New Geology Articles Published Online Ahead of Print in September
Boulder, Colo., USA: Article topics and locations include the use of
high-resolution three-dimensional seismic data to analyze the infill of
buried tunnel valleys in the North Sea; volcanic plumbing; rivers in
reverse; the 2014–2015 Holuhraun lava flow field, Iceland; Lake Magadi,
Kenya; and measuring wind via sand dunes on Mars. These Geology
articles are online at
https://geology.geoscienceworld.org/content/early/recent
.
Interaction bounding surfaces exposed in migrating transverse aeolian
ridges on Mars
Mackenzie Day
Abstract:
Wind-blown sand self-organizes into bedforms that have now been identified
on six different planetary bodies. These bedforms, including ripples and
dunes, exhibit patterns that are diagnostic of surface-atmosphere
interactions and can be used to interpret winds and sediment supply from
satellite images of planetary surfaces. Patterns in dune and ripple fields
change when one or more bedforms interact, for example, by linking,
colliding, or merging with one another. When two bedforms interact, the
cross-strata developed by the bedforms include a bounding surface where the
two bedforms combined. These “interaction bounding surfaces” have been
interpreted from ancient and modern strata in recent literature, but they
have not yet been identified beyond Earth. On Mars, aeolian dunes and
ripples form much as they do on Earth, but additional enigmatic bedform
types are also present. Transverse aeolian ridges are straight-crested
bedforms found abundantly on Mars, but with few analogs on Earth. Formation
mechanisms for these enigmatic bedforms range from dune-like migration and
construction to growth in place via wedge stacking or kinetic sieving. In
this work, I studied exposed stoss-slope stratification on these enigmatic
Martian bedforms to (1) identify the first in situ examples of
interaction bounding surfaces captured visually, and (2) demonstrate that
the transverse aeolian ridges must have been forward migrating.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49373.1/608087/Interaction-bounding-surfaces-exposed-in-migrating
The influence of rock uplift rate on the formation and preservation of
individual marine terraces during multiple sea-level stands
Luca C. Malatesta; Noah J. Finnegan; Kimberly L. Huppert; Emily I. Carreño
Abstract:
Marine terraces are a cornerstone for the study of paleo sea level and
crustal deformation. Commonly, individual erosive marine terraces are
attributed to unique sea-level high stands based on the reasoning that
marine platforms could only be significantly widened at the beginning of an
interglacial. However, this logic implies that wave erosion is
insignificant at other times. We postulate that the erosion potential at a
given bedrock elevation datum is proportional to the total duration of
sea-level occupation at that datum. The total duration of sea-level
occupation depends strongly on rock uplift rate. Certain rock uplift rates
may promote the generation and preservation of particular terraces while
others prevent them. For example, at rock uplift of ~1.2 mm/yr, the Marine
Isotope Stage (MIS) 5e (ca. 120 ka) high stand reoccupies the elevation of
the MIS 6d–e mid-stand, favoring creation of a wider terrace than at higher
or lower rock uplift rates. Thus, misidentification of terraces can occur
if each terrace in a sequence is assumed to form uniquely at successive
interglacial high stands and to reflect their relative elevations.
