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Kinematic and paleomagnetic restoration of the Semail ophiolite (Oman) reveals subduction initiation along an ancient Neotethyan fracture zone

Fri, 05/17/2019 - 19:10

Publication date: 15 July 2019

Source: Earth and Planetary Science Letters, Volume 518

Author(s): Douwe J.J. van Hinsbergen, Marco Maffione, Louise M.T. Koornneef, Carl Guilmette

Abstract

The archetypal Semail ophiolite of Oman has inspired much thought on the dynamics of initiation of intra-oceanic subduction zones. Current models invoke subduction initiation at a mid-oceanic ridge located sufficiently close to the Arabian passive margin to allow initiation of continental subduction below the ophiolite within ∼10–15 Myr after the 96–95 Ma age of formation of supra-subduction zone crust. Here, we perform an extensive paleomagnetic analysis of sheeted dyke sections across the Semail ophiolite to restore the orientation of the supra-subduction zone ridge during spreading. Our results consistently indicate that the ridge was oriented NNE–SSW, and we infer that the associated trench, close to the modern obduction front, had the same orientation. Our data are consistent with a previously documented ∼150° clockwise rotation of the ophiolite, and we reconstruct that the original subduction zone was WNW-ward dipping and NNE–SSW striking. Initial subduction likely occurred in the ocean adjacent and parallel to a transform margin of the part of the Arabian continent now underthrust below Iran that originally underpinned the nappes of the Zagros fold-thrust belt. Subduction thus likely initiated along an ancient, continental margin-parallel fracture zone, as also recently inferred from near-coeval ophiolites from the eastern Mediterranean and NW Arabian regions. Subduction initiation was therefore likely induced by (WN)W–(ES)E contraction and this constraint may help the future identification of the dynamic triggers of Neotethyan subduction initiation in the Late Cretaceous.

Quantitative impact of structural inheritance on present-day deformation and seismicity concentration in intraplate deformation zones

Fri, 05/17/2019 - 19:10

Publication date: 15 July 2019

Source: Earth and Planetary Science Letters, Volume 518

Author(s): Alizia Tarayoun, Stephane Mazzotti, Frédéric Gueydan

Abstract

Structural inheritance (i.e. paleo-tectonic) areas, acting as weakened domains, appear to be a key element localizing the seismicity in intraplate deformation zones. However, the impact of structural inheritance on the observed present-day seismicity and strain rate concentration remains to be quantified. In this study, we quantify through 2D numerical modeling the localization and amplification factor of upper crustal strain rates induced by structural inheritance. Our 2D models are constrained by intraplate velocity boundary conditions and include rheology laws that accounts for inherited strain weakening in both the brittle and ductile layers of the lithosphere. The role of structural inheritance is investigated for different localization of the weakened domain in the lithosphere. For an average intraplate geotherm (Moho temperature ca. 500 °C), brittle weakening (i.e. inherited faults) alone induces a limited amplification factor of upper crustal strain rates of ca. 4. Ductile weakening can increase the amplification factor to ca. 7 when localized in the lower crust, but has no effect when localized in the lithospheric mantle. Overall, the amplification factors of upper crustal strain rates vary between 1 and 27 depending on the location of the weakened area in the lithosphere and on the different possible net driving forces, crustal strengths, amounts of weakening, and geotherms. These model amplification factors are in reasonable agreement with those derived from GPS and seismicity data over large spatial scale (several hundreds of kilometers) in North America.

Hydroclimatic seasonality recorded by tree ring <em>δ</em><sup>18</sup>O signature across a Himalayan altitudinal transect

Fri, 05/17/2019 - 19:10

Publication date: 15 July 2019

Source: Earth and Planetary Science Letters, Volume 518

Author(s): Camilla Francesca Brunello, Christoff Andermann, Gerhard Helle, Francesco Comiti, Giustino Tonon, Achyut Tiwari, Niels Hovius

