippine Sea plate is an essential piece in
understanding the tectonic evolution of Southeast Asia, but it is still
unclear and controversial. We present the first geochemical data obtained
from lavas from the Gagua Ridge (GR) within the Philippine Sea. The GR
lavas exhibit geochemical signatures typical of subduction-related arc
magmatism. Plagioclase Ar-Ar ages of ca. 124–123 Ma and subduction-related
geochemical signatures support the formation of GR lavas in the vicinity of
an arc during the Early Cretaceous induced by subduction of the oceanic
plate along East Asia. The ages of trapped zircon xenocrysts within the GR
lavas cluster at 250 Ma, 0.75 Ga, and 2.45 Ga and match well the ages of
zircons recovered from the Cathaysian block, southern China. Our results
imply that the GR basement is partially composed of continental material
that rifted away from the Eurasian margin during opening and spreading of
the Huatung Basin. The depleted mantle wedge-derived magmas evolved and
picked up the continental zircons during ascent. The youngest zircon ages
and the GR lava Ar-Ar ages (ca. 124–123 Ma) presented in this study newly
constrain an Early Cretaceous age for the Huatung Basin. Our study provides
further evidence that the Huatung Basin is a remnant of a Mesozoic-aged
ocean basin that dispersed from southern China during the Cretaceous.
Transport of continental slivers by growth and closure of marginal seas
along the East Asia margin may have been more prevalent than previously
recognized.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48956.1/606272/First-identification-of-a-Cathaysian-continental
Skarn fluid sources as indicators of timing of Cordilleran arc
emergence and paleogeography in the southwestern United States
M. Gevedon; J.S. Lackey; J.D. Barnes
Abstract:
Oxygen isotope ratios of garnet provide well-established means to
investigate crustal fluid histories. Traditionally, δ18O values
from skarn garnets have been used to track the hydrothermal evolution of an
individual skarn body through time. We, however, use garnet from 14 skarns
from the Jurassic (ca. 175 to ca. 148 Ma) Cordilleran margin arc
(southwestern United States) to provide regional tectonic context to arc
magmatism and hydrothermal activity. We document arc-wide garnet δ18O variability of ~19‰ (–8.9‰ to +10.3‰, n = 159), providing a
record of contrasting meteoric fluid ingress between northern (Sierra
Nevada) and southern (Mojave Desert) arc segments. Strongly negative garnet
δ18O values (≤–3‰) are limited to the Mojave Desert arc segment
and can only form in the presence of meteoric fluid, requiring shallow
formation in subaerial crust. When combined with U-Pb garnet ages, the δ 18O data provide a minimum radiometric age of local subaerial
arc emergence and temporal constraint on the migration of the Jurassic
paleoshoreline in the Mojave Desert section of the arc.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49005.1/606266/Skarn-fluid-sources-as-indicators-of-timing-of
Central eastern China hydrological changes and ENSO-like variability
over the past 1800 yr
Shiwei Jiang; Xin Zhou; Julian P. Sachs; Wuhong Luo; Luyao Tu ...
Abstract:
Variations in East Asian summer monsoon (EASM) precipitation impact
agriculture, water resources, electricity generation, and economic
development in the densely populated region of central eastern China.
However, uncertainties remain in the history and driving mechanisms of
precipitation changes in this region over the past two millennia. We
present an 1800 yr multiproxy reconstruction of monsoon precipitation based
on hydroclimate variations inferred from Lake Nvshan sediments in the
Jiang-Huai region of central eastern China. We find that rainfall in the
Jiang-Huai region was higher during the Little Ice Age (LIA) than during
the Medieval Warm Period (MWP), and that these changes were modulated by
the mean state of the tropical Pacific Ocean on centennial time scales. We
infer that an El Niño–like (La Niña–like) mean state caused a strengthening
and westward displacement (weakening and eastward displacement) of the
Western Pacific Subtropical High (WPSH) and a weakening (strengthening) of
the EASM, causing more (less) rainfall in the Jiang-Huai region during the
LIA (MWP). These hydroclimate changes are likely to have been a response to
changes in effective radiative forcing.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48894.1/606267/Central-eastern-China-hydrological-changes-and
Earthquake doublet revealed by multiple pulses in lacustrine
seismo-turbidites
Katleen Wils; Maxim Deprez; Catherine Kissel; Morgan Vervoort; Maarten Van
Daele ...
