New Geology Articles Published Online Ahead of Print in November
Boulder, Colo., USA: Article topics and locations include insights into
Earth’s oldest stromatolites and prospects for the search for life on Mars;
rare earth elements; and the missing link in carbonatitic magma evolution.
These Geology articles are online at
https://geology.geoscienceworld.org/content/early/recent.
Climatic influence on the expression of strike-slip faulting
Nadine G. Reitman; Yann Klinger; Richard W. Briggs; Ryan D. Gold
Abstract:
Earthquakes on strike-slip faults are preserved in the geomorphic record by
offset landforms that span a range of displacements, from small offsets
created in the most recent earthquake (MRE) to large offsets that record
cumulative slip from multiple prior events. An exponential decay in the
number of large cumulative offsets has been observed on many faults, and a
leading hypothesis is that climate controls the rate of decay. We present
offset measurements compiled from 31 studies of strike-slip faults with
evidence of multiple paleoearthquakes and corresponding climatic and
tectonic information to test this hypothesis. Both the global compilation
and numerical landscape evolution modeling reveal that the decay rate in
large offsets is negatively correlated with mean annual precipitation.
Faults in dry regions with high drainage density more commonly preserve
small MRE offsets, and faults in wet regions with lower drainage density
more commonly preserve a mix of small MRE and large cumulative offsets.
Geomorphology of faults in different climates supports this result and
illustrates precipitation’s effect on the development and preservation of
offset channels. Our findings imply that current and past climate affect
how displacement on strike-slip faults is recorded and interpreted to
inform earthquake history.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50393.1/618746/Climatic-influence-on-the-expression-of-strike
Advanced two- and three-dimensional insights into Earth’s oldest
stromatolites (ca. 3.5 Ga): Prospects for the search for life on Mars
K. Hickman-Lewis; B. Cavalazzi; K. Giannoukos; L. D’Amico; S. Vrbaski ...
Abstract:
Paleoarchean stromatolites are among the oldest compelling evidence for
life. We present advanced two- and three-dimensional (2-D and 3-D)
reconstructions of the morphology, mineralogy, trace element geochemistry,
and taphonomy of permineralized stromatolites from the lowermost horizons
of the ca. 3.5 Ga Dresser Formation, Pilbara, Western Australia. Rare earth
element plus yttrium compositions suggest a restricted paleodepositional
setting influenced by marine influxes; this contrasts with other Dresser
stromatolites, which developed around terrestrial hot springs. Mineral
phase relationships and positive Eu anomalies denote syndepositional
hydrothermal influence and silicification promoting high-fidelity
microstructural preservation. Although no primary kerogen is preserved,
numerous 2-D and 3-D morphological characteristics denote a biogenic
origin, including the onlap of sedimentary layers onto stromatolitic
topography, fine-scale undulatory laminations, non-isopachous laminations
with crestal thickening, laminoid fenestrae, and subvertical pillar-like
fabrics interpreted as microbial palisade structure; these features suggest
that the stromatolite ecosystem was dominantly phototrophic. The deep
iron-rich weathering profile of the Dresser stromatolites makes them
pertinent analogues for potential microbialites in altered carbonates on
Mars. Were similar putative biogenic macro-, meso- and micromorphologies
identified in habitable Martian settings by rover imaging systems, such
materials would be compelling targets for sample return.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50390.1/618747/Advanced-two-and-three-dimensional-insights-into
Resetting of zircon inclusions in garnet: Implications for elastic
thermobarometry
Nicola Campomenosi; Ross J. Angel; Matteo Alvaro; Boriana Mihailova
Abstract:
Elastic thermobarometry of host-inclusion systems for back-calculating
pressure (P) and temperature (T) conditions of inclusion
entrapment relies on the assumption that the host-inclusion rheology is
purely elastic. In this study, we have explored both the elastic and
nonelastic behavior of zircon-in-garnet (ZiG) systems by in situ
Raman spectroscopy at high T and ambient P. We show that
upon heating, plastic relaxation takes place immediately after the zircon
inclusions experience tensile stress conditions with respect to a free
crystal at the same T. On subsequent cooling, the inclusions
develop a new stress state, and thus the inclusion pressures have been
reset from those corresponding to their original entrapment. Resetting of
inclusion pressures therefore strongly depends on the exhumation P
-T path. This explains why elastic thermobarometry using ZiG
systems is reliable when applied to low-P high-T rocks
where the cooling path after inclusion entrapment passes quickly into the
compression domain of the inclusion. On the other hand, high-P
