An experimental study of the role of partial melts of sediments versus mantle melts in the sources of potassic magmatism

Abstract Potassium-rich lavas with K/Na of >2 are common in orogenic and anorogenic intraplate magmatic provinces. However, in the primitive mantle, the concentration of Na exceeds that of K by 10 times. The source of K-rich lavas thus needs to be either K-enriched or Na-depleted to account for high K/Na ratios. The geochemical and isotopic compositions of high 87Sr/86Sr post-collisional lavas show that their mantle source contains a recycled crustal component. These highly K-enriched lavas with crustal like trace element patterns are termed “orogenic lamproites” and are compositionally distinct from K-rich “anorogenic lamproites” that show lower 87Sr/86Sr and a trace element pattern that r…

Facies variations in response to Holocene sea-level and climate change on Bora Bora, French Polynesia: Unravelling the role of synsedimentary siderite in a tropical marine, mixed carbonate-siliciclastic lagoon

International audience; Five mixed carbonate-siliciclastic sedimentary facies were identified in the barrier-reef lagoon of Bora Bora using microfacies and statistical analyses of 70 sediment samples taken at high resolution from two vibrocores. Fades and fades successions were interpreted with respect to Holocene sea-level and climate changes. The windward lagoon core is characterized by sideritic marly wackestones and foraminifera-sideritic wackestones, deposited around 7700 years BP (years before present) during the early-mid Holocene transgression. At that time, extensive weathering and erosion of iron-bearing minerals from the volcanic island, due to a wetter climate, were expressed in…

Origin of potassic postcollisional volcanic rocks in young, shallow, blueschist-rich lithosphere

Unusually high Th/La in K-rich orogenic rocks may indicate shallow blueschist-rich sources in accretionary settings.

The composition of near-solidus melts of peridotite in the presence of CO2 and H2O between 40 and 60 kbar

Abstract Partial melting experiments at 40, 50 and 60 kbar pressure on three peridotite compositions with 0.5–0.63 wt.% H2O and 2.0–3.2 wt.% CO2 added indicate melting temperatures only marginally above continental geotherms. Most experiments were performed on a composition with 1.5 wt.% K2O added, which causes a further decrease of about 40 °C in melting temperature. Melts progress gradually from carbonate-rich to carbonated silicate in composition: near-solidus melts have Ca/(Ca + Mg) of 0.46–0.53, which fall to  30 wt.%) and Al2O3 (  9 wt.%) and concomitant decrease in CaO (> 20 to Partial melts of peridotite with CO2 and H2O are too low in MgO to resemble kimberlites, but will act as ef…

Melting phlogopite-rich MARID: Lamproites and the role of alkalis in olivine-liquid Ni-partitioning

Abstract In this study, we show how veined lithospheric mantle is involved in the genesis of ultrapotassic magmatism in cratonic settings. We conducted high pressure experiments to simulate vein + wall rock melting within the Earth's lithospheric mantle by reacting assemblages of harzburgite and phlogopite-rich hydrous mantle xenoliths. These comprised a mica-, amphibole-, rutile-, ilmenite-, diopside (MARID) assemblage at 3–5 GPa and 1325–1450 °C. Melting of the MARID assemblages results in infiltration of melt through the harzburgite, leading to its chemical alteration. At 3 and 4 GPa, melts are high in K2O (> 9 wt%) with K2O/Na2O > > 2 comparable to anorogenic lamproites. Higher pressure…

Constraints on the sources of post-collisional K-rich magmatism: The roles of continental clastic sediments and terrigenous blueschists

Abstract The possible role of continental sediments in the generation of potassium-enriched lavas of the Alpine-Himalayan belt depends on their melting behaviour either during subduction or during post-collisional relaxation. Although usually classed as orogenic lavas, these volcanic rocks may result from re-melting of newly formed mantle lithosphere 30–40 million years after collision ends, and can thus be considered as the first stage of intraplate volcanism. The potassic component in these volcanics is characterized by a high Th/La signature for which there are two competing explanations: melting of subducted continental clastic sediments, and the involvement of lawsonite blueschists in …

Microanalytical methods for in-situ high-resolution analysis of rock varnish at the micrometer to nanometer scale

Abstract A wide range of analytical techniques were used to investigate rock varnish from different locations (Negev, Israel; Knersvlakte, South Africa; Death Valley and Mojave Desert, California): a 200 nm-femtosecond laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS), an electron probe microanalyzer (EPMA), focused ion beam (FIB) slicing, and scanning transmission X-ray microscopy–near edge X-ray absorption fine structure spectroscopy (STXM–NEXAFS). This combination enables comprehensive high-spatial-resolution analysis of rock varnish. Femtosecond LA-ICP-MS and EPMA were used for quantitative determination of element concentrations. In-situ measurements were conducte…