Abstract:
Earthquake doublets have been described in fault systems around the world
but have not yet been confidently resolved in paleoseismic records. Our
current knowledge is limited to historical occurrences, preventing
researchers from uncovering potential patterns or recognizing common fault
behavior. Identification of prehistoric doublets is thus of crucial
importance for adequate seismic hazard assessment and risk mitigation. We
developed a new methodology to reveal the sedimentary imprint of earthquake
doublets in lacustrine paleoseismic records based on flow direction
analysis in multipulsed turbidites, because the delayed arrival of
turbidity currents originating from the same source location demonstrates
the occurrence of individual triggering mechanisms. As grains tend to align
in the presence of a flow, we analyzed flow directions by determining the
dominant orientation of elongated grains using a combination of grain size,
paleomagnetism, and high-resolution X-ray computed tomography. This
methodology was applied to a turbidite deposited by the 2007 CE earthquakes
in West Sumatra (Mw 6.4 and 6.3, 2 h apart), and it provides the
first unmistakable sedimentary evidence for an earthquake doublet. We argue
that this methodology has great potential to be applied to multipulsed
turbidites in various subaquatic paleoseismic records and can reveal the
occurrence of unknown earthquake sequences.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G48940.1/606167/Earthquake-doublet-revealed-by-multiple-pulses-in
Emergent simplicity despite local complexity in eroding fluvial
landscapes
Gareth G. Roberts
Abstract:
Much of our current understanding of continental topographic evolution is
rooted in measuring and predicting the rates at which rivers erode the
landscape. Flume tank and field observations indicate that stochasticity
and local conditions play important roles in determining rates at small
scales (e.g., <10 km, thousands of years). Obversely, preserved river
profiles and common shapes of rivers atop uplifting topography indicate
that erosion rates are predictable at larger scales. These observations
indicate that the response of rivers to forcing can be scale dependent. I
demonstrate that erosional thresholds can provide an explanation for why
profile evolution can be very complicated and unique at small scales yet
simple and predictable at large scales.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48942.1/606168/Emergent-simplicity-despite-local-complexity-in
Quaternary sodic and potassic intraplate volcanism in northeast China
controlled by the underlying heterogeneous lithospheric structures
Xingli Fan; Qi-Fu Chen; Yinshuang Ai; Ling Chen; Mingming Jiang ...
Abstract:
The origin and mantle dynamics of the Quaternary intraplate sodic and
potassic volcanism in northeast China have long been intensely debated. We
present a high-resolution, three-dimensional (3-D) crust and upper-mantle
S-wave velocity (Vs) model of northeast China by combining ambient noise
and earthquake two-plane wave tomography based on unprecedented regional
dense seismic arrays. Our seismic images highlight a strong correlation
between the basalt geochemistry and upper-mantle seismic velocity
structure: Sodic volcanoes are all characterized by prominent low seismic
velocities in the uppermost mantle, while potassic volcanoes still possess
a normal but thin upper-mantle “lid” depicted by high seismic velocities.
Combined with previous petrological and geochemical research findings, we
propose that the rarely erupted Quaternary potassic volcanism in northeast
China results from the interaction between asthenospheric low-degree melts
and the overlying subcontinental lithospheric mantle. In contrast, the more
widespread Quaternary sodic volcanism in this region is predominantly
sourced from the upwelling asthenosphere without significant overprinting
from the subcontinental lithospheric mantle.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48932.1/606169/Quaternary-sodic-and-potassic-intraplate-volcanism
Staged formation of the supergiant Olympic Dam uranium deposit,
Australia
Kathy Ehrig; Vadim S. Kamenetsky; Jocelyn McPhie; Edeltraud Macmillan; Jay
Thompson ...
Abstract:
The origins of many supergiant ore deposits remain unresolved because the
factors responsible for such extreme metal enrichments are not understood.
