New Geology Articles Published Online Ahead of Print in December
Boulder, Colo., USA: Article topics and locations include methane seeps as
refugia; silver sources of Roman coinage; early Earth zircons; the
southwestern Laurentide Ice Sheet; the Siljan impact structure, Sweden; the
importance of oxbow lakes in the floodplain storage of pollutants; joint
earthquake ruptures of the San Andreas and San Jacinto faults; contemporary
and future dust sources; and soft-tissue preservation in ammonoids. These Geology articles are online at
https://geology.geoscienceworld.org/content/early/recent
.
When did the North Anatolian fault reach southern Marmara, Turkey?
Volkan Karabacak; Taylan Sançar; Gökhan Yildirim; I. Tonguç Uysal
Abstract:
We dated syntectonic calcites on fault planes from the southern branch of
the western North Anatolian fault (NAF) in northern Turkey using U-Th
geochronology. We selected strike-slip faults that are kinematically
related to the current regional strain field. The isotopic ages cluster
around different periods during the past ~700 k.y. The most prominent
cluster peak of 510.5 ± 9.5 ka (1σ) is consistent with the maximum
cumulative strike-slip offset data and tectonic plate motions measured by
GPS data, highlighting the fact that the present configuration of the NAF
in the southern Marmara region started at ca. 500 ka or earlier. These new
isotopic ages, combined with previous considerations of regional tectonics,
reveal that faulting along the western NAF initiated primarily in the
southern Marmara region at least a few hundred thousand years earlier than
the timing suggested for the northern branch of the western NAF. This study
presents an innovative approach to constrain the timing of initiation of
currently active fault segments along the NAF in southern Marmara. U-Th
geochronology of fault-hosted calcite thus has a wide application in
determining absolute ages of fault episodes in wider shear zones along
plate boundaries.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49726.1/610228/When-did-the-North-Anatolian-fault-reach-southern
Early Earth zircons formed in residual granitic melts produced by
tonalite differentiation
Oscar Laurent; Jean-François Moyen; Jörn-Frederik Wotzlaw; Jana Björnsen;
Olivier Bachmann
Abstract:
The oldest geological materials on Earth are Hadean (>4 Ga) detrital
zircon grains. Their chemistry and apparently low Ti-in-zircon temperatures
(≤700 °C) are considered to be inconsistent with crystallization in a magma
of the tonalite-trondhjemite-granodiorite (TTG) suite, although these are
the dominant Archean (4.0–2.5 Ga) silicic rocks. Using a new data set of
trace element contents in zircons from Paleoarchean Barberton TTGs (South
Africa) and thermodynamic modeling, we show that these zircons have
crystallized at near-solidus conditions from a compositionally uniform
granitic melt. This melt is residual from the crystallization of a less
evolved (tonalitic) parent and thereby shows major and trace element
compositions different from bulk TTG rocks. A global compilation reveals
that most Hadean detrital and Archean TTG-hosted grains share a peculiar
zircon trace element signature that is distinct from the chemical trends
defined by Phanerozoic zircons. Our model shows that the low Ti contents of
early Earth zircons reflect crystallization at higher temperatures (720–800
°C) than initially inferred due to lower modeled TiO2 activity
in the melt relative to previous estimates. We therefore propose that
near-solidus zircon crystallization from a chemically evolved melt in a
TTG-like magmatic environment was the dominant zircon-forming process on
the early Earth.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49232.1/610229/Early-Earth-zircons-formed-in-residual-granitic
Role of sediment in generating contemporaneous, diverse “type”
granitoid magmas
Shan Li; Calvin F. Miller; Wang Tao; Wenjiao Xiao; David Chew
Abstract:
Granite typology categorizes granitoid rocks based upon distinguishing
characteristics that are interpreted to indicate sources, conditions of
generation, and, by implication, tectonic setting. Complexities of
elemental and isotopic geochemistry, however, commonly preclude simple
typological interpretation and suggest more complex petrogenetic histories.
