Transcript Cenozoic Basaltic Volcanism in the Pacific Northwest

Cenozoic Basaltic Volcanism in the Pacific Northwest
Richard W. Carlson
DTM, Carnegie Institution of Washington
William K. Hart
Miami University
Timothy L. Grove
Massachusetts Institute of Technology
EAR-CD
A Long-Lived Volcanic Margin
Did not Start or Stop with the Flood Basalt Episode
Miocene and Younger
Eocene to Jurassic
Modified from Geologic Map of the United States by Phillip King and Helen Beikman with HTML implementation by William Menke
What is Causing all this Volcanism?
Subduction?
Colored Regions Show Volcanic Deposits Younger than 17 Million Years
(Smith and Luedke, 1984)
Cascades
Washington
< 5 5-10 10-17 Ma
Basalt
Andesite
Rhyolite Dacite
Montana
Idaho
Oregon
Wyoming
California
Utah
Nevada
Diamond Craters
Arizona
Newberry
What is Causing all this Volcanism?
The Yellowstone Plume?
Colored Regions Show Volcanic Deposits Younger than 17 Million Years
(Smith and Luedke, 1984)
Washington
< 5 5-10 10-17 Ma
Basalt
Andesite
Rhyolite Dacite
Montana
Idaho
Oregon
Wyoming
Plume-like characteristics of
SRP volcanism
1) Time progressive volcanism
2) Large volume volcanism
3) High 3He/4He
4) Topographic swell
5) Positive gravity anomaly
6) Enhanced seismicity
7) Seismically imaged plume conduit?
California
Utah
Nevada
Arizona
Yellowstone
Compositional Variation in Space and Time
Time Slices:
1) 0-14 Ma – Modern Cascades, HLP, SRP
2) 14-18 Ma – Flood basalt era
3) 19 – 195 Ma – Pre-flood basalt era
Modified from Geologic Map of the United States by Phillip King and Helen Beikman with HTML implementation by William Menke
Compositional Variation in Space and Time – Rock Type
Data Compiled
by NAVDAT
HLP
HLP
HLP
Compositional Variation in Space and Time – Mantle Input
Primitive
Basalts
8%<MgO<12%
HLP
Data Compiled
by NAVDAT
Compositional Variation in Space and Time - Subduction Contribution
HLP
Primitive
Basalts
8%<MgO<12%
Data
Compiled
by NAVDAT
Compositional Variation - Lithosphere Input
Open
C
a
s
c
a
d
e
s
Open > 70% SiO2
Filled < 56% SiO2
HLP
Precambrian
North America
Data compiled
by NAVDAT
Crustal Input Cannot be Neglected!
Steens, Picture Gorge
FC
AFC
Mantle
Grande Ronde
Crust
Carlson et al., 1981
What Causes the Lithophile Isotope Variation?
1)Subducted PNW offshore sediments
2)Archean lithosphere (mantle or crust?)
Data sources: Hawaii, Juan de Fuca, Cascades, HLP - EarthChem;
HAOT - Hart, Carlson; Newberry - Carlson, Grove, Donnelly-Nolan
The Subduction Contribution
87Sr/86Sr
87Sr/86Sr
= 0.70317
= 0.1518
187Os/188Os
87Sr/86Sr
= 0.70382
= 0.1922
187Os/188Os
= 0.703751
Helium
The Clearest (Only?) Chemical Signal of Deep Mantle Involvement
Province-specific He isotope
variation with SRP basalts outside
the range typical of upper mantle
(MORB, Arc) melts (Data and figures from
Graham et al. JVGR 2009)
Both He and Sr (and Nd, Hf, Pb) Show Longitudinal Variation
with a big Step at the PC border. To the East, Sr (and Nd, Hf,
Pb) Move Towards More Lithospheric Compositions, While He
Moves to More “Deep Mantle” Composition!
How Does the “Plume” Know the Location of the Lithospheric
Boundary?
Good Correspondence Between low Velocities and Sites of Recent HLP
Volcanism, but only in Uppermost Mantle.
SRP a Line of low Velocity to ~200 km
N = Newberry, D = Diamond
Crater, J = Jordan Crater
Conclusions:
Flood basalt and HLP magmatism show strong regional
isotopic and temporal compositional variation lithosphere is playing an important role in modifying
the composition of the melts. No geochemical signal
of a deeper mantle “plume” signature, but a
significant “subduction” compositional signature. Pb
isotopes consistent with addition of fluids from PNW
offshore sediments. Young volcanic centers
underlain by low seismic velocities to depths of 100150 km. HLP volcanism is a shallow phenomena.
Conclusions:
SRP basalts lack the subduction compositional
signal and have high 3He/4He suggestive of a
deeper mantle source. Low S-wave speeds
continue deeper beneath SRP than HLP. High Ti
and Ba contents suggest SRP basalts may be
relatively lower degree melts and low Al and
high Fe contents suggest deeper melting.
Lithophile-element isotope systems (Sr, Nd, Pb)
suggestive of a source in, or significant
overprinting by, Archean lithospheric mantle.