18 research outputs found
Deformation of feldspar at greenschist facies conditions – the record of mylonitic pegmatites from the Pfunderer Mountains, Eastern Alps
Deformation microstructures of albitic plagioclase and K-feldspar were investigated in
mylonitic pegmatites from the Austroalpine basement south of the western
Tauern Window by polarized light microscopy, electron microscopy and electron
backscatter diffraction to evaluate feldspar deformation mechanisms at
greenschist facies conditions. The main mylonitic characteristics are
alternating almost monophase quartz and albite layers, surrounding
porphyroclasts of deformed feldspar and tourmaline. The dominant deformation
microstructures of K-feldspar porphyroclasts are intragranular fractures at a
high angle to the stretching lineation. The fractures are healed or sealed by
polyphase aggregates of albite, K-feldspar, quartz and mica, which also occur
along intragranular fractures of tourmaline and strain shadows around other
porphyroclasts. These polyphase aggregates indicate dissolution–precipitation
creep. K-feldspar porphyroclasts are partly replaced by albite characterized
by a cuspate interface. This replacement is interpreted to take place by
interface-coupled dissolution–precipitation driven by a solubility difference
between K-feldspar and albite. Albite porphyroclasts are replaced at
boundaries parallel to the foliation by fine-grained monophase albite
aggregates of small strain-free new grains mixed with deformed fragments.
Dislocation glide is indicated by bent and twinned albite porphyroclasts with
internal misorientation. An indication of effective dislocation climb with
dynamic recovery, for example, by the presence of subgrains, is systematically
missing. We interpret the grain size reduction of albite to be the result of
coupled dislocation glide and fracturing (low-temperature plasticity).
Subsequent growth is by a combination of strain-induced grain boundary
migration and formation of growth rims, resulting in an aspect ratio of albite
with the long axis within the foliation. This strain-induced replacement by
nucleation (associated dislocation glide and microfracturing) and subsequent
growth is suggested to result in the observed monophase albite layers,
probably together with granular flow. The associated quartz layers show
characteristics of dislocation creep by the presence of subgrains, undulatory
extinction and sutured grain boundaries. We identified two endmember matrix
microstructures: (i) alternating layers of a few hundred micrometres' width,
with isometric, fine-grained feldspar (on average 15 µm in diameter)
and coarse-grained quartz (a few hundred micrometres in diameter),
representing lower strain compared to (ii) alternating thin layers of some
tens of micrometres' width composed of fine-grained quartz (<20 µm in diameter)
and coarse elongated albite grains (long axis of a few tens of
micrometres) defining the foliation, respectively. Our observations indicate
that grain size reduction by strain-induced replacement of albite (associated
dislocation glide and microfracturing) followed by growth and granular flow
simultaneous with dislocation creep of quartz are playing the dominating role
in formation of the mylonitic microstructure.</p
The microstructural record of porphyroclasts and matrix of partly serpentinized peridotite mylonites – from brittle and crystal-plastic deformation to dissolution–precipitation creep
We present microfabrics in high-pressure,
metamorphic, partly serpentinized peridotite mylonites from the Voltri
Massif, in which porphyroclasts and matrix record independent deformation
events. The microfabrics are analysed using polarization microscopy and
electron microscopy (SEM/EBSD, EMP). The mylonites contain diopside and
olivine porphyroclasts originating from the mantle protolith embedded in a
fine-grained matrix consisting mainly of antigorite and minor olivine and
pyroxene. The porphyroclasts record brittle and crystal-plastic deformation
of the peridotite at upper-mantle conditions and differential stresses of a
few hundred MPa. After the peridotites became serpentinized, deformation
occurred mainly by dissolution–precipitation creep resulting in a pronounced
foliation of the antigorite matrix, crenulation cleavages and newly
precipitated olivine and pyroxene from the pore fluid at sites of dilation,
i.e. in strain shadows next to porphyroclasts and folded fine-grained
