New Geology Articles Published Online Ahead of Print in April
Boulder, Colo., USA: Article topics and locations include “Pothole-like
depressions in the chamber floor of the Sudbury Igneous Complex, Canada”;
“Oceanic isostasy as a trigger for the rift-to-drift transition”; “Ancient
roots of tungsten in western North America”; and “Relationship between
glacial CO2 drawdown and mercury cycling in the western South Atlantic.”
These Geology articles are online at
https://geology.geoscienceworld.org/content/early/recent.
Cenozoic mountain building in eastern China and its correlation with
reorganization of the Asian climate regime
Jingxing Yu; Dewen Zheng; Jianzhang Pang; Chaopeng Li; Ying Wang ...
Abstract:
The Cenozoic Asian climate system experienced a transformation from a zonal
pattern to a monsoon-dominant pattern around the Paleogene-Neogene
boundary. A series of dynamic mechanisms, such as uplift of the Tibetan
Plateau, retreat of the Paratethys Sea, expansion of the South China Sea,
and decreasing atmospheric CO2 content, has been suggested to be
responsible for the transformation of the Asian climate pattern. However,
the role of topographic growth in eastern China has been rarely considered.
As the natural divides of geography, climate, and biology, the two most
distinct sets of topographic relief in eastern China, the Qinling and
Taihang Mountains, play an important role in shaping the Asian climate
pattern. We report low-temperature thermochronology data from the Qinling
and Taihang Mountains and use age-elevation relationships and thermal
history modeling to show that both mountain ranges experienced a phase of
rapid exhumation during the late Oligocene and early Miocene. The building
of the Qinling and Taihang Mountains around the Oligocene-Miocene boundary
temporally and spatially coincided with the reorganization of the Cenozoic
Asian climate regime, suggesting that the mountain building in eastern
China acted as a possible driving mechanism for the alleged reorganization
of the Cenozoic Asian climate regime.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49917.1/613381/Cenozoic-mountain-building-in-eastern-China-and
Thermal and compositional anomalies in a detailed xenolith-based
lithospheric mantle profile of the Siberian craton and the origin of
seismic midlithosphere discontinuities
Zhe Liu; Dmitri A. Ionov; Paolo Nimis; Yigang Xu; Pengli He ...
Abstract:
The fine structure and thermal state of >200-km-thick cratonic
lithosphere remain poorly explored because of insufficient sampling and
uncertainties in pressure (P) and temperature (T)
estimates. We report exceptionally detailed thermal and compositional
profiles of the continental lithospheric mantle (CLM) in the Siberian
craton based on petrographic, in situ chemical, and P- T data for 92 new garnet peridotite xenoliths from the Udachnaya
kimberlite, as well as literature data. The thermal profile is complex,
with samples indicating model conductive geotherms between 40 and 35 mW/m 2 at ~55–130 km, colder (35 mW/m2 geotherm) mantle
from 140 to 190 km, and hotter layers at the CLM base (190–230 km) and at
~135 km. The latter, previously unidentified, anomalous midlithospheric
horizon has rocks up to 150 °C hotter than the 35 mW/m2
geotherm, that are rich in garnet and clinopyroxene, have low Mg#, and have
melt-equilibrated rare earth element patterns. We posit that this horizon
formed in a depth range where ascending melts stall (e.g., via loss of
volatiles and redox change), heat wall-rock harzburgites, and transform
them to lherzolites or wehrlites. This may explain some seismic
midlithosphere discontinuities (MLDs) in cratons. By contrast, we found no
rocks rich in metasomatic volatile-rich amphibole, phlogopite, or carbonate
matching the MLD, nor layers composed of peridotites with distinct
melt-extraction degrees. The CLM below 190 km contains both coarse and
variably deformed rocks heated and reworked (Mg#Ol down to 0.86)
by localized lithosphere-asthenosphere interaction.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49947.1/613382/Thermal-and-compositional-anomalies-in-a-detailed
Possible bipolar global expression of the P3 and P4 glacial events of
eastern Australia in the Northern Hemisphere: Marine diamictites and
glendonites from the middle to upper Permian in southern Verkhoyanie,
Siberia
V.I. Davydov; I.V. Budnikov; R.V. Kutygin; N.G. Nurgalieva; A.S. Biakov ...
