3,652 research outputs found
Origin of Nepheline-normative High-K Ankaramites and the Evolution of Eastern Srednogorie Arc in SE Europe
Eastern Srednogorie is part of the Apuseni-Banat-Timok-Srednogorie magmatic belt in SE Europe, the main arc related to the Late Cretaceous subduction and closure of the Tethys Ocean between Africa and Europe. Extrusive and shallow intrusive magmatism in the Eastern Srednogorie is abundant and extremely diverse in composition, covering a wide range from ultramafic volcanic rocks to granites; this provides a unique opportunity to study processes of primitive melt formation and magma evolution in an arc environment. In contrast to other parts of the belt, relatively mafic lavas predominate here. Three magmatic regions are distinguished within Eastern Srednogorie from south to north: Strandzha, Yambol-Burgas and East Balkan. Systematic differences exist between these regions, notably the increased alkalinity of samples from the Yambol-Burgas region in the central part. All rocks display a clear subduction-like signature in their trace-element patterns, particularly the enrichment in large ion lithophile elements and light rare earth elements relative to high field strength elements. A distinct primitive nepheline-normative ankaramite magma type is recognized among the mafic volcanic rocks from the Yambol-Burgas region and melt inclusions entrapped in olivine and clinopyroxene from a cumulitic rock. Lower crustal clinopyroxene and amphibole cumulates carried to the surface as xenoliths in a mafic dike represent a possible source for the ankaramite. Modeling of the melting process suggests that low degrees of batch melting of a clinopyroxene-rich, amphibole-bearing source similar to the cumulate xenoliths at 1 GPa, temperatures of 1240-1300°C, oxidized conditions and a water content of 0·2 wt % reproduce accurately most of the observed major- and trace-element characteristics of the studied ankaramites. The elevated Rb, K2O, Th, Ba content and higher Pb isotope ratios of the predicted liquids compared with the ankaramites are explained by mixing of the ankaramite magma with lherzolite partial melts derived from the subduction-modified mantle wedge. Underplating of such mantle-derived magmas at the crust-mantle boundary in an extensional environment as a response to slab roll-back provides also the necessary heat to melt lower crustal cumulates. Fractional crystallization of mainly clinopyroxene plus olivine and Fe-Ti oxides in a deep (equivalent to 8 kbar pressure) magma chamber produced most of the observed range of shoshonitic basalts and basaltic andesites in Eastern Srednogorie. The more evolved intermediate varieties were probably formed by mixing and crystallization at lower temperatures in lower pressure magma chambers. Whole-rock Sr and Pb isotope compositions indicate variable degrees of admixing of basement rocks to generate the intermediate to acid Late Cretaceous magmas, but assimilation was minimal for magmas with less than 53 wt % SiO2. The proposed model for the evolution of the magmatism in Eastern Srednogorie involves initial formation of the calc-alkaline and high-K arc magmatism in the Strandzha and East Balkan regions, followed by roll-back induced intra-arc rifting and the formation of high-K, shoshonitic and ultra-high-K magmatism, including primitive ankaramites in the Yambol-Burgas regio
Adakite-like and Normal Arc Magmas: Distinct Fractionation Paths in the East Serbian Segment of the Balkan-Carpathian Arc
New age and whole-rock 87Sr/86Sr and 143Nd/144Nd isotopic data are used to assess petrogenetic and regional geodynamic processes associated with Late Cretaceous subvolcanic intrusions within the sparsely studied Timok Magmatic Complex (TMC) and Ridanj-Krepoljin Zone (RKZ) of eastern Serbia. The TMC and RKZ form part of the Apuseni-Banat-Timok-Srednogorie (ABTS) magmatic belt, a Cu-Au mineralized calc-alkaline magmatic arc related to closure of the Tethys Ocean that extends through Romania, Serbia, and Bulgaria in SE Europe. Zircon ages based on U-Pb laser ablation inductively coupled plasma mass spectrometry supplemented by existing isotope dilution thermal ionization mass spectrometry data respectively range from 89 to 79 Ma and from 76 to 71 Ma for the TMC and RKZ. This age pattern corresponds to cross-arc younging away from the European continent. Adakite-like trace element signatures (Y ≤18 ppm) are linked with samples that extend across the arc. These overlap in space and time with samples that conform to a normal arc differentiation trend. We performed energy-constrained assimilation-fractional crystallization (EC-AFC) modeling of Sr-La-Nd-Yb concentrations and Sr and Nd isotopic data. Results suggest that the two distinct fractionation trends may be explained in terms of a common mantle-derived parental magma but distinct fractionation and assimilation paths in the lower and upper crust. Petrogenesis of the adakite-like magmas is consistent with extensive high-pressure amphibole fractionation in the lower crust followed by ascent and plagioclase-dominant fractionation and assimilation in the upper crust. In contrast, normal arc signatures appear to have evolved exclusively via an upper-crustal differentiation process. Overall, our interpretation supports mantle wedge melting related to weak extension during progressive rollback of a subducting sla