rocks exhumed along quasi-isothermal paths take zircon inclusions into the
tensile domain where they are reset until significant cooling commences at
low P. ZiG systems in ultrahigh-P rocks therefore
commonly indicate pressures on clockwise exhumation paths instead of the
conditions of original entrapment.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50431.1/618749/Resetting-of-zircon-inclusions-in-garnet
Kovdor to Oldoinyo Lengai—The missing link in carbonatitic magma
evolution
Olga Vasyukova; Anastasia Kostyuk; Anthony Williams-Jones
Abstract:
Experiments were conducted to test the hypothesis that interaction of a
carbonatitic magma with quartz-rich rocks plays a key role in shaping
carbonatite complexes. The host rocks were represented by quartz, and the
magma was represented by synthetic mixtures of CaCO3, MgCO 3, and Na2CO3. With increasing distance
from the quartz, the reaction between the carbonate liquid and quartz
produced a domain of Na(Ca)-rich silicate glass, a domain of metasomatic
wollastonite, diopside, and forsterite, and a carbonate-rich domain. This
zonation reproduces that observed in many carbonatite complexes, e.g.,
Kovdor, Russia. The experiments provide strong evidence that carbonatitic
magma/host-rock interaction controls the evolution of carbonatite complexes
and explains how Mg-Ca-carbonatitic magmas from the mantle can evolve to
produce the natrocarbonatites and associated alkaline silicate rocks
observed at Oldoinyo Lengai, Tanzania.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50672.1/618716/Kovdor-to-Oldoinyo-Lengai-The-missing-link-in
Late Messinian submarine channel systems in the Levant Basin:
Challenging the desiccation scenario
Jimmy Moneron; Zohar Gvirtzman
Abstract:
The question of whether the Mediterranean Sea desiccated during the
Messinian salinity crisis (MSC) has been strongly debated for decades. In
the Levant Basin, this debate was recently reignited in relation to the
latest stage of the crisis after cessation of salt deposition. The
desiccation supporters argue that salt truncation—and its subsequent burial
by a latest Messinian, clastic-rich evaporitic unit—occurred subaerially on
a desiccated seafloor. However, we show that this latest Messinian unit
contains a dense net of channels with meanders, levees, and overspill
deposits and is very similar to the turbidite channels observed on the
modern seafloor. The aggradation characteristics of these buried channels
(levee height, channel depth, and channel-floodplain coupling) indicate a
marine rather than fluvial origin. Our conclusion adds to the findings of a
previous study that salt truncation occurred in deep waters by dissolution.
In a wider perspective, we suggest that the flush of clastics into the
basin during the last stage of the MSC indicates a combination of wet
climate and sea-level rise that started before the Zanclean (earliest
Pliocene).
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50583.1/618717/Late-Messinian-submarine-channel-systems-in-the
A tool to distinguish magmatic from secondarily recrystallized
carbonatites—Calcite/apatite rare earth element partitioning
Gino Sartori; Andrea Galli; Daniel Weidendorfer; Max W. Schmidt
Abstract:
Crustal geochemical signatures in carbonatites may arise from carbon
recycling through the mantle or from fluid-mediated interaction with the
continental crust. To distinguish igneous from fluid-mediated processes, we
experimentally determined rare earth element (REE) partitioning between
calcite/melt and apatite/melt at subvolcanic emplacement conditions (1–2
kbar, 750–1000 °C). Our data allow modeling of calcite-apatite (Cc/Ap)
partition coefficients (D), representing a new tool to bypass the
previously required but largely unknown carbonatite melt composition.
Experimentally determined magmatic calcite/apatite REE patterns are flat,
as DLaCc/Ap/DLu Cc/Ap is ~0.75, and they show a slight U-shape that becomes more
pronounced with temperature decreasing from 1000 to 750 °C. Application to
texturally well-equilibrated natural Ca-carbonatites and calcite-bearing
nephelinites shows that some calcite-apatite pairs follow this pattern and,
hence, confirm the magmatic nature of the carbonates. D LaCc/Ap/DLuCc/Ap
values of other mineral pairs range from 10–2 to 10–3
, which, together with a substantial light REE depletion in the calcite, is
interpreted as fluid-mediated light REE removal during secondary calcite
recrystallization. Calcite/apatite REE distributions are well suited to
evaluate whether a carbonatite mineralogy is primary and magmatic or has
been affected by secondary recrystallization. In this sense, our tool
provides information about the sample’s primary or secondary nature, which
is essential when assigning isotopic crustal signatures (in Ca, C, or Sr)
or REE patterns to related geologic processes.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50416.1/618718/A-tool-to-distinguish-magmatic-from-secondarily
Calcitic shells in the aragonite sea of the earliest Cambrian
Luoyang Li; Timothy P. Topper; Marissa J. Betts; Dorj Dorjnamjaa;
Gundsambuu Altanshagai ...
