New Geology Articles Published Online Ahead of Print in March
Boulder, Colo., USA: Article topics and locations include faulting in the
Imperial Valley, California; stagnation and tearing of the subducting
northwest Pacific slab; the vigor of Cenozoic ocean bottom currents; and
Greenland tidal water advance during the era of Norse settlement. These Geology articles are online at
https://geology.geoscienceworld.org/content/early/recent
.
Reconstructing source-to-sink systems from detrital zircon core and rim
ages
Li Liu; Daniel F. Stockli; Timothy F. Lawton; Jie Xu; Lisa D. Stockli ...
Abstract:
Grenville-age (1.3–0.9 Ga) zircons represent one of the most ubiquitous
detrital zircon (DZ) age modes on Earth. In North America, given the
widespread occurrence of Grenville basement, Grenville DZs are commonly
viewed as nondiagnostic with regard to source region in provenance studies.
Systematic recovery of DZ core-rim U-Pb ages makes it possible to identify
and differentiate previously indistinguishable basement source terranes by
leveraging their multistage tectono-magmatic evolution. Our analysis
demonstrates that Grenville DZs exhibit distinct rim ages in different
parts of the North American Paleozoic Appalachian-Ouachita-Marathon
foreland. Whereas Grenville DZ grains in the eastern foreland, sourced from
the southern Appalachian orogen in the eastern United States, exhibit
Taconian and Acadian (490–350 Ma) rims, grains in the western foreland,
derived from Mexico, mainly show Neoproterozoic (750–500 Ma) rim ages. This
difference permits differentiation of nondiagnostic core ages by their
distinctive rim ages. Furthermore, core-rim paired ages can illuminate
potential genetic relationships among coexisting age components in DZ
spectra, thereby indicating whether the DZs are derived from separate
sources or from a single source with multistage tectono-magmatic histories.
Thus, DZ rim-core ages can provide critical insights into reconstructing
global source-to-sink systems and elucidating genetic linkages within
multistage orogenic systems.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49904.1/612782/Reconstructing-source-to-sink-systems-from
Deformation and metasomatism recorded by single-grain apatite
petrochronology
Margaret L. Odlum; Drew A. Levy; Daniel F. Stockli; Lisa D. Stockli; Joel
W. DesOrmeau
Abstract:
The timing and processes of ductile deformation and metasomatism can be
documented using apatite petrochronology. We integrated microstructural,
U-Pb, and geochemical analyses of apatite grains from an exhumed mylonitic
shear zone in the St. Barthélémy Massif, Pyrenees, France, to understand
how deformation and metasomatism are recorded by U-Pb dates and geochemical
patterns. Electron backscatter diffraction (EBSD) analyses documents
crystal plastic deformation characterized by low-angle boundaries (<5°)
associated with dislocation creep and evidence of multiple slip systems.
Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS)
U-Pb maps indicate that dates in deformed grains reflect, and are governed
by, low-angle dislocation boundaries. Apatite rare earth element (REE) and
U-Pb behavior is decoupled in high-grade gneiss samples, suggesting REEs
record higher-temperature processes than U-Pb isotopic systems. Apatite
from (ultra)mylonitic portions of the shear zone showed evidence of
metasomatism, and the youngest dates constrain the age of metasomatism.
Collectively, these results demonstrate that crystal plastic
microstructures and fluid interactions can markedly change apatite isotopic
signatures, making single-grain apatite petrochronology a powerful tool for
dating and characterizing the latest major deformation and/or fluid events,
which are often not captured by higher-temperature chronometers.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49809.1/612783/Deformation-and-metasomatism-recorded-by-single
Groundwater seepage is a key driver of theater-headed valley formation
in limestone
Aaron Micallef; Nader Saadatkhah; Jurgen Spiteri; Enzo Rizzo; Luigi
Capozzoli ...
Abstract:
Groundwater seepage leads to the formation of theater-headed valleys (THVs)
in unconsolidated sediments. In bedrock, the role of groundwater in THV
development remains disputed. Here, we integrate field and remote-sensing
observations from Gnejna Valley (Maltese Islands) with numerical modeling
to demonstrate that groundwater seepage can be the main driver of THV
formation in jointed limestone overlying clays. The inferred erosion
mechanisms entail (1) widening of joints and fractures by fluid pressure
and dissolution and (2) creeping of an underlying clay layer, which lead to
slope failure at the valley head and its upslope retreat. The latter is
slower than the removal of the talus by creep and sliding on the valley
bed. The location and width of THVs are controlled by the location of the
master fault and the extent of the damage zone, respectively. The
variability of seepage across the fault zone determines the shape of the
valley head, with an exponential decrease in seepage away from the fault
giving rise to a theater-shaped head that best matches that of Gnejna
Valley. Our model may explain the formation of THVs by groundwater in
jointed, strong-over-weak chemical sedimentary lithologies, particularly in
arid terrestrial settings.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49938.1/612784/Groundwater-seepage-is-a-key-driver-of-theater
Greenland tidewater glacier advanced rapidly during era of Norse
settlement
Danni M. Pearce; James M. Lea; Douglas W.F. Mair; Brice R. Rea; J. Edward
Schofield ...
