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Pierre Cartigny - One of the best experts on this subject based on the ideXlab platform.
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Negligible sulfur isotope fractionation during partial melting: Evidence from Garrett Transform Fault basalts, implications for the late-veneer and the hadean matte
Earth and Planetary Science Letters, 2016Co-Authors: J Labidi,, Pierre CartignyAbstract:Abstract We report the quadruple sulfur isotope compositions, sulfur contents and speciation major and trace elements (including copper and chlorine abundances) of eleven basalts collected in the Garrett Transform Fault. We combine these data to discuss the absence of S isotopic fractionation along both partial melting and low-pressure fractional crystallization. The variations of K2O/TiO2 and La/SmN-ratios (respectively between 0.017 and 0.067, and between 0.31 and 0.59) suggest a range of depletion in Garrett lavas that includes ultra depleted samples (K2O/TiO20.1 that likely experienced hydrothermal sulfide assimilation, Garrett ITLs display homogeneous δ34Sδ34S, Δ33SΔ33S and Δ36SΔ36S values with averages of −0.68±0.08‰−0.68±0.08‰, +0.010±0.005‰+0.010±0.005‰ and −0.04±0.04‰−0.04±0.04‰, respectively (all 1σ , n=10n=10). The δ34Sδ34S values display no relationship with either K2O/TiO2 variations or extent sulfide fractionation. From these observations, we derive a 34S/32S fractionation factor between exsolved sulfides and sulfide dissolved in silicate melts of 1.0000±0.00031.0000±0.0003. The S isotopic fractionation during partial melting can thus be considered as negligible, and both MORBs and ITLs record the 34S/32S ratio of their mantle source. The concept of sulfide melts segregating from the mantle, sinking and being added to the core during planetary differentiation was termed the ‘Hadean Matte’. The segregation of sulfides from the mantle to the core during planetary differentiation could account for various geochemical features of the Earth's mantle. Based on S isotopic mass balance, we derive a lower and upper limit for the hadean matte. While the lower bound corresponds to a virtually negligible hadean matte, the upper limit is View the MathML source3.36×1024gS (i.e. ∼10% of the bulk terrestrial S), which remains 5 to 10 times lower than previous estimates. This upper bound nonetheless requires high mantle S content >1000 ppm S before the extraction of the hadean matte. This suggestion would have chronological requirements, requiring any sulfide melt to have formed after the core extraction but before late accretion of the highly siderophile elements
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Negligible sulfur isotope fractionation during partial melting: Evidence from Garrett Transform Fault basalts, implications for the late-veneer and the hadean matte
Earth and Planetary Science Letters, 2016Co-Authors: J Labidi,, Pierre CartignyAbstract:Abstract We report the quadruple sulfur isotope compositions, sulfur contents and speciation major and trace elements (including copper and chlorine abundances) of eleven basalts collected in the Garrett Transform Fault. We combine these data to discuss the absence of S isotopic fractionation along both partial melting and low-pressure fractional crystallization. The variations of K2O/TiO2 and La/SmN-ratios (respectively between 0.017 and 0.067, and between 0.31 and 0.59) suggest a range of depletion in Garrett lavas that includes ultra depleted samples ( K 2 O/TiO 2 0.03 ). The remarkable level of incompatible element depletion is consistent with re-melting of a depleted source. Contrasting with incompatible element depletion, all samples display similar S and Cu abundance (at a given major-element composition) to mid-ocean ridge basalts (MORB). This indicates that Garrett Intra Transform Lavas (ITL) are sulfide saturated as MORB are. Copper content for Garrett parental melts (MgO >8%) are ∼80 ppm, indistinguishable from MORBs. This requires their mantle sources, variably depleted in incompatible element, to host residual sulfide buffering the Cu content of all erupted melts. We calculate a minimum S content for the source of ultra-depleted Garrett lavas of 100 ± 40 ppm S , i.e. roughly a factor of 2 below the MORB mantle source. After exclusion of a single sample with Cl/K ratio >0.1 that likely experienced hydrothermal sulfide assimilation, Garrett ITLs display homogeneous δ 34 S , Δ 33 S and Δ 36 S values with averages of − 0.68 ± 0.08 ‰ , + 0.010 ± 0.005 ‰ and − 0.04 ± 0.04 ‰ , respectively (all 1σ, n = 10 ). The δ 34 S values display no relationship with either K2O/TiO2 variations or extent sulfide fractionation. From these observations, we derive a 34S/32S fractionation factor between exsolved sulfides and sulfide dissolved in silicate melts of 1.0000 ± 0.0003 . The S isotopic fractionation during partial melting can thus be considered as negligible, and both MORBs and ITLs record the 34S/32S ratio of their mantle source. The concept of sulfide melts segregating from the mantle, sinking and being added to the core during planetary differentiation was termed the ‘Hadean Matte’. The segregation of sulfides from the mantle to the core during planetary differentiation could account for various geochemical features of the Earth's mantle. Based on S isotopic mass balance, we derive a lower and upper limit for the hadean matte. While the lower bound corresponds to a virtually negligible hadean matte, the upper limit is 3.36 × 10 24 g S (i.e. ∼10% of the bulk terrestrial S), which remains 5 to 10 times lower than previous estimates. This upper bound nonetheless requires high mantle S content >1000 ppm S before the extraction of the hadean matte. This suggestion would have chronological requirements, requiring any sulfide melt to have formed after the core extraction but before late accretion of the highly siderophile elements.