Granitoids from the Songpan-Ganzi terrane in the eastern Tibetan Plateau
were emplaced within a short interval (~15 m.y.). They display
mineralogical and geochemical characteristics that are consistent with a
wide range of proposed typologies (I-, S-, and A-type; high Ba-Sr and
adakitic variants). Despite their close spatial and temporal association,
these granitoids exhibit diversity in geochemical characteristics that
indicates a broad spectrum of contributing sources. Radiogenic isotope data
reveal a continuum from primitive to evolved crustal compositions; i.e., 87Sr/86Sr(t) =
0.704–0.715 and εNd(t) = +2 to –11. All granitoid
“types” have variable but commonly high zircon δ18O (+4.1‰ to
+11.6‰) and low whole-rock Li-B-Mg isotopic ratios compared to mantle
and/or seawater (δ7Li = +5.1‰ to –3.2‰; δ11B = –10.7‰
to –16.5‰; δ26Mg = –0.23‰ to –0.59‰). These stable isotopic
compositions suggest that the Songpan-Ganzi granitic magmas of all “types”
had contributions from sediment, ranging from minor to dominant. The highly
variable isotopic compositions of the granitoids rule out a single
homogeneous source for these diverse yet contemporaneous granitoids. Their
compositional variability may have been significantly influenced by
sedimentary contributions, and these results demonstrate the difficulty of
straightforward assignment and interpretation of granitoids using
conventional typology.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49509.1/610230/Role-of-sediment-in-generating-contemporaneous
Crustal thickness of the Grenville orogen: A Mesoproterozoic Tibet?
Adam Brudner; Hehe Jiang; Xu Chu; Ming Tang
Abstract:
The Grenville Province on the eastern margin of Laurentia is a remnant of a
Mesoproterozoic orogenic plateau that comprised the core of the ancient
supercontinent Rodinia. As a protracted Himalayan-style orogen, its
orogenic history is vital to understanding Mesoproterozoic tectonics and
paleoenvironmental evolution. In this study, we compared two geochemical
proxies for crustal thickness: whole-rock [La/Yb]N ratios of
intermediate-to-felsic rocks and europium anomalies (Eu/Eu*) in detrital
zircons. We compiled whole-rock geochemical data from 124 plutons in the
Laurentian Grenville Province and collected trace-element and
geochronological data from detrital zircons from the Ottawa and St.
Lawrence River (Canada) watersheds. Both proxies showed several episodes of
crustal thickening and thinning during Grenvillian orogenesis. The thickest
crust developed in the Ottawan phase (~60 km at ca. 1080 Ma and ca. 1045
Ma), when the collision culminated, but it was still up to 20 km thinner
than modern Tibet. We speculate that a hot crust and several episodes of
crustal thinning prevented the Grenville hinterland from forming a high
Tibet-like plateau, possibly due to enhanced asthenosphere-lithosphere
interactions in response to a warm mantle beneath a long-lived
supercontinent, Nuna-Rodinia.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49591.1/610231/Crustal-thickness-of-the-Grenville-orogen-A
Methane seeps as refugia during ash falls in the Late Cretaceous
Western Interior Seaway of North America
Shannon K. Brophy; Matthew P. Garb; Jone Naujokaityte; James D. Witts; Neil
H. Landman ...
Abstract:
Methane seeps host rich biotic communities, forming patchy yet highly
productive ecosystems across the global ocean. Persistent hydrocarbon
emissions fuel chemosynthetic food webs at seeps. Methane seeps were
abundant in the Western Interior Seaway of North America during the Late
Cretaceous. This area also experienced intermittent ash falls, which
negatively impacted the marine fauna. We propose that methane seeps acted
as refugia during these environmental perturbations. We report a laterally
continuous bentonite within the upper Campanian Baculites compressus Zone of the Pierre Shale in southwestern
South Dakota (USA) that fortuitously cuts across a methane seep deposit. We
compare the macroinvertebrate record below and above the bentonite at seep
and non-seep sites. Our results reveal that the paleocommunity (measured by
abundance and diversity) was largely unaffected by the ash fall at the seep
site, whereas it was significantly altered at the non-seep site. Thus,
methane seeps in the Western Interior Seaway may have provided refuges or
served as oases in the aftermath of severe environmental perturbations.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49372.1/610232/Methane-seeps-as-refugia-during-ash-falls-in-the
Silver isotope and volatile trace element systematics in galena samples
from the Iberian Peninsula and the quest for silver sources of Roman
coinage
Jean Milot; Janne Blichert-Toft; Mariano Ayarzagüena Sanz; Chloé
Malod-Dognin
Abstract:
Silver played a key role in the progressive monetization of early
Mediterranean civilizations. We combine Pb and Ag isotopes with volatile
trace elements (Bi, Sb, and As) to assess whether, during the Roman
occupation of Iberia, galena constituted a significant source of silver. We
find that the Pb and Ag isotopic compositions of 47 samples of galena from
eight different Iberian mining provinces, many of them exploited during
Roman times, are uncorrelated. This indicates that their respective
isotopic variabilities depend on different petrogenetic processes.