One factor of critical importance is the timing of mineralization. However,
timing information is commonly confounded by the difficulty of dating ore
minerals. The world’s largest uranium resource at Olympic Dam, South
Australia, is exceptional because the high abundance of U allows U-Pb
dating of ore minerals. The Olympic Dam U(-Cu-Au-Ag) ore deposit is hosted
in ca. 1.59 Ga rocks, and the consensus has been that the supergiant
deposit formed at the same time. We argue that, in fact, two stages of
mineralization were involved. Paired in situ U-Pb and trace
element analyses of texturally distinct uraninite populations show that the
supergiant size and highest-U-grade zones are the result of U addition at
0.7–0.5 Ga, at least one billion years after initial formation. This
conclusion is supported by a remarkable clustering of thousands of
radiogenic 207Pb/206Pb model ages of Cu sulfide
grains at this time. Upgrading of the original ca. 1.59 Ga U deposit to its
present size at 0.7–0.5 Ga may have resulted from perturbation of regional
fluid flow triggered by global climatic (deglaciation) and tectonic
(breakup of Rodinia) events.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48930.1/606170/Staged-formation-of-the-supergiant-Olympic-Dam
Precession-driven climate cycles and time scale prior to the Hirnantian
glacial maximum
M. Sinnesael; P.I. McLaughlin; A. Desrochers; A. Mauviel; J. De Weirdt ...
Abstract:
Paleozoic astrochronologies are limited by uncertainties in past
astronomical configurations and the availability of complete stratigraphic
sections with precise, independent age control. We show it is possible to
reconstruct a robust Paleozoic ~104-yr-resolution
astrochronology in the well-preserved and thick Upper Ordovician reference
record of Anticosti Island (Canada). The clear imprint of astronomical
cycles, including ~18 k.y. precession, potential obliquity, and short and
long eccentricity, constrains the entire Vauréal Formation (~1 km thick) to
only ~3 m.y. in total, representing ~10 times higher accumulation rates
than previously suggested. This ~104 yr resolution represents an
order of magnitude increase in the current standard temporal resolution for
the Katian and even allows for the detection of sub-Milankovitch
climate-scale variability. The loss of a clear precession signal in the
uppermost Vauréal Formation might be related to contemporaneous global
cooling prior to the Hirnantian glacial maximum as indicated by the δ18O record. Complementary to the study of cyclostratigraphy of
longer and often simplified records, it is important to recognize
stratigraphic hiatuses and complexities on the ~104 yr scale to
achieve robust sub-eccentricity-scale Paleozoic astrochronologies.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49083.1/606171/Precession-driven-climate-cycles-and-time-scale
Magnetostratigraphy across the end-Permian mass extinction event from
the Meishan sections, southeastern China
Min Zhang; Hua-Feng Qin; Kuang He; Yi-Fei Hou; Quan-Feng Zheng ...
Abstract:
The end-Permian mass extinction (EPME) has been recorded as the most severe
biodiversity crisis in Earth’s history, although the timing of the marine
and terrestrial extinctions remains debatable. We present a new
high-resolution magnetostratigraphic succession across the EPME and the
Permian-Triassic boundary (PTB) from the Meishan sections in southeastern
China, which contain the global boundary stratotype section and point
(GSSP) for the base of the Triassic (also the Induan Stage) and the base of
the Changhsingian Stage. We identified five normal and five reverse
magnetozones, including MS1n to MS5n and MS1r to MS5r, from oldest to
youngest, in the Changhsingian and Induan Stages. The Induan Stage was
determined to consist of two polarity intervals, where the upper one is
reverse (MS5r), and the lower one is normal (MS5n). The Changhsingian Stage
is dominated by normal polarity, intercalated with four short-term reverse
magnetozones (MS1r to MS4r). Consequently, the PTB and the
Wuchiapingian-Changhsingian boundary are clearly located in MS5n and MS1n,
respectively. These new magnetostratigraphic results provide a potential
reference geomagnetic polarity pattern with which to refine the geomagnetic
polarity time scale for the EPME and the Permian-Triassic transition.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49072.1/606172/Magnetostratigraphy-across-the-end-Permian-mass
Anoxic depositional overprinting of 238U/235U in
calcite: When do carbonates tell black shale tales?