Abstract:
Our ability to improve prognostic modeling of the Greenland Ice Sheet
relies on understanding the long-term relationships between climate and
mass flux (via iceberg calving) from marine-terminating tidewater glaciers
(TWGs). Observations of recent TWG behavior are widely available, but
long-term records of TWG advance are currently lacking. We present glacial
geomorphological, sedimentological, archaeological, and modeling data to
reconstruct the ~20 km advance of Kangiata Nunaata Sermia (KNS; the largest
tidewater glacier in southwest Greenland) during the first half of the past
millennium. The data show that KNS advanced ~15 km during the 12th and 13th centuries CE at a rate of ~115 m a –1, contemporaneous with regional climate cooling toward the
Little Ice Age and comparable to rates of TWG retreat witnessed over the
past ~200 years. Presence of Norse farmsteads proximal to KNS demonstrates
their resilience to climate change, manifest as a rapidly advancing TWG in
a cooling climate. The results place limits on the magnitude of ice-margin
advance and demonstrate TWG sensitivity to climate cooling as well as
warming. These data combined with our grounding-line stability analysis
provide a long-term record that validates approaches to numerical modeling
aiming to link calving to climate.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49644.1/612785/Greenland-tidewater-glacier-advanced-rapidly
Can deepwater bottom currents generate clinothems? An example of a
large, asymmetric mounded drift in Upper Jurassic to Lower Cretaceous
sediments from northwestern Australia
O. Mantilla; F.J. Hernández-Molina; N. Scarselli
Abstract:
Clinoforms and clinothems are ubiquitous in shallow marine and shelf margin
environments, where they show remarkable seaward progradation trends.
Consensus holds that these features do not form in deepwater settings. This
study describes an example of a large, asymmetric mounded deposit formed in
Upper Jurassic to Lower Cretaceous sediments along the Exmouth Plateau
(offshore northwestern Australia). Although it formed in deepwater
environments, the deposit has previously been interpreted to reflect either
a deltaic or shelf margin system based on clinoform and clinothem
geometries. We support that this deposit shares similarities with a delta
drift that evolved into a large, mounded drift (~180 km in length, ~120 km
in width, and up to ~1.7 km in sedimentary thickness) that exhibits two
migration trends: one westward and the other northeastward. Three
evolutionary phases are proposed: (1) an onset drift stage (ca. 146.5–143.5
Ma); (2) a growth drift stage (ca. 143.5–138.2 Ma); and (3) a burial stage
(ca. 138.2 Ma), which marks the completion of the drift and a shift in
depositional style. The drift asymmetry and clinoform orientations indicate
the influence of a northward-flowing water mass with two main cores. Our
analysis thus suggests that bottom currents can create complex deposits
with geometries that resemble clinothems in deepwater environments.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G50068.1/612786/Can-deepwater-bottom-currents-generate-clinothems
Episodic fluid flow in an eclogite-facies shear zone: Insights from Li
isotope zoning in garnet
William F. Hoover; Sarah Penniston-Dorland; Lukas Baumgartner; Anne-Sophie
Bouvier; Besim Dragovic ...
Abstract:
Episodic fluid overpressure and escape is invoked as a cause or consequence
of many subduction-zone seismic phenomena but can be challenging to
constrain in exhumed high-pressure metamorphic rocks. In situ
measurements of lithium isotopes in garnet reveal evidence of episodic
fluid transport in a subduction shear zone now exposed in the Monviso
ophiolite (Western Alps). Garnet from an eclogite block and associated
metasomatic reaction rind was analyzed by secondary ion mass spectrometry
(SIMS). All analyzed garnet preserves core-rim zoning in δ7Li
and large negative δ7Li excursions (NEs) in mantles. These
excursions cannot be explained by instrumental mass fractionation during
analysis, equilibrium fractionation, or intracrystalline diffusion of Li
within garnet. Instead, NEs were produced by kinetic fractionation of Li
isotopes during bulk diffusion through a pore fluid, and the fractionated
isotopic compositions were incorporated into garnet by syn-diffusion
growth. Disequilibrium garnet growth textures associated with negative δ 7Li support this interpretation and suggest metasomatism drove
rapid garnet growth. Four distinct NEs were identified requiring that at
least four pulses of fluid were transported within the adjacent shear zone.