Developing a graphical proxy for the entire erosion potential of sea-level
history allows us to address creation and preservation biases at different
rock uplift rates.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49245.1/608088/The-influence-of-rock-uplift-rate-on-the-formation
Thermodynamic limits for assimilation of silicate crust in primitive
magmas
Jussi S. Heinonen; Frank J. Spera; Wendy A. Bohrson
Abstract:
Some geochemical models for basaltic and more primitive rocks suggest that
their parental magmas have assimilated tens of weight percent of crustal
silicate wall rock. But what are the thermodynamic limits for assimilation
in primitive magmas? We pursue this question quantitatively using a freely
available thermodynamic tool for phase equilibria modeling of open magmatic
systems—the Magma Chamber Simulator (https://mcs.geol.ucsb.edu)—and focus
on modeling assimilation of wall-rock partial melts, which is
thermodynamically more efficient compared to bulk assimilation of stoped
wall-rock blocks in primitive igneous systems. In the simulations, diverse
komatiitic, picritic, and basaltic parental magmas assimilate progressive
partial melts of preheated average lower, middle, and upper crust in
amounts allowed by thermodynamics. Our results indicate that it is
difficult for any subalkaline primitive magma to assimilate more than 20–30
wt% of upper or middle crust before evolving to compositions with higher
SiO2 than a basaltic magma (52 wt%). On the other hand, typical
komatiitic magmas have thermodynamic potential to assimilate as much as
their own mass (59–102 wt%) of lower crust and retain a basaltic
composition. The compositions of the parental melt and the assimilant
heavily influence both how much assimilation is energetically possible in
primitive magmas and the final magma composition given typical
temperatures. These findings have important implications for the role of
assimilation in the generation and evolution of, e.g., ultramafic to mafic
trans-Moho magmatic systems, siliceous high-Mg basalts, and massif-type
anorthosites.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49139.1/608089/Thermodynamic-limits-for-assimilation-of-silicate
New estimates of the magnitude of the sea-level jump during the 8.2 ka
event
Jonathan Obrist-Farner; Mark Brenner; Jeffery R. Stone; Marta
Wojewódka-Przybył; Thorsten Bauersachs ...
Abstract:
We analyzed sediment cores from coastal Lake Izabal, Guatemala, to infer
Holocene biogeochemical changes in the lake. At ca. 8370 calibrated yr B.P.
(cal. yr B.P.), marine waters entered the lake, which presently lies ~38 km
from the Caribbean coast. Temporal correlation between Early Holocene
drainage of high-latitude Lakes Agassiz and Ojibway (in North America) and
marine flooding of Lake Izabal suggests a causal link between the two
processes. Our data indicate a relative sea-level jump of 2.60 ± 0.88 m,
which is larger than previous estimates of sea-level rise during the 8.2 ka
event. The inferred sea-level jump, however, cannot be explained solely by
the volume of water released during drainage of Lakes Agassiz and Ojibway.
Instead, we propose that previous studies underestimated the magnitude of
Lakes Agassiz and Ojibway discharge, or that additional meltwater sources
contributed to global sea-level rise at that time.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49296.1/608090/New-estimates-of-the-magnitude-of-the-sea-level
Late Quaternary geochronologic record of soil formation and erosion:
Effects of climate change on Mojave Desert hillslopes (Nevada, USA)
L.P. Persico; L.D. McFadden; J.R. McAuliffe; T.M. Rittenour; T.E.
Stahlecker ...
Abstract:
Climate change is an often-cited control on geomorphic processes in the
arid southwestern United States, but links to direct climatic factors and
vegetation change remain under debate. Hillslopes at a site in the eastern
Mojave Desert in southern Nevada are mantled by 0–1.5 m of colluvial
deposits. Accumulation of weathered bedrock combined with eolian inputs of
fine sand and silt led to the formation of well-developed soil profiles.
Surface sediments from both sources were incorporated into colluvium,
allowing both processes to be dated with optically stimulated luminescence
(OSL). OSL ages indicate a period of increased colluviation in the Late
Pleistocene facilitated by enhanced bedrock weathering and dust deposition.
Hillslope aspect strongly controls predominant soil environments and
associated vegetation. Well-developed soils with dense grass cover
extensively mantle the mesic north-aspect hillslopes, while more xeric
south-aspect hillslopes are dominated by thin colluvium with minimal soil
development, extensive bedrock exposure, and desertscrub vegetation.
Remnants of older colluvium with moderately developed soils on south
aspects, however, indicate they were once more extensively mantled by
thicker colluvial deposits. The transition to drier conditions in the
Holocene diminished vegetation cover on more xeric south aspects,
triggering widespread erosion, whereas the more mesic north aspects
retained denser grass cover that minimized erosion. The transition to drier
conditions in the Holocene altered the vegetation; however, persistent
perennial grass cover minimized erosion into the middle Holocene.
