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Formation
of Low-d18O Rhyolites After
Caldera Collapse at Yellowstone, Wyoming, USA
Bindeman, Ilya N.
and Valley, John
W., (2000) Formation of low-d18O
rhyolites after caldera collapse at Yellowstone, Wyoming, USA. Geology,
v. 28, n. 8, p. 719-722
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Abstract
We present a new model for the genesis of low-d18O
rhyolites of the Yellowstone caldera based on analyses of zircons and
individual quartz phenocrysts. Low-d18O
rhyolites were erupted soon after the massive caldera-forming Lava Creek
Tuff eruption (602 k.y., ~1000 km3) and contain xenocrysts of quartz and
zircon, inherited from precaldera rhyolites. These zircons are isotopically
zoned and out of equilibrium with their host low-d18O
melts, and quartz. Diffusion modeling predicts that magmatic disequilibria
of oxygen isotopes persists for up to tens of thousands of years following
nearly total remelting of the hydrothermally altered igneous roots of
the depressed cauldron, in which the alteration resistant quartz and zircon
initially retained their d18O
values. These results link melting to caldera collapse, rule out rapid
or catastrophic magma-meteoric water interaction, and indicate wholesale
melting rather than assimilation or partial melting.
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Figure
1. Oxygen isotope ratios of larger and smaller zircons,
measured compositions of abraded cores and calculated compositions
of rims in low-d18O rhyolites
from Yellowstone Caldera, Wyoming. Boxes show ranges of quartz d18O
values. These data show (1) variablity of d18O
values among individual quartz phenocrysts, especially for low-d18O
lavas erupted after LCT at 550-450 k.y.; (2) variability of different
size zircons and air abraded cores; (3) disequilibria between average
quartz and zircon in most low-d18O
lavas. LCT - Lava Creek Tuff; CF - Canyon Flow, DRF -Dunraven Road
flow, SBBF, MBBF and NBBF are South, Middle and North Biscuit Basin
flows, SCLF - Scaup Lake flow, MLF - Mallard Lake flow, WYF- West
Yellowstone flow; ACF - Aster Creek flow. Ages are from Gansecki
et al. (1996, 1998) and Obradovich (1992). Plotted ages of post-LCT
lavas are not exact to prevent overlap of data points.
Note,
that oxygen isotopes permit making of stratigraphic distinctions.
Our results show that three separate outcrops (hills), southern,
middle, and northern, along Firehole River of the Upper Geyser Basin,
defined earlier as a single unit of Biscuit Basin flow represent
three independent lava flows, which have distinctly different isotope
compositions of phenocrysts. We hereby call them South, Middle and
North Biscuit Basin flows. Only Middle Biscuit Basin was sampled
by Hildreth et al. (1984). NBBF and SBBF ages were not determined. |
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Figure 2.
Zoning profile of d18O
in 105 µm zircons (52.5 µm radius) for magma residence times in Middle
Biscuit Basin flow based on successive air abrasions (thick curve).
Left-oriented horizontal marks on vertical lines at 40 50 µm
are measured core compositions; horizontal marks at the bottom are
the rim composition. Rim composition was calculated by mass balance
using measured d18O of
core, d18O of pre-abrasion
bulk, and percentage of abrasion (% of material removed). Thin curves
indicate calculated diffusion profiles (see text). Numbers in per
mil are d18O of the whole
grain, calculated (italic) and measured (bold). Stripped box is the
measured compositional range of host obsidian. Notice the steep isotopic
gradients predicted for residence times of 500-10 000 yr. Compositions
of rims, based on analyses of prismatic faces of four individual zircon
crystals by the ion microprobe, are shown as filled circle (±1s
). d18O values of measured
and predicted rims are 2 to 4 lower than those of the
bulk of zircon, the abraded cores and the smallest measured zircons,
but the rims are in equilibrium with the host obsidian (see Fig. 1). |
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Figure 3.
a: Calculations of isotopic exchange of normal zircon (d18O
=+4.0, thick lines) and quartz (+6.0, dotted lines) crystals
(sizes are diameters) immersed into a low-d18O
melt (0.0) predict a period of quartz-zircon disequilibrium
for residence times up to 50 k.y. b: Up to 5.5 disequilibrium
in D (Qz-Zrc) exists during the time
intervals up to 50 k.y. See Figure 1 for unit abbreviations and text
for discussion. Each core is the innermost 50% of crystal's radius.
Overgrowth implies constant rate of deposition of new rim (d18O
= -1) in equilibrium with melt at rate of 10-15cm/s (Watson,
1996). |
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Figure
4. Model of genesis of low-d18O
rocks by total fusion of hydrothermally altered caldera roots after
caldera collapse. Quartz and zircon were not altered by hydrothermal
fluids and retained precaldera higher-d18O
values.
Purely conductive
heating of D T=200-300 ° C of
the depressed cauldron with vertical dimension of ~500m, proceeding
at thermal diffusivity of 0.005 cm2/s would yield a heating on the
order of 10-2 degrees per yr (Carslaw and Jagger, 1959). At this
heating rate from Tsolidus to Tmax (700°
to 850 ° C) zircon growth rate is on the order of 10-16 to
10-17 cm/s (Watson, 1996). If zircon saturation temperature is exceeded
at higher temperatures (e.g., T~850 ° C), zircon dissolution
may proceed at <10-15 cm/s, followed by crystallization of overgrowth
during eruptive cooling. Given the short time scale involved (a
few hundreds to a few thousands of years, based on the diffusive
exchange alone) both overgrowth and dissolution are expected to
play a secondary role in oxygen exchange. |
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