Jeffrey J. Mcguire - One of the best experts on this subject based on the ideXlab platform.
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The geology of earthquake swarms
Nature Geoscience, 2019Co-Authors: Jeffrey J. McguireAbstract:Differences between earthquake sequences in the crust and adjacent uppermost mantle at oceanic Transform Faults are revealed by a seafloor seismic experiment at the Blanco Transform Fault.
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Millimeter-level precision in a seafloor geodesy experiment at the Discovery Transform Fault, East Pacific Rise
Geochemistry Geophysics Geosystems, 2013Co-Authors: Jeffrey J. Mcguire, John A. CollinsAbstract:[1] Direct-path acoustic ranging is a promising seafloor geodetic technique for continuous high-resolution monitoring of geodynamical process such as Fault slip and magma intrusion. Here we report on a yearlong acoustic ranging experiment conducted across the discovery Transform Fault at ∼4°S on the East Pacific Rise. The ranging instruments utilized a novel acoustic signal designed to enhance precision. We find that, after correcting for variations in sound speed at the path end-points, the ranging measurements have a precision of ∼1 mm over baselines approaching 1 km in length. The primary difficulty in this particular experiment was with the physical stability of the benchmarks, which were deployed free fall from a ship. Despite the stability issues, it appears that the portion of the Transform Fault that the array covered was locked during the year of our survey. The primary obstacle to continuous, high sample rate, high-precision geodetic monitoring of oceanic ridges and Transform Faults is now limited to the construction of geodetic monuments that are well anchored into bedrock.
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Variations in earthquake rupture properties along the Gofar Transform Fault, East Pacific Rise
Nature Geoscience, 2012Co-Authors: Jeffrey J. Mcguire, Margaret S. Boettcher, John A. Collins, Pierre Gouédard, Emily C. Roland, Daniel Lizarralde, Mark D. Behn, Robert D. Van Der HilstAbstract:Mid-ocean ridge Transform Faults experience more foreshocks than continental Faults, yet the mainshock rarely ruptures the entire Fault. Analysis of seismic data from the Gofar Transform Fault at the East Pacific Rise indicates that the foreshock region has different material properties from the mainshock region, and acts as a barrier to rupture propagation. On a global scale, seismicity on oceanic Transform Faults that link mid-ocean ridge segments is thermally controlled1,2. However, temperature cannot be the only control because the largest earthquakes on oceanic Transform Faults rupture only a small fraction of the area that thermal models predict to be capable of rupture3,4,5. Instead, most slip occurs without producing large earthquakes3,4,6. When large earthquakes do occur, they often repeat quasiperiodically7,8. Moreover, oceanic Transform Faults produce an order of magnitude more foreshocks than continental strike-slip Faults7,9. Here we analyse a swarm of about 20,000 foreshocks, recorded on an array of ocean-bottom seismometers, which occurred before a magnitude 6.0 earthquake on the Gofar Transform Fault, East Pacific Rise. We find that the week-long foreshock sequence was confined to a 10-km-long region that subsequently acted as a barrier to rupture during the mainshock. The foreshock zone is associated with a high porosity and undergoes a 3% decrease in average shear-wave speed during the week preceding the mainshock. We conclude that the material properties of Fault segments capable of rupturing in large earthquakes differ from those of barrier regions, possibly as a result of enhanced fluid circulation within the latter. We suggest that along-strike variations in Fault zone material properties can help explain the abundance of foreshocks and the relative lack of large earthquakes that occur on mid-ocean ridge Transform Faults.
J-m Kendall - One of the best experts on this subject based on the ideXlab platform.
