1 February 2023
The Geological Society of America
Release no. 23-06
Contact: Kea Giles
+1-303-357-1057
kgiles@geosociety.org
 

For Immediate Release

New Geology Articles Published Online Ahead of Print

Boulder, Colo., USA: Article topics and locations include submarine landslides; a fossil oceanic lithosphere preserved inside a continent; crater formation during the onset of mud volcanism; and kill dates from re-exposed black mosses in the northern Antarctic Peninsula. These Geology articles are online at https://geology.geoscienceworld.org/content/early/recent.

Slab damage and the pulsating retreat of the Ionian-Apennines subduction
Eugenio Carminati; Claudio Chiarabba
Episodes of slow and fast plate subduction, slab rollback, and backarc opening are widely documented; e.g., in the central Mediterranean region. Pervasive damage by fluids is emerging as a possible weakening mechanism that could lead to slab segmentation and breakoff. We show that low-velocity anomalies within the Ionian slab present in along-dip seismic tomography profiles are traces of past damaging events generated by water penetration into the oceanic lithosphere when it was at the trench. We propose that the stepwise evolution of the central Mediterranean over the past 21 m.y. has been governed by strength changes of the Ionian slab induced by damage. More generally, we speculate that the cyclicity of such a process in space and time in the different circum-Mediterranean subduction events may have led to the present shape of the Mediterranean basin.

Mantle serpentinization and associated hydrogen flux at North Atlantic magma-poor rifted margins
Zhonglan Liu; Marta Perez-Gussinye; Javier García-Pintado; Leila Mezri; Wolfgang Bach
Mantle serpentinization influences the rheology of altered peridotites and the global fluxes of energy and volatiles, the generation of seafloor and sub-seafloor chemolithotrophic life, and the carbon cycle. As a by-product of serpentinization, molecular hydrogen (H₂) is generated, which supports chemosynthetic communities, and this mechanism may have driven the origin of life on early Earth. At continent-ocean transition zones (COTs) of magma-poor rifted margins, the mantle is exposed and hydrated over hundreds of kilometers across the rift, but the H₂ fluxes associated with this process are poorly known. Here, we coupled a thermomechanical model with serpentinization reaction equations to estimate associated H₂ release during mantle exhumation at COTs. This reproduced a tectonic structure similar to that of the West Iberia margin, one of the best-studied magma-poor margins. We estimated the rate of H₂ production from mantle hydration at (7.5 ± 2.5) × 10 ⁷ mol/(yr × km). By estimating the area of exhumed mantle from wide-angle seismic profiles at North Atlantic magma-poor margins, we calculated that the accumulated H₂ production could have been as high as ~4.3 × 10¹⁸ mol (~8.6 × 10¹² metric tons) prior to opening of the North Atlantic Ocean, at a rate of ~1.4 × 10 ¹⁷ mol/m.y. This is one quarter of the total predicted flux produced by the global system of mid-ocean ridges, thus highlighting the significance of H₂ generation at magma-poor margins in global H ₂ fluxes, to hydrogenothropic microbial life, and, perhaps, as a potential energy source.

Slab window–related magmatism as a probe for pyroxenite heterogeneities in the upper mantle
Malcolm J. Hole; Sally A. Gibson; Matthew C. Morris
New high-precision trace-element analyses of magmatic olivines point to a pyroxenite-dominated source for recent alkali basalts erupted above slab windows formed along the Antarctic Peninsula. Melting occurred at ambient mantle temperature, and basalts have geochemical compositions that are indistinguishable from ocean-island basalts (OIBs). We propose that the pyroxenite component originally resided in the upper mantle beneath the subducted slab; formation of a slab window allowed limited decompression and the generation of melts of garnet-pyroxenite, but little or no melting of mantle peridotite. The pyroxenite component in the mantle formed ca. 550 Ma, an age that does not require long-term recycling of subducting slabs to the core-mantle boundary. Enriched mid-ocean ridge basalt (E-MORB) from the adjacent extinct Phoenix Ridge owes its enriched trace-element compositions to mixing between small melt fractions of pyroxenite and peridotite during a period of decreased spreading rate prior to the death of the ridge ca. 3.3 Ma. It is likely that the variable trace-element enrichment seen in East Pacific Rise E-MORB distal from hotspots results from the same process of interactions between small-melt-fraction (<~5%) melts of pyroxenite and peridotite.