antigorite layers. Antigorite reveals a pronounced associated shape
preferred orientation (SPO) and crystallographic preferred orientation (CPO)
with the basal (001) cleavage plane oriented in the foliation plane. In
monomineralic antigorite aggregates at sites of stress concentration around
porphyroclasts, a characteristically reduced grain size and deflecting CPO
as well as sutured grain boundaries indicate also some contribution of
crystal-plastic deformation and grain-boundary migration of antigorite. In
contrast, the absence of any intragranular deformation features in newly
precipitated olivine in strain shadows reveals that stresses were not
sufficiently high to allow for significant dislocation creep of olivine at
conditions at which antigorite is stable. The porphyroclast microstructures
are not associated with the microstructures of the mylonitic matrix, but are
inherited from an independent earlier deformation. The porphyroclasts record
a high-stress deformation of the peridotite with dislocation creep of
olivine in the upper mantle probably related to rifting processes, whereas
the serpentinite matrix records dominantly dissolution–precipitation creep
and low stresses during subduction and exhumation
Short-wavelength undulatory extinction in quartz recording coseismic deformation in the middle crust – an experimental study
Deformation experiments are carried out on natural vein quartz in a modified
Griggs-type solid medium apparatus to explore the preservation potential of
microfabrics created by crystal-plastic deformation at high stress,
overprinted during subsequent creep at lower stress. A corresponding stress
history is expected for the upper plastosphere, where fault slip during an
earthquake causes quasi-instantaneous loading to high stress, followed by
stress relaxation. The question is whether evidence of crystal-plastic
deformation at high stress, hence an indicator of past seismic activity, can
still be identified in the microstructure after overprint by creep at lower
stresses. First, quartz samples are deformed at a temperature of
400 °C and constant strain rate of 10<sup>−4</sup> s<sup>−1</sup> ("kick"),
and then held at 900 to 1000 °C at residual stress ("creep"). In
quartz exclusively subject to high-stress deformation, lamellar domains of
slightly differing crystallographic orientation (misorientation angle
< 2°) and a few tens of micrometres wide occur. In the
transmission electron microscope (TEM), these areas show a high density of
tangled dislocations and cellular structures. After "kick and creep"
experiments, pronounced short-wavelength undulatory extinction (SWUE) is
observed in the polarization microscope. The wavelength of SWUE is up to
10 μm, with oscillatory misorientation of up to a few degrees. TEM
inspection reveals domains with high density of dislocations and differing
diffraction contrast bound by poorly ordered dislocation walls. Only zones
with exceptional damage generated during high-stress deformation are replaced
by small new grains with a diameter of about 10 to 20 μm, forming
strings of recrystallized grains. For large original grains showing SWUE, the
Schmid factor for basal ⟨ <i>a</i> ⟩ glide is found to be high. SWUE
is taken to reflect high-stress crystal-plastic deformation, the modified
microstructure being sufficiently stable to be recognized after subsequent
creep as an indicator of past seismic activity
Polycrystalline apatite synthesized by hydrothermal replacement of calcium carbonates
Aragonite and calcite single crystals can be readily transformed into polycrystalline hydroxyapatite pseudomorphs by hydrothermal treatment in a (NH4)2HPO4 solution. Scanning electron microscopy of the reaction products showed that the transformation of aragonite to apatite is characterised by the formation of a sharp interface between the two phases and by the development of intracrystalline porosity in the hydroxyapatite phase. In addition, electron backscattered diffraction (EBSD) imaging showed that the c-axis of apatite is predominantly oriented perpendicular to the reaction front with no crystallographic relationship to the aragonite lattice. However, the Ca isotopic composition of the parent aragonite, measured by thermal ionization mass spectrometry was inherited by the apatite product.Hydrothermal experiments conducted with use of phosphate solutions prepared with water enriched in 18O (97%) further revealed that the 18O from the solution is incorporated in the