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GSA Annual Meeting in Seattle, Washington, USA -
2017
Paper No. 3-3
Presentation Time: 8:45 AM
TRIPLE OXYGEN AND HYDROGEN ISOTOPES IN SYN-GLACIAL
HYDROTHERMALLY ALTERED ROCKS: COMPARISON BETWEEN
MODERN ROCKS OF ICELAND AND SNOWBALL EARTH AGE
ROCKS FROM THE BALTIC SHIELD
ZAKHAROV, David1, BINDEMAN, Ilya N.1, FRIÐLEIFSSON,
Guðmundur Ó.2 and REED, Mark3, (1)Earth Sciences, University of
Oregon, Eugene, OR 97403, (2)HS Orka HF, Reykjanesbær, 260,
Iceland, (3)Geological Sciences, University of Oregon, Eugene, OR
97403, davidz@uoregon.edu
Hydrothermally altered rocks can be a powerful tool in studying
environments of the deep past when other paleoclimate proxies are not
available. In this study we use δ18O, Δ17O and δD of ancient
hydrothermally altered rocks from 2.4-2.3 Ga rift zones of the Baltic
Shield that are contemporaneous with the early Paleoproterozoic
snowball Earth glaciations to infer stable isotopic composition of
meteoric water and seawater at the time. The novel parameter Δ17O is
used to reconstruct water-to-rock ratios and δ18O of meteoric water when
original oxygen and hydrogen isotopic equilibria are lost. Using the early
Paleoproterozoic subglacial hydrothermally altered rocks from the
Belomorian Belt of the Baltic Shield we show that meteoric water had
δ18O of about −35 ‰ VSMOW at low latitudes during the snowball Earth
glaciations. The contemporaneous komatiitic basalts of the Vetreny Belt,
Baltic Shield exhibit pervasive and well-preserved submarine alteration
by seawater that had δ18O ≈ 0 ‰. To test the reliability of our findings,
we compare the Paleoproterozoic rocks with modern hydrothermally
altered rocks from continental and submarine hydrothermal systems
using samples extracted from drill cores in Iceland (Krafla and
Reykjanes) and modern seafloor (hole ODP 504B). We use isotopic
equilibrium fractionation as well as Δ17O approach to compute isotopic
composition of hydrothermal fluid and compare them to directly
measured isotopic composition of the fluids. Samples from Geitafell
eroded volcano which exhibits pervasive hydrothermal alteration with
involvement of 5-6 Myr meteoric water were also analyzed for this study.
Hydrothermally altered rocks from Iceland serve as a close analogue for
the Paleoproterozoic rocks formed in rifting zones of the Baltic Shield
that operated under glacial ice and seawater during snowball Earth
glaciations. We search for snowball Earth age hydrothermally altered
rocks elsewhere in the world. The early Paleoproterozoic Scourie dikes
(δ18O as low as −2 ‰), Scotland are contemporaneous with the snowball
Earth glaciations and their low-δ18O signature could be related to
interaction between magma and glacial melt waters.