Abstract:
Three intervals of glaciomarine diamictites with extensive glendonites in
middle to upper Permian sediments were found in the Kobyume River, southern
Verkhoyanie, Russia. The successions are biostratigraphically constrained
as middle to upper Permian. The middle Permian diamictite horizons extend
over a large area with a lateral distance of >1000 km. The upper Permian
diamictites developed only locally. The diamictites are interpreted as
glaciomarine sediments containing ice-rafted debris. Two glacial episodes
in Siberia temporally correspond to the P3 (middle Permian) and P4 (late
Permian) glacial events of eastern Australia, strongly suggesting a global
bipolar climate and well-developed climatic belts during the middle to late
Permian.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50165.1/613383/Possible-bipolar-global-expression-of-the-P3-and
Plagioclase archives of depleted melts in the oceanic crust
David A. Neave; Olivier Namur
Abstract:
Mid-ocean ridge and ocean-island basalts provide vital but incomplete
insights into mantle chemistry. For example, high-anorthite plagioclase is
generally too refractory and incompatible-element depleted to have
crystallized from the melts that carry it to the surface. Moreover, erupted
basalts rarely preserve the extreme isotopic and incompatible-element
depletions found in some primitive melt inclusions and residual abyssal
peridotites. By integrating experimental observations with analyses of
natural crystals and glasses, we infer that high-anorthite plagioclase
grows from high-degree melts of refractory but otherwise unexotic mantle
sources with depleted incompatible-element compositions. The widespread
occurrence of high-anorthite plagioclase in oceanic basalts and the oceanic
crust hence indicates that refractory melts pervade the uppermost mantle
and lower crust. We thus suggest that refractory melts play much a greater
role in crustal accretion than typically recognized and that refractory and
feasibly depleted peridotite is more prevalent in the upper mantle than
previously thought.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49840.1/613384/Plagioclase-archives-of-depleted-melts-in-the
Time scales of open-system processes in a complex and heterogeneous
mush-dominated plumbing system
Helena Albert; Teresa Trua; José Fonseca; Michael P. Marani; Fabiano
Gamberi ...
Abstract:
The architecture of a mush-dominated plumbing system in active volcanic
areas conditions the magma pathways feeding eruptions. Open-system
processes along these pathways and the associated time scales are directly
related to monitoring data and eruptive behavior. Despite crystal
mush–dominated systems being common in active volcanoes, previous studies
have not focused on the integration of data from the mush sectors feeding
different eruptions, supplying a partial view of the pre-eruptive magmatic
processes and hindering the interpretation of the monitoring signals during
unrest periods. We focus on the Marsili seamount (Tyrrhenian Sea), where
the mineral data document processes within a magmatic system vertically
extended throughout the local oceanic crust and made of a mush framework
spotted with eruptible melt- and crystal-rich pockets. We undertook a study
of Marsili olivine crystals that constrains the time scales of three
pre-eruptive scenarios, dominated by open-system processes: (1)
disaggregation of the deep Marsili volcano mush zone that occurred over a
time scale of years prior to the eruption; (2) rapid ascent (days) of
mantle-derived basaltic magma that, in some cases, intercepts shallow
plagioclase-rich pockets; and (3) multiple mixing events between melt- and
crystal-rich mush zones occurring approximately 1–2 mo and 0.5–3 yr before
the eruption. Our results highlight the importance of contemporaneously
studying eruptions in different locations on a volcano edifice for a better
comprehension on how mush-dominated plumbing systems work as a whole and
how this must be considered during the interpretation of monitoring data.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49934.1/613385/Time-scales-of-open-system-processes-in-a-complex
Plate tectonic–like cycles since the Hadean: Initiated or inherited?