Moreover, the range of Ag isotopic abundances is approximately six times
wider than that displayed worldwide by silver coins in general and Roman
silver coins in particular. Although galena from the Betics provides the
best fit for Pb isotopes with Roman coins, their fit with Ag isotopic
compositions is at best sporadic. We suggest that, together with Sb, Bi,
and As, silver is primarily derived from fluids boiled off from
differentiated mantle-derived magmas. These fluids, in turn, reacted with
preexisting galena and functioned as a silver trap. Lead sulfides with ε 109Ag of ~0 and unusually rich in Ag, Sb, Bi, and As were the
most probable sources of ancient silver, whereas samples with ε 109Ag departing significantly from ~0 reflect low-temperature
isotopic fractionation processes in the upper crust.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G49690.1/610233/Silver-isotope-and-volatile-trace-element
Rapid retreat of the southwestern Laurentide Ice Sheet during the
Bølling-Allerød interval
Sophie Norris; Lev Tarasov; Alistair J. Monteath; John C. Gosse; Alan J.
Hidy ...
Abstract:
The timing of Laurentide Ice Sheet deglaciation along its southwestern
margin controlled the evolution of large glacial lakes and has implications
for human migration into the Americas. Accurate reconstruction of the ice
sheet’s retreat also constrains glacial isostatic adjustment models and is
important for understanding ice-sheet sensitivity to climate forcing.
Despite its significance, retreat of the southwestern Laurentide Ice Sheet
(SWLIS) is poorly constrained by minimum-limiting 14C data. We
present 26 new cosmogenic 10Be exposure ages spanning the
western Interior Plains, Canada. Using a Bayesian framework, we combine
these data with geomorphic mapping, 10Be, and high-quality
minimum-limiting 14C ages to provide an updated chronology. This
dataset presents an internally consistent retreat record and indicates that
the initial detachment of the SWLIS from its convergence with the
Cordilleran Ice Sheet began by ca. 15.0 ka, concurrent with or slightly
prior to the onset of the Bølling-Allerød interval (14.7–12.9 ka) and
retreated >1200 km to its Younger Dryas (YD) position in ~2500 yr.
Ice-sheet stabilization at the Cree Lake Moraine facilitated a meltwater
drainage route to the Arctic from glacial Lake Agassiz within the YD, but
not necessarily at the beginning. Our record of deglaciation and new YD
constraints demonstrate deglaciation of the Interior Plains was ~60% faster
than suggested by minimum 14C constraints alone. Numerical
modeling of this rapid retreat estimates a loss of ~3.7 m of sea-level
equivalent from the SWLIS during the Bølling-Allerød interval.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49493.1/610234/Rapid-retreat-of-the-southwestern-Laurentide-Ice
Magmatic sill formation during dike opening
Zhonglan Liu; W. Roger Buck
Abstract:
The origin of horizontal magma-filled sills is disputed, particularly for
extensional settings where the opening of vertical dikes is the predicted
mode of magma intrusion. We simulate long-term extension followed by
short-term dike opening in a two-dimensional viscoelastic medium
representing a plate spreading center. We show that dike opening in
extensionally stressed lithosphere can reduce sublithospheric vertical
stresses enough for sill opening given three conditions: (1) the Maxwell
time of the asthenosphere is <5× the time interval between dike
episodes; (2) the average density of the lithosphere is not much greater
than the magma density; and (3) the depth of an axial valley is smaller
than a few hundred meters. This mechanism explains the presence of sills
along much of the axis of faster-spreading ridges and their absence along
slower-spreading centers where thick dense lithosphere and/or sizeable
axial valleys exist.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49400.1/610210/Magmatic-sill-formation-during-dike-opening
Empirical constraints on progressive shock metamorphism of magnetite
from the Siljan impact structure, Sweden
Sanna Holm-Alwmark; Timmons M. Erickson; Aaron J. Cavosie
Abstract:
Little is known about the microstructural behavior of magnetite during
hypervelocity impact events, even though it is a widespread accessory
mineral and an important magnetic carrier in terrestrial and
extraterrestrial rocks. We report systematic electron backscatter
diffraction crystallographic analysis of shock features in magnetite from a
transect across the 52-km-diameter ca. 380 Ma Siljan impact structure in
Sweden. Magnetite grains in granitoid samples contain brittle fracturing,
crystal-plasticity, and lamellar twins. Deformation twins along {111} with
shear direction of <112> are consistent with spinel-law twins.