Increasing aridity during the middle Holocene significantly reduced grass
cover on more xeric south aspects, triggering erosion and alluvial
deposition. OSL dates of dust incorporated into terrace sediments indicate
late Middle Holocene aggradation and soil development in the Late Holocene.
In contrast, maintenance of substantial perennial grass cover on mesic
north aspects minimized erosion from those hillslopes throughout the
Holocene.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49270.1/608091/Late-Quaternary-geochronologic-record-of-soil
Guttulatic calcite: A carbonate microtexture that reveals frigid
formation conditions
Eva L. Scheller; John Grotzinger; Miquela Ingalls
Abstract:
The paragenesis of carbonate pseudomorphic textures in the rock record that
are inferred to represent replaced metastable ikaite (CaCO3·6H 2O), which forms at frigid temperatures, is uncertain.
Petrographic analysis of Mono Lake (California, USA) Pleistocene tufas
allowed recognition of a distinctive calcite microtexture, termed
guttulatic calcite, that forms during carbonate dehydration and is
diagnostic for precursor ikaite. The texture is characterized by
pseudo-hexagonal or spherical low-Mg cores, which likely formed initially
as vaterite, with an ellipsoidal overgrowth, and a secondary high-Mg sparry
or micritic cement. Observations of Mono Lake ikaite pseudomorphs, combined
with a review of more ancient examples, indicate that guttulatic texture
records carbonate dehydration of precursor ikaite and can be used to infer
frigid paleotemperatures.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49312.1/607791/Guttulatic-calcite-A-carbonate-microtexture-that
Mantle heating at ca. 2 Ga by continental insulation: Evidence from
granites and eclogites
R. Tamblyn; D. Hasterok; M. Hand; M. Gard
Abstract:
Igneous and metamorphic rocks contain the mineralogical and geochemical
record of thermally driven processes on Earth. The generally accepted
thermal budget of the mantle indicates a steady cooling trend since the
Archean. The geological record, however, indicates this simple cooling
model may not hold true. Subduction-related eclogites substantially emerge
in the rock record from 2.1 Ga to 1.8 Ga, indicating that average mantle
thermal conditions cooled below a critical threshold for widespread
eclogite preservation. Following this period, eclogite disappeared again
until ca. 1.1 Ga. Coincident with the transient emergence of eclogite,
global granite chemistry recorded a decrease in Sr and Eu and increases in
yttrium and heavy rare earth element (HREE) concentrations. These changes
are most simply explained by warming of the thermal regime associated with
granite genesis. We suggest that warming was caused by increased
continental insulation of the mantle at this time. Ultimately, secular
cooling of the mantle overcame insulation, allowing the second emergence
and preservation of eclogite from ca. 1.1 Ga until present.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49288.1/607792/Mantle-heating-at-ca-2-Ga-by-continental
Rivers in reverse: Upstream-migrating dechannelization and flooding
cause avulsions on fluvial fans
Douglas A. Edmonds; Harrison K. Martin; Jeffery M. Valenza; Riley Henson;
Gary S. Weissmann ...
Abstract:
The process of river avulsion builds floodplains and fills alluvial basins.
We report on a new style of river avulsion identified in the Landsat
satellite record. We found 69 examples of retrogradational avulsions on
rivers of densely forested fluvial fans in the Andean and New Guinean
alluvial basins. Retrogradational avulsions are initiated by a channel
blockage, e.g., a logjam, that fills the channel with sediment and forces
water overbank (dechannelization), which creates a chevron-shaped flooding
pattern. Dechannelization waves travel upstream at a median rate of 387
m/yr and last on average for 13 yr; many rivers show multiple
dechannelizing events on the same reach. Dechannelization ends and the
avulsion is complete when the river finds a new flow path. We simulate
upstreammigrating dechannelization with a one-dimensional morphodynamic
model for open channel flow. Observations are consistent with model results
and show that channel blockages can cause dechannelization on steep (10–2 to 10–3), low-discharge (~101 m 3 s–1) rivers. This illustrates a new style of
floodplain sedimentation that is unaccounted for in ecologic and
stratigraphic models.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49318.1/607793/Rivers-in-reverse-Upstream-migrating
Propagating uplift controls on high-elevation, low-relief landscape
formation in the southeast Tibetan Plateau
X.P. Yuan; K.L. Huppert; J. Braun; X. Shen; J. Liu-Zeng ...
