Tellurium in Late Permian-Early Triassic Sediments as a Proxy for Siberian Flood Basalt Volcanism

We measured the concentrations of trace elements in Late Permian to Early Triassic sediments from Spitsbergen. High mercury concentrations in sediments from the level of the Permo-Triassic Mass Extinction (PTME) at this location were previously attributed to the emplacement of the Siberian Traps Large Igneous Province and used to link the timing of volcanism with the record of environmental change and extinction in these sediments. We investigated the use of the moderately to highly volatile, siderophile elements Ni, Zn, Cd, Sb, Te, Re, and Tl as proxies for the intensity of Siberian volcanism. These trace elements, like Hg, have high concentrations in volcanic gas compared to crustal rocks. Tellurium is highly enriched at the PTME, and Te/Th ratios increase by a factor of ∼20 across the PTME, similar to the variation in Hg/total organic carbon (TOC) in the same samples. Te/Th and Hg/TOC values imply that Siberian volcanism initiated at the onset of the PTME, coincident with the start of the δ13Corganic excursion and abrupt warming. Based on Te and Hg, most Siberian volcanism occurred between the two phases of the PTME boundary (a period of less than 100 ky), but also continued into the Early Triassic. The duration of Siberian volcanism inferred from Te/Th and Hg/TOC is shorter than that indicated by recent high-precision U-Pb ages of Siberian intrusive and extrusive rocks. Te concentrations and Te/Th ratios in sediments represent a useful new proxy for volcanism, which can be used to link the marine sedimentary record with large volcanic events on land

Geochemical mapping of a paleo-subduction zone beneath the Troodos Ophiolite

Supra-subduction zone ophiolites such as the Cretaceous Troodos Ophiolite of Cyprus are fragments of oceanic crust formed by seafloor spreading close to subduction zones. Their exact tectonic setting of origin has been intensively debated. Although many supra-subduction zone ophiolites are thought to represent fore-arc crust, created during subduction initiation, others may have formed at a subducting ridge, or in a back-arc, ridge-trench-trench/transform triple junction or ‘plate edge’ setting. We carried out major and trace element analyses of 515 fresh volcanic glasses from 7 detailed sections through the Troodos lava sequence in order to determine the regional and temporal variation in the composition of Troodos magmatism, and hence reconstruct the distance and orientation of the Troodos spreading axis relative to the former subduction zone. Troodos glasses range from boninite through tholeiitic basalt and andesite to dacite. All glasses are enriched in fluid-mobile trace elements, and variably depleted in the high-field strength elements compared to Mid-Ocean Ridge Basalt (MORB). None of these glasses therefore have compositions identical to Izu-Bonin-Mariana fore-arc lavas that have been proposed to be the prime example of lavas formed during subduction initiation. Boninites are apparently restricted to the southern margin of the Troodos Ophiolite, and glasses from the southeast margin of the ophiolite are the most depleted and contain the strongest input of subduction zone fluid and melt signature. These geographic variations in glass composition indicate that the Troodos Ophiolite formed by NW-SE directed spreading (at 91 Ma) approximately 100–120 km above an eastward-dipping subducting plate. The orientation of the Troodos spreading axis relative to the former trench could be explained if the Troodos Ophiolite formed in a fore-arc position by subduction initiation at a transform fault. However, the lack of glasses with fore-arc basalt composition, and similarities between the trace element compositions of Troodos glasses and those from the Fonualei basin and northern Lau Basin in the southwest Pacific suggest that the Troodos Ophiolite formed in a ridge-trench-trench or ridge-trench-transform triple junction setting, at a back-arc spreading centre that propagated into arc and fore-arc crust.Peer reviewe

Chemical Evolution of Calc-alkaline Magmas during the Ascent through Continental Crust: Constraints from Methana, Aegean Arc

M1 - egaa036Quaternary calc-alkaline andesitic to dacitic lavas effusively erupted on top of about 30 km thick accreted continental crust at Methana peninsula in the western Aegean arc. We present new data of major and trace element concentrations as well as of Sr-Nd-Pb isotope ratios along with mineral compositions of Methana lavas and their mafic enclaves. The enclaves imply a parental basaltic magma and fractional crystallization processes with relatively little crustal assimilation in the deep part of the Methana magma system. The composition of amphibole in some mafic enclaves and lavas indicates deeper crystallization at similar to 25km depth close to the Moho compared with the evolved lavas that formed atPeer reviewe