Xinming Chen; Stephen J. Romaniello; Michael McCormick; Alyssa Sherry; Jeff
R. Havig ...
Abstract:
The fidelity of uranium isotopes (δ238U) in marine carbonates as
a paleoredox proxy relies on whether carbonates can record and preserve
seawater δ238U. Although modern carbonate sediments deposited
under oxic conditions have been shown to track seawater δ238U,
it remains unknown whether this is true for carbonates deposited under
anoxic conditions. This is a crucial question because many ancient
carbonates were likely deposited or reworked under anoxic bottom waters. To
better understand the behavior of uranium isotopes under this scenario, we
investigated U isotope geochemistry in the meromictic Fayetteville Green
Lake (FGL; New York, USA), where primary calcite is precipitated from oxic
surface waters, sinks past the chemocline, and is deposited under anoxic
bottom waters. We observed significant depletions of dissolved U
concentration (from 2.7 to 0.9 ppb) and δ238U (from –0.55‰ to
–0.96‰) below the chemocline in FGL. Parallel with these depletions, δ238U of sediment traps increased progressively from –0.51‰ to
–0.16‰, suggesting that U(VI) reduction was occurring in the anoxic water
column. Carbonate sediments deposited under anoxic bottom waters were
enriched in U by 6–18× compared to primary calcite. Our data suggest that
such significant authigenic U enrichments resulted from U(VI) reduction in
the anoxic water column and below the sediment-water interface. The δ238U value in the top 0.25 cm of sediments was –0.29‰ ± 0.10‰,
overprinting original δ238U in primary calcite (–0.51‰ ± 0.02‰).
Future applications of carbonate δ238U as a paleoredox proxy
should consider depositional environments (oxic vs. anoxic) of carbonates.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48949.1/606038/Anoxic-depositional-overprinting-of-238U-235U-in
Early Neoproterozoic (870–820 Ma) amalgamation of the Tarim craton
(northwestern China) and the final assembly of Rodinia
Pan Zhao; Jinyou He; Chenglong Deng; Yan Chen; Ross N. Mitchell
Abstract:
In the paleogeographic configuration of the Neoproterozoic supercontinent
of Rodinia, the Tarim craton (northwestern China), traditionally seen as a
single block, is placed either on the periphery near northern Australia or
India or in a central position between Australia and Laurentia. To
distinguish between these possibilities, we present here new primary
paleomagnetic results from ca. 900 Ma volcanics in the Aksu region of the
northwestern Tarim craton. The data reveal a ~28° latitudinal difference
between the North Tarim and South Tarim blocks at ca. 900 Ma and constrain
the age of amalgamation of the Tarim craton to between 870 and 820 Ma.
Combining paleomagnetic poles from Tarim and major cratons of Rodinia with
geological evidence, a two-stage orogenic model is proposed for the
assembly of Rodinia. Late Mesoproterozoic orogenesis (1.3–1.0 Ga) led to
the assembly of Australia–East Antarctica, Baltica, Umkondia, South Tarim,
and Cathaysia with Laurentia, forming the core of Rodinia. Thereafter, the
Jiangnan–Central Tarim Ocean separating North Tarim and Yangtze from South
Tarim and Cathaysia was closed before ca. 820 Ma. This second
Jiangnan–Central Tarim orogeny caused nearly coeval amalgamation of the
peripheral Tarim and South China cratons by the welding of North Tarim and
Yangtze to South Tarim and Cathaysia, respectively. The supercontinent of
Rodinia was thus assembled by two orogenic phases separated by ~200 m.y.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48837.1/606039/Early-Neoproterozoic-870-820-Ma-amalgamation-of
Tectonic pressure gradients during viscous creep drive fluid flow and
brittle failure at the base of the seismogenic zone
Luca Menegon; Åke Fagereng
Abstract:
Fluid-pressure cycles are commonly invoked to explain alternating
frictional and viscous deformation at the base of the seismogenic crust.