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Initiation of a Proto-Transform Fault Prior to Seafloor Spreading
2020Co-Authors: Finnigan Illsley-kemp, Derek Keir, Taras Gerya, Carolina Pagli, Thomas M. Gernon, Atalay Ayele, Berhe Goitom, Jm Bull, Jos Hammond, J-m KendallAbstract:©2018. The Authors. Transform Faults are a fundamental tenet of plate tectonics, connecting offset extensional segments of mid-ocean ridges in ocean basins worldwide. The current consensus is that oceanic Transform Faults initiate after the onset of seafloor spreading. However, this inference has been difficult to test given the lack of direct observations of Transform Fault formation. Here we integrate evidence from surface Faults, geodetic measurements, local seismicity, and numerical modeling of the subaerial Afar continental rift and show that a proto-Transform Fault is initiating during the final stages of continental breakup. This is the first direct observation of proto-Transform Fault initiation in a continental rift and sheds unprecedented light on their formation mechanisms. We demonstrate that they can initiate during late-stage continental rifting, earlier in the rifting cycle than previously thought. Future studies of volcanic rifted margins cannot assume that oceanic Transform Faults initiated after the onset of seafloor spreading.
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Back-propagating super-shear rupture in the 2016 Mw7.1 Romanche Transform Fault earthquake
Nature Geoscience, 2020Co-Authors: Stephen Hicks, Ryo Okuwaki, Andreas Steinberg, Catherine A. Rychert, Nicholas Harmon, Rachel E. Abercrombie, Petros Bogiatzis, David Schlaphorst, Jiri Zahradnik, J-m KendallAbstract:How an earthquake rupture propagates strongly influences the potentially destructive ground shaking. Complex ruptures often involve slip along multiple Faults, which masks information on the frictional behaviour of Fault zones. Geometrically smooth ocean Transform Fault plate boundaries offer a favourable environment to study Fault dynamics, because strain is accommodated along a single, wide Fault zone that offsets the homogeneous geology. Here we present an analysis of the 2016 Mw 7.1 earthquake on the Romanche fracture zone in the equatorial Atlantic, using data from both nearby seafloor seismometers and global seismic networks. We show that this rupture had two phases: (1) upward and eastward propagation towards a weaker region where the Transform Fault intersects the mid-ocean ridge, and then (2) an unusual back-propagation westwards at a supershear speed towards the centre of the Fault. We suggest that deep rupture into weak Fault segments facilitated greater seismic slip on shallow locked zones. This highlights that even earthquakes along a single distinct Fault zone can be highly dynamic. Observations of back-propagating ruptures are sparse, and the possibility of reverse propagation is largely absent in rupture simulations and unaccounted for in hazard assessments. In one earthquake, an oceanic Transform Fault ruptured in one direction and then backwards at a speed exceeding that of shear-wave propagation, according to an analysis of data recorded by nearby seafloor and global seismometers.
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Initiation of a Proto-Transform Fault Prior to Seafloor Spreading
Geochemistry Geophysics Geosystems, 2018Co-Authors: Finnigan Illsley-kemp, Jonathan M. Bull, Derek Keir, Taras Gerya, Carolina Pagli, Thomas M. Gernon, Atalay Ayele, Berhe Goitom, James Hammond, J-m KendallAbstract:Transform Faults are a fundamental tenet of plate tectonics, connecting offset extensional segments of mid‐ocean ridges in ocean basins worldwide. The current consensus is that oceanic Transform Faults initiate after the onset of seafloor spreading. However, this inference has been difficult to test given the lack of direct observations of Transform Fault formation. Here, we integrate evidence from surface Faults, geodetic measurements, local seismicity, and numerical modelling of the subaerial Afar continental rift and show that a proto‐Transform Fault is initiating during the final stages of continental breakup. This is the first direct observation of proto‐Transform Fault initiation in a continental rift, and sheds unprecedented light on their formation mechanisms. We demonstrate that they can initiate during late‐stage continental rifting, earlier in the rifting cycle than previously thought. Future studies of volcanic rifted margins cannot assume that oceanic Transform Faults initiated after the onset of seafloor spreading.
J Labidi, - One of the best experts on this subject based on the ideXlab platform.