This evidence of episodic fluid transport along a subduction shear zone at
eclogite facies supports models of intermediate-depth seismicity that rely
on cyclic fluid overpressure and escape.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49737.1/612787/Episodic-fluid-flow-in-an-eclogite-facies-shear
Magmatism at oceanic core complexes on the ultraslow Southwest Indian
Ridge: Insights from near-seafloor magnetics
Fei Zhou; Jérôme Dyment; Chunhui Tao; Tao Wu
Abstract:
Oceanic core complexes (OCCs) and detachment faults play a key role in
crustal accretion at slow and ultraslow spreading centers. We investigated
the effect of different magma supply at three OCCs of the Southwest Indian
Ridge using high-resolution deep-sea bathymetric and magnetic data. The
average equivalent thickness of extrusive basalt deduced from the magnetic
anomalies, a proxy for magma supply, decreases from west to east, from the
Yuhuang (49.25°E) to Longqi (49.65°E) to Junhui (51.75°E) OCCs. Conversely,
serpentinite outcrops become more abundant, the domal OCC morphology
flattens as the footwall rotation (measured by the magnetization vector
inclination) increases, and hydrothermal evidence becomes sparse. Combined
with results from the amagmatic easternmost Southwest Indian Ridge, our
study shows that the magma supply controls the character and evolution of
the OCCs and detachment faults on the Southwest Indian Ridge.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49771.1/612788/Magmatism-at-oceanic-core-complexes-on-the
Deep-sea hiatuses track the vigor of Cenozoic ocean bottom currents
Adriana Dutkiewicz; Dietmar Müller
Abstract:
The deep-sea stratigraphic record is full of gaps. These hiatuses track
changes in ocean circulation and chemistry, but determining their timing
and causes has been limited by sparse data and incomplete knowledge of
ocean gateway evolution in earlier studies. We combine a significantly
expanded, age-calibrated deep-sea stratigraphic database with a global
tectonic and paleo–water depth model to investigate the distribution of
>400 Cenozoic hiatuses longer than ~0.2 m.y. We find that only a small
number of hiatuses are due to carbonate dissolution. The majority of
hiatuses were, by implication, caused by mechanical erosion and
redistribution of sediments by bottom currents into regions of increased
sedimentation such as contourite drifts. We link peaks in regional hiatuses
to changes in ocean circulation and intensification of deep-water
formation. Widespread hiatuses in the South Atlantic, South Pacific, and
southern Indian oceans between ca. 34 Ma and 30 Ma are attributed to the
coeval widening and deepening of the Drake Passage and the opening of the
deep Tasman Gateway. A peak in hiatuses in the Atlantic in the early
Miocene is linked to the initiation of a proto–Atlantic Meridional
Overturning Circulation driven by the complete opening of the deep Drake
Passage and the progressive closure of the Tethys seaway. A long-term 30%
decline in hiatus frequency since ca. 14 Ma is synchronous with
post–Miocene Climate Optimum cooling, suggesting the slowing of abyssal
circulation. Our synthesis of deep-sea hiatuses could be used to track the
fate of deep-sea sediments and to ground-truth deep-ocean circulation
models.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49810.1/612789/Deep-sea-hiatuses-track-the-vigor-of-Cenozoic
Groundwater age persistence in topography-driven groundwater flow over
paleohydrogeologic time scales
Yueqing Xie; Andrew J. Love; Craig T. Simmons; Adrian Costar; Jichun Wu
Abstract:
The Pleistocene-Holocene climate transition resulted in a dramatic
reduction in groundwater recharge in many aquifers in arid and semiarid
regions throughout the world. This study conducted numerical experiments to
compare the evolution of groundwater hydraulics and age patterns in arid
and semiarid aquifers in response to transient conditions associated with
recharge decline from the Pleistocene to the Holocene. Our results show
that after a rapid reduction in recharge, the amplitude of water-table
undulations and regional groundwater slope both reduced. This resulted in a
general, and relatively rapid, contraction of local flow systems and an
increase in the extent of intermediate and regional systems. The previous
hierarchy of local, intermediate, and regional flow systems was completely
replaced by largely horizontal and regional flow patterns after ~10,000 yr.
