Heavy Element Abundances in Presolar Silicon Carbide Grains from Low-Metallicity AGB Stars

Primitive meteorites contain small amounts of presolar minerals that formed in the winds of evolved stars or in the ejecta of stellar explosions. Silicon carbide is the best studied presolar mineral. Based on its isotopic compositions it was divided into distinct populations that have different origins: Most abundant are the mainstream grains which are believed to come from 1.5-3 Msun AGB stars of roughly solar metallicitiy. The rare Y and Z grains are likely to come from 1.5-3 Msun AGB stars as well, but with subsolar metallicities (0.3-0.5x solar). Here we report on C and Si isotope and trace element (Zr, Ba) studies of individual, submicrometer-sized SiC grains. The most striking results are: (1) Zr and Ba concentrations are higher in Y and Z grains than in mainstream grains, with enrichments relative to Si and solar of up to 70x (Zr) and 170x (Ba), respectively. (2) For the Y and Z grains there is a positive correlation between Ba concentrations and amount of s-process Si. This correlation is well explained by predictions for 2-3 Msun AGB stars with metallicities of 0.3-0.5x solar. This confirms low-metallicity stars as most likely stellar sources for the Y and Z grains

Stress resistance in two colonial ascidians from the Irish Sea: the recent invader Didemnum vexillum is more tolerant to low salinity than the cosmopolitan Diplosoma listerianum

It is widely believed that non-native species possess a high tolerance toward abiotic stress that allows them to survive transport and to colonize new habitats. However, to date empirical support for this concept is scarce. We therefore tested whether Didemnum vexillum, which recently invaded the Irish Sea, is more tolerant to fluctuations in salinity than the closely related Diplosoma listerianum, a cosmopolitan species of wide ranging distribution. The two fouling organisms exhibit similar habitat requirements and co-occur in many hard-bottom sites along the coasts of Ireland and Wales. We collected adult specimen of both taxa in Holyhead Marina, Wales, UK, and exposed them to different levels of reduced salinity ranging from 10 to 27 in short-term laboratory experiments. In separate trials, we mimicked chronic (2 weeks) as well as episodic (2 h) low-salinity stress to account for the natural temporal variability in osmotic fluctuations. We compared the ascidians' performances in the face of stress and found that the invasive D. vexillum showed higher growth and survival under low salinities than the cosmopolitan D. listerianum. This indicates that D. vexillum is generally better in adapting to fluctuating ion concentrations. This competitive advantage may facilitate the establishment and spread of D. vexillum, especially if osmotic conditions in coastal habitats will be altered by future climate-change induced shifts in local precipitation regimes. Highlights ► We tested for differential stress tolerances in two colonial ascidians. ► Ascidian growth and survival was measured under chronic and episodic osmotic stress. ► The cosmopolitan Diplosoma listerianum was susceptible to osmotic stress. ► The recent invader Didemnum vexillum exhibited a high tolerance to osmotic stress. ► Tolerance to salinity changes may add to the high invasiveness in D. vexillum

Amphibole in alkaline basalts from intraplate settings: implications for the petrogenesis of alkaline lavas from the metasomatised lithospheric mantle

Kaersutite to pargasite phenocrysts from Tertiary alkali basalts (Rhon, Central European Province, Germany) yield new high-precision Ar-40/Ar-39 ages of 24.0-24.1 Ma. Major and trace element compositions demonstrate that these high-TiO2 (4-7 wt%) amphiboles are in equilibrium with their host rock. Chemically, these amphibole phenocrysts resemble amphibole from magmatic veins in upper mantle rocks but differ from disseminated amphibole from peridotite. Most amphiboles have similar isotope characteristics to their alkaline basaltic host rocks (Sr-87/Sr-86(24) = 0.7035-0.736, epsilon Nd-24 = +3.8-+4.0, Pb-206/Pb-204(24) = 19.21-19.37, Pb-207/Pb-204(24) = 15.58-15.62, Pb-208/Pb-204(24) = 38.95-39.16), but two samples show contrasting isotopic compositions (epsilon Nd-24 = -4.0 and -2.9; Pb-206/Pb-204(24) = 17.08 and 18.11; Pb-207/Pb-204(24) = 15.51 and 15.58; Pb-208/Pb-204(24) = 37.41 and 37.99), indicating involvement of an ancient crust-derived component during melting. The O isotopic composition of the amphibole phenocrysts ranges from 5.4 to 7.5 parts per thousand, reflecting O isotope heterogeneity of the upper mantle sources. The contrasting isotopic composition of amphibole and host rock pairs furthermore indicates that phenocrysts record the early stages of the volcanic history of the Rhon volcanic field on a regional scale and at a different depth within the lithospheric mantle. Temperature and pressure estimates range from 1,010 and 1,080 degrees C and 0.7 and 1.0 GPa and are compatible with the experimental results on the stability of amphibole in alkaline rocks derived from the upper mantle. Geochemistry of major and trace elements and isotopic compositions shows that igneous amphibole from alkali basalts may preserve isotope heterogeneities suggesting that they sample heterogeneous upper mantle lithologies on a small scale