Abstract:
The initial acquisition of calcium carbonate polymorphs (aragonite and
calcite) at the onset of skeletal biomineralization by disparate metazoans
across the Ediacaran-Cambrian transition is thought to be directly
influenced by Earth’s seawater chemistry. It has been presumed that animal
clades that first acquired mineralized skeletons during the so-called
“aragonite sea” of the latest Ediacaran and earliest Cambrian
(Terreneuvian) possessed aragonite or high-Mg calcite skeletons, while
clades that arose in the subsequent “calcite sea” of Cambrian Series 2
acquired low-Mg calcite skeletons. Here, contrary to previous expectations,
we document shells of one of the earliest helcionelloid molluscs from the
basal Cambrian of southwestern Mongolia that are composed entirely of
low-Mg calcite and formed during the Terreneuvian aragonite sea. The
extraordinarily well-preserved Postacanthella shells have a simple
prismatic microstructure identical to that of their modern low-Mg calcite
molluscan relatives. High-resolution scanning electron microscope
observations show that calcitic crystallites were originally encased within
an intra- and interprismatic organic matrix scaffold preserved by
aggregates of apatite during early diagenesis. This indicates that not all
molluscan taxa during the early Cambrian produced aragonitic shells,
weakening the direct link between carbonate skeletal mineralogy and ambient
seawater chemistry during the early evolution of the phylum. Rather, our
study suggests that skeletal mineralogy in Postacanthella was
biologically controlled, possibly exerted by the associated prismatic
organic matrix. The presence of calcite or aragonite mineralogy in
different early Cambrian molluscan taxa indicates that the construction of
calcium carbonate polymorphs at the time when skeletons first emerged may
have been species dependent.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50533.1/618719/Calcitic-shells-in-the-aragonite-sea-of-the
Impact of stream power gradients on storage of sediment and carbon on
channel margins and floodplains
John D. Gartner; Carl E. Renshaw; Joshua Landis; Francis J. Magilligan
Abstract:
Spatial complexity impacts the resilience of river ecosystems by mediating
processes that control the sources and sinks of sediment and organic
material. Using four independent geochemical tracers and three morphometric
indices, we show that downstream spatial gradients in stream power (Ω)
predict storage of material in the channels and margins and/or floodplains.
A field test in a 48 km2 watershed demonstrates that reaches
with downstream decreases in Ω coincide with wider floodplains and elevated
inventories of 137Cs, 210Pbex (ex—excess),
and organic matter in locations of the ~3 to 20 yr floodplain. In contrast,
reaches with downstream increases in Ω coincide with narrower floodplains
and decreased inventories of 137Cs, 210Pb ex, and organic matter. The occurrence of in-channel bedrock
exposures and the activity of short-lived 7Be in within-channel
sediments also correlate with downstream Ω gradients, demonstrating a link,
over both short and long time scales, between within-channel processes and
floodplain-forming processes. The combined geochemical and physical
characteristics demonstrate the importance of downstream gradients in
sediment transport, characterized by downstream changes in stream power
rather than at-a-point stream power, in determining spatial complexity in
carbon and sediment storage at intermediate scales (102 to 10 3 m) in river systems.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50339.1/618720/Impact-of-stream-power-gradients-on-storage-of
Major-element geochemistry of pelites
Jacob B. Forshaw; David R.M. Pattison
Abstract:
Pelites (shales and mudstones) are arguably the most important rock type
for interpreting metamorphism. Their significance derives from their
widespread occurrence and the range of mineral assemblages they develop at
different conditions of pressure and temperature. We compiled a global
database of 5729 major-element whole-rock analyses of pelites from
different metamorphic grades (shales to granulite-facies paragneisses) to
(1) determine an average composition, (2) examine the range and variability
in their composition, and (3) assess if there is evidence for grade-related
geochemical changes. Median values are given instead of average values to
