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.
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.
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.
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.
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).
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/DLuCc/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.
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.
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.
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.
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.
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).
GEOLOGY articles are online at https://geology.geoscienceworld.org/content/early/recent