However, the stress conditions and geological environment of fluid-pressure
cycling are unclear. We address this problem by detailed structural
investigation of a vein-bearing shear zone at Sagelvvatn, northern
Norwegian Caledonides. In this dominantly viscous shear zone, synkinematic
quartz veins locally crosscut mylonitic fabric at a high angle and are
rotated and folded with the same sense of shear as the mylonite. Chlorite
thermometry indicates that both veining and mylonitization occurred at
~315–400 °C. The vein-filled fractures are interpreted as episodically
triggered by viscous creep in the mylonite, where quartz piezometry and
brittle failure modes are consistent with low (18–44 MPa) differential
stress. The Sagelvvatn shear zone is a stretching shear zone, where
elevated pressure drives a hydraulic gradient that expels fluids from the
shear zone to the host rocks. In low-permeability shear zones, this
hydraulic gradient facilitates buildup of pore-fluid pressure until the
hydrofracture criterion is reached and tensile fractures open. We propose
that hydraulic gradients established by local and cyclic pressure
variations during viscous creep can drive episodic fluid escape and result
in brittle-viscous fault slip at the base of the seismogenic crust.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49012.1/606040/Tectonic-pressure-gradients-during-viscous-creep
Construction of the Lesser Himalayan–Subhimalayan thrust belt: The
primary driver of thickening, exhumation, and high elevations in the
Himalayan orogen since the middle Miocene
Sean P. Long; Delores M. Robinson
Abstract:
Documenting the structural evolution of the Himalayan orogen is fundamental
for understanding the dynamics of collisional orogenesis. We argue that the
importance of deformation in the frontal, Lesser Himalayan–Subhimalayan
(LH-SH) portion of the Himalayan thrust belt for driving crustal thickening
over the past ~15–13 m.y. has long been overlooked. To quantify its
contribution to thickening, we measured parameters from 22 published cross
sections that span the length of the orogen. The mean structural uplift
accomplished by the LH-SH thrust belt increases from 10–15 km in the
eastern half of the orogen to 15–23 km in the western half. An antiformal
culmination constructed by LH duplexing is observed across the orogen and
increases in structural height (to as much as 15–20 km) and north-south
width moving westward. Construction of the culmination was the primary
mechanism for building and maintaining wedge taper. The westward scaling of
culmination size is accompanied by doubling and tripling of LH-SH
shortening and accretion magnitude, respectively; when combined with a
consistent orogen-wide modern taper angle (11° ± 2°), this indicates that
duplexing facilitated the growth of an overall larger orogenic wedge moving
westward. Following the initial southward propagation of deformation into
LH rocks at ca. 15–13 Ma, the Himalayan orogenic wedge has been
characterized by stacking of multiple thin, smalldisplacement thrust sheets
to develop a high-taper orogenic wedge. Thus, LH-SH deformation has had a
profound effect on driving thickening, exhumation, and the attainment of
high elevations since the middle Miocene.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48967.1/606041/Construction-of-the-Lesser-Himalayan-Subhimalayan
Earth’s new tectonic regime at the dawn of the Paleoproterozoic: Hf
isotope evidence for efficient crustal growth and reworking in the São
Francisco craton, Brazil
Henrique Bruno; Monica Heilbron; Rob Strachan; Mike Fowler; Claudio de
Morisson Valeriano ...
Abstract:
A zircon Hf isotope data set from Archean and Paleoproterozoic magmatic and
metasedimentary rocks of the southern São Francisco craton (Brazil) is
interpreted as evidence of accretionary and collisional plate tectonics
since at least the Archean-Proterozoic boundary. During the Phanerozoic,
accretionary and collisional orogenies are considered the end members of
different plate tectonic settings, both involving preexisting stable
continental lithosphere and consumption of oceanic crust. However,
mechanisms for the formation of continental crust during the Archean and
Paleoproterozoic are still debated, with the addition of magmatic rocks to
the crust being explained by different geodynamic models. Hf isotopes can
be used to quantify the proportion of magmatic addition into the crust:
positive εHf values are usually interpreted as indications of
magmatic input from the mantle, whereas crust-derived rocks show more
negative εHf. We show that the crust of the amalgamated
Paleoproterozoic tectonostratigraphic terranes that make up the southern
São Francisco craton were generated from different proportions of mantle
and crustal isotopic reservoirs. Plate tectonic processes are implied by a
