153 research outputs found
Can low metallicity binaries avoid merging?
Rapid mass transfer in a binary system can drive the accreting star out of
thermal equilibrium, causing it to expand. This can lead to a contact system,
strong mass loss from the system and possibly merging of the two stars. In low
metallicity stars the timescale for heat transport is shorter due to the lower
opacity. The accreting star can therefore restore thermal equilibrium more
quickly and possibly avoid contact.
We investigate the effect of accretion onto main sequence stars with
radiative envelopes with different metallicities. We find that a low
metallicity (Z<0.001), 4 solar mass star can endure a 10 to 30 times higher
accretion rate before it reaches a certain radius than a star at solar
metallicity. This could imply that up to two times fewer systems come into
contact during rapid mass transfer when we compare low metallicity. This factor
is uncertain due to the unknown distribution of binary parameters and the
dependence of the mass transfer timescale on metallicity. In a forthcoming
paper we will present analytic fits to models of accreting stars at various
metallicities intended for the use in population synthesis models.Comment: To appear in the proceedings of "First Stars III", Santa Fe, New
Mexico, July 16-20, 2007, 3 pages, 2 figure
Large-scale mantle discontinuity topography beneath Europe: Signature of akimotoite in subducting slabs
The mantle transition zone is delineated by seismic discontinuities around 410 and 660 km, which are generally related to mineral phase transitions. Study of the topography of the discontinuities further constrains which phase transitions play a role and, combined with their Clapeyron slopes, what temperature variations occur. Here we use P to S converted seismic waves or receiver functions to study the topography of the mantle seismic discontinuities beneath Europe and the effect of subducting and ponding slabs beneath southern Europe on these features. We combine roughly 28,000 of the highest quality receiver functions into a common conversion point stack. In the topography of the discontinuity around 660 km, we find broadscale depressions of 30 km beneath central Europe and around the Mediterranean. These depressions do not correlate with any topography on the discontinuity around 410 km. Explaining these strong depressions by purely thermal effects on the dissociation of ringwoodite to bridgmanite and periclase requires unrealistically large temperature reductions. Presence of several wt % water in ringwoodite leads to a deeper phase transition, but complementary observations, such as elevated Vp/Vs ratio, attenuation, and electrical conductivity, are not observed beneath central Europe. Our preferred hypothesis is the dissociation of ringwoodite into akimotoite and periclase in cold downwelling slabs at the bottom of the transition zone. The strongly negative Clapeyron slope predicted for the subsequent transition of akimotoite to bridgmanite explains the depression with a temperature reduction of 200–300 K and provides a mechanism to pond slabs in the first place.SC is funded by the Drapers’ Company Research Fellowship through Pembroke College, Cambridge, UK. AD was funded by the European Research Council under the European Community’s Seventh Framework Programme (FP7/20072013/ERC grant agreement 204995) and by a Philip Leverhulme Prize.This is the final version of the article. It first appeared from Wiley via http://dx.doi.org/10.1002/2015JB012452 The data used are freely available from the IRIS (www.iris.edu) and ORFEUS (http://www.orfeus-eu.org) databases
Morphology of seismically slow lower-mantle structures
Large low shear velocity provinces (LLSVPs), whose origin and dynamic implication remain enigmatic, dominate the lowermost mantle. For decades, seismologists have created increasingly detailed pictures of the LLSVPs through tomographic models constructed with different modeling methodologies, data sets, parametrizations and regularizations. Here, we extend the cluster analysis methodology of Lekic , to classify seismic mantle structure in five recent global shear wave speed () tomographic models into three groups. By restricting the analysis to moving depth windows of the radial profiles of , we assess the vertical extent of features. We also show that three clusters are better than two (or four) when representing the entire lower mantle, as the boundaries of the three clusters more closely follow regions of high lateral gradients. Qualitatively, we relate the anomalously slow cluster to the LLSVPs, the anomalously fast cluster to slab material entering the lower mantle and the neutral cluster to ‘background’ lower mantle material. We obtain compatible results by repeating the analysis on recent global -wave speed () models, although we find less agreement across models. We systematically show that the clustering results, even in detail, agree remarkably well with a wide range of local waveform studies. This suggests that the two LLSVPs consist of multiple internal anomalies with a wide variety of morphologies, including shallowly to steeply sloping, and even overhanging, boundaries. Additionally, there are indications of previously unrecognized meso-scale features, which, like the Perm anomaly, are separated from the two main LLSVPs beneath the Pacific and Africa. The observed wide variety of structure size and morphology offers a challenge to recreate in geodynamic models; potentially, the variety can result from various degrees of mixing of several compositionally distinct components. Finally, we obtain new, much larger estimates of the volume/mass occupied by LLSVPs— 8.0 per cent ±0.9 ( ± 1) of whole mantle volume and 9.1 per cent ±1.0 ( ± 1) of whole mantle mass—and discuss implications for associating the LLSVPs with the hidden reservoir enriched in heat producing elements.National Science Foundation (EAR1352214), Packard Foundation, Pembroke College, Cambridge (Drapers’ Company Research Fellowship