Ross N. Mitchell; Christopher J. Spencer; Uwe Kirscher; Simon A. Wilde
Abstract:
Interpretation of Earth’s oldest preserved crustal archive, the Jack Hills
zircon of Western Australia, has been controversial in terms of the onset
of plate tectonics. We conduct time-series analysis on hafnium isotopes of
the Jack Hills zircon and reveal an array of statistically significant
cycles that are reminiscent of plate-tectonic subduction. At face value,
such cycles may suggest early Earth conditions similar to those of
today—the uniformitarian hypothesis that plate tectonics was essentially
operational since “day one”. On the other hand, in the context of expected
secular changes due to planetary evolution and geological observations, the
cycles could instead imply that modern plate-tectonic subduction inherited
mantle convective harmonics already facilitated by an early phase of
stagnant-lid delamination—the “lid-to-plates” hypothesis. Either way, any
model for the nature of plate tectonics must incorporate conditions
operating during Hadean time, either by initiation of plate tectonics then
or by later inheritance of preexisting cycles of mantle convection.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G49939.1/613386/Plate-tectonic-like-cycles-since-the-Hadean
Atypical landslide induces speedup, advance, and long-term slowdown of
a tidewater glacier
Maximillian Van Wyk de Vries; Andrew D. Wickert; Kelly R. MacGregor; Camilo
Rada; Michael J. Willis
Abstract:
Atmospheric and oceanic warming over the past century have driven rapid
glacier thinning and retreat, destabilizing hillslopes and increasing the
frequency of landslides. The impact of these landslides on glacier dynamics
and resultant secondary landslide hazards are not fully understood. We
investigated how a 262 ± 77 × 106 m3 landslide
affected the flow of Amalia Glacier, Chilean Patagonia. Despite being one
of the largest recorded landslides in a glaciated region, it emplaced
little debris onto the glacier surface. Instead, it left a series of
landslide-perpendicular ridges, landslide-parallel fractures, and an apron
of ice debris—with blocks as much as 25 m across. Our observations suggest
that a deep-seated failure of the mountainside impacted the glacier flank,
propagating brittle deformation through the ice and emplacing the bulk of
the rock mass below the glacier. The landslide triggered a brief
downglacier acceleration of Amalia Glacier followed by a slowdown of as
much as 60% of the pre-landslide speed and increased suspended-sediment
concentrations in the fjord. These results highlight that landslides may
induce widespread and long-lasting disruptions to glacier dynamics.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49854.1/613387/Atypical-landslide-induces-speedup-advance-and
Pothole-like depressions in the chamber floor of the Sudbury Igneous
Complex, Canada
S.Yu. Chistyakova; R.M. Latypov
Abstract:
The magmatic stratigraphy of the Sudbury Igneous Complex (Canada) is
thought to have resulted from closed-system differentiation of an initially
homogeneous impact melt sheet. The topography of its upward-growing chamber
floor is therefore thought to have been planar and subhorizontal. However,
we report on the discovery of a large pothole-like depression (~300 m in
depth and ~550 m in width) in the chamber floor of this complex. The
depression has been revealed through two-dimensional mapping of igneous
layering that is defined by systematic vertical changes in cumulus
assemblages and bulk rock chemistry. Although the formation of the
depression by syn- to post-magmatic folding and/or slumping of chamber
floor cumulates cannot be completely excluded, we favor an alternative
explanation that follows from the recent recognition that the Sudbury
Igneous Complex melt sheet crystallized concurrently from the floor and
roof inward. The roof sequence was subsequently disrupted and collapsed as
large discrete blocks onto the floor sequence. This may have resulted in
local irregularities in topography of the upward-growing chamber floor so
that crystal deposition onto and between the neighboring blocks produced
pothole-like depressions. The phenomenon of physical disruption of roof
sequences appears to provide a reasonable explanation for the common lack
of the rocks that grew from the roof downward in layered intrusions.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49928.1/613388/Pothole-like-depressions-in-the-chamber-floor-of
Seismic anisotropy in the central Tien Shan unveils rheology-controlled
deformation during intracontinental orogenesis
Bingfeng Zhang; Xuewei Bao; Yixian Xu
Abstract:
The initiation and evolution of compressional intracontinental orogens are
favored by rheologically weak lithosphere underneath; however, how this
weakened lithosphere responds to the regional stress regime remains
vigorously debated. The Tien Shan mountains in central Asia provide the
best example to illustrate the deep deformational responses to
intracontinental orogenesis. We present new constraints on the nature of
seismic anisotropy in the crust and upper mantle of the central Tien Shan
through shear-wave splitting analyses. Our results reveal a sharp change in
the orientations of crustal anisotropic fabrics on two sides of the
mountains. The convergence-parallel fast orientations in the northern
segment are closely related to the lower-crustal simple-shear deformation
caused by the underthrusting of the Kazakh Shield, whereas the
depth-independent orogen-parallel fast orientations in the southern segment
suggest vertically coherent pure-shear thickening of the Tien Shan
lithosphere in response to the northward indentation of the Tarim Basin.