eliminate the effect of extremes. The median worldwide pelite is as follows
(anhydrous, values in wt%): SiO2 = 64.13, TiO2 =
0.91, Al2O3 = 19.63, FeOtotal = 6.85, MnO
= 0.08, MgO = 2.41, CaO = 0.65, Na2O = 1.38, and K2O
= 3.95. The median XMg = MgO/(MgO + FeOtotal
) in moles is 0.39. The median XFe3+ = 2 × Fe 2O3/ (2 × Fe2O3 + FeO) in moles
was measured in 1964 samples and is 0.23. On an Al2O3
-FeO-MgO (AFM) diagram, the median worldwide pelite plots within a strong
clustering of analyses between XMgproj =
projected molar MgO/(MgO + FeOtotal) = 0.30–0.55 (median = 0.42)
and AMs = molar [Al2O3 – (3 × K2O)]/[Al 2O3 – (3 × K2O) + FeOtotal +
MgO] = 0.0–0.4 (median = 0.19). Pelites show a continuous decrease in
volatile content with increasing metamorphic grade and a decrease in XFe3+ from the diagenetic to biotite zone. Lower median
SiO2 values and higher median Al2O3 and
AMs values in the porphyroblast and subsolidus sillimanite or K-feldspar
zones, as well as higher median MnO values in the garnet zone, may reflect
sampling bias or metasomatism.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50542.1/618721/Major-element-geochemistry-of-pelites
Heavy Mo isotope composition of northern Bataan adakites, Philippines:
Evidence for fore-arc subduction erosion?
Hai-Quan Liu; Jie Li; Yi-Gang Xu; Graciano P. Yumul, Jr.; Ulrich Knittel
...
Abstract:
The identification of an eroded fore-arc crust component in arc magmas is
challenging due to the combined effects of mantle metasomatism and crustal
assimilation–fractional crystallization. In this study, molybdenum (Mo)
isotope compositions are used in conjunction with Sr-Nd-Hf isotopic and
elemental data to identify eroded fore-arc crust components in adakites
from the Cuyapo and Balungao volcanoes of the northern Bataan segment of
the Luzon arc (Philippines). The Mo isotopic ratios (δ98/95Mo,
relative to the NIST SRM 3134 standard) of these adakites increase with
increasing εNd (+4.3 to +5.6) and Ba/Nb (206–286). The low δ 98/95Mo (–0.36‰ to –0.26‰) in the Cuyapo adakites coupled with
low Sr-Nd-Hf isotopic ratios suggests contributions from the residual slab,
which lost isotopically heavy Mo during dehydration. Interestingly, the
high δ98/95Mo (–0.18‰ to 0.00‰) Balungao adakites have
Mo-Sr-Nd-Hf isotopic ratios similar to those of the Luzon basement.
Fractionated Nb/Ta (16–18) and high Sr/Y indicate the coexistence of melt
with residual rutile and garnet ± amphibole assemblages, corresponding to a
source region (>~45 km) below the present Luzon crust (~33 km). This
thus suggests an origin of heavy Mo from partial melting of eroded crust in
the mantle wedge rather than in the upper-plate crust. Our work not only
demonstrates that Mo isotopes may be a potential tracer of eroded crust but
also highlights that lavas with combined high δ98/95Mo, ε Nd, and Ba/Nb emplaced at subduction zones with juvenile arc
crust may be a result of subduction erosion.
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50465.1/618722/Heavy-Mo-isotope-composition-of-northern-Bataan
Rapid megaflood-triggered base-level rise on Mars
Joshua Ahmed; Jeffrey Peakall; Matthew R. Balme; Daniel R. Parsons
Abstract:
The existence of ancient fluvial systems on Mars is widely accepted, but
little is known about how quickly they formed, or what environmental
conditions controlled their evolution. We analyzed a sequence of
well-preserved inner-bank bar deposits within the meander bends of a
multistacked sinuous fluvial ridge in Aeolis Dorsa and compared them to
similar features on Earth to establish the conditions required for their
formation. Our results reveal that these Martian channels were highly
aggradational, rising an order of magnitude higher than terrestrial rivers.
This evolution occurred over very rapid time scales, with our estimates
suggesting that some entire inner-bar set deposits, and therefore the
aggradational channel, may have formed in less than a single Martian year,
with upper bounds of a few decades. We suggest that this unique channel
topography was created by a rapidly rising downstream water body, triggered
by a sequence of externally sourced megafloods (e.g., crater lake
breaches).
View article:
https://pubs.geoscienceworld.org/gsa/geology/article-abstract/doi/10.1130/G50277.1/618748/Rapid-megaflood-triggered-base-level-rise-on-Mars
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