However, in stark contrast, we observed that the original Pleistocene age
patterns have remained almost unchanged throughout the 10,000 yr Holocene
period. Thus, groundwater age is more likely to be indicative of past
rather than current flow systems. Consequently, due to this age
persistence, the use of modern groundwater age data to calibrate models or
compute recharge with methods that do not account for this potentially
significant spatial and temporal mismatch between age and hydraulics will
be misleading and erroneous. This has significant implications for
hydrogeologic analyses. The findings of this study may also apply to areas
that have undergone dramatic changes in land cover or land use that
strongly influence transient groundwater recharge processes.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49842.1/612790/Groundwater-age-persistence-in-topography-driven
Nanoscale trace-element zoning in pyrite framboids and implications for
paleoproxy applications
Daniel D. Gregory; Libor Kovarik; Sandra D. Taylor; Daniel E. Perea; Jeremy
D. Owens ...
Abstract:
Pyrite framboids (spherical masses of nanoscale pyrite) are among the
earliest textures of pyrite to form in sediments. It has been proposed that
their trace-element (TE) contents can be used to track the TE composition
of the water column in which they formed. However, it is not clear how
these TEs are associated with the framboidal pyrite grains. For instance,
it is important to know whether they are incorporated uniformly or are
enriched in different regions of the framboid. We used high-resolution
scanning transmission electron microscopy to identify chemical zoning
within pyrite framboids. We found that initial, nanoscale pyrite euhedral
crystals, which make up the volumetric majority of the framboids, are
covered/infilled by later pyrite that templates on the earlier pyrite.
Further, this later pyrite is enriched in TEs, suggesting that many TEs are
incorporated in pyrite relatively late (during early diagenesis; not in the
water column). This observation suggests that although chemical analyses of
pyrite framboids may provide ocean-water chemistry trends through time, the
details are complex. Specifically, the TEs found in pyrite may be linked to
adsorption onto organic matter, detrital material, and authigenic minerals
such as Fe- and Mn-oxide phases followed by desorption in the sediments or
release via dissolution and incorporation into pyrite as overgrowths on the
initial nanoscale euhedral crystals that make up framboids. While the use
of pyrite chemistry to understand past ocean conditions remains promising,
and even diagenetic additions may not preclude the utility of pyrite for
reconstructing ancient ocean conditions, care must be taken in
interpretations because the end concentration may be influenced by
diagenesis.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49890.1/612791/Nanoscale-trace-element-zoning-in-pyrite-framboids
How old are the Jack Hills metasediments really?: The case for
contamination of bedrock by zircon grains in transported regolith
P.D. Kinny; C. Clark; C.L. Kirkland; M. Hartnady; J. Gillespie ...
Abstract:
The enigmatic occurrence of Neoarchean- and Proterozoic-aged zircon grains
in some samples of metasedimentary rocks from the Jack Hills, Western
Australia, that otherwise appear to have been deposited before 3.0 Ga is
explained by contamination with embedded zircon grains sourced from
transported regolith. Zircon U-Pb age spectra obtained by laser
ablation–inductively coupled plasma mass spectrometry (LA-ICPMS) analysis
of modern alluvium in creek beds draining the metasedimentary belts of
Mount Narryer and Jack Hills are dominated by these exotically sourced
younger components, as is the red soil that partly covers and infiltrates
fractures in the exposed Hadean zircon-bearing bedrock at Jack Hills. The
consistency of the age patterns found in superficial deposits sampled over
some 100 km implies a dominance of colluvium transported from the wider
Yilgarn craton, adjacent Capricorn orogen, and more distal sources rather
than material produced from the local (Mesoarchean and older) bedrocks. The
potential for similar cases of “environmental contamination” of surface
rocks by exogenous zircons needs to be considered more widely, particularly
in situations where interpreted maximum deposition ages are based on only a
small number of outlying youngest analyzed grains.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49822.1/612792/How-old-are-the-Jack-Hills-metasediments-really
Re-Os geochronology for the Cambrian SPICE event: Insights into euxinia
and enhanced continental weathering from radiogenic isotopes
Alan D. Rooney; Alexie E.G. Millikin; Per Ahlberg
Abstract:
The late Cambrian Steptoean positive carbon isotope excursion (SPICE)
represents a major perturbation to the global carbon cycle and was
associated with trilobite extinctions and expansion of anoxic and/or
euxinic water masses during episodes of eustatic sea-level change. We
present a new Re-Os age together with Os and Nd isotope stratigraphy and
major- and trace-element data from the Alum Shale Formation (Scania,
Sweden). The Re-Os age of 494.6 ± 2.9 Ma is from the interval of peak δ 13Corg values, providing the first radiometric age
constraint for this Cambrian carbon isotope excursion, interpreted as a
possible pre-Mesozoic ocean anoxia event, and the timing of biomere-level
