Experimental production of K-rich metasomes through sediment recycling at the slab-mantle interface in the fore-arc.

Sediment contribution to the mantle is the key step for the generation of orogenic magmatism to produce its isotopic and geochemical inventory. Even though they are exceptional for the post-collisional settings, there are worldwide examples of arc-related ultrapotassic mafic magmas which require complex multi-stage processes along with sediment melting e.g. in Italy or Pontides of Türkiye. To understand the metasomatism leading mantle to produce ultrapotassic mafic melts, we simulated the reactions of depleted (harzburgite) and fertile (lherzolite) mantle with subducted carbonate-rich sediment at relatively cold (800-850 °C) and shallow (2 GPa, 60-80 km) slab-mantle interfaces. The melting of sediments can trigger the formation of immiscible and conjugate carbonatitic and silicic melts which flux the mantle to develop hydrous minerals and dolomitic melt. The metasomatic growth product is a wehrlite composed of clinopyroxene, phlogopite, carbonate minerals and amphibole, representing a source of choice for Si-undersaturated ultrapotassic lavas. The occurrence of conjugate carbonatitic and silicic melts and their potential physical separation, offer a possibility for fractionation of several canonical trace element ratios such as Th/La, observed in Si-saturated ultrapotassic lavas. The synergy between peridotite-melt interaction and the physical separation of the carbonatitic and extremely K-enriched silicic melts are essential for the compositional evolution of ultrapotassic orogenic magmas and their mantle sources.

Rapid quench piston cylinder apparatus: An improved design for the recovery of volatile-rich geological glasses from experiments at 0.5-2.5 GPa.

The accurate and precise determination of the compositions of silicate glasses formed from melts containing volatile components H2O and CO2 recovered from high-pressure, high-temperature experiments is essential to our understanding of geodynamic processes taking place within the planet. Silicate melts are often difficult to analyze chemically because the formation of quench crystals and overgrowths on silicate phases is rapid and widespread upon quenching of experiments, preventing the formation of glasses in low-SiO2 and volatile-rich compositions. Here, we present experiments conducted in a novel rapid quench piston cylinder apparatus on a series of partially molten low-silica alkaline rock compositions (lamproite, basanite, and calk-alkaline basalt) with a range of water contents between 3.5 and 10 wt %. Quench modification of the volatile-bearing silicate glasses is significantly reduced compared to those produced in older piston cylinder apparatuses. The recovered glasses are almost completely free of quench modification and facilitate the determination of precise chemical compositions. We illustrate significantly improved quench textures and provide an analytical protocol that recovers accurate chemical compositions from both poorly quenched and well-quenched silicate glasses.

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

Potassium-rich volcanism occurring throughout the Alpine-Himalayan belt from Spain to Tibet is characterized by unusually high Th/La ratios, for which several hypotheses have brought no convincing solution. Here, we combine geochemical datasets from potassic postcollisional volcanic rocks and lawsonite blueschists to explain the high Th/La. Source regions of the volcanic melts consist of imbricated packages of blueschist facies mélanges and depleted peridotites, constituting a new mantle lithosphere formed only 20 to 50 million years earlier during the accretionary convergence of small continental blocks and oceans. This takes place entirely at shallow depths (<80 km) without any deep subduction of continental materials. High Th/La in potassic rocks may indicate shallow sources in accretionary settings even where later obscured by continental collision as in Tibet. This mechanism is consistent with a temporal trend in Th/La in potassic postcollisional magmas: The high Th/La signature first becomes prominent in the Phanerozoic, when blueschists became widespread.