Unravelling biotic versus abiotic processes in the development of large sulfuric-acid karsts
D. Laurent; G. Barré; C. Durlet; P. Cartigny; C. Carpentier ...
In carbonate rocks, natural production of sulfuric acid can form karstic cavities. Where both epigenic and hypogenic speleogeneses have taken place, these processes are challenging to constrain, especially if there is more than one source of sulfur involved. Thanks to an innovative approach coupling geomorphology with measurements of multiple sulfur, oxygen, and strontium isotopes, our study of two French Pyrenean caves quantifies the relative influence of both microbial and thermochemical processes implied in sulfuric-acid production. Multiple sulfur isotopes reveal that sulfate speleothems derived from a mixing of microbial H₂S in hydrothermal water and fossil thermochemical H₂S previously trapped within the cave host rock. We quantify the percentages of biotic and abiotic sulfuric-acid speleogeneses that have taken place in these caves, paving the way for similar studies of other sulfuric-acid caves where usually only microbial activity has been considered.

Diamonds reveal subducted slab harzburgite in the lower mantle
Nicole A. Meyer; Thomas Stachel; D. Graham Pearson; Richard A. Stern; Jeffrey W. Harris ...
Characterizing compositional heterogeneity in Earth’s lower mantle is critical to understanding its dynamics. Three low-nitrogen diamonds from Koffiefontein (South Africa), containing inclusion assemblages of ferropericlase ± orthopyroxene ± magnesite, constrain diamond formation in an Mg-rich lower-mantle environment. Ferropericlase inclusions have Mg# 82.7–88.5 and orthopyroxene inclusions (retrogressed bridgmanite) have Mg# 95.0–95.1 and mantle-like δ¹⁸O of +5.6‰ ± 0.2‰. Magnesite included in one diamond implicates carbonated fluids in diamond formation. High Mg# and low Ca, Al, and Na of the assemblage indicate a melt-depleted meta-harzburgitic environment, in contrast to more fertile compositions expected for primitive lower mantle. Extremely low Ca in orthopyroxene inclusions may reflect a combination of melt depletion and low equilibration temperatures at the time of trapping. Inclusion compositions implicate subducted oceanic slab meta-harzburgite as the host for diamond growth. Mantle-like δ¹⁸O of the orthopyroxene inclusions indicates unaltered oceanic lithosphere. Similar melt-depleted characteristics in lower-mantle inclusion assemblages worldwide support that meta-harzburgite is the dominant host of lower-mantle diamonds.

Concordant changes in late Holocene hydroclimate across southern Patagonia modulated by westerly winds and the El Niño–Southern Oscillation
Julie Loisel; Kristen Sarna; Zhengyu Xia; Yongsong Huang; Zicheng Yu
The southern westerly winds influence weather patterns and water resources across the southern high-latitude regions, with important socioeconomic impacts. The strengthening and poleward migration of these winds since the late 20^th century also have implications for regional environmental change, including drought, wildfire, and sea-ice loss. However, it is challenging to recognize the natural variability of the westerlies and predict their future behavior, as those recent changes have been influenced by anthropogenic factors. We present a 4200-yr-long record from a southern Patagonian peatland in a location that is sensitive to changes in the position and/or strength of the westerlies. Our δ ¹³C record shows a 6‰ increasing trend from 4200 to 1200 cal. yr B.P., indicating a progressive, millennial-scale increase in peatland moisture. This long-term trend is attributed to an increase in moisture induced by strengthening southern westerly winds associated with a change in the mean state of the El Niño–Southern Oscillation (ENSO) system. Superimposed on this millennial trend, centennial-scale shifts in hydroclimate persist into modern times. We suggest that a “paleo”–Southern Annular Mode, which is linked to tropical Pacific climate, with dry events contemporaneous with positive phases and La Niña–like conditions, is responsible for this enhanced hydroclimate variability. Overall, our results point to millennial- and centennial-scale changes in hydroclimate during the late Holocene that link tropical Pacific climate variability with the Southern Annular Mode and the southern westerlies, with far-reaching implications for future changes in the southern high latitudes, including CO₂ ventilation from the Southern Ocean.