extinctions. The Os isotope chemostratigraphic profile can be explained by
an increase in terrigenous weathering prior to the SPICE, potentially
driven by sea-level fall, and in agreement with enhanced nutrient supply,
primary productivity, and organic matter burial as the driver of the SPICE
event. Post-SPICE, the Os isotopes become increasingly unradiogenic;
however, invariant εNd(t) values argue
against a change in provenance and instead support a decrease in the
continental weathering flux, possibly related to eustatic sea-level rise.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49833.1/612793/Re-Os-geochronology-for-the-Cambrian-SPICE-event
Nanoscale isotopic evidence resolves origins of giant Carlin-type ore
deposits
E.A. Holley; A. Fulton; C. Jilly-Rehak; C. Johnson; M. Pribil
Abstract:
The western North American Great Basin’s Carlin-type deposits represent the
largest accumulation of gold in the Northern Hemisphere. The controversy
over their origins echoes the debate between Neptunists and Plutonists at
the birth of modern geology: were the causative processes meteoric or
magmatic? Sulfur isotopes have long been considered key to decoding metal
cycling in the Earth’s crust, but previous studies of Carlin-type pyrite
lacked the spatial resolution to quantify differences among the numerous
generations of sulfide mineralization. We developed a new dual-method,
nanoscale approach to examine the fine-grained ore pyrite. The δ 34S of the ore pyrite varies systematically with Au
concentration at the nanoscale, indicating that both magmatic and meteoric
fluids contributed during mineralization, but the magmas brought the gold.
Repeated oscillations in fluid ratios upgraded the metal content, resulting
in high gold endowment. Our results demonstrate that
high-spatial-resolution studies are key to elucidate the spatiotemporal
evolution of complex hydrothermal systems.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49888.1/612794/Nanoscale-isotopic-evidence-resolves-origins-of
Calcite U-Pb ages constrain petroleum migration pathways in tectonic
complex basins
Fuyun Cong; Jinqiang Tian; Fang Hao; Andrew R.C. Kylander-Clark; Wenqing
Pan ...
Abstract:
Tracing secondary oil migration pathways is critical for understanding
petroleum system evolution histories. Traditional tools (e.g., molecular
indicators and numerical modeling) utilized for evaluating oil migration
processes either lead to ambiguous interpretations or only provide
qualitative estimates. We quantitatively constrain secondary oil migration
processes under an absolute time frame by integrating oil-inclusion
fluorescence and in situ calcite U-Pb dating on calcite veins and
cements hosting primary oil inclusions. Fluorescence spectra of oil
inclusions and U-Pb ages were obtained on samples from ultra-deep
Ordovician reservoirs along two major faults in the Halahatang oilfield,
Tarim Basin (northwestern China). Absolute U-Pb ages suggest two main oil
charge events during 475–433 Ma and 294–262 Ma, respectively, and revealed
a northward-decreasing trend for oil maturity during single charge events.
Vertical migration of oil from in situ source rock through active
(or reactivated) faults is believed to be the key process inducing the
spatial maturity variation in charged oils and considered as the main
mechanism of secondary migration, with brecciated fault zones and dilatant
fractures along faults acting as major vertical oil-migration pathways. The
successful application of this approach has wider implications for
elucidating petroleum migration processes in tectonic complex basins
worldwide.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49750.1/612418/Calcite-U-Pb-ages-constrain-petroleum-migration
Significance of U-Pb detrital zircon geochronology for mudstone
provenance
Paul J. Sylvester; A. Kate Souders; Rui Liu
Abstract:
Detrital zircon U-Pb studies of mudstone provenance are rare but may
preferentially fingerprint distal zircon sources. To examine this issue,
Pierre Shale and Trinidad Sandstone deposited in a Late Cretaceous deltaic
environment in the Raton Basin, Colorado (USA), were measured for detrital
zircon U-Pb age by laser ablation–inductively coupled plasma–mass
spectrometry. Two major detrital zircon age peaks at ca. 70 and 1690 Ma are
found in both Pierre Shale and Trinidad Sandstone but in inversely varying
proportions: 68% and 16%, respectively, for the finest zircon fraction
(~15–35 μm) in the shale, and 25% and 32%, respectively, for the coarsest
zircon fraction (~60–80 μm) in the sandstone. Proximal sources in the
Sangre de Cristo Mountains, directly west of the Raton Basin, contain
coarse-grained, ca. 1690 Ma zircon, whereas distal sources in Laramide
uplifts and basins in Colorado, New Mexico, and Arizona contain
fine-grained, ca. 70 Ma zircon. The results indicate that U-Pb zircon
provenance of mudstone reflects availability of volcanic and other
fine-grained source rocks rather than simply distal sources. U-Pb zircon
provenance studies should routinely include mudstone units because these
units may identify fine-grained zircon sources more reliably than
sandstones alone.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49684.1/612419/Significance-of-U-Pb-detrital-zircon-geochronology
Cumulate granites: A perspective from new apatite MgO partition
coefficients
Qiong-Yao Zhan; Di-Cheng Zhu; Roberto F. Weinberg; Qing Wang; Jin-Cheng Xie
...