The thickened lithosphere has partly foundered into the deep mantle,
contributing to the accelerated shortening deformation in the late
Cenozoic. Our observations demonstrate the complex tectonic processes in
the Tien Shan and suggest that the rheological properties of bounding
blocks can play a significant role in shaping the lithospheric structures
of intracontinental orogens.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49633.1/613051/Seismic-anisotropy-in-the-central-Tien-Shan
Sulfide-associated hydrothermal dolomite and calcite reveal a shallow
burial depth for Alpine-type Zn-(Pb) deposits
M. Giorno; L. Barale; C. Bertok; M. Frenzel; N. Looser ...
Abstract:
Difficulties in dating Mississippi Valley–type (MVT) mineral deposits and
the often closely associated dolomitization have led to controversy
regarding their origin. We report the first radiometric ages for the Gorno
mining district in northern Italy, an example of the Alpine subclass of MVT
deposits. U-Pb ages of hydrothermal carbonates pre- and postdating the
ore-forming event show that base-metal mineralization occurred shortly
after the deposition of the Carnian host rocks. This implies that the Gorno
ore deposits formed at shallow burial depth prior to the Early Jurassic
western Tethys rifting phase. Contemporaneous Triassic magmatism and
extensional tectonics likely contributed to the high geothermal heat fluxes
required to drive the mineralizing system. Our study reinforces the need
for reliable geochronological data for metallogenic models and warns
against a general application of classic North American MVT models to
similar deposits worldwide.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49812.1/613052/Sulfide-associated-hydrothermal-dolomite-and
Cenozoic sediment bypass versus Laramide exhumation and erosion of the
Eagle Ford Group: Perspective from modelling of organic and inorganic
proxy data (Maverick Basin, Texas, USA)
Alexandra S. Robinson; Annabel Dale; Thierry Adatte; Cédric M. John
Abstract:
The Cenozoic unconformity above the Late Cretaceous carbonates within the
Maverick Basin is a unique feature of Texas (USA). Hypotheses accounting
for the unconformity include (1) Cenozoic sediment bypass, and (2) ~6400 m
of erosion during the Laramide orogeny. Both hypotheses have different
implications for the burial history of the Eagle Ford Group (EFG) and for
our understanding of the Laramide orogeny. We generated clumped isotope
data and organic maturation proxies from the same location. Carbonate
clumped isotope temperatures obtained (113 ± 9 °C) represent
recrystallization during burial and a minimum estimate of the maximum
burial temperature. This constraint is significantly warmer than apparent
organic temperatures derived using an Arrhenius equation (40–55 °C).
Organic matter transformation and carbonate recrystallization respond to
temperature over different time scales and therefore capture snapshots of
the thermal history particular to the chemical reactions that control the
respective processes. Using numerical forward modeling on the combination
of two different temperature proxies, we derived a new hypothesis: ~2800 m
of Cenozoic sediments were accumulated and then eroded during late Laramide
compression. This is significantly less erosion than previous estimates,
indicating the impact of the Laramide orogeny in the basin may have been
less severe than previously thought.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49886.1/613053/Cenozoic-sediment-bypass-versus-Laramide
Oceanic isostasy as a trigger for the rift-to-drift transition
James A. Conder:
Abstract:
A long-standing missing link in our understanding of the Wilson cycle is
how a continental rift transitions to seafloor spreading. The variety of
rift structures and transition timings at rift margins do not easily lend
themselves to some specific degree of strain and/or magmatism as the
tipping point. Invariably ignored in the process, but a potential key to
the conundrum, is the isostatic response that comes with ocean loading
during and after inundation. Ocean mass redistribution on variably
subsiding crust drives flow in the asthenosphere in much the same way a
growing icecap drives a corresponding outward mantle flow. This flow alters
mantle tractions of the rift system, with disappearance of basal
resistance, and even adds a push to the rifting process. Evidence for ocean
inundation facilitating self-sustained seafloor spreading is observed in
the Atlantic, around the Afar triple junction, and elsewhere, indicating
that the ocean should not be considered simply incidental to the creation
of oceanic basins.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49914.1/613054/Oceanic-isostasy-as-a-trigger-for-the-rift-to
Relationship between glacial CO2 drawdown and mercury
cycling in the western South Atlantic: An isotopic insight
Thiago S. Figueiredo; Bridget A. Bergquist; Thiago P. Santos; Ana Luiza S.
Albuquerque; Emmanoel Vieira Silva-Filho
Abstract:
Pronounced changes in the deep Atlantic circulation occurred during glacial
stages, which affected the global carbon distribution and the
biogeochemical cycles of other elements. Previous studies demonstrated that
oceanic mercury is sensitive to the same kind of processes that affect the
carbon cycle in glacial–interglacial time scales. We used Hg isotopes to
elucidate Hg cycling during the last two glacial–interglacial transitions
in the subtropical western South Atlantic. Mass-dependent fractionation
(MDF, δ202Hg) and mass-independent fractionation (MIF, Δ 199Hg) show significant variations and shift toward higher and
lower values during the penultimate and Last Glacial Maximum periods,
respectively. δ202Hg variability is in-phase with periods of
lower atmospheric CO2 and benthic δ13C, suggesting
that MDF is affected by the same mechanisms that led to atmospheric CO 2 drawdown during glacial stages. In contrast, terrestrial Hg
input to our site, boosted by lower sea level and exposure of the
continental shelf, dominates the Δ199Hg variability.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49942.1/613055/Relationship-between-glacial-CO2-drawdown-and
Laser-ablation Lu-Hf dating reveals Laurentian garnet in subducted
rocks from southern Australia
Dillon A. Brown; Alexander Simpson; Martin Hand; Laura J. Morrissey; Sarah
Gilbert ...