Abstract

Water stable isotope ratios of tropical precipitation predominantly reflect moisture source and precipitation intensity. Trees can incorporate the isotopic signals into annual tree-ring cellulose records, permitting reconstruction of the temporal changes of hydroclimate over decades to millennia. This is especially valuable in the Himalayas where the understanding of monsoon dynamics is limited by the lack of a dense and representative observational network. We have analyzed tree ring δ18O records from two distinct physiographic sites along the upper Kali Gandaki valley in the central Nepal Himalayas, representing the wet High-Himalayas and the Trans-Himalayan dryland to the north. Empirical correlations and regression analyses were compared to an in-situ calibrated oxygen isotope fractionation model, exploring the relationships between tree ring δ18O and seasonal-mean variability of hydroclimatic forcing at the different locations. For this purpose, gridded precipitation data from the Asian rain gauge dataset APHRODITE, as well as high resolution onsite observations (relative humidity, air temperature, δ18O of precipitation and radial tree growth) were used. We found that two distinct sets of meteorological values, reflecting pre-monsoon and monsoon conditions, are needed to reproduce the measured tree ring δ18O values from the High-Himalayan site, but that a single set of monsoonal values performs best for the Trans-Himalayan site. We conclude that Trans-Himalayan trees capture long-term changes in strength of the Indian summer monsoon. In contrast, High-Himalayan tree ring δ18O records a more complex hydro-climatic signal reflecting both pre-monsoon and monsoon seasons with very contrasting isotopic signatures of precipitation. This difference in the two hydroclimatic proxy records offers an opportunity to reconstruct first-order hydroclimate conditions, such as local precipitation rates, and to gain new insights into monsoon timing and seasonal water source determination across the Himalayan orographic region.

Geochronology and geochemistry of the northern Scotia Sea: A revised interpretation of the North and West Scotia ridge junction

Fri, 05/17/2019 - 19:10

Publication date: 15 July 2019

Source: Earth and Planetary Science Letters, Volume 518

Author(s): Teal R. Riley, Andrew Carter, Philip T. Leat, Alex Burton-Johnson, Joaquin Bastias, Richard A. Spikings, Alex J. Tate, Charlie S. Bristow

Abstract

Understanding the tectonic evolution of the Scotia Sea is critical to interpreting how ocean gateways developed during the Cenozoic and their influence on ocean circulation patterns and water exchange between the Atlantic and Southern oceans. We examine the geochronology and detrital age history of lithologies from the prominent, submerged Barker Plateau of the North Scotia Ridge. Metasedimentary rocks of the North Scotia Ridge share a strong geological affinity with the Fuegian Andes and South Georgia, indicating a common geological history and no direct affinity to the Antarctic Peninsula. The detrital zircon geochronology indicates that deposition was likely to have taken place during the mid – Late Cretaceous. A tonalite intrusion from the Barker Plateau has been dated at 49.6 ± 0.3 Ma and indicates that magmatism of the Patagonian–Fuegian batholith continued into the Eocene. This was coincident with the very early stages of Drake Passage opening, the expansion of the proto Scotia Sea and reorganization of the Fuegian Andes. The West Scotia Ridge is an extinct spreading center that shaped the Scotia Sea and consists of seven spreading segments separated by prominent transform faults. Spreading was active from 30–6 Ma and ceased with activity on the W7 segment at the junction with the North Scotia Ridge. Reinterpretation of the gravity and magnetic anomalies indicate that the architecture of the W7 spreading segment is distinct to the other segments of the West Scotia Ridge. Basaltic lava samples from the eastern flank of the W7 segment have been dated as Early – mid Cretaceous in age (137–93 Ma) and have a prominent arc geochemical signature indicating that seafloor spreading did not occur on the W7 segment. Instead the W7 segment is likely to represent a downfaulted block of the North Scotia Ridge of the Fuegian Andes continental margin arc, or is potentially related to the putative Cretaceous Central Scotia Sea.

Abrupt mid-Holocene ice loss in the western Weddell Sea Embayment of Antarctica

Fri, 05/17/2019 - 19:10

Publication date: 15 July 2019

Source: Earth and Planetary Science Letters, Volume 518

Author(s): Joanne S. Johnson, Keir A. Nichols, Brent M. Goehring, Greg Balco, Joerg M. Schaefer

Abstract

The glacial history of the westernmost Weddell Sea sector of Antarctica since the Last Glacial Maximum is virtually unknown, and yet it has been identified as critical for improving reliability of glacio-isostatic adjustment models that are required to correct satellite-derived estimates of ice sheet mass balance. Better knowledge of the glacial history of this region is also important for validating ice sheet models that are used to predict future contribution of the Antarctic ice sheet to sea level rise. Here we present a new Holocene deglacial chronology from a site on the Lassiter Coast of the Antarctic Peninsula, which is situated in the western Weddell Sea sector. Samples from 12 erratic cobbles and 18 bedrock surfaces from a series of presently-exposed ridges were analysed for cosmogenic 10Be exposure dating, and a smaller suite of 7 bedrock samples for in situ 14C dating. The resulting 10Be ages are predominantly in the range 80–690 ka, whereas bedrock yielded much younger in situ 14C ages, in the range 6.0–7.5 ka for samples collected from 138–385 m above the modern ice surface. From these we infer that the ice sheet experienced a period of abrupt thinning over a short time interval (no more than 2700 years) in the mid-Holocene, resulting in lowering of its surface by at least 250 m. Any late Holocene change in ice sheet thickness — such as re-advance, postulated by several modelling studies — must lie below the present ice sheet surface. The substantial difference in exposure ages derived from 10Be and 14C dating for the same samples additionally implies ubiquitous 10Be inheritance acquired during ice-free periods prior to the last deglaciation, an interpretation that is consistent with our glacial-geomorphological field observations for former cold-based ice cover. The results of this study provide evidence for an episode of abrupt ice sheet surface lowering in the mid-Holocene, similar in rate, timing and magnitude to at least two other locations in Antarctica.