Inferred bulk shock pressures for the investigated samples, as constrained
by planar deformation features (PDFs) in quartz and shock twins in zircon,
range from 0 to 20 GPa; onset of shock-induced twinning in magnetite is
observed at >5 GPa. These results highlight the utility of magnetite to
record shock deformation in rocks that experience shock pressures >5
GPa, which may be useful in quartz-poor samples. Despite significant
hydrothermal alteration and the variable transformation of host magnetite
to hematite, shock effects are preserved, which demonstrates that magnetite
is a reliable mineral for preserving shock deformation over geologic time.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49498.1/610211/Empirical-constraints-on-progressive-shock
Whole-lithosphere shear during oblique rifting
Brandon M. Lutz; Gary J. Axen; Jolante W. van Wijk; Fred M. Phillips
Abstract:
Processes controlling the formation of continental whole-lithosphere shear
zones are debated, but their existence requires that the lithosphere is
mechanically coupled from base to top. We document the formation of a
dextral, whole-lithosphere shear zone in the Death Valley region (DVR),
southwest United States. Dextral deflections of depth gradients in the
lithosphere-asthenosphere boundary and Moho are stacked vertically,
defining a 20–50-km-wide, lower lithospheric shear zone with ~60 km of
shear. These deflections underlie an upper-crustal fault zone that accrued
~60 km of dextral slip since ca. 8–7 Ma, when we infer that
whole-lithosphere shear began. This dextral offset is less than net dextral
offset on the upper-crustal fault zone (~90 km, ca. 13–0 Ma) and total
upper-crustal extension (~250 km, ca. 16–0 Ma). We show that, before ca.
8–7 Ma, weak middle crust decoupled upper-crustal deformation from
deformation in the lower crust and mantle lithosphere. Between 16 and 7 Ma,
detachment slip thinned, uplifted, cooled, and thus strengthened the middle
crust, which is exposed in metamorphic core complexes collocated with the
whole-lithosphere shear zone. Midcrustal strengthening coupled the layered
lithosphere vertically and therefore enabled whole-lithosphere dextral
shear. Where thick crust exists (as in pre–16 Ma DVR), midcrustal
strengthening is probably a necessary condition for whole-lithosphere
shear.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49603.1/610212/Whole-lithosphere-shear-during-oblique-rifting
The importance of oxbow lakes in the floodplain storage of pollutants
Summer-Solstice Thomas; José Antonio Constantine; David Dethier; John W.
Thoman, Jr.; Jason Racela ...
Abstract:
Oxbow lakes are important stores for fine-grained sediment, which
potentially makes them critical sinks for sediment-associated pollutants.
We leverage an exhaustive public archive of coring data, supplemented by
our data collection, to provide a quantitative assessment of the role of
oxbows as off-channel sinks. We investigated loading trends of
sediment-sorbed polychlorinated biphenyls (PCBs) within oxbows of the
Housatonic River, an actively meandering river in western Massachusetts,
USA. Our results reveal the efficiency of oxbows as sinks, with average PCB
concentrations (14.8 ppm) that are nearly twice that of the surrounding
floodplain (7.56 ppm). Even though the 5.83 km2 floodplain is
the largest sink of PCB-laden material, storing as much as 14.1 t of PCBs
or 2.42 g m–2, oxbows store more than 20% of all PCBs (3.63 t of
PCBs or 11.2 g m–2) while making up just over 5% of the
floodplain surface area. Nearly 85% of the oxbow storage of PCBs occurs
within the first 50 m of floodplain, making clear the significance of
regular oxbow production to the off-channel storage of sediment-associated
pollutants.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49427.1/610213/The-importance-of-oxbow-lakes-in-the-floodplain
Can coseismic static stress changes sustain postseismic degassing?
Marco Bonini
Abstract:
Earthquakes can trigger increased degassing in hydrogeological systems.