Insights Into Deep Mantle Thermochemical Contributions to African Magmatism From Converted Seismic Phases
The contribution of mantle upwellings of varying spatial extent to Cenozoic magmatism across Africa is debated because geochemical and seismological tools used to interrogate them are primarily sensitive to either composition or temperature. Thermochemical conditions control the depth at which mantle materials undergo phase changes, which cause seismic discontinuities. Mapping seismic discontinuities across the mantle transition zone (MTZ) and below provides insight into the variable thermochemical nature of upwellings. We present observations of seismic discontinuities beneath Africa obtained from a compilation of P-to-s receiver functions (using Pds, PPds, and PKPds phases), recorded at seismograph networks across Africa between 1990-2019. We exploit a recent high-resolution African continental P-wavespeed model to migrate our receiver functions to depth in a common conversion point stack. Cenozoic magmatism along the East African Rift is largely underlain by a thin MTZ implying a contribution to rift magmatism from sources at or below MTZ depths. The Ethiopian rift is underlain by a depressed d410 and uplifted d660 indicating a moderate positive thermal anomaly at MTZ depths (~100-150K). The southern East African Rift displays a greater d410 depression and a regional d660 depression, suggesting a stronger thermochemical anomaly at MTZ depths. Here, seismic conversions at ~1025km depth are collocated with slow wavespeeds within the African Superplume, corroborating evidence for a compositional anomaly. We suggest that the contribution of a purely thermal plume directly below Ethiopia augments conditions for mantle melting and rifting. Distinct upwellings may also affect the MTZ below Cenozoic magmatism in Cameroon and Madagascar
Observations of changing anisotropy across the southern margin of the African LLSVP
We present evidence for the presence of complex anisotropy in the lowermost mantle from 3-D waveform modelling of observed core-diffracted shear waves that sample the southern edge of the African Large Low Shear Velocity Province (LLSVP). The anomalously strong amplitude of the SV component for the shear core-diffracted phase at large distances indicates the presence of anisotropy. We measure shear wave splitting parameters to determine which part of the elastic tensor is constrained by this particular data set. The modelling is performed using the spectral element method. The anisotropy is strong outside the LLSVP, weakens or rotates close to its boundary, and appears to be absent inside the LLSVP. The presence of the LLSVP margin may cause flow in the mantle to change direction. The occurrence of strong anisotropy in the region of fast seismic velocities is compatible with lattice-preferred orientation in post-perovskite due to accommodation of flow through dislocation creep
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Converted phases from sharp 1000 km depth mid-mantle heterogeneity beneath Western Europe
Until recently, most of the lower mantle was generally considered to be well-mixed with strong heterogeneity restricted to the lowermost several hundred kilometres above the core–mantle boundary, known as the ″ layer. However several recent studies have started to hint at a potential change in Earth's structure at mid-mantle depths beneath the transition zone.
Here we present a continental-wide search of Europe and the North Atlantic for mid-mantle P-to-s wave converted phases. Our data set consists of close to 50,000 high quality receiver functions. These are combined in slowness and depth stacks to identify seismic discontinuities in the range of 800–1400 km depth to determine at which depths and in which tectonic settings these features exist. Receiver functions are computed in different frequency bands to resolve the sharpness of the observed discontinuities. We find most seismic velocity jumps are observed between 975–1050 km depth, localised beneath western Europe and Iceland. The shear wave velocity jumps are roughly 1–2.5% velocity increase with depth occurring over less than 8 km in width. The most robust observations are coincident with areas of active upwelling (under Iceland) and an elongate lateral low velocity anomaly imaged in recent tomographic models which has been interpreted as diverted plume material at depth.
The lack of any suggested phase change in a normal pyrolitic mantle composition at around 1000 km depth indicates the presence of regional chemical heterogeneity within the mid-mantle, potentially caused by diverted plume material. We hypothesise that our observations represent either a phase change within chemically distinct plume material itself, or are caused by small scale chemical heterogeneities entrained within the upwelling plume, either in the form of recycled basaltic material or deep sourced chemically distinct material from LLSVPs.