product apatite, as measured by micro-Raman spectroscopy. Monitoring the distribution of 18O with Raman spectroscopy was possible because the incorporation of 18O in the PO4 group of apatite generates four new Raman bands at 945.8, 932, 919.7 and 908.8cm-1, in addition to the ?1(PO4) symmetric stretching band of apatite located at 962cm-1, which can be assigned to four 18O-bearing PO4 species. The relative intensities of these bands reflect the 18O content in the PO4 group of the apatite product. By using equilibrated and non-equilibrated solutions, with respect to the 18O distribution between aqueous phosphate and water, we could show that the concentration of 18O in the apatite product is linked to the degree of 18O equilibration in the solution. The textural and chemical observations are indicative of a coupled mechanism of aragonite dissolution and apatite precipitation taking place at a moving reaction interface
Rare earth elements and Sm-Nd isotope redistribution in apatite and accessory minerals in retrogressed lower crust material (Bergen Arcs, Norway)
In the Bergen Arcs (Norway), Grenvillian granulite retrogressed under eclogite- and amphibolite-facies conditions during Caledonian subduction/collision offer a unique opportunity to investigate rare earth elements (REE) and Sm-Nd isotope redistribution in accessory minerals during fluid-assisted metamorphism. Our sampling targeted apatite-bearing REE-rich protoliths (mangerite and jotunite) that preserve distinct mineral assemblages, depending on the external fluid availability and metamorphic conditions. REE concentrations in apatite are the highest in the granulite. Two populations are present: magmatic apatite (Ap1) relics that occur as inclusions in ilmenite-hematite, and intergranular apatite (Ap2) formed under granulite-facies conditions. The presence of abundant needle-like monazite and sulphide inclusions in Ap2 indicate that granulite reactions were fluid assisted. A thin (typically < 10 μm) rim of REE-rich epidote (Ep1) commonly surrounds Ap2. In these accessory minerals, U and Th contents are too low, or grains are too small, for in situ U-Th-Pb dating. Sm-Nd isotope data of Ap2, monazite and Ep1 give an isochron age of 601 ± 69 Ma, which is interpreted to represent a partially reset Grenvillian age, affected by Caledonian fluid-assisted mineral growth. In amphibolitized samples, granulite Ap2 is replaced by apatite (Ap3) with lower REE contents and no monazite inclusions. The REE released by this replacement are redistributed in a corona of epidote group minerals (Ep2) surrounding Ap3. The in situ Sm-Nd isotope data for Ep2 and titanite, found in replacement of ilmenite-hematite, return an isochron age of 395 ± 65 Ma, recording the timing of amphibolite-facies mineral growth when fluids were introduced into the rock. In eclogitized samples, eclogitic apatite (Ap4) occurs as polycrystalline aggregates, suggesting for a complex replacement process during deformation. REE contents of Ap4 are low, as REE originally contained in the precursor apatite were redistributed mainly into zoisite. Apatite shielded as inclusions in ilmenite and garnet preserve the REE-rich signature of the initial magmatic (Ap1) and granulite (Ap2) apatite, indicating these grains did not undergo further re-equilibration during Caledonian metamorphism. The resistance of apatite to compositional re-equilibration in this case confirms the petrological potential of apatite inclusions shielded in chemically inert minerals to track early magmatic, or metamorphic, crystallisation stages.Emilie Janots, Håkon Austrheim, Carl Spandler, Johannes Hammerli, Claudia A. Trepmann, Jasper Berndt, Valérie Magnin, Anthony I.S. Kem
Evaluating the influence of meteorite impact events on global potassium feldspar availability to the atmosphere since 600 Ma
Potassium feldspar present in global mineral aerosol (30%. By combining crater size and tectonic reconstructions, we are able to provide a quantitative and self-consistent assessment of changes to global potassium feldspar availability. Considerable differences in potassium feldspar availability following meteorite impact events are revealed. Different impact events generated dust containing different amounts of potassium feldspar. Differing levels of influence upon climate are hypothesized, and should now be tested by looking at stratigraphic records of these events to reveal the sensitivity of climate to different dust mineralogy