Abstract:
Garnet is a fundamental expression of metamorphism and one of the most
important minerals used to constrain the thermal conditions of the crust.
We used innovative in situ laser-ablation ICP-MS/MS Lu-Hf
geochronology to demonstrate that garnet in metapelitic rocks enclosing
Cambrian eclogite in southern Australia formed during Laurentian
Mesoproterozoic metamorphism. Garnet porphyroblasts in amphibolite-facies
metapelitic rocks yielded Lu-Hf ages between 1286 ± 58 Ma and 1241 ± 16 Ma,
revealing a record of older metamorphism that was partially obscured by
metamorphic overprinting during ca. 510 Ma Cambrian subduction along the
East Gondwana margin. Existing detrital zircon age data indicate the
protoliths to the southern Australian metapelitic rocks were sourced from
western Laurentia. We propose that the metapelitic rocks of southern
Australia represent a fragment of western Laurentian crust, which was
separated from Laurentia in the Neoproterozoic and incorporated into the
East Gondwana subduction system during the Cambrian. The ability to obtain
Lu-Hf isotopic data from garnet at acquisition rates comparable to those
for U-Pb analysis of detrital zircon means, for the first time, the
metamorphic parentage of rocks as expressed by garnet can be efficiently
accessed to assist paleogeographic reconstructions.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49784.1/613056/Laser-ablation-Lu-Hf-dating-reveals-Laurentian
Apparent preservation of primary foraminiferal Mg/Ca ratios and
Mg-banding in recrystallized foraminifera
Philip T. Staudigel; Eleanor H. John; Ben Buse; Caroline H. Lear
Abstract:
Trace element and δ18O values of foraminifera are widely used to
reconstruct oceanic temperatures throughout the Cenozoic and beyond.
Previous work evaluating the geochemistry of foraminifera with differing
degrees of physical preservation have shown that Mg/Ca and δ18O
paleothermometers give discrepant values in recrystallized tests, with
planktonic oxygen isotopes often yielding significantly lower temperatures
than Mg/Ca ratios. To study the mobility of elements during diagenesis, we
performed microspatial trace element analyses in Eocene Morozovella. Element maps show that trace element banding is
readily identifiable and preserved, to an extent, in texturally
recrystallized tests. A reaction-diffusion model was used to test whether
the preservation of Mg-banding and the decoupling of δ18O and
Mg/Ca values could be the result of diffusively limited “closed-system”
recrystallization. Results show that, in a closed system, internal features
(such as Mg-banding) will dissipate prior to changes in bulk Mg/Ca
composition, while the bulk δ18O value will typically change
faster than Mg/Ca. This is observed regardless of what partitioning
coefficient is used for Mg and demonstrates that the planktonic Mg/Ca proxy
is more diagenetically robust than the δ18O proxy. Thus, this
model can explain the observed decoupling of these two proxies.
Furthermore, the preservation of intra-test Mg-banding shows potential for
use in evaluating the preservation of primary Mg/Ca values and hence the
accuracy of paleotemperature reconstructions.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49984.1/613057/Apparent-preservation-of-primary-foraminiferal-Mg
Metastable olivine within oceanic lithosphere in the uppermost lower
mantle beneath the eastern United States
Fansheng Kong; Stephen S. Gao; Kelly H. Liu; Yinxia Fang; Hejun Zhu ...
Abstract:
Approximately two-thirds of Earth’s outermost shell is composed of oceanic
plates that form at spreading ridges and recycle back to Earth’s interior
in subduction zones. A series of physical and chemical changes occur in the
subducting lithospheric slab as the temperature and pressure increase with
depth. In particular, olivine, the most abundant mineral in the upper
mantle, progressively transforms to its high-pressure polymorphs near the
mantle transition zone, which is bounded by the 410 km and 660 km
discontinuities. However, whether olivine still exists in the core of slabs
once they penetrate the 660 km discontinuity remains debated. Based on SKS
and SKKS shear-wave differential splitting times, we report new evidence
that reveals the presence of metastable olivine in the uppermost lower
mantle within the ancient Farallon plate beneath the eastern United States.