Kill dates from re-exposed black mosses constrain past glacier advances in the northern Antarctic Peninsula
Dulcinea V. Groff; David W. Beilman; Zicheng Yu; Derek Ford; Zhengyu Xia
Glaciers are receding in the northern Antarctic Peninsula and exposing previously entombed soils and plants. We used 39 black (dead) mosses collected from rapidly retreating ice margins at four sites along the Antarctic Peninsula to determine the kill dates using radiocarbon measurements and to constrain the timing of past glacier advances over the last 1500 yr. We established strict new criteria for sample collection to promote robust estimates of plant death. We found distinct phases of ice advance during ca. 1300, 800, and 200 calibrated years before 1950 (cal yr B.P.). We report estimates of the rate of glacier advance at ca. 800 cal yr B.P. at Gamage and Bonaparte Points (southern Anvers Island) of 2.0 and 0.3 m/yr, respectively. Although the range of kill dates is relatively narrow within a region, suggesting multiple glaciers advanced simultaneously, the rates of local advances can vary by almost an order of magnitude and are much less than retreat rates. Our kill dates coincide with evidence for glacier advances from other studies in the northern Antarctic Peninsula at ca. 1300, 800, and 200 cal yr B.P. and for penguin colony abandonment at several sites in the region ranging from 450 to 0 cal yr B.P. The combination of our new terrestrial evidence for glacier advances with other lines of evidence shows the regional synchroneity of glacial dynamics and cryosphere-biosphere connections during rapid climate shifts and the sensitivity of terrestrial ecosystems to climate cooling.

A power-based abrasion law for use in landscape evolution models
D.D. Hansen; J.P. Brooks; L.K. Zoet; N.T. Stevens; L. Smith ...
Subglacial abrasion drives erosion for many glaciers, inundating forefields and proglacial marine environments with glaciogenic sediments. Theoretical treatments of this process suggest that bedrock abrasion rates scale linearly with the energy expended through rock-on-rock friction during slip, but this assumption lacks an empirical basis for general implementation. To test this approach, we simulated abrasion by sliding debris-laden ice over rock beds under subglacial conditions in a cryo-ring shear and a direct shear device. Miniscule volumes of erosion that occurred during each run were mapped with a white-light profilometer, and we measured the rock mechanical properties needed to constrain the energy expended through abrasion. We find that abraded volume per unit area increases linearly with average shear force at the bed and that abrasion rates increase linearly with basal power for plane beds. Lastly, only a small percentage (1%) of the energy partitioned to basal slip is dissipated by abrasion. These results confirm the basal-power abrasion rule is viable to implement in landscape evolution models.

Crater formation during the onset of mud volcanism
Evan Pryce; Chris Kirkham; Joe Cartwright
Three-dimensional (3-D) seismic imaging was used to reveal >2.5-km-wide and >150-m-deep craters at the basal surface of 64 mud volcanoes out of a suite of 86, offshore Egypt. The craters were infilled soon after they formed by successive mud extrusions that combined to build mud volcanoes, as evidenced by onlap fill geometries of the earliest mud flows. We propose that the craters formed as the earliest manifestation of mud volcano formation. We infer that the energy required to excise in situ clays and sands buried and consolidated to depths over 150 m below the seafloor was provided by the highly vigorous venting of a dominantly gas and water mix during the initial eruption, in which gas column height was the critical factor. This primary phase of mud volcanism is rarely observed, and the findings presented here have significant implications for interpretations of the dynamism during this fundamental stage of mud volcano genesis.

Fragmentation, rafting, and drowning of a carbonate platform margin in a rift-basin setting
Alexander Petrovic; Thomas Lüdmann; Abdulkader M. Afifi; Yannick Saitz; Christian Betzler ...
High-resolution bathymetric and shallow seismic data along the northeast Red Sea margin reveal a previously disregarded mechanism for carbonate platform drowning at a steep-flanked rift basin. At the seafloor, salt extrusions highlight the influence of extensional salt tectonics, with a salt flow from the southern flank of the Al Wajh carbonate platform that likely originates from below. Salt-flow direction, morphology, and kilometer-sized slumps and rotated blocks indicate platform-margin disintegration and rafting of platform blocks toward the southwest. The outlines of several smaller detached or semi-detached carbonate platforms to the south of the main platform can be refitted to the larger platform margin by counter-moving the direction of mass wasting. Several platforms, reaching heights above the seafloor of up to 650 m, are partially or fully submerged in the mesophotic zone and appear to be in danger of drowning. We conclude that the southern outer rim of the Al Wajh platform is breaking apart owing to salt withdrawal, which indicates that carbonate platforms on top of salt sequences grow on mobile ground, leading to platform disintegration, basinward rafts, and the demise of broken-off, smaller pieces of platform. Salt displacement also controls the growth geometries of individual platform rafts, with keep-up reef growth (growth rate equal to sea-level rise) and drowning occurring in close spatial proximity. Therefore, the interplay between salt diapirism and platform growth is not limited to platforms growing on the apexes of diapirs and is more complex than previously thought.