Single-crystal elasticity of (Al,Fe)-bearing bridgmanite and seismic shear wave radial anisotropy at the topmost lower mantle

Fri, 05/17/2019 - 19:10

Publication date: 15 July 2019

Source: Earth and Planetary Science Letters, Volume 518

Author(s): Suyu Fu, Jing Yang, Noriyoshi Tsujino, Takuo Okuchi, Narangoo Purevjav, Jung-Fu Lin

Abstract

In this study, we investigated the single-crystal elasticity of (Al,Fe)-bearing bridgmanite (Bgm) with chemical compositions of Mg0.95Fe0.0332+Fe0.0273+Al0.04Si0.96O3 (Fe6-Al4-Bgm) and Mg0.89Fe0.0242+Fe0.0963+Al0.11Si0.89O3 (Fe12-Al11-Bgm) using combined experimental results from Brillouin light scattering (BLS), impulsive stimulated light scattering (ISLS), and X-ray diffraction (XRD) measurements in diamond anvil cells at 25 and 35 GPa. Based on experimentally measured compressional and shear wave velocities (VP, VS) as a function of azimuthal angles within selected crystal platelets that are sensitive to derivation of nine elastic constants for each composition, we reliably derived the full elastic constants of Fe6-Al4-Bgm and Fe12-Al11-Bgm at the two experimental pressures. Our results show that the combined Fe and Al substitution results in a reduction of both VS and VP in Fe12-Al11-Bgm up to 2.6(±0.5)% and 1.5(±0.3)%, respectively, compared with those in Fe6-Al4-Bgm at the experimental pressures. In particular, we observed strong combined Fe and Al effects on VS splitting anisotropy of (Al,Fe)-bearing Bgm at the two experimental pressures: Fe6-Al4-Bgm exhibits the highest VS splitting anisotropy of ∼8.23-9.0% along the [001] direction, while the direction shifts to the midway between [100] and [001] directions for Fe12-Al11-Bgm with VS splitting anisotropy of ∼7.68-11.06%. Combining the single-crystal elasticity data of Fe6-Al4-Bgm and Fe12-Al11-Bgm with the crystallographic preferred orientation (CPO) results of deformed Bgm at relevant lower-mantle pressure-temperature (P-T) conditions from literature, we modeled the seismic VS radial anisotropy of deformed (Al,Fe)-bearing Bgm near a subducting slab at conditions relevant to the topmost lower mantle. Taking into account the Fe and Al contents in (Al,Fe)-bearing Bgm with depth in the Earth's topmost lower mantle, the results of our model show that the deformation of Fe6-Al4-Bgm and Fe12-Al11-Bgm crystals would produce ∼0.9% and ∼0.8% VS radial anisotropy at depths of ∼670 and ∼920 km, respectively. These findings provide mineral physics explanations to the distinct seismically-detected VS radial anisotropies at the topmost lower mantle near subducted slabs, especially in the Tonga-Kermadec subduction region.

Ocean bottom geophysical array studies may reveal the cause of seafloor flattening

Fri, 05/17/2019 - 19:10

Publication date: 15 July 2019

Source: Earth and Planetary Science Letters, Volume 518

Author(s): Hisashi Utada

Abstract

Flattening of old seafloor as a result of the time evolution of oceanic lithosphere and asthenosphere has been studied for several decades and remains one of the unresolved fundamental problems of geodynamics. The phenomenon of seafloor flattening at ages greater than 70-80 Ma can be explained by a simple model of a cooling plate with a fixed thickness, but the physical cause of flattening is not fully understood. Here, a simple model of lithosphere rejuvenation is considered as an alternative mechanism and is also shown to represent similar features of seafloor flattening. We suggest that constraints on mantle structure from use of data such as variations of seafloor depth and heat flow are weak, but that first order differences in mantle thermal structure are introduced by these two models. This suggests that probing of the mantle by geophysical arrays on the seafloor, particularly those sensitive to mantle temperature such as magnetotelluric imaging, should be able to constrain the cause of seafloor flattening. In the present study, recent results of seafloor electromagnetic explorations are compiled and estimated depths to the high conductivity layer (HCL) are used as a proxy for 1,300 °C isotherm depths. For the data obtained at sites well away from plate boundaries in the Pacific Ocean, a model of lithosphere rejuvenation was found to better explain the correlation between HCL depths and seafloor depths than a standard plate cooling model.

