Transcript Isotopic compositional changes across space, time, and bulk rock

Isotopic Compositional Changes Across Space, Time,
and Bulk Rock Composition in the High Lava Plains
and Northwestern Basin and Range, Oregon
GSA 2009
abs. #224-5
Mark T. Ford
Richard Carlson
Anita L. Grunder
Oregon State University
[email protected]
Dept. of Terrestrial Magnetism
[email protected]
Oregon State University
[email protected]
HLP
NWBR
0
50
100 Miles
0
80
160 km
Overview:
 Volcanic episodes and estimated volumes
 Focus on the 12 Ma to Recent rhyolites
 Time-transgressive nature
 Bulk rock composition
 Isotope composition
 Implications of heat flux on petrogenesis in the HLP and NWBR
12 – 0 Ma HLP and NWBR volcanism
Volume estimate 2,000 km3 to 2,500 km3
Basalts < 20
Ma in gray
Rhyolites in
purple
Ash flow tuffs
in yellow
Age progression in rhyolites
HLP Rhyolite
• Volume declines in time
• Heightened activity 7-7.5
Ma, just after basalt pulse at
7.5-8 Ma (Jordan et al., 2004)
NWBR rhyolites
• not younger than ~5 Ma
One post-progression
rhyolite: Iron Mt.
Black ages - measured
Colored ages – interpolated:
2.89 Ma
Number
Comparison to suites: Cascades, SRP, Iceland
Tholeiitic vs. Calc-alkaline suites
Clearly separated on FeO – SiO2 diagram, except at highest silica
Tholeiitic vs. Calc-alkaline suites
Clearly separated on FeO – SiO2 diagram, except at highest silica
Can we use this to help separate NWBR and HLP samples?
FeO – SiO2 diagram from the study area
Nearly all NWBR are “Low FeO”, HLP is variable to high FeO
• High Fe/Si focused along a belt in the HLP
• Variability in composition to the East in the HLP
• All tuffs high Fe/Si, large-volume tuffs in East
Glass Buttes
Juniper Ridge
Within suite variation relative to FeO vs. SiO2
High Fe/Si
Zero line
Low Fe/Si
Within suite Fe/Si enrichment
High Fe/Si
Juniper Ridge and Glass Buttes
Fe-Si zero line
Low Fe/Si
1.2 Ma
0.7 Ma
30 km
What might this be telling us about the role of crust in making
the rhyolites – or about the thermal inputs into the system?
Lets examine isotopic systems to gain some insights…
Nd- and Sr-isotopic variations of the rhyolites
– some with elevated Sr isotopic ratios
143Nd/144Nd
crustal addition
87Sr/86Sr
(i)
Comparison to basalts
Some of elevated Sr ratios may be due to parental magmas with high ratios
143Nd/144Nd
crustal addition
87Sr/86Sr
(i)
Longitude vs. Sr isotopic ratios:
Will the real crustal signature please stand up
87Sr/86Sr
(i)
Crustal
addition
“Basalt-like”
West
East
Longitude
vs. 207Pb/204Pb correlation diagram
207Pb/204Pb
206Pb/204Pb
Pelagic sediments
or continental crust
206Pb/204Pb
vs. 207Pb/204Pb correlation diagram
207Pb/204Pb
206Pb/204Pb
Pelagic sediments
or continental crust
206Pb/204Pb
87Sr/86Sr
(i)
Magmatic d18O vs. 87Sr/86Sr correlation diagram
d18O
Matrix of crustal influence
HLP
NWBR
16
5
8 + Iron Mt
6
1 or more crustal signatures (Sr, Pb, O isotopes)
No crustal factors (potential fractionates)
Within the HLP:
high Fe/Si
low Fe/Si
1 or more crustal signatures
11
5
No crustal factors (potential fractionates)
5
3 + Iron Mt
Conclusions:
 HLP and NWBR are a single bimodal province with
time-transgressive rhyolitic volcanism from 12 Ma to
Recent
 NWBR rhyolites are dominantly low FeO/SiO2
 HLP rhyolites have more chemical diversity,
especially to the east with high FeO/SiO2 along the
axis of the plain
 Within suite temporal evolution to higher FeO/SiO2
and greater crustal contribution
 High heat flux creates a feedback in the crust that
yields both a more mafic crust and more crustal melt
in the HLP, including voluminous ignimbrites
Time for a short movie?…
Acknowledgements: NSF funding; Ilya Bindeman: Oxygen isotopes; Jenda Johnson: animation