Large-scale submarine landslide drives long-lasting regime shift in slope sediment deposition
Marina Dottore Stagna; Vittorio Maselli; Arjan van Vliet
Submarine landslides and associated mass-transport deposits (MTDs) modify the physiography of continental margins and influence the evolution of submarine sediment routing systems. Previous studies highlighted the control of landslides and MTDs on subsequent sedimentary processes and deposits at spatial scales ranging from tens of centimeters to few kilometers, leaving a knowledge gap on how and for how long large-scale submarine landslides (i.e., headscarps wider than 50–100 km) may affect the stratigraphic evolution of continental margins. To fill this gap, we used three-dimensional seismic reflection data tied to an exploration well to investigate the impact of one of the largest submarine landslides discovered on Earth, the Mafia mega-slide (Mms) offshore Tanzania, on slope sediment deposition. Seismic data interpretation indicates that turbidite lobes/lobe complexes and coalescent mixed turbidite-contourite systems formed the pre-Mms stratigraphy between 38 and ca. 21 Ma (age of the Mms), whereas coarser-grained sheet turbidites and debrites accumulated after the Mms for ~15 m.y., primarily on the topographic lows generated by the emplacement of the landslide. We interpret this drastic and long-lasting regime shift in sediment deposition to be driven by the increase in seafloor gradient and the capture and focus of feeding systems within the broad failed area. We propose that the extensive evacuation zones associated with such giant landslides can generate major “conveyor belts”, trapping land-derived material or sediments transported by along-slope processes such as bottom currents. During the progressive healing of the landslide escarpments, which may last for several million years, sand-prone facies are deposited primarily in the upper slope, filling up the accommodation space generated by the landslide, while deeper-water environments likely remain sediment starved or experience accumulation of finer-grained deposits. Our study provides new insights into the long-term response of slope depositional systems to large-scale submarine landslides, with implications for the transfer of coarse-grained sediments that can be applied to continental margins worldwide.

The role of surface processes in basin inversion and breakup unconformity
Luke S. Mondy; Patrice F. Rey; Guillaume Duclaux
In the context of continental extension, transient compressional episodes (stress inversion) and phases of uplift (depth inversion) are commonly recorded with no corresponding change in plate motion. Changes in gravitational potential energy during the rifting process have been invoked as a possible source of compressional stresses, but their magnitude, timing, and relationship with depth inversions remain unclear. Using high-resolution two-dimensional numerical experiments of the full rifting process, we track the dynamic interplay between the far-field tectonic forces, loading and unloading of the surface via surface processes, and gravitational body forces. Our results show that rift basins tend to localize compressive stresses; they record transient phases of compressional stresses as high as 30 MPa and experience a profound depth inversion, 2 km in magnitude, when sediment supply ceases, providing an additional driver for the breakup unconformity, a well-documented phase of regional uplift typically associated with continental breakup.

Quantitative record of the Neoarchean water cycle from a 2.67 Ga magmatic-hydrothermal system, Fennoscandian Shield
D.O. Zakharov; D.R. Zozulya; D.P. Colòn
Given the scarcity of reliable paleoclimate record, the surface temperatures of the first half of Earth’s history remain poorly constrained. Here we show how the climate-sensitive δ¹⁸O value of surface precipitation recorded in Archean igneous and hydrothermal formations can help to resolve the state of early Earth climate. The Keivy complex, Kola craton (Fennoscandian Shield), formed via the intrusion of granitic and mafic magmas in the shallow crust at 2.67 Ga, where circulation of meteoric water created a distinct archive of the contemporaneous water cycle. Using whole rock data, mineral separates, and in situ zircon δ¹⁸O measurements, we disentangle the reaction mechanisms between the shallow magma and local precipitation. Syn-emplacement hydrothermal alteration produced near-contact lithologies with δ¹⁸O values as low as –8‰ recorded in amphiboles, while igneous zircons from granites crystallized from melts with δ¹⁸O from +1‰ to +4.5‰. High-precision U-Pb geochronology constrains the granite intrusion at 2673.5 ± 0.3 Ma. Using the Δ¹⁷O approach, these rocks reveal that the precipitation had a δ¹⁸O value 18‰ lower than the hydrosphere, providing one of the earliest quantitative records of continental precipitation generally compatible with a cold climate at high latitudes.