Abstract:
High-elevation, low-relief surfaces are widespread in many mountain belts.
However, the origin of these surfaces has long been debated. In particular,
the southeast Tibetan Plateau has extensive low-relief surfaces perched
above deep valleys and in the headwaters of three of the world’s largest
rivers (Salween, Mekong, and Yangtze Rivers). Various geologic data and
geodynamic models show that many mountain belts grow first to a certain
height and then laterally in an outward propagation sequence. By
translating this information into a kinematic propagating uplift function
in a landscape evolution model, we propose that the high-elevation,
low-relief surfaces in the southeast Tibetan Plateau are simply a
consequence of mountain growth and do not require a special process to
form. The propagating uplift forms an elongated river network geometry with
broad high-elevation, low-relief headwaters and interfluves that persist
for tens of millions of years, consistent with the observed geochronology.
We suggest that the low-relief interfluves can be long-lived because they
lack the drainage networks necessary to keep pace with the rapid incision
of the large main-stem rivers. The propagating uplift also produces spatial
and temporal exhumation patterns and river profile morphologies that match
observations. Our modeling therefore reconciles geomorphic observations
with geodynamic models of uplift of the southeast Tibetan Plateau, and it
provides a simple mechanism to explain the low-relief surfaces observed in
several mountain belts on Earth.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49022.1/607794/Propagating-uplift-controls-on-high-elevation-low
Diversification of Archean tonalite-trondhjemite-granodiorite suites in
a mushy middle crust
Jillian Kendrick; Manuel Duguet; Chris Yakymchuk
Abstract:
Tonalite-trondhjemite-granodiorite (TTG) suites are the dominant component
of Earth’s first continents, but their origins are debated. The trace
element concentrations of these rocks are conventionally linked to their
source depths and inferred geodynamic settings with the implicit assumption
that TTG compositions are source-controlled. Alternatively, their variable
compositions may be caused by fractional crystallization in TTG plutons
after emplacement and less clearly linked to source depth. Most TTGs in
Archean mid-crustal exposures are the dominant component of igneous gray
gneiss complexes; the processes that influence the evolution of TTG magmas
in this setting are poorly understood. We present a
petrological–geochemical model that explains the coexistence of TTGs in the
middle crust with low-pressure and high-pressure geochemical trends,
irrespective of tectonic setting or depth of the TTG source. We propose
that mid-crustal TTGs were long-lived crystal mushes with compositions
controlled by the separation of early-crystallizing plagioclase and melt.
Using phase equilibrium modeling, we demonstrate that a suite of TTGs from
the southern Superior Province in Canada represents variably
plagioclase-rich and melt-rich fractions from a common parent magma. The
behavior of plagioclase may have a strong influence on the geochemical
trends of TTGs, including the degree of rare earth element fractionation.
Our results suggest that trace element compositions of TTGs may not
primarily reflect the depth of the source and cannot be used alone to infer
Archean geodynamic settings.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49287.1/607795/Diversification-of-Archean-tonalite-trondhjemite
Linking lava morphologies to effusion rates for the 2014–2015 Holuhraun
lava flow field, Iceland
Joana R.C. Voigt; Christopher W. Hamilton; Gregor Steinbrügge; Ármann
Höskuldsson; Ingibjörg Jónsdottir ...
Abstract:
Determining the parameters that control fissure-fed lava morphologies is
critical for reconstructing the complex emplacement histories of eruptions
on Earth and other planetary bodies. We used a geomorphological map of the
2014–2015 Holuhraun lava flow field, in combination with new constraints on
lava emplacement chronology and two independently derived time-averaged
discharge rate (TADR) data sets, to analyze correlations between lava
morphology and effusion rate. Results show that lava morphologies are
dominantly controlled by effusion rate at the vent during the early phases
of the eruption and by lava transport processes as the system evolves.