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Negligible sulfur isotope fractionation during partial melting: Evidence from Garrett Transform Fault basalts, implications for the late-veneer and the hadean matte
Earth and Planetary Science Letters, 2016Co-Authors: J Labidi,, Pierre CartignyAbstract:Abstract We report the quadruple sulfur isotope compositions, sulfur contents and speciation major and trace elements (including copper and chlorine abundances) of eleven basalts collected in the Garrett Transform Fault. We combine these data to discuss the absence of S isotopic fractionation along both partial melting and low-pressure fractional crystallization. The variations of K2O/TiO2 and La/SmN-ratios (respectively between 0.017 and 0.067, and between 0.31 and 0.59) suggest a range of depletion in Garrett lavas that includes ultra depleted samples (K2O/TiO20.1 that likely experienced hydrothermal sulfide assimilation, Garrett ITLs display homogeneous δ34Sδ34S, Δ33SΔ33S and Δ36SΔ36S values with averages of −0.68±0.08‰−0.68±0.08‰, +0.010±0.005‰+0.010±0.005‰ and −0.04±0.04‰−0.04±0.04‰, respectively (all 1σ , n=10n=10). The δ34Sδ34S values display no relationship with either K2O/TiO2 variations or extent sulfide fractionation. From these observations, we derive a 34S/32S fractionation factor between exsolved sulfides and sulfide dissolved in silicate melts of 1.0000±0.00031.0000±0.0003. The S isotopic fractionation during partial melting can thus be considered as negligible, and both MORBs and ITLs record the 34S/32S ratio of their mantle source. The concept of sulfide melts segregating from the mantle, sinking and being added to the core during planetary differentiation was termed the ‘Hadean Matte’. The segregation of sulfides from the mantle to the core during planetary differentiation could account for various geochemical features of the Earth's mantle. Based on S isotopic mass balance, we derive a lower and upper limit for the hadean matte. While the lower bound corresponds to a virtually negligible hadean matte, the upper limit is View the MathML source3.36×1024gS (i.e. ∼10% of the bulk terrestrial S), which remains 5 to 10 times lower than previous estimates. This upper bound nonetheless requires high mantle S content >1000 ppm S before the extraction of the hadean matte. This suggestion would have chronological requirements, requiring any sulfide melt to have formed after the core extraction but before late accretion of the highly siderophile elements
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Negligible sulfur isotope fractionation during partial melting: Evidence from Garrett Transform Fault basalts, implications for the late-veneer and the hadean matte
Earth and Planetary Science Letters, 2016Co-Authors: J Labidi,, Pierre CartignyAbstract:Abstract We report the quadruple sulfur isotope compositions, sulfur contents and speciation major and trace elements (including copper and chlorine abundances) of eleven basalts collected in the Garrett Transform Fault. We combine these data to discuss the absence of S isotopic fractionation along both partial melting and low-pressure fractional crystallization. The variations of K2O/TiO2 and La/SmN-ratios (respectively between 0.017 and 0.067, and between 0.31 and 0.59) suggest a range of depletion in Garrett lavas that includes ultra depleted samples ( K 2 O/TiO 2 0.03 ). The remarkable level of incompatible element depletion is consistent with re-melting of a depleted source. Contrasting with incompatible element depletion, all samples display similar S and Cu abundance (at a given major-element composition) to mid-ocean ridge basalts (MORB). This indicates that Garrett Intra Transform Lavas (ITL) are sulfide saturated as MORB are. Copper content for Garrett parental melts (MgO >8%) are ∼80 ppm, indistinguishable from MORBs. This requires their mantle sources, variably depleted in incompatible element, to host residual sulfide buffering the Cu content of all erupted melts. We calculate a minimum S content for the source of ultra-depleted Garrett lavas of 100 ± 40 ppm S , i.e. roughly a factor of 2 below the MORB mantle source. After exclusion of a single sample with Cl/K ratio >0.1 that likely experienced hydrothermal sulfide assimilation, Garrett ITLs display homogeneous δ 34 S , Δ 33 S and Δ 36 S values with averages of − 0.68 ± 0.08 ‰ , + 0.010 ± 0.005 ‰ and − 0.04 ± 0.04 ‰ , respectively (all 1σ, n = 10 ). The δ 34 S values display no relationship with either K2O/TiO2 variations or extent sulfide fractionation. From these observations, we derive a 34S/32S fractionation factor between exsolved sulfides and sulfide dissolved in silicate melts of 1.0000 ± 0.0003 . The S isotopic fractionation during partial melting can thus be considered as negligible, and both MORBs and ITLs record the 34S/32S ratio of their mantle source. The concept of sulfide melts segregating from the mantle, sinking and being added to the core during planetary differentiation was termed the ‘Hadean Matte’. The segregation of sulfides from the mantle to the core during planetary differentiation could account for various geochemical features of the Earth's mantle. Based on S isotopic mass balance, we derive a lower and upper limit for the hadean matte. While the lower bound corresponds to a virtually negligible hadean matte, the upper limit is 3.36 × 10 24 g S (i.e. ∼10% of the bulk terrestrial S), which remains 5 to 10 times lower than previous estimates. This upper bound nonetheless requires high mantle S content >1000 ppm S before the extraction of the hadean matte. This suggestion would have chronological requirements, requiring any sulfide melt to have formed after the core extraction but before late accretion of the highly siderophile elements.