Plumbing the depths of magma crystallization using ¹⁷⁶ Lu/¹⁷⁷Hf in zircon as a pressure proxy
Hugo Moreira; Anda Buzenchi; Chris J. Hawkesworth; Bruno Dhuime
Extensional tectonics are marked by shallow magma crystallization depths, whereas compressional tectonics are associated with deeper crystallization depths. This implies that variations in crystallization depths can be used to track changes in Earth’s dominant tectonic regimes through time. We therefore developed a new “pressure of crystallization” proxy based on the variation of the ¹⁷⁶Lu/¹⁷⁷Hf ratio in zircon. This ratio is controlled by zircon fractionation and residual garnet, and it can be used to monitor the evolution of a crystallizing magma ascending within the crust. The secular evolution of the ¹⁷⁶Lu/¹⁷⁷Hf ratio in zircon is characterized by cyclical oscillations that are broadly in tune with the δ¹⁸O record in zircon and with periods of continental collision and supercontinent amalgamation. The apparent mean depth of crystallization of zircon-bearing igneous rocks has decreased with time over the last ~3.0 b.y. This can be linked to shallowing of the primary crystallization depths and/or to the effect of time-integrated erosion in the geologic record. Prior to ca. 3.0 Ga, crystallization depth maxima and oscillations in apparent depth are less clear, perhaps suggesting that the nature of tectonic interactions was different in the Mesoarchean and earlier.

Triggering of episodic back-arc extensions in the northeast Asian continental margin by deep mantle flow
Yu Dong; Shuai Xiong; Feng Wang; Zheng Ji; Yi-Bing Li ...
Back-arc extension has been well documented in subduction plate tectonic regimes. However, the reasons why back-arc extensions are associated with some subduction systems but not others have remained elusive. Here, spatio-temporal variations in the composition of Cenozoic basalts in the northeast Asian continental margin are used to constrain the mechanism for episodic back-arc extensions. Using geochemical data sensitive to tectonic affinity, we show that typical volcanic arc compositions are located in the eastern margin of northeast Asia, whereas coeval intraplate volcanic compositions are located in the western part of northeast Asia, and that the intraplate and arc volcanism exhibit two eastward shifts, from 52 Ma to 33 Ma and from 33 Ma to 21 Ma. Intraplate basalts dated at ca. 11 Ma display a weak, arc-like geochemical signature, which suggests that the upwelling of asthenospheric mantle resulted in the remelting of previously melt-extracted lithospheric mantle modified by slab-derived fluids and the cessation of back-arc extensions. Thus, we propose that the eastward mantle flow resulted in eastward shifts of back-arc extensions that led to the development of extensive Cenozoic arc and intraplate volcanism in the northeast Asian continental margin.

Ancient deep ocean as a harbor of biotic innovation revealed by Carboniferous ophiuroid microfossils
Ben Thuy; Larry Knox; Lea D. Numberger-Thuy; Nicholas S. Smith; Colin D. Sumrall
Fossil-informed molecular phylogenies have emerged as the most powerful tool for correlating biotic evolution and Earth history. The accuracy of these trees, however, depends on the completeness of fossil sampling. For most organismal clades, the available fossil record is insufficiently sampled. This is especially true for groups with a multi-element skeleton, such as echinoderms and vertebrates, where sampling efforts focus largely on rare finds of intact skeletons. For these groups, inconspicuous but informative skeletal fragments are commonly neglected. This sampling bias excludes the numerous paleoenvironments in which preservation of intact skeletons is extremely unlikely, in particular deep-water settings. We describe new finds of brittle-star, or ophiuroid, fossils retrieved from sieving residues of sediments deposited during the Atokan (Upper Carboniferous) on the deep shelf to upper slope of the Ardmore Basin in present-day southern Oklahoma, USA. Although preserved as disarticulated, microscopic ossicles, the pristine preservation of the skeletal microstructure allows for precise identification of the remains. Comparative anatomical and phylogenetic analyses confirm the presence of basal representatives of the extant ophiuroid orders Ophioscolecida and Amphilepidida. Our finds provide the first unambiguous fossil evidence that the early crown-group diversification of the Ophiuroidea was well under way long before the end-Permian mass extinction, and that a significant part of this diversification took place in deep-water settings, as previously predicted by molecular evidence.