Many of these systems return to preseismic conditions after months, but
sometimes postseismic degassing lasts for years. The factors controlling
such long-lasting degassing are poorly known. I explored the potential role
of diverse triggering mechanisms (i.e., dynamic and static stress changes,
volumetric strain) for three large earthquakes that induced postseismic
degassing (the Wenchuan [China], Maule [Chile], and Gorkha [Nepal]
earthquakes). The lessons from this study suggest that hydrogeological
systems can respond to earthquakes in various ways, and different causal
mechanisms can play a role. Persistent increased CO2 flux from
hot springs has been documented after the Gorkha earthquake. These hot
springs had their feeder systems dominantly unclamped, suggesting that
sufficiently large normal stress changes may sustain late postseismic
degassing. The results of this study are twofold: (1) they show a spatial
correlation between unclamping stress and increased gas flow, and (2) they
provide an explanation for protracted increased degassing.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49465.1/610214/Can-coseismic-static-stress-changes-sustain
Joint earthquake ruptures of the San Andreas and San Jacinto faults,
California, USA
Alba M. Rodriguez Padilla; Michael E. Oskin; Thomas K. Rockwell; Irina
Delusina; Drake M. Singleton
Abstract:
Large, multi-fault earthquakes increase the threat of strong ground shaking
and reshape the probability of future events across a system of faults.
Fault junctions act as conditional barriers, or earthquake gates, that stop
most earthquakes but permit junction-spanning events when stress conditions
are favorable. Constraining the physical conditions that favor multi-fault
earthquakes requires information on the frequency of isolated events versus
events that activate faults through the junction. Measuring this frequency
is challenging because dating uncertainties limit correlation of
paleoseismic events at different faults, requiring a direct approach to
measuring rupture through an earthquake gate. We show through documentation
and finite-element modeling of secondary fault slip that co-rupture of the
San Andreas and San Jacinto faults (California, USA) through the Cajon Pass
earthquake gate occurred at least three times in the past 2000 yr, most
recently in the historic 1812 CE earthquake. Our models show that
gate-breaching events taper steeply and halt abruptly as they transfer slip
between faults. Comparison to independent chronologies shows that 20%–23%
of earthquakes on the San Andreas and the San Jacinto faults are
co-ruptures through Cajon Pass.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49415.1/610118/Joint-earthquake-ruptures-of-the-San-Andreas-and
Flexural strike-slip basins
Derek Neuharth; Sascha Brune; Anne Glerum; Chris K. Morley; Xiaoping Yuan
...
Abstract:
Strike-slip faults are classically associated with pull-apart basins where
continental crust is thinned between two laterally offset fault segments.
We propose a subsidence mechanism to explain the formation of a new type of
basin where no substantial segment offset or syn-strike-slip thinning is
observed. Such “flexural strike-slip basins” form due to a sediment load
creating accommodation space by bending the lithosphere. We use a two-way
coupling between the geodynamic code ASPECT and surface-processes code
FastScape to show that flexural strike-slip basins emerge if sediment is
deposited on thin lithosphere close to a strike-slip fault. These
conditions were met at the Andaman Basin Central fault (Andaman Sea, Indian
Ocean), where seismic reflection data provide evidence of a laterally
extensive flexural basin with a depocenter located parallel to the
strike-slip fault trace.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49351.1/610119/Flexural-strike-slip-basins
Correlative tomography of an exceptionally preserved Jurassic ammonite
implies hyponome-propelled swimming
Lesley Cherns; Alan R.T. Spencer; Imran A. Rahman; Russell J. Garwood;
Christopher Reedman ...
Abstract:
The extreme rarity of soft-tissue preservation in ammonoids has meant
there are open questions regarding fundamental aspects of their biology. We
report an exceptionally preserved Middle Jurassic ammonite with unrivaled
information on soft-body organization interpreted through correlative
neutron and X-ray tomography. Three-dimensional imaging of muscles and
organs of the body mass for the first time in this iconic fossil group
provides key insights into functional morphology. We show that paired
dorsal muscles withdrew the body into the shell, rather than acting with
the funnel controlling propulsion as in Nautilus. This suggests a
mobile, retractable body as a defense strategy and necessitates a distinct
swimming mechanism of hyponome propulsion, a trait that we infer evolved
early in the ammonoid-coleoid lineage.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49551.1/610120/Correlative-tomography-of-an-exceptionally
Moho carbonation at an ocean-continent transition
Rémi Coltat; Philippe Boulvais; Yannick Branquet; Antonin Richard;
Alexandre Tarantola ...