Our observations, which cannot be directly linked to an area of either active or ancient subduction, along with observations in other hotspot regions, suggest that such mid-mantle seismic features are not unique to subduction zones despite the large number of observations that have previously been made in such settings.The facilities of IRIS Data Services, and specifically the IRIS Data Management Center, as well as the ORFEUS data centre were used for access to waveforms, related metadata, and/or derived products used in this study. IRIS Data Services are funded through the Seismological Facilities for the Advancement of Geoscience and EarthScope (SAGE) Proposal of the National Science Foundation under Cooperative Agreement EAR-1261681. For full citation list of all FDSN networks please see the Supplementary Material. Seismometers for the Cambridge seismic network in Iceland were borrowed from the Natural Environment Research Council (NERC) SEIS-UK (loans 857, 968 and 1022), and funded by research grants from the NERC and the European Community's Seventh Framework Programme Grant No. 308377 (Project FUTUREVOLC), to Robert S. White. J.J. was funded by a graduate studentship from NERC (LBAG/148 Task 5). S.C. is funded by the Drapers' Company Research Fellowship through Pembroke College, Cambridge. Thanks are also extended to the Icelandic Meteorological office for sharing data that were used in this study. A.D. and J.J. were funded by the European Research Council under the European Community's Seventh Framework Programme (FP7/2007–2013/ERC grant agreement 204995) and by a Philip Leverhulme Prize. Data was downloaded from IRIS DMC and figures made using GMT (Wessel and Smith, 2001)
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Receiver function mapping of mantle transition zone discontinuities beneath Alaska using scaled 3-D velocity corrections
SUMMARYThe mantle transition zone is the region between the globally observed major seismic velocity discontinuities around depths of 410 and 660 km and is important for determining the style of convection and mixing between the upper and the lower mantle. In this study, P-to-S converted waves, or receiver functions, are used to study these discontinuities beneath the Alaskan subduction zone, where the Pacific Plate subducts underneath the North American Plate. Previous tomographic models do not agree on the depth extent of the subducting slab, therefore improved imaging of the Earth structure underneath Alaska is required. We use 27 800 high quality radial receiver functions to make common conversion point stacks. Upper mantle velocity anomalies are accounted for by two recently published regional tomographic S-wave velocity models. Using these two tomographic models, we show that the discontinuity depths within our CCP stacks are highly dependent on the choice of velocity model, between which velocity anomaly magnitudes vary greatly. We design a quantitative test to show whether the anomalies in the velocity models are too strong or too weak, leading to over- or undercorrected discontinuity depths. We also show how this test can be used to rescale the 3-D velocity corrections in order to improve the discontinuity topography maps.After applying the appropriate corrections, we find a localized thicker mantle transition zone and an uplifted 410 discontinuity, which show that the slab has clearly penetrated into the mantle transition zone. Little topography is seen on the 660 discontinuity, indicating that the slab has probably not reached the lower mantle. In the southwest, P410s arrivals have very small amplitudes or no significant arrival at all. This could be caused by water or basalt in the subducting slab, reducing the strength at the 410, or by topography on the 410 discontinuity, preventing coherent stacking. In the southeast of Alaska, a thinner mantle transition zone is observed. This area corresponds to the location of a slab window, and thinning of the mantle transition zone may be caused by hot mantle upwellings.</jats:p
Depressed mantle discontinuities beneath Iceland: Evidence of a garnet controlled 660 km discontinuity?
The presence of a mantle plume beneath Iceland has long been hypothesised to explain its high volumes of crustal volcanism. Practical constraints in seismic tomography mean that thin, slow velocity anomalies representative of a mantle plume signature are difficult to image. However it is possible to infer the presence of temperature anomalies at depth from the effect they have on phase transitions in surrounding mantle material. Phase changes in the olivine component of mantle rocks are thought to be responsible for global mantle seismic discontinuities at 410 and 660 km depth, though exact depths are dependent on surrounding temperature conditions. This study uses P to S seismic wave conversions at mantle discontinuities to investigate variation in topography allowing inference of temperature anomalies within the transition zone. We employ a large data set from a wide range of seismic stations across the North Atlantic region and a dense network in Iceland, including over 100 stations run by the University of Cambridge. Data are used to create over 6000 receiver functions. These are converted from time to depth including 3D corrections for variations in crustal thickness and upper mantle velocity heterogeneities, and then stacked based on common conversion points. We find that both the 410 and 660 km discontinuities are depressed under Iceland compared to normal depths in the surrounding region. The depression of 30km observed on the 410 km discontinuity could be artificially deepened by un-modelled slow anomalies in the correcting velocity model. Adding a slow velocity conduit of -1.44% reduces the depression to 18 km; in this scenario both the velocity reduction and discontinuity