|
|
William Peck
Associate Professor of Geology
Research in the Adirondacks
Oxygen isotope thermometry in the Adirondack Highlands
Supported by grant EAR-0106890 from the National Science Foundation
|
The Adirondack Highlands are an exposed section of the mid-crust that dates
from 1.0-1.3 Billion years ago. This study focuses on quantifying the
metamorphic conditions that these rocks experienced by using the fractionation
of oxygen isotopes between garnet and quartz in quartzites.
The results of oxygen isotope thermometry yields metamorphic temperatures
700-800°C, consistent with granulite-facies mineral assemblages. Samples
from the Irving Pond quartzite record D(Qtz-Grt)=2.58±0.58‰, corresponding
to peak metamorphic conditions of 733±37°C. This agrees well with some new
estimates from garnet-biotite exchange thermometry, and similar quartz-garnet
temperatures are obtained from three other localities on the periphery of the
Adirondack Highlands. All of these temperatures are higher than previous
regional temperature estimates. Peak metamorphic temperatures are preserved
in rocks with varying quartz:feldspar ratios and garnet sizes. Similar
fractionations from all of these slowly-cooled rocks suggest slow rates
of oxygen diffusion in quartz, feldspar, and garnet, consistent with the
results of anhydrous diffusion experiments and low water fugacities in
Adirondack quartzites. These results suggest low water activity in most
(if not all) samples of this suite during cooling after metamorphism.
Related References:
Peck, WH, and Valley, JW, 2004, Garnet-quartz oxygen isotope thermometry in the
southern Adirondack Highlands (Grenville Province, New York): Journal of
Metamorphic Geology, v. 22, p. 763-773
Peck, WH, and Valley, JW, 2003 Oxygen isotope thermometry of quartzites,
southern Adirondack Highlands, Geological Society of America Abstracts
with Programs, v.35, n. 7., p. 592.
Peck, WH, Valley, JW, and Graham, CM, 2003, Slow oxygen diffusion rates in
igneous zircons from metamorphic rocks: American Mineralogist, v. 88, p.
1003-1014.
|
|
|
|
Temperatures determined in this study, and regional
isotherms from previous work.
|
|
|
|
Quartzite outcrops on the shore of Irving Pond, southern Adirondacks
|
|
|
