35 research outputs found
Grain boundary partitioning of Ar and He
An experimental procedure has been developed that permits measurement of the partitioning of Ar and He between crystal interiors and the intergranular medium (ITM) that surrounds them in synthetic melt-free polycrystalline diopside aggregates. ^(37)Ar and ^(4)He are introduced into the samples via neutron irradiation. As samples are crystallized under sub-solidus conditions from a pure diopside glass in a piston cylinder apparatus, noble gases diffusively equilibrate between the evolving crystal and intergranular reservoirs. After equilibration, ITM Ar and He is distinguished from that incorporated within the crystals by means of step heating analysis. An apparent equilibrium state (i.e., constant partitioning) is reached after about 20 h in the 1450 °C experiments. Data for longer durations show a systematic trend of decreasing ITM Ar (and He) with decreasing grain boundary (GB) interfacial area as would be predicted for partitioning
controlled by the network of planar grain boundaries (as opposed to ITM gases distributed in discrete micro-bubbles or melt).
These data yield values of GB-area-normalized partitioning, KÂŻ^(Ar)_(ITM), with units of (Ar/m^3 of solid)/(Ar/m^2 of GB) of 6.8 x 10^3 â 2.4 x 104 m^(-1). Combined petrographic microscope, SEM, and limited TEM observation showed no evidence that a residual glass phase or grain boundary micro-bubbles dominated the ITM, though they may represent minor components. If a nominal GB thickness (ÎŽ) is assumed, and if the density of crystals and the grain boundaries are assumed equal, then a true grain boundary partition coefficient (K^(Ar)_(GB) = X^(Ar)_(crystals)/X^(Ar)_(GB) may be determined. For reasonable values of ÎŽ, K^(Ar)_(GB) is at least an order of magnitude lower than the Ar partition coefficient between diopside and melt. Helium partitioning data provide a less robust constraint with KÂŻ^(He)_(ITM) between 4 x 10^3 and 4 x 10^4 cm^(-1), similar to the Ar partitioning data. These data suggest that an ITM consisting of nominally melt free, bubble free, tight grain boundaries can constitute a significant but not infinite reservoir, and therefore bulk transport pathway, for noble gases in fine grained portions of the crust and mantle where aqueous or melt fluids are non-wetting and of very low abundance (i.e., <0.1% fluid). Heterogeneities in grain size within dry equilibrated systems will correspond to significant differences in bulk rock noble gas content
Refined Tectonic Evolution of the Betic-Rif Orogen Through Integrated 3-D Microstructural Analysis and Sm-Nd Dating of Garnet Porphyroblasts
Acknowledgments
The authors would like to thank Ăngel PerandrĂ©s-Villegas for preparing the thin sections used for this study, FĂĄtima Linares Ordóñez for X-ray computed micro-tomography scanning, Mike Tappa for assistance with thermal ionization mass spectrometry analysis, Francisco Alonso-Chaves and Fernando Simancas for sending structural data, Whitney Behr, Johannes Glodny, Sean Mulcahy, and an anonymous reviewer for providing helpful reports, and editors Laurent Jolivet and Federico Rossetti for additional comments that helped improve two earlier manuscript versions. EFB gratefully acknowledges support from the NSF Grants EAR-1250497 and PIRE-1545903 as well as start up funds from the Boston College. DA and ARF gratefully acknowledge financial support through Spanish project âDAMAGEâ (CGL2016-80687-R AEI/FEDER), and Junta de AndalucĂa project RNM148, both directed by JesĂșs Galindo Zaldivar. ARF acknowledges an PhD Grant (FPU) from the Spanish government. Open access fees have been funded by Universidad de Granada /CBUA.High-resolution microstructural analysis of porphyroblast inclusion trails integrated with Sm-Nd
garnet geochronology has provided new insight into the tectonic history of the Betic-Rif orogen. Three principal
age groups of porphyroblasts are demonstrated with distinctly oriented inclusion-trails. Inclusion-trail curvature
axes or âFIAâ (Foliation Inflexion/Intersection Axes) are shown to represent âfossilizedâ crenulation axes
from which a succession of different crustal shortening directions can be deduced. The regional consistency
of microstructural orientations and their geometric relationship with multiple sets of macroscopic folds
reveal the composite character of the Gibraltar Arc formed by a superposition of different folding directions