Initially, TADR and its variance, which reflect pulsation in the lava
supply rate from the vent, directly affect lava emplacement styles.
However, as the eruption progresses, the lava transport system exerts a
stronger control with channels and ponds that can either dampen variation
in local effusion rate or create surges during sudden drainage events. The
Holuhraun eruption predominantly produced rubbly lava in its earlier
eruption phases and transitioned into the production of spiny lava toward
the end of the eruption. However, a drop of TADR during the first phase of
the eruption correlates with a decrease in rubbly lava formation and an
increase in spiny lava production. This suggests that a change in effusion
rate caused the observed transition in lava type. Our findings show that
rubbly lava is formed under relatively high local effusion rates with
pulsating supply conditions, whereas spiny lava is formed under lower local
effusion rates and steadier supply.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49251.1/607796/Linking-lava-morphologies-to-effusion-rates-for
Orbital control of Pleistocene euxinia in Lake Magadi, Kenya
D.M. Deocampo; R.B. Owen; T.K. Lowenstein; R.W. Renaut; N.M. Rabideaux ...
Abstract:
Lake Magadi is an internally drained, saline and alkaline terminal sump in
the southern Kenya Rift. Geochemistry of samples from an ~200 m core
representing the past ~1 m.y. of the lake’s history shows some of the
highest concentrations of transition metals and metalloids ever reported
from lacustrine sediment, including redox-sensitive elements molybdenum,
arsenic, and vanadium. Elevated concentrations of these elements represent
times when the lake’s hypolimnion was euxinic—that is, anoxic, saline, and
sulfide-rich. Euxinia was common after ca. 700 ka, and after that tended to
occur during intervals of high orbital eccentricity. These were likely
times when high-frequency hydrologic changes favored repeated episodes of
euxinia and sulfide precipitation. High-amplitude environmental
fluctuations at peak eccentricity likely impacted water balance in
terrestrial habitats and resource availability for early hominins. These
are associated with important events in human evolution, including the
first appearance of Middle Stone Age technology between ca. 500 and 320 ka
in the southern Kenya Rift.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49140.1/607797/Orbital-control-of-Pleistocene-euxinia-in-Lake
Unraveling the histories of Proterozoic shales through in situ Rb-Sr
dating and trace element laser ablation analysis
Darwinaji Subarkah; Morgan L. Blades; Alan S. Collins; Juraj Farkaš; Sarah
Gilbert ...
Abstract:
Authigenic components in marine sediments are important archives for past
environment reconstructions. However, defining reliable age constraints and
assessing the effects of post-depositional overprints in Precambrian
sequences are challenging. We demonstrate a new laser-based analytical
approach that has the potential to rapidly and accurately evaluate the
depositional and alteration histories of Proterozoic shales. Our study
employs a novel application of in situ Rb-Sr dating coupled with
simultaneous trace-element analysis using reaction-cell laser
ablation–inductively coupled plasma–tandem mass spectrometry (LA-ICPMS/MS).
We present results from shales sourced from two wells in the Proterozoic
McArthur Basin, northern Australia. These rocks have been widely used by
previous studies as a key section for ancient biogeochemical and
paleo-redox reconstructions. Shales from well UR5 yielded initial 87Sr/86Sr ratios, Rb-Sr ages, and rare earth element
plus yttrium (REEY) patterns similar to those of a dolerite sampled from
the same core. We propose that the UR5 samples chronicle hydrothermal
alteration instigated by the dolerite intrusion. In contrast, a correlative
shale from well UR6 yielded an age consistent with the expected
depositional age (1577 ± 56 Ma) with REEY and initial 87Sr/ 86Sr ratios similar to ca. 1.5 Ga seawater. We suggest that this
sample records the minimum depositional age and early marine diagenetic
history for this unit. This new technique can date Proterozoic shales
quickly, cheaply, and with minimum sample preparation. Importantly, ages
are triaged to differentiate between those recording primary marine versus
secondary processes. This novel approach provides a potentially powerful
tool for dating and fingerprinting the vast array of ancient marine shales
for further studies of Earth systems through deep time.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49187.1/607798/Unraveling-the-histories-of-Proterozoic-shales
Metamorphism obscures primary taphonomic pathways in the early Cambrian
Sirius Passet Lagerstätte, North Greenland
Morten Lunde Nielsen; Mirinae Lee; Hong Chin Ng; Jeremy C. Rushton;
Katharine R. Hendry ...