From mantle plume to rift-related volcanism of an oceanic plateau: The complex magmatic evolution of the Rio Grande Rise, South Atlantic

The Rio Grande Rise in the western South Atlantic Ocean has been interpreted as either an oceanic plateau related to the Tristan-Gough mantle plume, or a fragment of detached continental crust. Here we present new major and trace element data for volcanic rocks from the western and eastern Rio Grande Rise and the adjacent Jean Charcot Seamount Chain. The eastern Rio Grande Rise and older parts of the western Rio Grande Rise are comprised of tholeiitic basalt with moderately enriched trace element compositions and likely formed above the Tristan-Gough mantle plume close to the southern Mid-Atlantic Ridge. Younger alkalic lavas from the western Rio Grande Rise and the Jean Charcot Seamount Chain were formed by lower degrees of melting beneath thicker lithosphere in an intraplate setting possibly during rifting of the plateau. There is no clear geochemical evidence that remnants of continental crust are present beneath the Rio Grande Rise

Effects of the Hydrous Domain in the Mantle Wedge on Magma Formation and Mixing at the Northeast Lau Spreading Center, SW Pacific

Abundant volcanic activity occurs in the back-arc region of the northern Tofua island arc where the Northeast Lau Spreading Center (NELSC) propagates southwards into older crust causing the formation of numerous seamounts at the propagating rift tip. An off-axis volcanic diagonal ridge (DR) occurs at the eastern flank of the NELSC, linking the large rear-arc volcano Niuatahi with the NELSC. New geochemical data from the NELSC, the southern propagator seamounts, and DR reveal that the NELSC lavas are tholeiitic basalts whereas the rear-arc volcanoes typically erupt lavas with boninitic composition. The sharp geochemical boundary probably reflects the viscosity contrast between off-axis hydrous harzburgitic mantle and dry fertile mantle beneath the NELSC. The new data do not indicate an inflow of Samoa plume mantle into the NELSC, confirming previously published He isotope data. The NELSC magmas form by mixing of an enriched and a depleted Indian Ocean-type upper mantle end-member implying a highly heterogeneous upper mantle composition in this area. Most NELSC lavas are little affected by a slab component implying that melting is adiabatic beneath the spreading center. The DR lavas show the influence of a component from the subducted Louisville Seamount Chain, which was previously thought to be restricted to the nearby arc volcanoes Niuatoputapu and Tafahi. This signature is rarely detected along the NELSC implying little mixing of melts from the low-viscosity hydrous portion of the mantle wedge beneath the rear-arc volcanoes into the melting region of the dry mantle beneath the NELSC.Peer reviewe

The origin of short-lived radionuclides and the astrophysical environment of solar system formation

Based on early solar system abundances of short-lived radionuclides (SRs), such as $^{26}$Al (T$_{1/2} = 0.74$ Myr) and $^{60}$Fe (T$_{1/2} = 1.5$ Myr), it is often asserted that the Sun was born in a large stellar cluster, where a massive star contaminated the protoplanetary disk with freshly nucleosynthesized isotopes from its supernova (SN) explosion. To account for the inferred initial solar system abundances of short-lived radionuclides, this supernova had to be close ($\sim$ 0.3 pc) to the young ($\leqslant$ 1 Myr) protoplanetary disk. Here we show that massive star evolution timescales are too long, compared to typical timescales of star formation in embedded clusters, for them to explode as supernovae within the lifetimes of nearby disks. This is especially true in an Orion Nebular Cluster (ONC)-type of setting, where the most massive star will explode as a supernova $\sim$ 5 Myr after the onset of star formation, when nearby disks will have already suffered substantial photoevaporation and/or formed large planetesimals. We quantify the probability for {\it any} protoplanetary disk to receive SRs from a nearby supernova at the level observed in the early solar system. Key constraints on our estimate are: (1) SRs have to be injected into a newly formed ($\leqslant$ 1 Myr) disk, (2) the disk has to survive UV photoevaporation, and (3) the protoplanetary disk must be situated in an enrichment zone permitting SR injection at the solar system level without disk disruption. The probability of protoplanetary disk contamination by a supernova ejecta is, in the most favorable case, 3 $\times$ 10$^{-3}$

Mantle plume and rift-related volcanism during the evolution of the Rio Grande Rise