Abstract:
Carbonation of mantle rocks during mantle exhumation is reported in
present-day oceanic settings, both at mid-ocean ridges and ocean-continent
transitions (OCTs). However, the hydrothermal conditions of carbonation
(i.e., fluid sources, thermal regimes) during mantle exhumation remain
poorly constrained. We focus on an exceptionally well-preserved fossil OCT
where mantle rocks have been exhumed and carbonated along a detachment
fault from underneath the continent to the seafloor along a tectonic Moho.
Stable isotope (oxygen and carbon) analyses on calcite indicate that
carbonation resulted from the mixing between serpentinization-derived
fluids at ~175 °C and seawater. Strontium isotope compositions suggest
interactions between seawater and the continental crust prior to
carbonation. This shows that carbonation along the tectonic Moho occurs
below the continental crust and prior to mantle exhumation at the seafloor
during continental breakup.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49363.1/610089/Moho-carbonation-at-an-ocean-continent-transition
Quantifying metasomatic high-field-strength and rare-earth element
transport from alkaline magmas
Krzysztof Sokół; Adrian A. Finch; William Hutchison; Jonathan Cloutier;
Anouk M. Borst ...
Abstract:
Alkaline igneous rocks host many global high-field-strength element (HFSE)
and rare-earth element (REE) deposits. While HFSEs are commonly assumed to
be immobile in hydrothermal systems, transport by late-stage hydrothermal
fluids associated with alkaline magmas is reported. However, the magnitude
of the flux and the conditions are poorly constrained and yet essential to
understanding the formation of REE-HFSE ores. We examined the alteration of
country rocks (“fenitization”) accompanying the emplacement of a syenite
magma at Illerfissalik in Greenland, through analysis of changes in rock
chemistry, mineralogy, and texture. Our novel geochemical maps show a
400-m-wide intrusion aureole, within which we observed typically tenfold
increases in the concentrations of many elements, including HFSEs. Textures
suggest both pervasive and structurally hosted fluid flow, with initial
reaction occurring with the protolith’s quartz cement, leading to increased
permeability and enhancing chemical interaction with a mixed Ca-K-Na
fenitizing fluid. We estimated the HFSE masses transferred from the syenite
to the fenite by this fluid and found ~43 Mt of REEs were mobilized (~12%
of the syenite-fenite system total rare-earth-oxide [TREO] budget), a mass
comparable to the tonnages of some of the world’s largest HFSE resources.
We argue that fenite can yield crucial information about the tipping points
in magma evolution because retention and/or loss of volatile-bonded alkali
and HFSEs are key factors in the development of magmatic
zirconosilicate-hosted HFSE ores (e.g., Kringlerne, at Ilímaussaq), or the
formation of the syenite-hosted Nb-Ta-REE (Motzfeldt-type) roof-zone
deposits.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49471.1/610090/Quantifying-metasomatic-high-field-strength-and
Olivine in komatiite records origin and travel from the deep upper
mantle
Allan Wilson; Robert Bolhar
Abstract:
The deep upper mantle is the main source of high-temperature magmatism, but
the only known naturally occurring samples of high-pressure mantle
constituents are mineral inclusions in diamonds. Trace elements in olivine
crystals from the 3.33 Ga Commondale Greenstone Belt in South Africa reveal
that these crystals formed in the deep upper mantle as high-pressure
phenocrysts, and some perhaps even formed in the mantle transition zone
(410–600 km) where they began as wadsleyite. The crystals were entrained
within ascending komatiite magma and conveyed to the surface. The olivine
crystals have the highest contents of Al2O3 (0.3 wt%)
recorded in any terrestrial olivine, which is indicative of formation at
high pressure. The deep mantle gave rise to Archean komatiites,
extraordinarily hot magmas (up to 1700 °C), which provide insight into
Earth’s early mantle evolution and the formation of most ancient
continental and oceanic crust. In spite of extensive research since their
discovery over 50 years ago, the origins of komatiites have remained
contentious. Plumes—thermochemical instabilities originating at the
core-mantle boundary—are the most likely source, but no direct evidence of
a deep mantle origin of komatiite has yet been recognized.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49523.1/610091/Olivine-in-komatiite-records-origin-and-travel
Could the Réunion plume have thinned the Indian craton?