We estimate that the low-density olivine layer in the subducted Farallon
slab may compensate the high density of the rest of the slab associated
with the low temperature, leading to neutral buoyancy and preventing
further sinking of the slab into the deeper part of the lower mantle.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49879.1/612989/Metastable-olivine-within-oceanic-lithosphere-in
Ancient roots of tungsten in western North America
V. Elongo; H. Falck; K.L. Rasmussen; L.J. Robbins; R.A. Creaser ...
Abstract:
The highly irregular and localized distribution of tungsten deposits
worldwide constitutes a supply challenge for basic industries such as steel
and carbides. Over Earth’s history, tungsten has preferentially accumulated
at paleocontinental margins formed during the breakup of supercontinents.
Later crustal thickening of these paleogeographic regions and the magmas
they produce are associated with large tungsten districts. However, all of
the largest tungsten deposits in the modern North American Cordillera,
which preserves over 3 b.y. of geologic record in a paleocontinental margin
with abundant crustal magmatism, are limited to the narrow Canadian
Tungsten Belt in northwestern Canada. We use neodymium isotopic
compositions of scheelite (CaWO4) from the Canadian Tungsten
Belt and the paleogeographic distribution of tungsten deposits in the North
American Cordillera to constrain the factors that control tungsten
distribution. We document that tungsten is specifically associated with
materials that, on average, were derived from the mantle during the
Mesoarchean to Paleoproterozoic. Weathering and erosion of the
supercontinents Columbia and Rodinia favored pre-enrichment of tungsten in
sediments. The orogenic heating of pre-enriched sediments produced reduced
melts that were capable of efficiently scavenging tungsten and formed the
largest deposits in North America.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49801.1/612990/Ancient-roots-of-tungsten-in-western-North-America
Origin of Tibetan post-collisional high-K adakitic granites: Anatexis
of intermediate to felsic arc rocks
Jian-Kang Yi; Di-Cheng Zhu; Roberto F. Weinberg; Qing Wang; Jin-Cheng Xie
...
Abstract:
Potassium-rich adakitic rocks have been used to infer high-pressure (HP)
melting of thickened or foundered mafic lower crust in post-collisional
settings. However, their origin remains debated because of their potassic
rather than sodic nature. We address this debate by investigating the
source of post-collisional Oligocene high-K adakitic granites in southern
Tibet, which are widely attributed to melting of overthickened mafic lower
crust. Our new data provide evidence for the generation of these high-K
adakitic granites by anatexis of exposed migmatitic intermediate to felsic
arc orthogneisses. These granites contain high-Th/U Oligocene magmatic
zircons (30–22 Ma), and inherited zircons (66–48 Ma) with the same ε Hf(t) (0 to +12) as the coeval migmatites. The migmatites have
in turn low-Th/U metamorphic Oligocene zircon rims around 66–48 Ma magmatic
zircon cores recording the anatectic event at 29–25 Ma. Phase equilibrium
modeling yields an anatectic temperature of ~740 °C and pressure (P) of
~0.9 GPa and shows that garnet is stable at P > 0.7 GPa in the
melt-present field. The results indicate that high-K adakitic magmas are
derived from melting of older intermediate to felsic arc rocks at
intermediate pressures, in the garnet stability field, without any
involvement of HP melting of metabasaltic rocks. We propose that
hybridization between such purely crustal magmas and subcontinental
lithospheric mantle–derived shoshonitic magmas results in the hybridized
post-collisional Oligocene–Miocene mafic microgranular enclave–bearing
potassic adakitic granitic rocks in southern Tibet.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49818.1/612991/Origin-of-Tibetan-post-collisional-high-K-adakitic
Looking upstream with clumped and triple oxygen isotopes of estuarine
oyster shells in the early Eocene of California, USA
Julia R. Kelson; Sierra V. Petersen; Nathan A. Niemi; Benjamin H. Passey;
Allison N. Curley
Abstract:
The δ18O of carbonate minerals that formed at Earth’s surface is
widely used to investigate paleoclimates and paleo-elevations. However, a
multitude of hydrologic processes can affect δ18O values,
including mixing, evaporation, distillation of parent waters, and carbonate
growth temperatures. We combined traditional carbon and oxygen isotope
analyses with clumped (Δ47) and triple oxygen isotopes (Δ′ 17O) analyses in oyster shells (Acutostrea idriaensis)
of the Goler Formation in southern California (USA) to obtain insights into
surface temperatures and δ18O values of meteoric waters during
the early Eocene hothouse climate. The Δ47-derived temperatures