The relationship between mantle potential temperature and oceanic lithosphere buoyancy

Fri, 05/17/2019 - 19:10

Publication date: 15 July 2019

Source: Earth and Planetary Science Letters, Volume 518

Author(s): O.M. Weller, A. Copley, W.G.R. Miller, R.M. Palin, B. Dyck

Abstract

The Earth's mantle potential temperature (TP) is thought to have cooled by ∼250 ∘C since the Archean, causing a progressive change in both the structure and composition of oceanic lithosphere. These variables affect the negative buoyancy of subducting slabs, which is known to be an important force in driving plate motions. However, the relationship between TP and slab buoyancy remains unclear. Here, we model the formation and subduction of oceanic lithosphere as a function of TP, to investigate how TP influences the buoyancy of subducting slabs, and by extension how buoyancy forces may have changed through time. First, we simulate isentropic melting of peridotite at mid-ocean ridges over a range of TP (1300–1550 ∘C) to calculate oceanic lithosphere structure and composition. Second, we model the thermal evolution of oceanic plates undergoing subduction for a variety of scenarios (by varying lithospheric thickness, slab length and subduction velocity). Finally, we integrate the structural, compositional and thermal constraints to forward model subduction metamorphism of oceanic plates to determine down-going slab density structures. When compared with ambient mantle, these models allow us to calculate buoyancy forces acting on subducting slabs. Our results indicate that oceanic lithosphere derived from hotter mantle has a greater negative buoyancy, and therefore subduction potential, than lithosphere derived from cooler mantle for a wide range of subduction scenarios. With respect to the early Earth, this conclusion supports the viability of subduction, and models of subduction zone initiation that invoke the concept of oceanic lithosphere being primed to subduct. However, we also show that decreases to lithosphere thickness and slab length, and reduced crustal hydration, progressively reduce slab negative buoyancy. These results highlight the need for robust estimates of early Earth lithospheric properties when considering whether subduction was operative at this time. Nevertheless, our findings suggest that subduction processes on the early Earth may have been uniformitarian.

Atmospheric flow deflection in the late Cenozoic Sierra Nevada

Fri, 05/17/2019 - 19:10

Publication date: 15 July 2019

Source: Earth and Planetary Science Letters, Volume 518

Author(s): Hari T. Mix, Jeremy K. Caves Rugenstein, Sean P. Reilly, Andrea J. Ritch, Matthew J. Winnick, Tyler Kukla, C. Page Chamberlain

Abstract

Given the intimate links between topography, tectonics, climate, and biodiversity, considerable effort has been devoted to developing robust climate and elevation histories of orogens. In particular, quantitative geochemical reconstructions using stable oxygen and hydrogen isotopes have been applied to many of the world's mountain belts. Recent advances in atmospheric modeling have suggested that such stable isotope records from leeward sites can be affected by the complicating role that sufficiently elevated topography such as the southern (High) Sierra plays in diverting atmospheric circulation. While such “terrain blocking” effects are a hallmark feature of modern atmospheric circulation in the Sierra, their evolution remains poorly constrained. In order to examine the history of these terrain blocking effects, we developed stable isotope records from three late Cenozoic sedimentary basins in the Eastern Sierra and Basin and Range: 1) Authigenic clay minerals in the Mio-Pliocene Verdi Basin (VB) near present-day Reno, Nevada, 2) Fluvial and lacustrine carbonates from the Plio-Pleistocene Coso Basin (CB) in the southern Owens Valley, and 3) Miocene to Holocene pedogenic, fluvial and lacustrine carbonates of Fish Lake Valley (FLV). Whereas both the VB and CB are proximal to the Sierra crest, FLV is a distal leeward site east of the White and Inyo Mountains in the Basin and Range. The CB oxygen isotope record exhibits an increase of 1-2‰ over the last 6 Myr while VB and FLV show no significant change. These results suggest that terrain blocking around the southern Sierra initiated prior to the late Cenozoic, though it may have been modestly enhanced during the last 6 Ma.