The Rio Grande Rise in the western South Atlantic Ocean has been interpreted as either an oceanic plateau related to the Tristan-Gough mantle plume, or a fragment of detached continental crust. Here we present new major and trace element data for volcanic rocks from the western and eastern Rio Grande Rise and the adjacent Jean Charcot Seamount Chain. The eastern Rio Grande Rise and older parts of the western Rio Grande Rise are comprised of tholeiitic basalt with moderately enriched trace element compositions and likely formed above the Tristan-Gough mantle plume close to the southern Mid-Atlantic Ridge. Younger alkalic lavas from the western Rio Grande Rise and the Jean Charcot Seamount Chain were formed by lower degrees of melting beneath thicker lithosphere in an intraplate setting possibly during rifting of the plateau. There is no clear geochemical evidence that remnants of continental crust are present beneath the Rio Grande Rise

Insights into mantle composition and mantle melting beneath mid‐ocean ridges from postspreading volcanism on the fossil Galapagos Rise

New major and trace element and Sr, Nd, and Pb isotope data, together with ³⁹ Ar-⁴⁰Ar ages for lavas from the extinct Galapagos Rise spreading center in the eastern Pacific reveal the evolution in magma compositions erupted during slowdown and after the end of active spreading at a mid-ocean ridge. Lavas erupted at 9.2 Ma, immediately prior to the end of spreading are incompatible element depleted mid-ocean ridge tholeiitic basalts, whereas progressively younger (7.5 to 5.7 Ma) postspreading lavas are increasingly alkalic, have higher concentrations of incompatible elements, higher La/Yb, K/Ti, ⁸⁷Sr/⁸⁶Sr, and lower ¹⁴³Nd/¹⁴⁴Nd ratios and were produced by smaller degrees of mantle melting. The large, correlated variations in trace element and isotope compositions can only be explained by melting of heterogenous mantle, in which incompatible trace element enriched lithologies preferentially contribute to smaller degree mantle melts. The effects of variable degrees of melting of heterogeneous mantle on lava compositions must be taken into account when using mid-ocean ridge basalt (MORB) to infer the conditions of melting beneath active spreading ridges. For example, the stronger “garnet signature” inferred from Sm/Nd and ¹⁴³Nd/¹⁴⁴Nd ratios for postspreading lavas from the Galapagos Rise results from a larger contribution from enriched lithologies with high La/Yb and Sm/Yb, rather than from a greater proportion of melting in the stability field of garnet peridotite. Correlations between ridge depth and Sm/Yb and fractionation-corrected Na concentrations in MORB worldwide could result from variations in mantle fertility and/or variations in the average degree of melting, rather than from large variations in mantle temperature. If more fertile mantle lithologies are preferentially melted beneath active spreading ridges, then the upper mantle may be significantly more “depleted” than is generally inferred from the compositions of MORB.Keywords: radiogenic isotope, geochemistry, magmatis

Insights into mantle composition and mantle melting beneath mid-ocean ridges from postspreading volcanism on the fossil Galapagos Rise

New major and trace element and Sr, Nd, and Pb isotope data, together with 39Ar-40Ar ages for lavas from the extinct Galapagos Rise spreading center in the eastern Pacific reveal the evolution in magma compositions erupted during slowdown and after the end of active spreading at a mid-ocean ridge. Lavas erupted at 9.2 Ma, immediately prior to the end of spreading are incompatible element depleted mid-ocean ridge tholeiitic basalts, whereas progressively younger (7.5 to 5.7 Ma) postspreading lavas are increasingly alkalic, have higher concentrations of incompatible elements, higher La/Yb, K/Ti, 87Sr/86Sr, and lower 143Nd/144Nd ratios and were produced by smaller degrees of mantle melting. The large, correlated variations in trace element and isotope compositions can only be explained by melting of heterogenous mantle, in which incompatible trace element enriched lithologies preferentially contribute to smaller degree mantle melts. The effects of variable degrees of melting of heterogeneous mantle on lava compositions must be taken into account when using mid-ocean ridge basalt (MORB) to infer the conditions of melting beneath active spreading ridges. For example, the stronger “garnet signature” inferred from Sm/Nd and 143Nd/144Nd ratios for postspreading lavas from the Galapagos Rise results from a larger contribution from enriched lithologies with high La/Yb and Sm/Yb, rather than from a greater proportion of melting in the stability field of garnet peridotite. Correlations between ridge depth and Sm/Yb and fractionation-corrected Na concentrations in MORB worldwide could result from variations in mantle fertility and/or variations in the average degree of melting, rather than from large variations in mantle temperature. If more fertile mantle lithologies are preferentially melted beneath active spreading ridges, then the upper mantle may be significantly more “depleted” than is generally inferred from the compositions of MORB