Abstract:
Apatite is a powerful research tool because it is common in various rocks
and incorporates many chemical elements. Understanding how elements
partition between apatite and melt and the controls on the partition are
critical for applications of apatite. It has been recently proposed that
MgO content in magmatic apatite is proportional to that of the melt,
highlighting the potential importance of this oxide as a critical
parameter. A comprehensive compilation of experimental data in this study
shows that the relationship between apatite MgO and melt MgO defines two
distinct equilibrium trends, indicating that melt composition or, more
specifically, melt polymerization strongly controls the partition of MgO.
These findings have great implications for using apatite in the study of
magma petrogenesis and sediment provenance. We show that when apatite MgO
is plotted against host-rock MgO, volcanic and plutonic systems have
different behaviors. The volcanic data plot around the experimental
apatite-melt equilibrium trends, while apatites in most plutonic rocks have
lower MgO than expected if they were in equilibrium with the host bulk
rock. For granites with high apatite saturation temperatures, this
indicates that apatites crystallized from low-MgO parent melts but later
became part of a cumulate with higher MgO than the parental melt, possibly
due to extraction of evolved interstitial melt. The contrast between
apatite MgO in volcanic and plutonic rocks therefore can provide a new
perspective on the debate about volcanic-plutonic connection.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49781.1/612420/Cumulate-granites-A-perspective-from-new-apatite
Millennial pulses of ore formation and an extra-high Tibetan Plateau
Yang Li; Mark B. Allen; Xian-Hua Li
Abstract:
Quantifying the rhythms and rates of magmatic-hydrothermal systems is
critical for a better understanding of their controls on ore formation and
the dynamics of magmatic reservoirs that feed them. We reconstructed the
evolution of ore-forming fluids using hydrothermal quartz from the 17.4 Ma
Zhibula skarn, Tibet. Ion probe analysis reveals sharp and dramatic changes
in quartz δ18O values between 5‰ and –9.3‰, with fluid δ 18O values varying between 2.8‰ and –18.2‰, which are best
explained by transient meteoric water incursion into a hydrothermal system
dominated by magmatic fluids. Two pulses of magmatic fluids and a meteoric
water incursion event are inferred, which operated at the millennium scale
(760–1510 yr) as constrained by the aluminum diffusion chronometer. Our
results indicate that magmatic reservoirs are likely water unsaturated for
most of their lifetime (>105–106 yr), with
transient and episodic fluid exsolutions (~103 yr) being driven
by magma replenishment or crystallization-induced water saturation. With
focused and efficient metal deposition, multiple pulses of metalliferous
fluids favor the formation of giant deposits with high grade. Meteoric
water δ18O values (–25.4 ± 2.3‰) derived from Zhibula quartz
further suggest a paleo-elevation of 5.9 ± 0.3 km; this transient early
Miocene surface uplift plausibly was due to break-off of the oceanic slab
attached to the Indian Plate. Our research highlights that ubiquitous
hydrothermal quartz in orogenic belts can probe the dynamics of
magmatic-hydrothermal systems and also quantify paleo-elevations, which has
significant tectonic implications.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49911.1/612421/Millennial-pulses-of-ore-formation-and-an-extra
Linking metamorphism and plate boundaries over the past 2 billion years
Yebo Liu; Ross N. Mitchell; Michael Brown; Tim E. Johnson; Sergei
Pisarevsky
Abstract:
Since the Jurassic, there has been a clear spatiotemporal correlation
between different types of metamorphism and active convergent plate
margins. However, the extent to which this relationship extends into the
past is poorly understood. We compared paleogeographic reconstructions and
inferred plate kinematics with the age and thermobaric ratio
(temperature/pressure [T/P]) of metamorphism over the
past 2 b.y. The null hypothesis—that there is no spatiotemporal link
between inferred plate margins and metamorphism—can be rejected. Low- T/P metamorphism is almost exclusively located near plate
margins, whereas intermediate- and high-T/P metamorphism
skews toward increasingly greater distances from these margins, consistent
with three different tectonic settings: the subduction zone, the mountain
belt, and the orogenic hinterland, respectively. However, paleogeographic
reconstructions suggest that so-called “paired metamorphic belts” are rare