Céline Rommevaux-jestin - One of the best experts on this subject based on the ideXlab platform.
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Segment-scale and intrasegment lithospheric thickness and melt variations near the Andrew Bain megaTransform Fault and Marion hot spot: Southwest Indian Ridge, 25.5°E-35°E
Geochemistry Geophysics Geosystems, 2010Co-Authors: Christopher Takeuchi, John Sclater, Nancy Grindlay, John Madsen, Céline Rommevaux-jestinAbstract:We analyze bathymetric, gravimetric, and magnetic data collected on cruise KN145L16 between 25.5°E and 35°E on the ultraslow spreading Southwest Indian Ridge, where the 750 km long Andrew Bain Transform domain separates two accretionary segments to the northeast from a single segment to the southwest. Similar along-axis asymmetries in seafloor texture, rift valley curvature, magnetic anomaly amplitude, magnetization intensity, and mantle Bouguer anomaly (MBA) amplitude within all three segments suggest that a single mechanism may produce variable intrasegment lithospheric thickness and melt delivery. However, closer analysis reveals that a single mechanism is unlikely. In the northeast, MBA lows, shallow axial depths, and large abyssal hills indicate that the Marion hot spot enhances the melt supply to the segments. We argue that along-axis asthenospheric flow from the hot spot, dammed by major Transform Faults, produces the inferred asymmetries in lithospheric thickness and melt delivery. In the southwest, strong rift valley curvature and nonvolcanic seafloor near the Andrew Bain Transform Fault indicate very thick subaxial lithosphere at the end of the single segment. We suggest that cold lithosphere adjacent to the eastern end of the ridge axis cools and thickens the subaxial lithosphere, suppresses melt production, and focuses melt to the west. This limits the amount of melt emplaced at shallow levels near the Transform Fault. Our analysis suggests that the Andrew Bain divides a high melt supply region to the northeast from an intermediate to low melt supply region to the southwest. Thus, this Transform Fault represents not only a major topographic feature but also a major melt supply boundary on the Southwest Indian Ridge.
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Tectonic interpretation of the Andrew Bain Transform Fault: Southwest Indian Ocean
Geochemistry Geophysics Geosystems, 2005Co-Authors: John Sclater, Nancy Grindlay, John Madsen, Céline Rommevaux-jestinAbstract:[1] Between 25°E and 35°E, a suite of four Transform Faults, Du Toit, Andrew Bain, Marion, and Prince Edward, offsets the Southwest Indian Ridge (SWIR) left laterally 1230 km. The Andrew Bain, the largest, has a length of 750 km and a maximum Transform domain width of 120 km. We show that, currently, the Nubia/Somalia plate boundary intersects the SWIR east of the Prince Edward, placing the Andrew Bain on the Nubia/Antarctica plate boundary. However, the overall trend of its Transform domain lies 10° clockwise of the predicted direction of motion for this boundary. We use four Transform-parallel multibeam and magnetic anomaly profiles, together with relocated earthquakes and focal mechanism solutions, to characterize the morphology and tectonics of the Andrew Bain. Starting at the southwestern ridge-Transform intersection, the relocated epicenters follow a 450-km-long, 20-km-wide, 6-km-deep western valley. They cross the Transform domain within a series of deep overlapping basins bounded by steep inward dipping arcuate scarps. Eight strike-slip and three dip-slip focal mechanism solutions lie within these basins. The earthquakes can be traced to the northeastern ridge-Transform intersection via a straight, 100-km-long, 10-km-wide, 4.5-km-deep eastern valley. A striking set of seismically inactive NE-SW trending en echelon ridges and valleys, lying to the south of the overlapping basins, dominates the eastern central section of the Transform domain. We interpret the deep overlapping basins as two pull-apart features connected by a strike-slip basin that have created a relay zone similar to those observed on continental Transforms. This Transform relay zone connects three closely spaced overlapping Transform Faults in the southwest to a single Transform Fault in the northeast. The existence of the Transform relay zone accounts for the difference between the observed and predicted trend of the Andrew Bain Transform domain. We speculate that between 20 and 3.2 Ma, an oblique accretionary zone jumping successively northward created the en echelon ridges and valleys in the eastern central portion of the domain. The style of accretion changed to that of a Transform relay zone, during a final northward jump, at 3.2 Ma.