ranged from 9 °C to 20 °C. We found a correlation between the δ 18O of growth water (δ18Ogw) (calculated
using Δ47 temperatures and δ18O of carbonate) and the
δ13C values of shells. The Δ′17O values of shell
growth waters (0.006‰–0.013‰ relative to Vienna standard mean ocean
water–standard light Antarctic precipitation [VSMOW-SLAP]) calculated from
Δ′17O of carbonate (–0.087‰ to –0.078‰ VSMOW-SLAP) were lower
than typical meteoric waters. These isotopic compositions are consistent
with oyster habitation in an estuary. We present a new triple oxygen
isotope mixing model to estimate the δ18O value of freshwater
supplying the estuary (δ18Ofw). The reconstructed δ 18Ofw of –11.3‰ to –14.7‰ (VSMOW) is significantly
lower than the δ18Ogw of –4.4‰ to –9.9‰ that would
have been calculated using “only” Δ47 and δ18O values
of carbonate. This δ18Ofw estimate supports
paleogeographic reconstructions of a Paleogene river fed by high-elevation
catchments of the paleo–southern Sierra Nevada. Our study highlights the
potential for paired Δ47 and Δ′17O analyses to
improve reconstructions of meteoric water δ18O, with
implications for understanding ancient climates and elevations.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49634.1/612992/Looking-upstream-with-clumped-and-triple-oxygen
Late Holocene increase of winter precipitation in midcontinental North
America from a seasonally resolved speleothem record
Cameron J. Batchelor; Shaun A. Marcott; Ian J. Orland; Kouki Kitajima
Abstract:
Subannual climate reconstructions of the Holocene are rare despite the
ability of such records to provide a better understanding of the underlying
factors that drive subannual climate variability. We used specialized
confocal laser fluorescent microscope imaging and automated secondary ion
mass spectrometry microanalysis to resolve a seasonal oxygen isotope (δ 18O) record of a late Holocene–aged (2.7–2.1 ka) speleothem from
mid-continental North America. We did this by measuring intra-band δ 18O variability (Δ18O) within 117 annual bands over a
600 yr span of the late Holocene. We interpret a change in Δ18O
values after 2.4 ± 0.1 ka to reflect an increase in the amount of winter
precipitation. Our study produced direct measurements of past seasonality,
offers new insights into shifting seasonal precipitation patterns that
occurred during the late Holocene in central North America, and adds a new
tool for understanding the complex precipitation and temperature histories
of this region.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50096.1/612993/Late-Holocene-increase-of-winter-precipitation-in
Oxygen isotope evidence for progressively assimilating trans-crustal
magma plumbing systems in Iceland
A. Caracciolo; S.A. Halldórsson; E. Bali; E.W. Marshall; H. Jeon ...
Abstract:
The oxygen isotope composition of mantle-derived melts can place important
constraints on how magmas are processed as they traverse the crust.
Assimilation of crustal material is a crucial aspect of basalt
petrogenesis, as it affects the chemical and rheological characteristics of
eruptive magmas at active volcanoes. We report oxygen isotope (δ 18O) and trace element (TE) data from a suite of
well-characterized basaltic melt inclusions and groundmass glasses from the
Bárðarbunga volcanic system in Iceland to assess how and where in the
plumbing system crustal rocks interact with ascending magmas. While both
melt inclusions and groundmass glasses record a large range in δ 18O values (+3.2‰ to +6.4‰ and +2.6‰ to +5.5‰, respectively)
groundmass glasses record lower values on average. Relationships between
incompatible trace element (e.g., Zr/Nb) and oxygen isotope ratios are best
explained with three-component mixing, where primary melts derived from
depleted and enriched mantle components with distinct δ18O
values mix and acquire a low-δ18O character upon progressive
contamination with altered Icelandic crust. The majority (60%) of melt
inclusions require 10–30% exchange of oxygen with the Icelandic crust. In
addition, for the first time, we link the extent of oxygen isotope exchange
with melt equilibration depths, showing that most of the contamination
occurs at 1–2 kbar (3–7 km depth). We propose that a progressively
assimilating, multi-tiered plumbing system is a characteristic feature of
the Bárðarbunga volcanic system, whereby chemical modifications resulting
from interaction with the crust systematically increase as melts migrate
through higher crustal levels. We show that similar processes may also
occur across the active rift zone in Iceland.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49874.1/612994/Oxygen-isotope-evidence-for-progressively
Five million years of high atmospheric CO2 in the aftermath
of the Permian-Triassic mass extinction
Michael M. Joachimski; Johann Müller; Timothy M. Gallagher; Gregor Mathes;
Daoliang L. Chu ...