and associated lineations. Bulk-garnet ages of 35â22 Ma obtained from five micaschist samples of the
Alpujarride-Sebtide complex (ASC) and of 35â13 Ma from four micaschists of the Nevado-Filabride complex
(NFC) allow to deduce NNE-SSW directed shortening in the Late Eocene changing to NW-SE shortening in
the early Oligocene, alternating with suborthogonal NE-SW shortening during the Miocene. These directions
can be related to a major swing in the direction of relative Africa-Iberia plate-motion known from kinematic
modeling of magnetic seafloor anomalies, and subsequent dynamic interference between plate convergence and
suborthogonal âtectonic escapeâ of the Alboran Domain. Coupled to previously established P-T-t paths, the
new garnet ages support a common tectono-metamorphic evolution of the ASC and NFC as laterally equivalent
orogenic domains until, in the Miocene, the second became re-buried under the first.NSF Grant EAR-1250497PIRE-1545903Spanish project âDAMAGEâ (CGL2016-80687-R AEI/FEDER)Junta de AndalucĂa project RNM148PhD Grant (FPU) from the Spanish governmentOpen access fees have been funded by Universidad de Granada /CBU
Rapid development of spiral garnets during subduction zone metamorphism revealed from high-resolution Sm-Nd garnet geochronology
Multiple studies have applied zoned garnet geochronology to place temporal constraints on
the rates of metamorphism and deformation during orogenesis. We report new high-resolution
isotope dilutionâthermal ionization mass spectrometry Sm-Nd isochron ages on concentric
growth zones from microstructurally and thermodynamically characterized garnets from
the Betic Cordillera, southern Spain. Our ages for the garnet core (13.64 ± 0.31 Ma), mantle
(13.41 ± 0.37 Ma), and rim (13.34 ± 0.45 Ma) indicate rapid garnet growth and are consistent
with published garnet ages interpreted to reflect high-pressure metamorphism in the
region. Thermodynamic analysis indicates garnets grew during subduction at âŒ1.5â2.0 GPa
and 570â600 °C. The core to rim duration of spiral garnet growth was just a few hundred
thousand years. While other zoned garnet studies have shown similar rapid growth in subduction
zone settings, this is the first documentation of such rapid growth of a spiral garnet.
Combining this garnet growth duration with the magnitude of spiral inclusion trail curvature,
we compute a strain rate of âŒ10â13 sâ1, an order of magnitude faster than all previous spiral
garnet studies. We interpret that these spiral garnets recorded a rapid pulse of deformation
and strain during the final stages of subduction and incipient exhumation.Spanish grants CGL2015â65602-R (AEI-FEDER), P18-RT-3275, and B-RNM-301-UGR18 (Junta de AndaucĂa/FEDER)U.S. National Science Foundation grants PIRE-1545903 and EAR-194665
Release of oxidizing fluids in subduction zones recorded by iron isotope zonation in garnet
Subduction zones are key regions of chemical and mass transfer between the Earthâs surface and mantle. During subduction, oxidized material is carried into the mantle and large amounts of water are released due to the breakdown of hydrous minerals such as lawsonite. Dehydration accompanied by the release of oxidizing species may play a key role in controlling redox changes in the subducting slab and overlying mantle wedge. Here we present measurements of oxygen fugacity, using garnetâepidote oxybarometry, together with analyses of the stable iron isotope composition of zoned garnets from Sifnos, Greece. We find that the garnet interiors grew under relatively oxidized conditions whereas garnet rims record more reduced conditions. Garnet ÎŽ56Fe increases from core to rim as the system becomes more reduced. Thermodynamic analysis shows that this change from relatively oxidized to more reduced conditions occurred during lawsonite dehydration. We conclude that the garnets maintain a record of progressive dehydration and that the residual mineral assemblages within the slab became more reduced during progressive subduction-zone dehydration. This is consistent with the hypothesis that lawsonite dehydration accompanied by the release of oxidizing species, such as sulfate, plays an important and measurable role in the global redox budget and contributes to sub-arc mantle oxidation in subduction zones
Evaluating the Effects of SARS-CoV-2 Spike Mutation D614G on Transmissibility and Pathogenicity.