Abstract:
Correct interpretation of soft-bodied fossils relies on a thorough
understanding of their taphonomy. While the focus has often been on the
primary roles of decay and early diagenesis, the impacts of deeper burial
and metamorphism on fossil preservation are less well understood. We
document a sequence of late-stage mineral replacements in panarthropod
fossils from the Sirius Passet Lagerstätte (North Greenland), an important
early Cambrian Burgess Shale–type (BST) biota. Muscle and gut diverticula
were initially stabilized by early diagenetic apatite, prior to being
pervasively replaced by quartz and then subordinate chlorite, muscovite,
and chloritoid during very low- to low-grade metamorphism. Each new mineral
replicates the soft tissues with different precision and occurs in
particular anatomical regions, imposing strong biases on the biological
information retained. Muscovite and chloritoid largely obliterate the
tissues’ original detail, suggesting that aluminum-rich protoliths may have
least potential for conserving mineralized soft tissues in metamorphism.
Overall, the fossils exhibit a marked shift toward mineralogical
equilibration with the matrix, obscuring primary taphonomic modes.
Sequential replacement of the phosphatized soft tissues released phosphorus
to form new accessory monazite (and apatite and xenotime), whose presence
in other BST biotas might signal the prior, more widespread, occurrence of
this primary mode of preservation. Our results provide critical context for
interpreting the Sirius Passet biota and for identifying late-stage
overprints in other biotas.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G48906.1/607799/Metamorphism-obscures-primary-taphonomic-pathways
Carbon isotope excursions during the late Miocene recorded by lipids of
marine Thaumarchaeota, Piedmont Basin, Mediterranean Sea
Mathia Sabino; Daniel Birgel; Marcello Natalicchio; Francesco Dela Pierre;
Jörn Peckmann
Abstract:
Group I mesophilic Thaumarchaeota fix dissolved inorganic carbon (DIC),
accompanied by a biosynthetic fractionation factor of ~20‰. Accordingly,
the δ13C signature of their diagnostic biomarker crenarchaeol
was suggested as a potential δ13CDIC proxy in marine
basins if input from nonmarine Thaumarchaeota is negligible. Semi-enclosed
basins are sensitive to carbon-cycle perturbations, because they tend to
develop thermohaline stratification. Water column stratification typified
the semi-enclosed basins of the Mediterranean Sea during the late Miocene
(Messinian) salinity crisis (5.97–5.33 Ma). To assess how the advent of the
crisis affected the carbon cycle, we studied sediments of the Piedmont
Basin (northwestern Italy), the northernmost Mediterranean subbasin. A
potential bias of our δ13CDIC reconstructions from
the input of soil Thaumarchaeota is discarded, since high and increasing
branched and isoprenoid tetraether (BIT) index values do not correspond to
low and decreasing δ13C values for thaumarchaeal lipids, which
would be expected in case of high input from soil Thaumarchaeota. Before
the onset of the crisis, the permanently stratified distal part of the
basin hosted a water mass below the chemocline with a δ13C DIC value of approximately –3.5‰, while the well-mixed proximal
part had a δ13CDIC value of approximately –0.8‰. The
advent of the crisis was marked by 13C enrichment of the DIC
pool, with positive δ13CDIC excursions up to +5‰ in
the upper water column. Export of 12C to the seafloor after
phytoplankton blooms and limited replenishment of remineralized carbon due
to the stabilization of thermohaline stratification primarily caused such 13C enrichment of the DIC pool.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49360.1/607640/Carbon-isotope-excursions-during-the-late-Miocene