High-resolution records of Oceanic Anoxic Event 2: Insights into the timing, duration and extent of environmental perturbations from the palaeo-South Pacific Ocean

Fri, 05/17/2019 - 19:10

Publication date: 15 July 2019

Source: Earth and Planetary Science Letters, Volume 518

Author(s): S.K. Gangl, C.M. Moy, C.H. Stirling, H.C. Jenkyns, J.S. Crampton, M.O. Clarkson, C. Ohneiser, D. Porcelli

Abstract

Oceanic Anoxic Event 2 (OAE 2), which took place around the Cenomanian–Turonian boundary (∼94 Ma), is associated with extreme perturbations to the global carbon cycle, affected ocean basins worldwide and was associated with significant biological turnover. Although this event has been well studied in the northern hemisphere, the evolution and character of OAE 2, particularly in terms of the vertical and lateral extent of anoxia, is poorly constrained in the palaeo-Pacific Ocean. Furthermore, the precise timing, duration and character of this event, and the exact mechanisms driving OAE 2 environmental changes, are still being debated. Here, we present the first high-resolution records of carbon isotopes, total organic carbon and magnetic susceptibility from the southern palaeo-Pacific Ocean during OAE 2, sampled at two sections in New Zealand. The carbon isotope records from both localities reveal a ∼2‰ positive excursion that represents the global change in the carbon cycle associated with OAE 2. When combined with a cyclostratigraphic age model, these new records constrain the duration of the OAE 2 carbon isotope excursion to at least 930 ± 25 ky and indicate a minimum duration of 200 ± 25 ky for the ‘Plenus Cold Event’ that took place during OAE 2. The lithologies and low organic-carbon contents of the New Zealand sections imply that oxic conditions prevailed along, at least parts of, the margins of the palaeo-Pacific Ocean at mid- to high southern latitudes during OAE 2 while, contemporaneously, conditions were locally anoxic in the mid-water column of the equatorial Pacific Ocean. Despite these apparently oxic conditions in the New Zealand region, there was a partial collapse of benthic ecosystems leading up to, and during, OAE 2, suggesting environmental deterioration caused by intermittent oxygen deprivation, or other chemical or biological disturbances in the South Pacific region that remain to be elucidated.

Corrigendum to “Axial Seamount: Periodic tidal loading reveals stress dependence of the earthquake size distribution (<em>b</em> value)” [Earth Planet. Sci. Lett. 512 (2019) 39–45]

Fri, 05/17/2019 - 19:10

Publication date: Available online 15 May 2019

Source: Earth and Planetary Science Letters

Author(s): Yen Joe Tan, Felix Waldhauser, Maya Tolstoy, William S.D. Wilcock

Deep ocean <sup>14</sup>C ventilation age reconstructions from the Arctic Mediterranean reassessed

Fri, 05/17/2019 - 19:10

Publication date: 15 July 2019

Source: Earth and Planetary Science Letters, Volume 518

Author(s): Mohamed M. Ezat, Tine L. Rasmussen, Luke C. Skinner, Katarzyna Zamelczyk

Abstract

The present-day ocean ventilation in the Arctic Mediterranean (Nordic Seas and Arctic Ocean), via transformation of northward inflowing warm Atlantic surface water into cold deep water, affects regional climate, atmospheric circulation and carbon storage in the deep ocean. Here we study the glacial evolution of the Arctic Mediterranean circulation and its influence on glacial climate using radiocarbon reservoir-age reconstructions on deep-sea cores from the Fram Strait that cover the late glacial period (33,000–20,000 yr ago; 33–20 ka). Our results show high Benthic-Planktic 14C age differences of ∼1500 14C years 33–26.5 ka suggesting significant water column stratification between ∼100–2600 m water depth, and reduction and/or shoaling of deep-water formation. This phase was followed by break-up of the stratification during the Last Glacial Maximum (LGM; 26–20 ka), with Benthic-Planktic 14C age differences of ∼250 14C years, likely due to enhanced upwelling. These ocean circulation changes potentially contributed to the final intensification phase of glaciation via positive cryosphere-atmosphere-ocean circulation-carbon cycle feedbacks. Our data also do not support ‘extreme aging’ of >6000 14C years in the deep Arctic Mediterranean, and appear to rule out the proposed outflow of very old Arctic Ocean water to the Nordic Seas during the LGM and to the subpolar North Atlantic Ocean during the deglacial period.