Two-Stage Origin of K-Enrichment in Ultrapotassic Magmatism Simulated by Melting of Experimentally Metasomatized Mantle

The generation of strongly potassic melts in the mantle requires the presence of phlogopite in the melting assemblage, while isotopic and trace element analyses of ultrapotassic rocks frequently indicate the involvement of subducted crustal lithologies in the source. However, phlogopite-free experiments that focus on melting of sedimentary rocks and subsequent hybridization with mantle rocks at pressures of 1&ndash

Ancient xenocrystic zircon in young volcanic rocks of the southern Lesser Antilles island arc

Abstract The Lesser Antilles arc is one of the best global examples in which to examine the effects of the involvement of subducted sediment and crustal assimilation in the generation of arc crust. Most of the zircon recovered in our study of igneous and volcaniclastic rocks from Grenada and Carriacou (part of the Grenadines chain) is younger than 2 Ma. Within some late Paleogene to Neogene (~ 34–0.2 Ma) lavas and volcaniclastic sediments however, there are Paleozoic to Paleoarchean (~ 250–3469 Ma) xenocrysts, and Late Jurassic to Precambrian zircon (~ 158–2667 Ma) are found in beach and river sands. The trace element characteristics of zircon clearly differentiate between different types o…

Late Pleistocene Eifel eruptions: insights from clinopyroxene and glass geochemistry of tephra layers from Eifel Laminated Sediment Archive sediment cores

Metamorphism and melting of picritic crust in the early Earth

Abstract Partial melting experiments with models of Archean oceanic crust (MAOC; with 11, 13 and 15 wt.% MgO) are used to investigate the role of metamorphism and melting of primary picritic compositions in the formation of TTG-like melts and continental crust on the early Earth. The approach investigates the possibility that the average early crust composition was comparatively MgO-rich and evolved to lower magnesium content during the secular cooling of the Earth. High-pressure partial melting experiments indicate a transition of melt compositions from aluminous basaltic melts in MAOC 15 to predominantly tonalitic melts in MAOC 11 and higher melting temperatures with increasing magnesium …

Melting of sediments in the deep mantle produces saline fluid inclusions in diamonds.

We demonstrate the formation of highly saline mantle fluids by the reaction of subducted sediment with peridotite.

Variation of olivine composition in the volcanic rocks in the Songliao basin, NE China: lithosphere control on the origin of the K-rich intraplate mafic lavas

Abstract Lithospheric thickness and the heterogeneity of the mantle lithosphere are two major parameters that play a role in determining the final composition of the mafic melts and their minerals. The Songliao basin in northeast China represents an ideal natural laboratory to study the effect of these two parameters on early Pliocene to Holocene K-rich mafic lavas (K2O > 4 wt.%; K2O/Na2O > 1). A series of Cenozoic volcanic edifices (Erkeshan, Wudalianchi, Keluo and Xiaogulihe) are tentatively divided into three groups (Group 1 — thin, Group 2 — middle, and Group 3 — thick) according to the lithosphere thickness. They are located in the northern region of the Songliao basin extending in a n…

Experimental organic matter maturation at 2kbar: Heat-up effect to low temperatures on vitrinite reflectance

Abstract An experimental study was performed to evaluate the effect of heat-up to various low temperatures on vitrinite reflectance ( VR ) at 2 kbar employing the same previously used heat-up procedures, starting material and apparatus. Heat-up is the isobaric experimental procedure consisting of the increase in temperature of the laboratory vitrinite maturation from room temperature to the desired run temperature T end of heat-up at which the isobaric-isothermal maturation starts. Experiments employed cold-seal pressure vessels with classical heat-up devices and were carried out on wet (water added) xylite of swamp cypress. Confined system maturation experiments were conducted at 2 kbar an…

Peri-Gondwanan Ordovician arc magmatism in southeastern Ireland and the Isle of Man: Constraints on the timing of Caledonian deformation in Ganderia

The timing of and tectonic controls on Ordovician magmatism and deformation within accreted terranes are key elements in reconstructing the generation of the Caledonian-Appalachian orogen. This pap ...