Global dispersal and increasing frequency of the SARS-CoV-2 spike protein variant D614G are suggestive of a selective advantage but may also be due to a random founder effect. We investigate the hypothesis for positive selection of spike D614G in the United Kingdom using more than 25,000 whole genome SARS-CoV-2 sequences. Despite the availability of a large dataset, well represented by both spike 614 variants, not all approaches showed a conclusive signal of positive selection. Population genetic analysis indicates that 614G increases in frequency relative to 614D in a manner consistent with a selective advantage. We do not find any indication that patients infected with the spike 614G variant have higher COVID-19 mortality or clinical severity, but 614G is associated with higher viral load and younger age of patients. Significant differences in growth and size of 614G phylogenetic clusters indicate a need for continued study of this variant
Evaluating the Effects of SARS-CoV-2 Spike Mutation D614G on Transmissibility and Pathogenicity
Global dispersal and increasing frequency of the SARS-CoV-2 spike protein variant D614G are suggestive of a selective advantage but may also be due to a random founder effect. We investigate the hypothesis for positive selection of spike D614G in the United Kingdom using more than 25,000 whole genome SARS-CoV-2 sequences. Despite the availability of a large dataset, well represented by both spike 614 variants, not all approaches showed a conclusive signal of positive selection. Population genetic analysis indicates that 614G increases in frequency relative to 614D in a manner consistent with a selective advantage. We do not find any indication that patients infected with the spike 614G variant have higher COVID-19 mortality or clinical severity, but 614G is associated with higher viral load and younger age of patients. Significant differences in growth and size of 614G phylogenetic clusters indicate a need for continued study of this variant
Can metamorphic reactions proceed faster than bulk strain?
Available constraints on metamorphic reaction rates derived from the study of natural systems are similar to, or slightly lower than, the bulk strain rates measured in the same rocks. Here, we explore whether this apparent relationship is merely coincidence or due to a more fundamental mechanistic link between reaction and strain. Grain boundary migration accommodated dislocation creep (GBMDC) or grain boundary diffusion creep (GBDC) (i.e. pressure solution), both of which involve dissolution-precipitation as we define it, will occur simultaneously with mineral reactions involving dissolution-precipitation in the presence of a non-zero deviatoric stress. The exact relationships between reaction and strain are different depending on whether GBMDC or GBDC is controlling strain, but the mechanistic link exists in both cases. We present theoretical arguments which show that bulk strain by GBMDC or GBDC, which may additionally be accommodated by processes not involving dissolution-precipitation, such as dislocation glide and climb or grain boundary sliding, should in most cases be somewhat faster than the bulk reaction rates as observed. With few exceptions, for natural metamorphic systems undergoing plastic deformation, strain rates provide an upper limit for bulk reaction rates occurring simultaneously in the same rocks. The data suggest that mineral reaction rates may typically be within one order of magnitude of the strain rate
Can metamorphic reactions proceed faster than bulk strain?
Available constraints on metamorphic reaction rates derived from the study of natural systems are similar to, or slightly lower than, the bulk strain rates measured in the same rocks. Here, we explore whether this apparent relationship is merely coincidence or due to a more fundamental mechanistic link between reaction and strain. Grain boundary migration accommodated dislocation creep (GBMDC) or grain boundary diffusion creep (GBDC) (i.e. pressure solution), both of which involve dissolution-precipitation as we define it, will occur simultaneously with mineral reactions involving dissolution-precipitation in the presence of a non-zero deviatoric stress. The exact relationships between reaction and strain are different depending on whether GBMDC or GBDC is controlling strain, but the mechanistic link exists in both cases. We present theoretical arguments which show that bulk strain by GBMDC or GBDC, which may additionally be accommodated by processes not involving dissolution-precipitation, such as dislocation glide and climb or grain boundary sliding, should in most cases be somewhat faster than the bulk reaction rates as observed. With few exceptions, for natural metamorphic systems undergoing plastic deformation, strain rates provide an upper limit for bulk reaction rates occurring simultaneously in the same rocks. The data suggest that mineral reaction rates may typically be within one order of magnitude of the strain rate