Lithological control on the post-orogenic topography and erosion history of the Pyrenees

Fri, 05/17/2019 - 19:10

Publication date: 15 July 2019

Source: Earth and Planetary Science Letters, Volume 518

Author(s): Thomas Bernard, Hugh D. Sinclair, Boris Gailleton, Simon M. Mudd, Mary Ford

Abstract

Numerous studies on active mountain ranges have demonstrated the interaction between tectonics and climate in shaping topography. Here we explore how variations in rock types have affected the topographic development of the Pyrenees since cessation of orogenesis ca. 20 Ma. Our study is based on a multidisciplinary approach and integrates topographic analyses, rock strength measurements and thermal modelling of low-temperature thermochronological data published across the Central Pyrenees. Results indicate a strong influence of rock strength in determining the post-orogenic morphology of the Pyrenees. We observe a correlation between rock strength and the normalized channel steepness index (ksn) of the different lithologies. Moreover, the highest topography is dominated by the Variscan plutonic massifs which have highest rock strength. Consequently, the drainage divide appears to track the position of these massifs. Abrupt deceleration of exhumation recorded in inverse modelling of low-temperature thermochronologic data suggests that the exhumation of the Variscan massifs also played role in lowering in erosion rates over the massifs during orogenesis.

Metal segregation in planetesimals: Constraints from experimentally determined interfacial energies

Fri, 05/17/2019 - 19:10

Publication date: 15 July 2019

Source: Earth and Planetary Science Letters, Volume 518

Author(s): A. Néri, J. Guignard, M. Monnereau, M.J. Toplis, G. Quitté

Abstract

High temperature experiments have been performed to constrain interfacial energies in a three-phase system (metal–forsterite–silicate melt) representative of partially differentiated planetesimals accreted early in the solar system history, with the aim of providing new insights into the factors affecting the interconnection threshold of metal-rich phases. Experiments were run under controlled oxygen fugacity (ΔNi-NiO=−3) at 1440 °C, typically for 24 h. Quantification of the true dihedral angles requires a resolution of at least 30 nm per pixel in order to reveal small-angle wedges of silicate melt at crystal interfaces. At this level of resolution, dihedral angle distributions of silicate melt and olivine appear asymmetric, an observation interpreted in terms of anisotropy of olivine crystals. Based upon the theoretical relation between dihedral angles and interfacial energies in a three-phase system, the relative magnitudes of interfacial energies have been determined to be: γMelt-Ol<γMelt-Ni<γOl-Ni. This order differs from that obtained with experiments using an iron sulfide liquid close to the Fe–FeS eutectic for which γMelt-Sulfide<γMelt-Ol<γOl-Sulfide, implying a lower interconnection threshold for sulfur-rich melts than for pure metallic phases. This dependence of the interconnection threshold on the sulfur content will affect the drainage of metallic phases during melting of small bodies. Assuming a continuous extraction of silicate melt, evolution of the metal volume fraction has been modeled. Several sulfur-rich melts extraction events are possible over a range of temperatures relevant with thermometric data on primitive achondrites (1200–1400 °C and 25% of silicate melt extracted). These successive events provide novel insight into the variability of sulfur content in primitive achondrites, which are either representative of a region that experienced sulfide extraction or from a region that accumulated sulfide melt from overlying parts of the parent body.

Origins of the terrestrial Hf-Nd mantle array: Evidence from a combined geodynamical-geochemical approach

Fri, 05/17/2019 - 19:10

Publication date: 15 July 2019

Source: Earth and Planetary Science Letters, Volume 518

Author(s): Rosemary E. Jones, Peter E. van Keken, Erik H. Hauri, Jonathan M. Tucker, Jeffrey Vervoort, Chris J. Ballentine

Abstract

The formation and segregation of oceanic and continental crust from the mantle, and its return to the mantle via subduction and/or delamination, leads to the development of distinct geochemical reservoirs in the terrestrial mantle. Fundamental questions remain regarding the location, nature, and residence time of these reservoirs, as well as the respective roles of oceanic and continental crust in the development of the mantle's geochemical endmembers. The Lu-Hf and Sm-Nd isotope systems behave similarly in magmatic systems and together form the terrestrial mantle Hf-Nd isotopic array. Here we combine a geodynamic model of mantle convection with isotope and trace element (TE) geochemistry to investigate the evolution of the Hf-Nd mantle array. This study examines the sensitivity to: TE partition coefficients used in the formation of oceanic crust; density contrasts between subducting oceanic crust and the mantle; and the formation and recycling of continental crust. We show that the fractionation between the parent (Lu and Sm) and daughter (Hf and Nd) species needs to be higher than is indicated by partition coefficients determined from the present-day melting environment. This is consistent with the suggestion of deeper mantle melting earlier in Earth history and an increased role for residual garnet. Subduction and accumulation of dense oceanic crust produces a large mass of incompatible TE enriched material in the deep mantle. This deep mantle enrichment appears to play a more significant role than the extraction and recycling of continental crust in developing the Hf and Nd isotope and TE compositions of the mid-ocean ridge mantle source. The corollary of this result is that the formation of the continental crust plays a secondary role, contrary to the currently accepted paradigm. Nevertheless, the inclusion of continental crust formation and recycling produces a broader model mantle array, which better reproduces the spread in the natural data set. This model also produces the Hf and Nd isotope and TE compositions of the upper mantle and continental crust, as well as deep mantle compositions similar to those of plume-fed ocean island basalts. Our model is consistent with continental growth models based on the Lu-Hf isotopic composition of zircon, which suggest that 50–70% of the present-day mass of the continental crust is produced prior to 3 Ga, and that the recycling of continental crust becomes more prevalent after this time.