Volcanic plumbing filters on ocean-island basalt geochemistry
Teresa Ubide; Patricia Larrea; Laura Becerril; Carlos Galé
Abstract:
Ocean-island basalts (OIBs) are considered to be messengers from the deep
mantle, yet the filtering effect of the plumbing systems that bring OIB
melts to the surface remains poorly assessed. We investigated volcanic
products from El Hierro island (Canary Islands) from textural and chemical
perspectives. The majority of geochemical data cluster at relatively
fractionated basaltic compositions of 5 wt% MgO. Compositions ≥10 wt% MgO
are porphyritic whole rocks that accumulate mafic minerals. Near-primary
melts do not erupt. Instead, we show that carrier melts (crystal-free whole
rocks, glasses, and melt inclusions) are consistently buffered to low-MgO
compositions during passage through the plumbing system. We tested our
model of melt fractionation and crystal accumulation on a global
compilation of OIBs. Similar to El Hierro, the majority of data cluster at
evolved compositions of 5 wt% MgO (alkaline) to 7 wt% MgO (tholeiitic).
Modeling the fractionation of OIB parental melts, we show that with 50%
crystallization, OIB melts reach 5 wt% MgO with reduced density, increased
volatile content, and overall low viscosity, becoming positively buoyant
relative to wall rocks and highly eruptible when reaching volatile
saturation at depths around the crust-mantle boundary. Under these
conditions, 5 wt% MgO OIB “sweet spot” melts are propelled to the surface
and erupt carrying an assortment of recycled crystals. This mechanism is
consistent with the petrography and chemistry of erupted products and
suggests OIB volcanoes are dominated by low-MgO basaltic melts.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49224.1/607641/Volcanic-plumbing-filters-on-ocean-island-basalt
Boninitic blueschists record subduction initiation and subsequent
accretion of an arc–forearc in the northeast Proto-Tethys Ocean
Dong Fu; Bo Huang; Tim E. Johnson; Simon A. Wilde; Fred Jourdan ...
Abstract:
Subduction of oceanic lithosphere is a diagnostic characteristic of plate
tectonics. However, the geodynamic processes from initiation to termination
of subduction zones remain enigmatic mainly due to the scarcity of
appropriate rock records. We report the first discovery of early Paleozoic
boninitic blueschists and associated greenschists from the eastern
Proto-Tethyan North Qilian orogenic belt, northeastern Tibet, which have
geochemical affinities that are typical of forearc boninites and island arc
basalts, respectively. The boninitic protoliths of the blueschists record
intra-oceanic subduction initiation at ca. 492–488 Ma in the eastern North
Qilian arc/forearc–backarc system, whereas peak blueschist facies
metamorphism reflects subsequent subduction of the arc/forearc complex to
high pressure at ca. 455 Ma. These relations therefore record the life
circle of an intra-oceanic subduction zone within the northeastern
Proto-Tethys Ocean. The geodynamic evolution provides an early Paleozoic
analogue of the early development of the Izu–Bonin–Mariana arc and its
later subduction beneath the extant Japanese arc margin. This finding
highlights the important role of subduction of former upper plate island
arc/forearcs in reducing the likelihood of preservation of initial
subduction-related rock records in ancient orogenic belts.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49457.1/607464/Boninitic-blueschists-record-subduction-initiation
Detrital signals of coastal erosion and fluvial sediment supply during
glacio-eustatic sea-level rise, Southern California, USA
G.R. Sharman; J.A. Covault; D.F. Stockli; Z.T. Sickmann; M.A. Malkowski ...