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

Diamonds growing in the Earth's mantle often trap inclusions of fluids that are highly saline in composition. These fluids are thought to emerge from deep in subduction zones and may also be involved in the generation of some of the kimberlite magmas. However, the source of these fluids and the mechanism of their transport into the mantle lithosphere are unresolved. Here, we present experimental results showing that alkali chlorides are stable solid phases in the mantle lithosphere below 110 km. These alkali chlorides are formed by the reaction of subducted marine sediments with peridotite and show identical K/Na ratios to fluid inclusions in diamond. At temperatures >1100°C and low pressures, the chlorides are unstable; here, potassium is accommodated in mica and melt. The reaction of subducted sediments with peridotite explains the occurrence of Mg carbonates and the highly saline fluids found in diamonds and in chlorine-enriched kimberlite magmas.

In vitro study on dental erosion caused by different vinegar varieties using an electron microprobe.

BACKGROUND: Among health-conscious individuals, including vegetarians, salads dressed with vinegar are frequently consumed. Dental erosion can also be caused by an acidic diet, and occurs with increasing tendency. The aim of this study was to analyze the erosive potential of vinegar varieties on human enamel samples. METHODS: A total of 30 vinegar varieties were selected. Enamel samples were prepared from human wisdom teeth, and the specially prepared enamel slices were incubated with 5 selected vinegars (Bio vinegar, pH = 3.1; raspberry vinegar, pH = 2.7; Condimento Balsamico, pH = 3.95; Ortalli Bianco Modena, pH = 2.7; Vinaigre de Jerez, pH = 2.9) for up to 8 hours. Controls were incubated with a 0.9% sodium chloride solution. The quantitative analysis of CaO, P2O5, F, MgO, Cl, and O in the enamel samples (incubation: 4, 8 hours) in various depths ranging from 7.5 - 105 microm was carried out using an electron probe micro-analyser (Jeol JXA 8900RL). Linear mixed models were fitted to analyze statistically relevant differences between the different vinegars at various depth levels. RESULTS: Incubating the enamel slices with the selected vinegars caused a release of minerals, which was dependent on time and type of vinegar. The vinegar Ortalli Bianco Modena led to a slight loss (1%) of the mineral CaO in a depth up to 20 microm, while the loss of minerals caused by raspberry vinegar in a depth of up to 30 microm was about 20% (4 hours). The greatest loss of the minerals was detected for the Bio Vinegar. After 8 hours incubation, a loss of minerals of about 20% in a depth of 45 microm and in a depth of 60 microm of 16% could be observed. Both, the Bio Vinegar and the raspberry vinegar led to a significantly higher loss of minerals (p < 0.0001) than all other tested vinegars. CONCLUSIONS: In this in vitro study, the erosive potential of different vinegar varieties on human enamel samples could be demonstrated. However, it must be considered that numerous modifying factors influence the enamel surface in vivo; therefore, a direct translation to in vitro conditions can only be done with caution.

Evolution of the Archaean crust by delamination and shallow subduction.

The Archaean oceanic crust was probably thicker than present-day oceanic crust owing to higher heat flow and thus higher degrees of melting at mid-ocean ridges. These conditions would also have led to a different bulk composition of oceanic crust in the early Archaean, that would probably have consisted of magnesium-rich picrite (with variably differentiated portions made up of basalt, gabbro, ultramafic cumulates and picrite). It is unclear whether these differences would have influenced crustal subduction and recycling processes, as experiments that have investigated the metamorphic reactions that take place during subduction have to date considered only modern mid-ocean-ridge basalts. Here we present data from high-pressure experiments that show that metamorphism of ultramafic cumulates and picrites produces pyroxenites, which we infer would have delaminated and melted to produce basaltic rocks, rather than continental crust as has previously been thought. Instead, the formation of continental crust requires subduction and melting of garnet-amphibolite--formed only in the upper regions of oceanic crust--which is thought to have first occurred on a large scale during subduction in the late Archaean. We deduce from this that shallow subduction and recycling of oceanic crust took place in the early Archaean, and that this would have resulted in strong depletion of only a thin layer of the uppermost mantle. The misfit between geochemical depletion models and geophysical models for mantle convection (which include deep subduction) might therefore be explained by continuous deepening of this depleted layer through geological time.