Fibrous calcite veins record stepwise, asymmetric opening and episodic hydrocarbon expulsion from organic-rich shales
Miao Wang; Yong Chen; Richard A. Stern; Ashley Went; Yaoqi Zhou ...
Episodic fluid expulsion through fractures is widely expected during hydrocarbon generation, yet direct evidence for this process is lacking in the case of organic-rich shales. We investigated the formation of antitaxial, bed-parallel fibrous calcite veins hosted in organic-rich shales of the Eocene Dongying Depression, Bohai Bay Basin, China. Our results from detailed, in situ geochemical traverses show that while some symmetric veins exhibit broadly synchronous and steady-state opening, other asymmetric veins consist of two geochemically distinct generations of calcite to either side of the median zone, suggesting asymmetric and asynchronous growth in two discrete episodes during hydrocarbon expulsion. Thus, we argue that each asymmetric vein recording two stages of opening implies that hydrocarbons were expelled from shales episodically.

Phanerozoic cratonization by plume welding
Xi Xu; Hanlin Chen; Andrew V. Zuza; An Yin; Peng Yu ...
Deformation-resistant cratons comprise >60% of the continental landmass on Earth. Because they were formed mostly in the Archean to Mesoproterozoic, it remains unclear if cratonization was a process unique to early Earth. We address this question by presenting an integrated geological-geophysical data set from the Tarim region of central Asia. This data set shows that the Tarim region was a deformable domain from the Proterozoic to early Paleozoic, but deformation ceased after the emplacement of a Permian plume despite the fact that deformation continued to the north and south due to the closure of the Paleo-Asian and Tethyan Oceans. We interpret this spatiotemporal correlation to indicate plume-driven welding of the earlier deformable continents and the formation of Tarim’s stable cratonic lithosphere. Our work highlights the Phanerozoic plume-driven cratonization process and implies that mantle plumes may have significantly contributed to the development of cratons on early Earth.

A fossil oceanic lithosphere preserved inside a continent
Shucheng Wu; Yingjie Yang; Yixian Xu; Juan Carlos Afonso; Anqi Zhang
The recycling of oceanic lithosphere into the deep mantle at subduction zones is one of the most fundamental geodynamic processes on Earth. During the closure of an ocean, ancient oceanic slabs are thought to be consumed entirely in subduction zones due to their negative buoyancy. Yet, it is recently suggested that small pieces of oceanic slabs could be trapped along paleo-subduction zones. What remains far more enigmatic is whether significant portions of paleo-oceanic lithosphere could eventually avoid the fate of subduction and be accreted to continental lithosphere, thus contributing to continental growth through time. We present seismic evidence for a preserved paleo-oceanic lithosphere beneath the Junggar region in northwestern China. We show that unsubducted oceanic lithosphere in the West Junggar has been preserved beneath the Junggar Basin, becoming a piece of the Eurasian continent. This scenario is likely to have occurred in other continents throughout Earth’s history, providing an additional and commonly underestimated contribution to the growth of continental lithosphere.

Natural growth of gold dendrites within silica gels
Thomas Monecke; T. James Reynolds; Tadsuda Taksavasu; Erik R. Tharalson; Lauren R. Zeeck ...
High-grade ores in low-sulfidation epithermal precious metal deposits include banded quartz veins that contain gold dendrites. The processes by which dendrite growth takes place have been subject to debate for decades, especially given that these deposits are known to form from dilute thermal liquids that contain only trace amounts of gold. It is shown here that growth of gold dendrites in epithermal veins at the McLaughlin deposit in California (western USA) originally took place within bands of gel-like noncrystalline silica. The gel provided a framework for the delicate dendrites to form. The high permeability of the gel allowed the diffusion and advection of gold from the thermal liquids flowing across the top of the silica layers to the sites of crystal growth within the gel. Over time, the gel hardened to form opal-AG. This silica phase is thermodynamically unstable and recrystallized to quartz that has a distinct mosaic texture.

GEOLOGY articles are online at https://geology.geoscienceworld.org/content/early/recent . Representatives of the media may obtain complimentary articles by contacting Kea Giles at the e-mail address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

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