Jyotirmoy Paul; Attreyee Ghosh
Abstract:
Thick and highly viscous roots are the key to cratonic survival.
Nevertheless, cratonic roots can be destroyed under certain geological
scenarios. Eruption of mantle plumes underneath cratons can reduce root
viscosity and thus make them more prone to deformation by mantle
convection. It has been proposed that the Indian craton could have been
thinned due to eruption of the Réunion plume underneath it at ca. 65 Ma. In
this study, we constructed spherical time-dependent forward mantle
convection models to investigate whether the Réunion plume eruption could
have reduced the Indian craton thickness. Along with testing the effect of
different strengths of craton and its surrounding asthenosphere, we
examined the effect of temperature-dependent viscosity on craton
deformation. Our results show that the plume-induced thermomechanical
erosion could have reduced the Indian craton thickness by as much as ~130
km in the presence of temperature-dependent viscosity. We also find that
the plume material could have lubricated the lithosphere-asthenosphere
boundary region beneath the Indian plate. This could be a potential reason
for acceleration of the Indian plate since 65 Ma.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49492.1/610092/Could-the-Reunion-plume-have-thinned-the-Indian
Contemporary and future dust sources and emission fluxes from gypsum-
and quartz-dominated eolian systems, New Mexico and Texas, USA
Mark R. Sweeney; Steven L. Forman; Eric V. McDonald
Abstract:
Recent research on dust emissions from eolian dunes seeks to improve
regional and global emissions estimates and knowledge of dust sources,
particularly with a changing climate. Dust emissions from dune fields can
be more accurately estimated when considering the whole eolian system
composed of active to stabilized dunes, interdunes, sand sheets, and
playas. Each landform can emit different concentrations of dust depending
on the supply of silt and clay, soil surface characteristics, and the
degree to which the landforms are dynamic and interact. We used the
Portable In Situ Wind Erosion Laboratory (PI-SWERL) to measure PM10
(particulate matter <10 μm) dust emission potential from landforms in
two end-member eolian systems: the White Sands dune field in New Mexico
(USA), composed of gypsum, and the Monahans dune field in west Texas,
composed of quartz. White Sands is a hotspot of dust emissions where dunes
and the adjacent playa yield high dust fluxes up to 8.3 mg/m2/s.
In contrast, the active Monahans dunes contain 100% sand and produce low
dust fluxes up to 0.5 mg/m2/s, whereas adjacent stabilized sand
sheets and dunes that contain silt and clay could produce up to 17.7 mg/m 2/s if reactivated by climate change or anthropogenic
disturbance. These findings have implications for present and future dust
emission potential of eolian systems from the Great Plains to the
southwestern United States, with unrealized emissions of >300 t/km 2/yr.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49488.1/610056/Contemporary-and-future-dust-sources-and-emission
Appearance and disappearance rates of Phanerozoic marine animal
paleocommunities
A.D. Muscente; Rowan C. Martindale; Anirudh Prabhu; Xiaogang Ma; Peter Fox
...