Abstract:
Coastal erosion, including sea-cliff retreat, represents both an important
component of some sediment budgets and a significant threat to coastal
communities in the face of rising sea level. Despite the importance of
predicting future rates of coastal erosion, few prehistoric constraints
exist on the relative importance of sediment supplied by coastal erosion
versus rivers with respect to past sea-level change. We used detrital
zircon U-Pb geochronology as a provenance tracer of river and deep-sea fan
deposits from the Southern California Borderland (United States) to
estimate relative sediment contributions from rivers and coastal erosion
from late Pleistocene to present. Mixture modeling of submarine canyon and
fan samples indicates that detrital zircon was dominantly (55%–86%)
supplied from coastal erosion during latest Pleistocene (ca. 13 ka)
sea-level rise, with lesser contributions from rivers, on the basis of
unique U-Pb age modes relative to local Peninsular Ranges bedrock sources.
However, sediment that was deposited when sea level was stable at its
highest and lowest points since the Last Glacial Maximum was dominantly
supplied by rivers, suggesting decreased coastal erosion during periods of
sea-level stability. We find that relative sediment supply from coastal
erosion is strongly dependent on climate state, corroborating predictions
of enhanced coastal erosion during future sea-level rise.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49430.1/607465/Detrital-signals-of-coastal-erosion-and-fluvial
Extreme event-driven sediment aggradation and erosional buffering along
a tectonic gradient in southern Taiwan
Clarke DeLisle; Brian J. Yanites; Chia-Yu Chen; Bruce H. Shyu; Tammy M.
Rittenour
Abstract:
In most landscape evolution models, extreme rainfall enhances river
incision. In steep landscapes, however, these events trigger landslides
that can buffer incision via increased sediment delivery and aggradation.
We quantify landslide sediment aggradation and erosional buffering with a
natural experiment in southern Taiwan where a northward gradient in
tectonic activity drives increasing landscape steepness. We find that
landscape response to extreme rainfall during the 2009 typhoon Morakot
varied along this gradient, where steep areas experienced widespread
channel sediment aggradation of >10 m and less steep areas did not
noticeably aggrade. We model sediment export to estimate a sediment removal
timeline and find that steep, tectonically active areas with the most
aggradation may take centuries to resume bedrock incision. Expected
sediment cover duration reflects tectonic uplift. We find that despite high
stream power, sediment cover may keep steep channels from eroding bedrock
for up to half of a given time period. This work highlights the importance
of dynamic sediment cover in landscape evolution and suggests a mechanism
by which erosional efficiency in tectonically active landscapes may
decrease as landscape steepness increases.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49304.1/607466/Extreme-event-driven-sediment-aggradation-and
Tunnel valley infill and genesis revealed by high-resolution 3-D
seismic data
James D. Kirkham; Kelly A. Hogan; Robert D. Larter; Ed Self; Ken Games ...
Abstract:
Landforms produced beneath former ice sheets offer insights into
inaccessible subglacial processes and present analogues for how current ice
masses may evolve in a warming climate. Large subglacial channels cut by
meltwater erosion (tunnel valleys [TVs]) have the potential to provide
valuable empirical constraints for numerical ice-sheet models concerning
realistic melt rates, water routing, and the interplay between basal
hydrology and ice dynamics. However, the information gleaned from these
features has thus far been limited by an inability to adequately resolve
their internal structures. We use high-resolution three-dimensional (HR3-D)
seismic data (6.25 m bin size, ~4 m vertical resolution) to analyze the
infill of buried TVs in the North Sea. The HR3-D seismic data represent a
step-change in our ability to investigate the mechanisms and rates at which
TVs are formed and filled. Over 40% of the TVs examined contain buried
glacial landforms including eskers, crevasse-squeeze ridges, glacitectonic
structures, and kettle holes. As most of these landforms had not previously
been detected using conventional 3-D seismic reflection methods, the
mechanisms that formed them are currently absent from models of TV genesis.
The ability to observe such intricate internal structures opens the
possibility of using TVs to reconstruct the hydrological regimes of former
mid-latitude ice sheets as analogues for contemporary ones.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G49048.1/607467/Tunnel-valley-infill-and-genesis-revealed-by-high
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