Abstract:
The end-Permian mass extinction, the largest biological crisis in Earth
history, is currently understood in the context of Siberian Traps volcanism
introducing large quantities of greenhouse gases to the atmosphere,
culminating in the Early Triassic hothouse. In our study, the late Permian
and Early Triassic atmospheric CO2 history was reconstructed by
applying the paleosol pCO2 barometer. Atmospheric pCO2 shows an approximate 4× increase from mean
concentrations of 412–919 ppmv in the late Permian (Changhsingian) to
maximum levels between 2181 and 2610 ppmv in the Early Triassic (late
Griesbachian). Mean CO2 estimates for the later Early Triassic
are between 1261–1936 ppmv (Dienerian) and 1063–1757 ppmv (Spathian).
Significantly lower concentrations ranging from 343 to 634 ppmv are
reconstructed for the latest Early to Middle Triassic (Anisian). The 5 m.y.
episode of elevated pCO2 suggests that negative
feedback mechanisms such as silicate weathering were not effective enough
to reduce atmospheric pCO2 to precrisis levels and that
marine authigenic clay formation (i.e., reverse weathering) may have been
an important component of the global carbon cycle keeping atmospheric pCO2 at elevated levels.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49714.1/612995/Five-million-years-of-high-atmospheric-CO2-in-the
Fluid inclusion evidence for extreme overpressure induced by gas
generation in sedimentary basins
Xiaolin Wang; Wenxuan Hu; Ye Qiu; Yifeng Liu; Dong Jia ...
Abstract:
Formation and preservation of extremely high-pressure fluid in sedimentary
basins is critical to understanding petroleum accumulation and basin
evolution; however, this issue remains poorly understood due to a lack of
key evidence. We present fluid inclusion evidence from the Paleozoic black
shales in the eastern Sichuan Basin (South China block) that suggest that
gas generation can form methane-saturated fluids with pressures
significantly exceeding the lithostatic pressure. The fluid inclusion
internal pressure is so high (e.g., ~77 MPa) that gas hydrate formed at
ambient temperature, aqueous-vapor homogenization could not be achieved
under ambient pressure, and the homogenization pressure was beyond the
scope of the current equation of state for the NaCl-H2O-CH 4 system. We infer that the formation of high-pressure (e.g.,
>230 MPa), methane-saturated fluids induces hydrofracture under weak
compressional tectonic regimes, where the excess lithostatic fluids can be
accumulated and sustained. Fluid inclusion records with an abnormally high
magnitude of overpressure indicate small differential stress and thus can
serve as a microscale indicator for tectonic quiescence.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49848.1/612996/Fluid-inclusion-evidence-for-extreme-overpressure
Infaunalization and resource partitioning during the Mesozoic marine
revolution
Luis A. Buatois; M. Gabriela Mángano; Bhawanisingh Desai; Noelia B.
Carmona; Fiona Burns ...
Abstract:
Infaunalization has been regarded as representing a response to increased
predation pressures and is therefore central to the Mesozoic marine
revolution, which gives pre-eminence to the role that enemy-directed
evolution has played as a driving force of biotic change. Our ichnologic
compilation from 39 Middle Triassic to Late Jurassic shallow-marine
siliciclastic units allows us to evaluate the vertical partitioning of the
infaunal ecospace through the application of the ichnoguild concept. This
study shows that infaunal communities experienced a marked increase in
ecospace utilization during the Early Jurassic, reaching a plateau during
the rest of the Jurassic. This trend is expressed by an increase in the
maximum number of ichnoguilds per community and per tier and in the number
of ichnotaxa per ichnoguild. This pattern shows important partitioning of
the infaunal ecospace into a series of tiers and that several organisms
were able to exploit the same resources available at discrete sediment
zones below the sea bottom. The increase in the maximum number of
ichnoguilds per community and per tier suggests that niche partitioning was
a key factor in a more efficient use of the infaunal ecospace and in
driving alpha diversity. However, the increase in the number of ichnotaxa
per ichnoguild indicates that ichnoguilds were packed with organisms
exploiting similar resources, arguing against the role of competitive
exclusion in structuring communities. Because several episodes of predation
increase took place during the Mesozoic, an unequivocal link between
predation pressures and infaunalization cannot be demonstrated empirically.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50088.1/612997/Infaunalization-and-resource-partitioning-during
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