consistent sequence of events involving (1) the generation of juvenile
subduction-related magmatic arc rocks, followed by (2) collisional
orogenesis and remelting of older crust, and (3) post-collisional bimodal
magmatism.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49024.1/606042/Earth-s-new-tectonic-regime-at-the-dawn-of-the
Phanerozoic record of mantle-dominated arc magmatic surges in the
Zealandia Cordillera
M.F. Ringwood; J.J. Schwartz; R.E. Turnbull; A.J. Tulloch
Abstract:
We integrated new and existing bedrock and detrital zircon dates from the
Zealandia Cordillera to explore the tempo of Phanerozoic arc magmatism
along the paleo-Pacific margin of southeast Gondwana. We found that
episodic magmatism was dominated by two high-magma-addition-rate (MAR)
events spaced ~250 m.y. apart in the Devonian (370–368 Ma) and the Early
Cretaceous (129–105 Ma). The intervening interval between high-MAR events
was characterized by prolonged, low-MAR activity in a geographically stable
location for more than 100 m.y. We found that the two high-MAR events in
Zealandia have distinct chemistries (S-type for the Devonian and I-type for
the Cretaceous) and are unlikely to have been related by a repeating,
cyclical process. Like other well-studied arc systems worldwide, the
Zealandia Cordillera high-MAR events were associated with upper-plate
deformation; however, the magmatic events were triggered by enhanced
asthenospheric mantle melting in two distinct arc-tectonic settings—a
retreating slab and an advancing slab, respectively. Our results
demonstrate that dynamic changes in the subducting slab were primary
controls in triggering mantle flare-up events in the Phanerozoic Zealandia
Cordillera.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48916.1/606043/Phanerozoic-record-of-mantle-dominated-arc
Contamination of the Bushveld Complex (South Africa) magmas by basinal
brines: Stable isotopes in phlogopite from the UG2 chromitite
Haoyang Zhou; Robert B. Trumbull; Ilya V. Veksler; Ilya N. Bindeman;
Johannes Glodny ...
Abstract:
There is abundant evidence for significant H2O in evolved melts
from the platinum-rich UG2 chromitite and the Merensky Reef of the Bushveld
Complex (South Africa), but there is no consensus about the source of H 2O. We report triple-oxygen and hydrogen isotope ratios of
interstitial, late-magmatic phlogopite from three localities of the UG2
layer. The phlogopite yielded δD values of –43‰ to –23‰, which is >30‰
higher than previously known from Bushveld rocks and far above the mantle
values of ~–75‰. The phlogopite triple-oxygen isotope ratios are the first
to be reported for Bushveld rocks, with values of Δ′17O 0.5305 (17O excess relative to the reference line
0.5305) from –0.069‰ to –0.044‰ (δ18O 5.2‰–6.2‰). The oxygen
data support existing models of as much as 30%–40% contamination of
mantlederived magmas in the lower to middle crust. However, the high δ
values require a second step of contamination, which we attribute to brines
from the marine sediments in the Transvaal Basin at the emplacement level.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49173.1/606044/Contamination-of-the-Bushveld-Complex-South-Africa
Deformation-induced Japan twinning in quartz during incipient
mylonitization
Michel Bestmann; Giorgio Pennacchioni; Bernhard Grasemann
Abstract:
Many crystalline rocks of the continental crust contain coarse-grained
quartz as a main mineral (e.g., granitoids). Incipient deformation of
coarse quartz, which likely controls the accumulation of bulk strain in
heterogeneously deformed crustal rock volumes, commonly develops microshear
zones (MSZs) of localized recrystallization. At mid-crustal conditions,
where quartz deformation is mostly accomplished by subgrain rotation
recrystallization, grains of MSZs can show an abrupt change in
crystallographic orientation (large misorientation angle) with respect to
the host quartz that is still not fully understood. We analyzed MSZs
(20–200 μm thick) from deformed coarse-grained (millimeter grain size)
quartz veins in the Austroalpine Schobergruppe (Eastern Alps). Electron
backscatter diffraction analysis reveals that the MSZs are characterized by
a nearly 90° misorientation angle between the c-axes of the host
and new grains, which also share one {m} and one {1122} pole,
compatible with Japan twinning. This abrupt switch of the c-axis
orientation can promote geometrical softening and shear localization. So
far, Japan twinning has been interpreted as a growth feature. We show that
deformation-induced twinning in quartz, including Japan and Dauphiné
twinning, can play an important role in initiation of crystal-plastic
deformation within the crust.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49077.1/606045/Deformation-induced-Japan-twinning-in-quartz
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