and that high- and low-T/P localities more commonly occur
along strike from each other. The observation that bimodal metamorphism is
largely a function of distance from the trench and that end-member T/P types rarely occur in the same place can be explained
if the style of orogenesis has evolved from hotter to colder, consistent
with the abrupt emergence of low-T/P metamorphism in the
Cryogenian. The widespread development of high-T/P rocks
in orogenic hinterlands in the Proterozoic was followed by the production
and efficient exhumation of low-T/P rocks in subduction
channels in the Phanerozoic.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49637.1/612422/Linking-metamorphism-and-plate-boundaries-over-the
Stagnation and tearing of the subducting northwest Pacific slab
Muchen Sun; Youqiang Yu; Stephen S. Gao; Kelly H. Liu
Abstract:
Despite numerous observational and geodynamic modeling studies, the
presence of the northwest Pacific slab tear and its influence on mantle
dynamics remain controversial. By imaging the mantle transition zone (MTZ)
discontinuities beneath the Japan Sea and adjacent areas, we demonstrate an
ESE-WNW elongated zone with significant MTZ thinning extending from central
Honshu, Japan, to the Korean Peninsula, which provides additional
supporting evidence for the existence and distribution of a large-scale
slab tear. Our results, when combined with other geophysical and
geochemical evidence, indicate that the hot mantle material oceanward of
the slab may flow through the slab tear and contribute to surface
volcanism. Substantial MTZ thickening is widely observed in the region
south of the slab tear and suggests the existence of slab stagnation,
possibly related to the strong resistance at the bottom of the MTZ.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49862.1/612423/Stagnation-and-tearing-of-the-subducting-northwest
Hot atmospheric formation of carbonate accretionary lapilli at the
Cretaceous-Paleogene boundary, Brazos River, Texas, from clumped
isotope thermometry
David G. Burtt; Gregory A. Henkes; Thomas E. Yancey; Daniel Schrag
Abstract:
The Chicxulub impact (in the northern Yucatan Penninsula, Mexico) marks the
Cretaceous-Paleogene (K-Pg) boundary and is implicated in one of the five
major extinctions. Researchers have examined ejecta from the Chicxulub
impact, and most recently a drill core from the crater itself, yet the
processes and chemical reactions occurring in the impact vapor plume are
poorly constrained. Rounded carbonate particles, identified as accretionary
lapilli, have been found thousands of kilometers from the impact crater and
may be a unique record of plume conditions. We present carbon (δ 13C), oxygen (δ18O), and clumped (Δ47)
isotope ratios of lapilli from the Brazos River, Texas (USA), as well as
from foraminifera and a mudstone. Unaltered lapilli δ13C and δ 18O values covary, ranging from –9.38‰ to –2.10‰ and from –7.72‰
to –5.36‰, respectively, and they are distinct from mudstones,
foraminifera, and secondarily altered lapilli in the same section. Clumped
isotope temperatures [T(Δ47)] from the lapilli range
from 66 °C to 539 °C and average 155 ± 46 °C (1 standard deviation), with
sedimentary and fossil carbonates recording clement, shallow ocean–like T(Δ47). These data are consistent with petrography and
hypothesized vapor plume formation, and we argue that the δ13C
and δ18O values result from target rock decarbonation.
Atmospheric temperatures >100 °C extending >1800 km from the
Chicxulub crater imply an uninhabitable zone within seconds to minutes of
the impact that was 10× larger in diameter than the crater itself.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49674.1/612424/Hot-atmospheric-formation-of-carbonate
Shallow distributed faulting in the Imperial Valley, California,
USA
Valerie J. Sahakian; Boe J. Derosier; Thomas K. Rockwell; Joann M. Stock
Abstract:
In the tectonically complex Imperial Valley, California (USA), the Imperial
fault (IF) is often considered to be the primary fault at the U.S.-Mexico
border; however, its strain partitioning and interactions with other faults
are not well understood. Despite inferred evidence of other major faults
(e.g., seismicity), it is difficult to obtain a holistic view of this
system due to anthropogenic surface modifications. To better define the
structural configuration of the plate-boundary strain in this region, we
collected high-resolution shallow seismic imaging data in the All American
Canal, crossing the Imperial, Dixieland, and Michoacán faults. These data
image shallow (<25 m) structures on and near the mapped trace of the
Imperial fault, as well as the Michoacán fault and adjacent stepover.