Multiple intrusive phases in the Leinster Batholith, Ireland: geochronology, isotope geochemistry and constraints on the deformation history

The formation of granite batholiths, commonly by incremental assembly of small magma batches, and their correlation with tectonic events, on both local and regional scales, is crucial to understanding the evolution of the Earth9s continental crust. However, these correlations often rely on assumptions about the detailed relationship and timing of mapped units. Here we report how an integrated geochronological, structural and isotope geochemical study in only one key locality from the late Caledonian Leinster Batholith in SE Ireland provides the potential for unravelling essential questions on batholith petrogenesis. The Northern Unit of the Leinster Batholith intruded incrementally, as demo…

Partitioning of nitrogen during melting and recycling in subduction zones and the evolution of atmospheric nitrogen

Abstract The subduction of sediment connects the surface nitrogen cycle to that of the deep Earth. To understand the evolution of nitrogen in the atmosphere, the behavior of nitrogen during the subduction and melting of subducted sediments has to be estimated. This study presents high-pressure experimental measurements of the partitioning of nitrogen during the melting of sediments at sub-arc depths. For quantitative analysis of nitrogen in minerals and glasses, we calibrated the electron probe micro-analyzer on synthetic ammonium feldspar to measure nitrogen concentrations as low as 500 μg g−1. Nitrogen abundances in melt and mica are used together with mass balance calculations to determi…

Chemical heterogeneities in the mantle: The equilibrium thermodynamic approach

Abstract This study attempts to answer a simple and yet fundamental question in relation to our understanding of the chemical evolution of deep Earth and planetary interiors. Given two initially separate assemblages (lithologies) in chemical equilibrium can we predict the chemical and mineralogical compositions of the two assemblages when they are put together to form a new equilibrated system? Perhaps a common perception is that given sufficient time, the two assemblages will homogenize chemically and mineralogically, however from a chemical thermodynamic point of view, this is not the case. Certain petrological differences in terms of bulk composition, mineralogy and mineral abundance rem…

Zircon Dates Long-Lived Plume Dynamics in Oceanic Islands

In this contribution we report the first systematic study of zircon U-Pb geochronology and δ 18O-εHf(t) isotope geochemistry from 10 islands of the hot-spot related Galapagos Archipelago. The data extracted from the zircons allow them to be grouped into three types: (a) young zircons (0–∼4 Ma) with εHf(t) (∼5–13) and δ 18O (∼4–7) isotopic mantle signature with crystallization ages dating the islands, (b) zircons with εHf(t) (∼5–13) and δ 18O (∼5–7) isotopic mantle signature (∼4–164 Ma) which are interpreted to date the time of plume activity below the islands (∼164 Ma is the minimum time of impingement of the plume below the lithosphere), and (c) very old zircons (∼213–3,000 Ma) with mostly…

Pressure dependence of density and structural relaxation of glass near the glass transition region

International audience; A simplified and effective pressure cell together with an experimental procedure has been developed and applied to compress samples of SCHOTT N-BK7 glass under static high pressures in a piston-cylinder apparatus. Results from the density and volume recovery measurements show that, the glass samples were effectively densified in piston-cylinder apparatus with the density at room temperature increasing linearly with frozen-in pressure. To explain the experimental data, we developed a mathematical model based on a suggestion by Gupta (1988) with two internal parameters, named fictive temperature (Tf) and fictive pressure (Pf), which fits the experimental data well.

Morphological and chemical evolution of corundum (ruby and sapphire): Crystal ontogeny reconstructed by EMPA, LA-ICP-MS, and Cr3+Raman mapping

The term “ ontogeny ,” which is commonly used in biology, was introduced into the Earth sciences in 1961 to include the genesis and evolution of single crystals and crystal aggregates. The term encompasses nucleation , growth , alteration , and destruction . We present results of studies concerning the ontogeny of natural corundum (rubies and sapphires), and the chemical and morphological evolution of corundum crystals from deposits in Africa (Kenya, Tanzania, Madagascar) and Southeast Asia (Vietnam). Trace-element compositions indicative for different corundum habits were determined by rim-to-rim LA-ICP-MS and electron microprobe analyses. Raman spectroscopy was applied for Cr 3+ photolumi…

Melting and dynamic metasomatism of mixed harzburgite + glimmerite mantle source: Implications for the genesis of orogenic potassic magmas

Abstract Tectonically young, orogenic settings are commonly the sites of post-collisional silica-rich ultrapotassic magmas with extreme K2O-contents of up to 9 wt% and K2O/Na2O > 2. Many experimental studies investigating the generation of these melts have concentrated on melting of homogenous phlogopite bearing peridotites, whereas geochemical signatures indicate the involvement of at least two types of source rocks: ultra-depleted and K and trace elements-enriched ones. We report the results of melting experiments at 1–2 GPa of mixed glimmerite and harzburgite, in which these rock types make up two halves each capsule. Melting begins in the glimmerite, and its metasomatic effects on the h…