High-K ankaramitic melt inclusions and lavas in the Upper Cretaceous Eastern Srednogorie continental arc, Bulgaria: Implications for the genesis of arc shoshonites
Application of precise 142Nd/144Nd analysis of small samples to inclusions in diamonds (Finsch, South Africa) and Hadean Zircons (Jack Hills, Western Australia)
146Sm-142Nd and 147Sm-143Nd systematics were investigated in garnet inclusions in diamonds from Finsch (S. Africa) and Hadean zircons from Jack Hills (W. Australia) to assess the potential of these systems as recorders of early Earth evolution. The stud
Split-grain 40Ar/39Ar dating : integrating temporal and geochemical data from crystal cargoes
NERC is acknowledged for continued funding of AIF at SUERC, East Kilbride. This work was supported by an ETH research grant (ETH-05 13-2) and funds from Swiss National Science Foundation research grants (SNSF 200021-146268 and SNSF 200021-155923/1) and US National Science Grant 1425491.Large sanidine crystals from the Mesa Falls Tuff (MFT), Yellowstone volcanic field, have been split and individually dated via high-precision 40Ar/39Ar geochronology with the undated portions further analysed for major elements, trace elements, Pb and Sr isotopes in the sanidine and trace elements in the melt inclusions. This allows the geochemical and geochronological identity of an individual sanidine to be combined. Our MFT sanidines return a preferred eruption age of 1.3011 ± 0.0015/0.0016 Ma (2-sigma, n = 56, MSWD 0.8, analytical/full external) with a significant component of subtly older (up to ~ 2 Ma) crystals. Combined with recent results (Rivera et al., 2016, Journal of Petrology 57, 9, 1677–1704) our data define a global mean sanidine 40Ar/39Ar age for the MFT of 1.3022 ± 0.0006/0.0008 Ma (2 sigma, analytical/full external) relative to Alder Creek sanidine at 1.1891 Ma and total λ40Ar 5.5305e-10, which gives RACsMFT: 1.09542 ± 0.00050. The ability to couple geochemistry and geochronology from a single grain allows us, for the first time, to evaluate the origin of the subtly older sanidines present in the same pumices as juvenile sanidines. Melt inclusions from all Mesa Falls sanidines represent extremely fractionated melts with low Sr contents (max. 12 ppm, n = 39), and rare earth element patterns which require that they be formed from an A-type magma rather than the preceding subduction-related Eocene volcanism as previously suggested. 87Sr/86Sr from juvenile and subtly older sanidines shows the same range of 0.7073 to 0.7096, illustrating the susceptibility of such low-Sr melts to slight degrees of assimilation. Pb isotopic compositions are more restricted and identical between the juvenile and subtly older sanidines in the Mesa Falls Tuff (207Pb/206Pb 0.900–0.903, 208Pb/206Pb 2.217–2.226, n = 83) and these compositions rule out the underlying Huckleberry Ridge Tuff member B as a potential source for the subtly older sanidine. LA-ICPMS 206Pb/238U dating of Mesa Falls zircons supports no role for the Huckleberry Ridge Tuff. Rather, these subtly older sanidines are interpreted as containing excess mantle-derived Ar. The ability to couple the geochemical and geochronological records within individual sanidine crystals that we demonstrate here has potential to provide new insights for a variety of petrological studies such as diffusional modelling.PostprintPeer reviewe
Resolving the timescales of magmatic and hydrothermal processes associated with porphyry deposit formation using zircon U–Pb petrochronology
Understanding the formation of economically important porphyry Cu–Au
deposits requires knowledge of the magmatic-to-hydrothermal processes
that act within the much larger magmatic system and the timescales on which
they occur. We apply high-precision zircon geochronology (chemical abrasion–isotope dilution–thermal ionisation mass spectrometry; CA–ID–TIMS) and
spatially resolved zircon geochemistry (laser ablation inductively coupled plasma mass
spectrometry; LA-ICP-MS) to constrain the magmatic
evolution of the underlying magma reservoir at the Pliocene Batu Hijau
porphyry Cu–Au deposit. We then use this extensive dataset to assess the
accuracy and precision of different U–Pb dating methods of the same zircon
crystals.
Emplacement of the oldest pre- to syn-ore tonalite (3.736±0.023 Ma)
and the youngest tonalite porphyry to cross-cut economic Cu–Au mineralisation
(3.646±0.022 Ma) is determined by the youngest zircon grain from
each sample, which constrains the duration of metal precipitation to fewer
than 90±32 kyr. Overlapping spectra of single zircon crystallisation
ages and their trace element distributions from the pre-, syn and post-ore
tonalite porphyries reveal protracted zircon crystallisation together with
apatite and plagioclase within the same magma reservoir over >300 kyr. The presented petrochronological data constrain a protracted early
>200 kyr interval of melt differentiation and cooling within a
large heterogeneous magma reservoir, followed by magma storage in a highly
crystalline state and chemical and thermal stability over several tens of thousands of years
during which fluid expulsion formed the ore deposit. Irregular trace element
systematics suggest magma recharge or underplating during this final short
time interval.
