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| Titel |
Solid inclusion thermobarometry under fire: Heating experiments on encapsulated quartz inclusions in garnet |
| VerfasserIn |
Kyle T. Ashley, Matthew Steele-MacInnis, Robert J. Bodnar, Robert S. Darling |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250122636
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| Publikation (Nr.) |
 EGU/EGU2016-1725.pdf |
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| Zusammenfassung |
| Internal pressures of mineral inclusions (Pincl) result from differences in elastic properties of
the inclusion mineral and its host mineral. Recent studies utilize pressure-sensitive Raman
spectroscopic waveshifts to determine the retained Pincl and apply elastic theory to estimate
pressure (or temperature) conditions of entrapment. Quartz inclusions are commonly utilized
because quartz is a “soft,” compressible mineral that is ubiquitous as an inclusion phase
in continental metamorphic rocks. Garnet is a commonly used host because it is
rigid and isotropic. Quartz inclusions trapped in garnet at high-P, low-T conditions
will retain high Pincl; whereas those trapped at low-P, high-T conditions yield
negative waveshifts equating to “negative” pressure, or net tensile stress on the
inclusion.
While Pincl can be accurately calculated from Raman data, barometry relies also upon
the quality of the elastic model, which fundamentally depends on the quality of the
P −V −T equations of state (EOS) applied. For quartz, EOS modeling is challenging due to
the spontaneous strain that develops close to the lambda transition. In this study we conduct
heating experiments on quartz inclusions in garnet from natural samples to assess the
response of inclusion pressure to varying temperature (at ambient external pressure), and to
evaluate predictions based on commonly applied EOS. Experiments were conducted
on two quartz standards (a Herkimer “diamond” and Brazillian quartz) and four
completely encapsulated inclusions of quartz in garnet from three tectonically diverse
terranes, including: (i) a dilated quartz inclusion (Pincl = -4.3 kbar) from Port Leyden,
Adirondack Mountains, New York, (ii) a Barrovian-sequence quartz inclusion from
northern Scotland (Pincl = 3.1 kbar), and (iii) two high-pressure (blueschist) quartz
inclusions from Sifnos, Greece (Pincl = 7.7 and 8.9 kbar). Standards were heated
in 25 ˚ C increments with smaller increments near the lambda transition. Quartz
inclusions were heated in 50 ˚ C increments. The standards were used to correct for
thermal perturbations to the Δν464 Raman band for quartz upon heating, allowing for
determination of Pincl (Δν464,measured = Δν464,heating + Δν464,pressure). Measurements
were taken upon cooling to test for irreversible plastic deformation (which was not
observed).
All samples exhibited a decrease in Δν464 upon heating. The dilated inclusions from
Port Leyden experienced the largest increase in Pincl (>2 kbar from 25 ˚ C to
500 ˚ C). The inclusion from Scotland developed moderate Pincl increase when
heated to 500 ˚ C (∼1.4 kbar), whereas the Sifnos inclusion experienced little to
no change in Pincl upon heating. These results reflect the anomalous increase in
thermal expansivity of quartz near the lambda transition. For the low-P samples, the
increased thermal expansivity during heating results in an increase in pressure as the
inclusion expands more than the void space. For the Sifnos sample, the void space and
inclusion expand by nearly the same amount, resulting in no additional pressurization.
While the Pincl − T trends match predictions, the magnitude of Pincl increase
is notably less than that predicted by numerical modeling. These results suggest
that improvements in P − V − T EOS or more sophisticated elastic models are
required to optimize quartz inclusion barometry for formation pressure constraints. |
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