Abstract:
Ecological observations and paleontological data show that communities of
organisms recur in space and time. Various observations suggest that
communities largely disappear in extinction events and appear during
radiations. This hypothesis, however, has not been tested on a large scale
due to a lack of methods for analyzing fossil data, identifying
communities, and quantifying their turnover. We demonstrate an approach for
quantifying turnover of communities over the Phanerozoic Eon. Using network
analysis of fossil occurrence data, we provide the first estimates of
appearance and disappearance rates for marine animal paleocommunities in
the 100 stages of the Phanerozoic record. Our analysis of 124,605 fossil
collections (representing 25,749 living and extinct marine animal genera)
shows that paleocommunity disappearance and appearance rates are generally
highest in mass extinctions and recovery intervals, respectively, with
rates three times greater than background levels. Although taxonomic change
is, in general, a fair predictor of ecologic reorganization, the variance
is high, and ecologic and taxonomic changes were episodically decoupled at
times in the past. Extinction rate, therefore, is an imperfect proxy for
ecologic change. The paleocommunity turnover rates suggest that efforts to
assess the ecological consequences of the present-day biodiversity crisis
should focus on the selectivity of extinctions and changes in the
prevalence of biological interactions.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49371.1/610057/Appearance-and-disappearance-rates-of-Phanerozoic
Possible shock-induced crystallization of skeletal quartz from
supercritical SiO2-H2O fluid: A case study of impact melt from Kamil
impact crater, Egypt
Agnese Fazio; Luigi Folco; Falko Langenhorst
Abstract:
Since its discovery, the Kamil crater (Egypt) has been considered a natural
laboratory for studying small-scale impact cratering. We report on a
previously unknown shock-related phenomenon observed in impact melt masses
from Kamil; that is, the shock-triggered formation of skeletal quartz
aggregates from silica-rich fluids. These aggregates are unshocked and
characterized by crystallographically oriented lamellar voids and rounded
vesicles. The distribution of the aggregates can be correlated with former
H2O- and impurity-rich heterogeneities in precursor quartz;
i.e., fluid inclusions. The heterogeneities acted as hot spots for local
melting. Due to the presence of H2O and the high impact pressure
and temperature, the formation of a localized supercritical fluid is
plausible. Below the upper critical end point of the SiO2–H2O system (temperature <1100 °C and pressure <1 GPa), SiO 2 melt and H2O fluid become immiscible, leading to
the rapid and complete crystallization of skeletal quartz.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49476.1/610058/Possible-shock-induced-crystallization-of-skeletal
Monthly insolation linked to the time-transgressive nature of the
Holocene East Asian monsoon precipitation maximum
Xin Zhou; Tao Zhan; Luyao Tu; John P. Smol; Shiwei Jiang ...
Abstract:
More than 10% of the world’s population lives in the East Asian monsoon
(EAM) region, where precipitation patterns are critical to agricultural and
industrial activities. However, the dominant forcing mechanisms driving
spatiotemporal changes in the EAM remain unclear. We selected Holocene
records tracking monsoon precipitation in the EAM region reconstructed from
pollen data to explore the spatiotemporal patterns of monsoon precipitation
changes. Our analysis shows a time-transgressive pattern of maximum
precipitation, with earlier occurrence in the southern area and later
occurrence in the northern area. The monthly insolation changes force
monsoon precipitation in different parts of the EAM region through a shift
in the Western Pacific Subtropical High. We conclude that low-latitude
monthly insolation changes (rather than average summer insolation changes)
were the main forcing mechanisms of the spatiotemporal patterns of the
monsoon precipitation maximum during the Holocene.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49550.1/610059/Monthly-insolation-linked-to-the-time
Preferential dissolution of uranium-rich zircon can bias the hafnium
isotope compositions of granites
Peng Gao; Chris Yakymchuk; Jian Zhang; Changqing Yin; Jiahui Qian ...
Abstract:
Hafnium (Hf) isotopes in zircon are important tracers of granite
petrogenesis and continental crust evolution. However, zircon in granites
generally shows large Hf isotope variations, and the reasons for this are
debated. We applied U-Pb geochronology, trace-element, and Hf isotope
analyses of zircon from the Miocene Himalayan granites to address this
issue. Autocrystic zircon had εHf values (at 20 Ma) of –12.0 to
–4.3 (median = –9). Inherited zircon yielded εHf values (at 20
Ma) of –34.8 to +0.3 (median = –13); the majority of εHf values
were lower than those of autocrystic zircon. The εHf values of
inherited zircon with high U concentrations resembled those of autocrystic
zircon. Geochemical data indicates that the granites were generated during
relatively low-temperature (<800 °C) partial melting of metasedimentary
rocks, which, coupled with kinetic hindrance, may have led to the
preferential dissolution of high-U zircon that could dissolve more
efficiently into anatectic melt due to higher amounts of radiation damage.
Consequently, Hf values of autocrystic zircon can be biased toward the
values of U-rich zircon in the source. By contrast, literature data
indicate that granites generated at high temperatures (<820–850 °C)
generally contain autocrystic and inherited zircons with comparable Hf
isotope values. During higher-temperature melting, indiscriminate
dissolution of source zircon until saturation is reached will result in
near-complete inheritance of Hf isotope ratios from the source. Our results
impose an extra layer of complexity to interpretation of the zircon Hf
isotope archive that is not currently considered.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49656.1/610060/Preferential-dissolution-of-uranium-rich-zircon
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