The comparison of high-precision CA–ID–TIMS results with in situ LA-ICP-MS
and a sensitive high-resolution ion microprobe (SHRIMP) U–Pb geochronology data from the same zircon grains allows a
comparison of the applicability of each technique as a tool to constrain
dates and rates on different geological timescales. All techniques provide
accurate dates but with different precision. Highly precise dates derived by
the calculation of the weighted mean and standard error of the mean of
the zircon dates obtained by in situ techniques can lead to ages of unclear
geological significance that are older than the maximum ages of emplacement
given by the CA–ID–TIMS ages of the youngest zircons in each sample. This
lack of accuracy of the weighted means is due to the protracted nature of
zircon crystallisation in upper crustal magma reservoirs, suggesting that
standard errors should not be used as a means to describe the uncertainty in
those circumstances. We conclude from this and similar published studies
that the succession of magma and fluid pulses forming a single porphyry
deposit and similarly rapid geological events are too fast to be reliably
resolved by in situ U–Pb geochronology and that assessing the tempo of ore
formation requires CA–ID–TIMS geochronology.</p
Colour reconnection in e+e- -> W+W- at sqrt(s) = 189 - 209 GeV
The effects of the final state interaction phenomenon known as colour
reconnection are investigated at centre-of-mass energies in the range sqrt(s) ~
189-209 GeV using the OPAL detector at LEP. Colour reconnection is expected to
affect observables based on charged particles in hadronic decays of W+W-.
Measurements of inclusive charged particle multiplicities, and of their angular
distribution with respect to the four jet axes of the events, are used to test
models of colour reconnection. The data are found to exclude extreme scenarios
of the Sjostrand-Khoze Type I (SK-I) model and are compatible with other
models, both with and without colour reconnection effects. In the context of
the SK-I model, the best agreement with data is obtained for a reconnection
probability of 37%. Assuming no colour reconnection, the charged particle
multiplicity in hadronically decaying W bosons is measured to be (nqqch) =
19.38+-0.05(stat.)+-0.08 (syst.).Comment: 30 pages, 9 figures, Submitted to Euro. Phys. J.
Determination of alpha_s using Jet Rates at LEP with the OPAL detector
Hadronic events produced in e+e- collisions by the LEP collider and recorded
by the OPAL detector were used to form distributions based on the number of
reconstructed jets. The data were collected between 1995 and 2000 and
correspond to energies of 91 GeV, 130-136 GeV and 161-209 GeV. The jet rates
were determined using four different jet-finding algorithms (Cone, JADE, Durham
and Cambridge). The differential two-jet rate and the average jet rate with the
Durham and Cambridge algorithms were used to measure alpha(s) in the LEP energy
range by fitting an expression in which order alpah_2s calculations were
matched to a NLLA prediction and fitted to the data. Combining the measurements
at different centre-of-mass energies, the value of alpha_s (Mz) was determined
to be
alpha(s)(Mz)=0.1177+-0.0006(stat.)+-0.0012$(expt.)+-0.0010(had.)+-0.0032(theo.)
\.Comment: 40 pages, 17 figures, Submitted to Euro. Phys. J.
Tests of model of color reconnection and a search for glueballs using gluon jets with a rapidity gap
Gluon jets with a mean energy of 22 GeV and purity of 95% are selected from
hadronic Z0 decay events produced in e+e- annihilations. A subsample of these
jets is identified which exhibits a large gap in the rapidity distribution of
particles within the jet. After imposing the requirement of a rapidity gap, the
gluon jet purity is 86%. These jets are observed to demonstrate a high degree
of sensitivity to the presence of color reconnection, i.e. higher order QCD
processes affecting the underlying color structure. We use our data to test
three QCD models which include a simulation of color reconnection: one in the
Ariadne Monte Carlo, one in the Herwig Monte Carlo, and the other by Rathsman
in the Pythia Monte Carlo. We find the Rathsman and Ariadne color reconnection
models can describe our gluon jet measurements only if very large values are
used for the cutoff parameters which serve to terminate the parton showers, and
that the description of inclusive Z0 data is significantly degraded in this
case. We conclude that color reconnection as implemented by these two models is
disfavored. The signal from the Herwig color reconnection model is less clear
and we do not obtain a definite conclusion concerning this model. In a separate
study, we follow recent theoretical suggestions and search for glueball-like
objects in the leading part of the gluon jets. No clear evidence is observed
for these objects.Comment: 42 pages, 18 figure
Search for right-handed W bosons in top quark decay
We present a measurement of the fraction f+ of right-handed W bosons produced
in top quark decays, based on a candidate sample of events in the
lepton+jets decay mode. These data correspond to an integrated luminosity of
230pb^-1, collected by the DO detector at the Fermilab Tevatron
Collider at sqrt(s)=1.96 TeV. We use a constrained fit to reconstruct the
kinematics of the and decay products, which allows for the
measurement of the leptonic decay angle for each event. By comparing
the distribution from the data with those for the expected
background and signal for various values of f+, we find
f+=0.00+-0.13(stat)+-0.07(syst). This measurement is consistent with the
standard model prediction of f+=3.6x10^-4.Comment: Submitted to Physical Review D Rapid Communications 7 pages, 3
figure
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