Editorial Board

Fri, 05/17/2019 - 19:10

Publication date: 1 July 2019

Source: Earth and Planetary Science Letters, Volume 517

Author(s):

Duration, evolution, and implications of volcanic activity across the Ordovician–Silurian transition in the Lower Yangtze region, South China

Fri, 05/17/2019 - 19:10

Publication date: 15 July 2019

Source: Earth and Planetary Science Letters, Volume 518

Author(s): Shengchao Yang, Wenxuan Hu, Xiaolin Wang, Baoyu Jiang, Suping Yao, Funing Sun, Zhicheng Huang, Feng Zhu

Abstract

Volcanism provides a reliable record of local and global tectonic events and substantially influences both modern and ancient environments, climates, and the evolution of life. The Ordovician–Silurian (O–S) transition is a special period because intensive volcanism occurred globally, including in the Yangtze region of South China. Volcanic events during this period are a symptom of plate tectonic behaviour and are thought to be responsible for the remarkable changes in climate in the early Palaeozoic, though the relationships between these events remain unclear and controversial. Coeval igneous rocks and volcanic sediments (VS) are primarily used to resolve this issue. However, limited studies have been performed on VS from the O–S transition in South China. Recently, a typical VS-bearing section was found in the Lower Yangtze region, which contains ∼100 thin, interbedded volcanic ash layers across the O–S transition. Detailed petrographic and geochemical analyses of the volcanic ashes were conducted to determine their isotopic ages, magma sources, evolutionary processes, and tectonic settings. Our preliminary results suggest that volcanic eruptions in South China lasted for more than 22 Ma across the O–S boundary, from ∼449.3 ± 3.6 to 427.6 ± 4.1 Ma, where 445.14 Ma is the lowermost graptolite biozone for Metabolograptus extraordinarius, as well as the initiation of the Late Ordovician mass extinction (LOME) event in the Yangtze region. The evolutionary history of the parental magma was constructed from a depleted mantle source in the early stage and from a crustal source in the late stage, with several transitional features in the middle. The mantle source and arc-related geochemical indicators for the volcanic ashes support the disputed “subduction-collision orogeny” model. We propose that the strong volcanism in South China, accompanied by volcanism in numerous other regions worldwide, was an important trigger for the LOME and was likely responsible for oceanic 87Sr/ 86Sr fractionation and other climatic changes during the O–S transition.

Triple oxygen isotope signatures of evaporation in lake waters and carbonates: A case study from the western United States

Fri, 05/17/2019 - 19:10

Publication date: 15 July 2019

Source: Earth and Planetary Science Letters, Volume 518

Author(s): Benjamin H. Passey, Haoyuan Ji

Abstract

Evaporation can increase the δ18O values of lake waters and carbonates by several per mil. If not accounted for in geological studies, this can lead to substantial misinterpretation of δ18O values in terms of paleoclimate and paleoelevation. Evaporation also leads to a lowering in residual waters of Δ17O, a measure of the departure of δ′17O from a characteristic relationship with δ′18O. We present new triple oxygen isotope data from waters and carbonates from lakes and their source rivers in the western United States (Bear Lake, Great Salt Lake, Lake Tahoe, Mono Lake, and Pyramid Lake). Consistent with predictions from steady-state isotopic mass balance models, the data illustrate marked lowering of Δ17O in closed basin lakes and freshwater lakes relative to their source rivers. The evaporation slope in triple oxygen isotope space (λlake) is similar for these lakes, averaging 0.5229 and ranging between 0.5219 and 0.5239. Moreover, models and data both show that the evaporation slope correlates with Δ17O, meaning that the slope can be estimated on the basis of the measured Δ17O value of the carbonate. We show how triple oxygen isotopes in lake waters and carbonates and ‘clumped isotopes’ (Δ47) in carbonates can be combined to reconstruct the δ18O values of primary (unevaporated) catchment precipitation (δ18Orucp). We use our lacustrine carbonate data as a test case for this approach, and find that δ18Orucp values closely approximate independently-measured δ18O values of catchment precipitation. However, the δ18Orucp values are consistently higher than δ18O of catchment precipitation by ∼2‰, which may reflect present incomplete understanding of a number of triple oxygen isotope parameters used in the calculation, such as the fractionation exponent for carbonate-water equilibrium, the evaporation slope λlake, and the Δ17O values of unevaporated meteoric waters. In conclusion, triple oxygen isotope analysis of lake waters and lacustrine carbonates is a promising new method for studying evaporation in fossil lake systems, but will benefit from additional research into triple oxygen isotope systematics relevant to meteoric waters, lake systems, and carbonate–water fractionation.

