128 research outputs found
Compositional characteristics and spatial distribution of enriched Icelandic mantle components
Author Posting. © The Authors, 2010. This is the author's version of the work. It is posted here by permission of Oxford University Press for personal use, not for redistribution. The definitive version was published in Journal of Petrology 51 (2010): 1447-1475, doi:10.1093/petrology/egq025.We present compositional data on a suite of 18 primitive neovolcanic alkali basalts from three
flank zone regions in Iceland (Vestmannaeyjar in the south, Snæfell in the east, and Snæfellsnes
in the west) that are peripheral to the main rift zones that are dominated by tholeiitic basalts. This
study integrates He isotope data with radiogenic isotope data (Sr-Nd-Pb-Hf), stable isotope data
(δ18O), and trace element data to characterise the compositional features of the trace-elementenriched
components of the Icelandic mantle. We also present high-precision Pb isotope data on
an additional 57 lava samples from the flank zones (including Öræfajökull in the south-east) and
the Northern and Eastern rift zones. Most Icelandic lavas have negative Δ207Pb (–4 to –1), with
higher values (–1 to +4) found only in samples from Öræfajökull, Snæfell, and parts of the
Reykjanes Peninsula. At Snæfell, this EM1-type component is characterised by a low δ18Oolivine
signature (+4.1‰ to +4.6‰), moderate 206Pb/204Pb values (18.4-18.6) and MORB-like 3He/4He
(6.9-7.5 R/RA). Samples from Vestmannaeyjar and Snæfellsnes have mantle-like δ18Oolivine
(+4.9‰ to +5.0‰), and radiogenic 206Pb/204Pb values (18.9-19.3) that fall on the NHRL for
208Pb/204Pb (Δ208Pb –5 to +5). Compared to the Vestmannaeyjar lavas, Snæfellsnes lavas have
higher La/YbN (5-11 vs. 3-5), lower εNd (5.5-6.5 vs. 6.8-7.6) and lower 3He/4He (6.3-8.6 R/RA vs.
11.4-13.5 R/RA). Therefore, the most trace element enriched components in the Icelandic mantle
are not the carriers of the high 3He/4He values (> 15 R/RA) found in some lavas on Iceland and
the adjacent ridges, and instead are consistent with degassed, recycled components. Even after
excluding the EM1-type high Δ207Pb samples, high-precision Pb isotope data produce a kinked
array on an 206Pb/204Pb vs. 208Pb/204Pb plot, which is not consistent with simple binary mixing
between two end-members. This requires significant lateral heterogeneity within the Icelandic
mantle and the presence of more than just two compositionally-distinct local mixing end-member
components. Samples from each of the main axial rift zones define different trends. Despite the
tectonic continuity between the Northern Volcanic Zone and the Eastern Volcanic Zone, lavas
from these two rift zones define separate sub-parallel linear arrays. Lavas from the adjacent
Western Volcanic Zone and the Eastern Volcanic Zone define oblique linear arrays that converge
on a common local end-member that is not involved in the magmatism of the Northern Volcanic
Zone. Therefore, there is a distinct NE-SW compositional heterogeneity within the Icelandic
mantle.work was funded primarily by the Danish National Research
Foundation through a grant to the former Danish Lithosphere Centre, with additional funding
from the University of Iowa for the oxygen isotope analyses
Nonconventional amide bond formation catalysis: programming enzyme specificity with substrate mimetics
The evolution and storage of primitive melts in the Eastern Volcanic Zone of Iceland: the 10 ka Grímsvötn tephra series (i.e. the Saksunarvatn ash)