Integration of our data with nearby terrestrial cores provides age
constraints on Imperial fault deformation. These data suggest that the
Michoacán fault, unmapped in the United States, is active and likely
produces dynamic or off-fault deformation within its stepover to the
Dixieland fault. Together, these data support more strain partitioning than
previously documented in this region.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49572.1/612425/Shallow-distributed-faulting-in-the-Imperial
A Tonian volcano-sedimentary succession in Newfoundland, eastern North
America: A post-Grenvillian link to the Asgard Sea?
Susan Strowbridge; Aphrodite Indares; Greg Dunning; Markus Wälle
Abstract:
Early Tonian bimodal volcanic and sedimentary rocks (the Pine Pond
succession) were identified in the Appalachians of eastern Laurentia for
the first time, with U-Pb ages of igneous zircon at 951.4 ± 9 Ma for a
felsic tuff and 952 ± 10 Ma for a crosscutting granite sheet. This
Neoproterozoic succession is part of the East Pond metamorphic suite on the
Baie Verte Peninsula (Newfoundland, Canada), which also contains 1491 Ma
orthogneiss and is consistent with deposition over Mesoproterozoic basement
in a pre-Iapetan extensional setting. Collectively, the data suggest a
correlation with the southern Labrador Pinware terrane of the Grenville
Province, where 1.52–1.46 Ga bedrock is intruded by 975–950 Ma granite. In
addition, the age and location of this 952 Ma succession are consistent
with deposition on the western margin of the Asgard Sea. Thus, the Pine
Pond succession provides a link between key events in the assembly of the
supercontinent Rodinia: the end of the Grenville orogeny, which led to the
amalgamation of Laurentia and Amazonia, and the early Neoproterozoic
evolution of the Asgard Sea, which separated Laurentia and Baltica.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49885.1/612426/A-Tonian-volcano-sedimentary-succession-in
Quantifying large-scale continental shelf margin growth and dynamics
across middle-Cretaceous Arctic Alaska with detrital zircon U-Pb dating
Richard O. Lease; David W. Houseknecht; Andrew R.C. Kylander-Clark
Abstract:
Sequence stratigraphy provides a unifying framework for integrating diverse
observations to interpret sedimentary basin evolution; however, key time
assumptions about stratigraphic elements spanning hundreds of kilometers
are rarely quantified. We integrate new detrital zircon U-Pb (DZ) dates
from 28 samples with seismic mapping to establish a chronostratigraphic
framework across 800 km and ~20 m.y. for the middle-Cretaceous
Torok-Nanushuk clinothem of Arctic Alaska (USA). Shelf-margin DZ dates
indicate continent-scale sediment routing with Russian Chukotka provenance
and provide reliable maximum depositional ages derived from arc volcanism.
Shelf-margin advance rates display a clear relationship to toplap
trajectories and provide empirical support for long-held inferences linking
sediment supply to margin architecture. Two distinct shelf-margin growth
regimes are evident: (1) a ca. 115–107 Ma phase of rapid ~50 km/m.y. shelf
advance rates with mainly progradational trajectories; and (2) a ca. 107–98
Ma phase of moderate ~13 km/m.y. shelf advance rates with
progradational-retrogradational-aggradational trajectories. We established
a subsequent shelf–to–deep water correlation by independently dating ca.
98–95 Ma low shelf accommodation and basin-floor deposition as far as 240
km east that indicate lowstand shedding and a change to localized routing
with Brooks Range provenance. Finally, we dated a ca. 95 Ma basin-wide
transgression at deep-water to shelfal settings across 350 km that exhibits
apparent synchroneity consistent with an event-significant surface. In one
of the world’s largest foreland-basin clinothems, our work constrains the
timing and duration of key depositional elements to test large-scale
sequence stratigraphic assumptions, enables reliable correlation and
quantification of sediment dynamics across 800 km, and captures the
chronology of a giant regressive-transgressive cycle.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49118.1/612427/Quantifying-large-scale-continental-shelf-margin
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