From catastrophic collapse to multi-phase deposition: Flow transformation, seafloor interaction and triggered eruption following a volcanic-island landslide

Fri, 05/17/2019 - 19:10

Publication date: 1 July 2019

Source: Earth and Planetary Science Letters, Volume 517

Author(s): Sebastian F.L. Watt, Jens Karstens, Aaron Micallef, Christian Berndt, Morelia Urlaub, Melanie Ray, Anisha Desai, Maddalena Sammartini, Ingo Klaucke, Christoph Böttner, Simon Day, Hilary Downes, Michel Kühn, Judith Elger

Abstract

The current understanding of tsunamis generated by volcanic-island landslides is reliant on numerical models benchmarked against reconstructions of past events. As the largest historical event with timed tsunami observations, the 1888 sector collapse of Ritter Island, Papua New Guinea provides an outstanding opportunity to better understand the linked process of landslide emplacement and tsunami generation. Here, we use a combination of geophysical imaging, bathymetric mapping, seafloor observations and sampling to demonstrate that the Ritter landslide deposits are spatially and stratigraphically heterogeneous, reflecting a complex evolution of mass-flow processes. The primary landslide mass was dominated by well-bedded scoriaceous deposits, which rapidly disintegrated to form an erosive volcaniclastic flow that incised the substrate over much of its pathway. The major proportion of this initial flow is inferred to have been deposited up to 80 km from Ritter. The initial flow was followed by secondary failure of seafloor sediment, over 40 km from Ritter. The most distal part of the 1888 deposit has parallel internal boundaries, suggesting that multiple discrete units were deposited by a series of mass-flow processes initiated by the primary collapse. The last of these flows was derived from a submarine eruption triggered by the collapse. This syn-collapse eruption deposit is compositionally distinct from pre- and post-collapse eruptive products, suggesting that the collapse immediately destabilised the underlying magma reservoir. Subsequent eruptions have been fed by a modified plumbing system, constructing a submarine volcanic cone within the collapse scar through at least six post-collapse eruptions. Our results show that the initial tsunami-generating landslide at Ritter generated a stratigraphically complex set of deposits with a total volume that is several times larger than the initial failure. Given the potential for such complexity, there is no simple relationship between the volume of the tsunamigenic phase of a volcanic-island landslide and the final deposit volume, and deposit area or run-out cannot be used to infer primary landslide magnitude. The tsunamigenic potential of prehistoric sector-collapse deposits cannot, therefore, be assessed simply from surface mapping, but requires internal geophysical imaging and direct sampling to reconstruct the event.

A small, unextractable melt fraction as the cause for the low velocity zone

Fri, 05/17/2019 - 19:10

Publication date: 1 July 2019

Source: Earth and Planetary Science Letters, Volume 517

Author(s): Kate Selway, J.P. O'Donnell

Abstract

Throughout the oceanic asthenosphere there exists a zone of seismic velocities that are lower than would be predicted. The cause of this low velocity zone (LVZ), whether partial melt or solid-state mechanisms, has been debated for decades. We investigate the LVZ by considering seismic and magnetotelluric data from tectonically stable, ∼70 Myr old lithosphere in the central Pacific Ocean. We utilise recent experimental advances on the influence of partial melt and seismic attenuation. Results show that the LVZ is characterised by a small volume of interconnected melt and by low hydrogen contents. Beneath the LVZ, the asthenosphere does not contain partial melt but has high hydrogen contents and low attenuation values, indicating large grain sizes and/or low oxygen fugacities. To explain these observations, we propose that a small amount of unextractable melt is trapped in the asthenosphere after melting at mid-ocean ridges.

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