Major, trace and volatile elements were measured in a suite of primitive macrocrysts and melt inclusions from the thickest layer of the 10 ka Grímsvötn tephra series (i.e. Saksunarvatn ash) at Lake Hvítárvatn in central Iceland. In the absence of primitive tholeiitic eruptions (MgO > 7 wt.%) within the Eastern Volcanic Zone (EVZ) of Iceland, these crystal and inclusion compositions provide an important insight into magmatic processes in this volcanically productive region. Matrix glass compositions show strong similarities with glass compositions from the AD 1783–84 Laki eruption, confirming the affinity of the tephra series with the Grímsvötn volcanic system. Macrocrysts can be divided into a primitive assemblage of zoned macrocryst cores (An_78–An_92, Mg#_cpx = 82–87, Fo_79.5–Fo_87) and an evolved assemblage consisting of unzoned macrocrysts and the rims of zoned macrocrysts (An_60–An_68, Mg#_cpx = 71–78, Fo_70–Fo_76). Although the evolved assemblage is close to being in equilibrium with the matrix glass, trace element disequilibrium between primitive and evolved assemblages indicates that they were derived from different distributions of mantle melt compositions. Juxtaposition of disequilibrium assemblages probably occurred during disaggregation of incompatible trace element-depleted mushes (mean La/Yb_melt = 2.1) into aphyric and incompatible trace element-enriched liquids (La/Yb_melt = 3.6) shortly before the growth of the evolved macrocryst assemblage. Post-entrapment modification of plagioclase-hosted melt inclusions has been minimal and high-Mg# inclusions record differentiation and mixing of compositionally variable mantle melts that are amongst the most primitive liquids known from the EVZ. Coupled high field strength element (HFSE) depletion and incompatible trace element enrichment in a subset of primitive plagioclase-hosted melt inclusions can be accounted for by inclusion formation following plagioclase dissolution driven by interaction with plagioclase-undersaturated melts. Thermobarometric calculations indicate that final crystal-melt equilibration within the evolved assemblage occurred at ~1140°C and 0.0–1.5 kbar. Considering the large volume of the erupted tephra and textural evidence for rapid crystallisation of the evolved assemblage, 0.0–1.5 kbar is considered unlikely to represent a pressure of long-term magma accumulation and storage. Multiple thermometers indicate that the primitive assemblage crystallised at high temperatures of 1240–1300°C. Different barometers, however, return markedly different crystallisation depth estimates. Raw clinopyroxene-melt pressures of 5.5–7.5 kbar conflict with apparent melt inclusion entrapment pressures of 1.4 kbar. After applying a correction derived from published experimental data, clinopyroxene-melt equilibria return mid-crustal pressures of 4±1.5 kbar, which are consistent with pressures estimated from the major element content of primitive melt inclusions. Long-term storage of primitive magmas in the mid-crust implies that low CO_2 concentrations measured in primitive plagioclase-hosted inclusions (262–800 ppm) result from post-entrapment CO_2 loss during transport through the shallow crust. In order to reconstruct basaltic plumbing system geometries from petrological data with greater confidence, mineral-melt equilibrium models require refinement at pressures of magma storage in Iceland. Further basalt phase equilibria experiments are thus needed within the crucial 1–7 kbar range.D.A.N. was supported by a Natural Environment Research Council studentship (NE/1528277/1) at the start of this project. SIMS analyses were supported by Natural Environment Research Council Ion Microprobe Facility award (IMF508/1013).This is the final version of the article. It first appeared from Springer via http://dx.doi.org/10.1007/s00410-015-1170-
Transcript expression plasticity as a response to alternative larval host plants in the speciation process of corn and rice strains of Spodoptera frugiperda
The evolution and storage of primitive melts in the Eastern Volcanic Zone of Iceland: the 10 ka Grímsvötn tephra series (i.e. the Saksunarvatn ash)
Carboxypeptidase S-1 from Penicillium janthinellum: Enzymatic properties in hydrolysis and aminolysis reactions
Determination of C-terminal sequences by digestion with serine carboxypeptidases: The influence of enzyme specificity
Chemical modifications of a cysteinyl residue introduced in the binding site of carboxypeptidase Y by site-directed mutagenesis
- …