topography reflect a temperature anomaly of 210 K. We find that much larger velocity reductions would be required to remove all depression on the 660 km discontinuity, and therefore correlated discontinuity depressions appear to be a robust feature of the data. While it is not possible to definitively rule out the possibility of uncorrected velocity anomalies causing the observed correlated topography we show that this is unlikely. Instead our preferred interpretation is that the 660 km discontinuity is controlled by a garnet phase transition described by a positive Clapeyron slope, such that depression of the 660 is representative of a hot anomaly at depth.Seismometers for the Cambridge network in Iceland were borrowed from the Natural Environment Research Council (NERC) SEIS-UK (loans 857 and 968), and funded by research grants from the NERC to RSW. Thanks are also extended to the Icelandic Meteorological office for sharing data that were used in this study. A.D. and J.J. were funded by the European Research Council under the European Communitys Seventh Framework Programme (FP7/20072013/ERC grant agreement 204995) and by a Philip Leverhulme Prize. SC is funded by the Drapers’ Company Research Fellowship through Pembroke College, Cambridge, UK. Data was downloaded from IRIS DMC and figures made using GMT (Wessel and Smith, 2001). The authors would like to thank all the PhD students and technicians who aid in the running and maintenance of the University of Cambridge seismic network. Dept. Earth Sciences, Cambridge contribution no ESC.3452.This is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.epsl.2015.10.05
Multigenetic origin of the X-discontinuity below continents: insights from African receiver functions
Constraints on chemical heterogeneities in the upper mantle may be derived from studying the seismically observable impedance contrasts that they produce. Away from subduction zones, several causal mechanisms are possible to explain the intermittently observed X-discontinuity (X) at 230–350 km depth: the coesite-stishovite phase transition, the enstatite to clinoenstatite phase transition, and/or carbonated silicate melting, all requiring a local enrichment of basalt. Africa hosts a broad range of terranes, from Precambrian cores to Cenozoic hotspots with or without lowermost mantle origins. With the absence of subduction below the margins of the African plate for >0.5 Ga, Africa presents an ideal study locale to explore the origins of the X. Traditional receiver function (RF) approaches used to map seismic discontinuities, such as common conversion-point stacking, ignore slowness information crucial for discriminating converted upper mantle phases from surface multiples. By manually assessing depth and slowness stacks for 1° radius overlapping bins, normalized vote mapping of RF stacks is used to robustly assess the spatial distribution of converted upper mantle phases. The X is mapped beneath Africa at 233–340 km depth, revealing patches of heterogeneity proximal to mantle upwellings in Afar, Canaries, Cape Verde, East Africa, Hoggar, and Réunion with further observations beneath Cameroon, Madagascar, and Morocco. There is a lack of an X beneath southern Africa and strikingly, the magmatic eastern rift branch of the southern East African Rift. With no relationships existing between depth and amplitudes of observed X and estimated mantle temperatures, multiple causal mechanisms are required across a range of continental geodynamic settings
X-discontinuity and transition zone structure beneath Hawaii suggests a heterogeneous plume
The Hawaiian Island chain in the middle of the Pacific Ocean is a well-studied example of hotspot volcanism caused by an underlying upwelling mantle plume. The thermal and compositional nature of the plume alters the mantle phase transitions, which can be seen in the depth and amplitude of seismic discontinuities. This study utilises 5000 high quality receiver functions from Hawaiian island stations to detect P-to-s converted phases to image seismic discontinuities between 200 to 800 km depth. Common-conversion point stacks of the data are used to map out lateral variations in converted phase observations, while slowness stacks allow differentiation between true conversions from discontinuities and multiples. We find that the 410 discontinuity is depressed by 20 km throughout our study region, while the main 660 is around average depth throughout most of the area. To the southwest of the Big Island we observe splitting of the 660, with a major peak at 630 km, and a minor peak appearing at 675 km depth. This is inferred to represent the position of the hot plume at depth, with the upper discontinuity caused by an olivine phase transition and the lower by a garnet phase transition. In the upper mantle, a discontinuity is found across the region at depths varying between 290 to 350 km. Identifying multiples from this depth confirms the presence of a so-called X-discontinuity. To the east of the Big Island the X-discontinuity lies around 336 km and the associated multiple is particularly coherent and strong in amplitude. Strikingly, the discontinuity around 410 km disappears in this area. Synthetic modelling reveals that such observations can be explained by a silica phase transition from coesite to stishovite, consistent with widespread ponding of silica-saturated material at these depths around the plume. This material could represent eclogite enriched material, which is relatively silica-rich compared to pyrolite, spreading out from the plume to the east as a deep eclogite pool, a hypothesis which is consistent with dynamical models of thermochemical plumes. Therefore these results support the presence of a significant garnet and eclogite component within the Hawaiian mantle plume
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