Download Using Igneous Rocks to Probe the Evolution of the Lithosphere Terry

Using Igneous Rocks to Probe the Evolution of the Lithosphere
Terry Plank
Boston University, Dept. of Earth Sciences, 685 Commonwealth Ave., Boston, MA 02215, [email protected]
The Contribution of Geology and Geochemistry to USArray
There are two ways in which geological studies of rocks can complement the seismic imaging planned for USArray.
One is to assist in basic interpretation of the images - by providing geologic maps of surface exposures to correlate with
seismic structures at depth, and by providing information on possible rock types in the crust and their density and
seismic velocity. The other potential of geologic studies is to provide complementary information to seismic images where melting occurs in the crust and mantle, where the lithosphere/asthenosphere boundary occurs, and what
processes lead to creation of continental crust. Thus collection of petrologic and geochemical data on rocks is needed
both in the basic interpretation of the seismic data as well as the processes probed. The rolling seismic footprint across
the country should be accompanied by a campaign to analyze and archive rocks within each grid.
Geochemical studies of igneous rocks - including geochronology, major and trace element geochemistry, and
isotope geochemistry - are in a similar state of maturity to seismic imaging. The recent advances in analytical
instrumentation and technique development mean that high quality geochemical data are within reach of many labs
across the country. There are no real impediments to dating and analyzing every exposed igneous rock at the surface of
the continent, other than labor, cost, and organization. The geological community as a whole could also benefit from
the type of systematic data collection and archiving that is more typical of geophysical programs. For example, no
database currently exists of geochronological or geochemical data on volcanic rocks in the US. But more to the point,
little high quality data exist. The density of seismic stations in any given "BigFoot" grid is currently greater than the
density of well-studied volcanic/plutonic centers (with age dates, major and trace elements, and isotopic data), even in
well-studied parts of the country, such as the Western US. The sweep of BigFoot across the country would provide an
impetus to organize the geochemical community to systematically study and database the continent's igneous products.
Like the seismic component to USArray, such an effort would lead to a new clarity in mapping the ages of magmatism,
the conditions of magmagenesis, and the deformation events that build the continent.
Constraints on Mantle Melting and Thermal Structure from Inversion of Basalt Compositions
One fruitful avenue of combined seismic and geochemical studies is in regions of recent volcanism and tectonism
(in the US, this includes the Basin and Range and the Cascadia arc). In these regions, seismic images of the current
state of the mantle and crust can be combined with constraints from geochemical studies of volcanic products. Basalt
compositions can be used to infer the shape of the mantle melting regime (e.g., Rogers, et al., 1995; DePaolo and
Daley, 2000; Wang, et al., 2001) and so complement seismic observations on the in situ state of the sub-continental
mantle. While it is often difficult to uniquely constrain temperature, melt content, and composition with seismic or
petrologic data alone, the combination is potentially powerful.
Like a genetic code, the conditions of formation are embedded in the major, trace element and isotopic
concentrations of primitive basalts. Experimental studies of mantle melting have progressed to the point where it is
possible to read this code, and estimate the pressures and temperatures of mantle melting. These estimates provide
independent constraints to seismic velocity and attenuation for deducing the temperature and melt content of the
mantle. Although inversions were first developed for mid-ocean ridge basalts (McKenzie and Bickle, 1988; Langmuir,
et al., 1992; Kinzler, 1997; Hirschmann, et al., 1999), they are robust for anhydrous mantle melting, and so are
applicable to hot spots (e.g., Fram and Lesher, 1997) and continental basalts (and see below). Recent developments in
studying the effect of H2O (Gaetani and Grove, 1998) and CO2 (Hirose, 1997) mean that we will soon be able to
develop quantitative melting models for subduction zones as well.
An Example: Mantle Melting Profile across the Basin and Range, SW USA
We have recently studied late Cenozoic basalts (0-10 Ma) across the Basin and Range province (B&R), which
preserve a signal of mantle melting depth variations in their Fe, rare earth element (REE), Si and Al concentrations
(Wang, et al., 2001). These variations are qualitatively consistent with shallower melting beneath the Western Great
Basin (WGB) than in the central B&R. In order to quantify the depth range and percent of decompression melting, we
invert primary Na 2O and FeO contents of basalts using a melting model based on the partitioning of FeO and MgO in
olivine, and Na 2O in clinopyroxene. Independent inversions, using the REE and Si, corroborate the Fe-Na depths, and
place most of the melting in the central B&R in the garnet-peridotite stability field. We find that the shape of the
melting region across the B&R closely mimics the shape of the mantle lithosphere, as inferred from geological and
geophysical observations. Melting across the study area occurs largely within the asthenosphere and generally stops at
the base of the mantle lithosphere. In the WGB, melting paths are shallow, from 75-50 km, and in some cases extend
almost to the base of the crust. These melting paths are consistent with adiabatic melting in normal-temperature
asthenosphere, beneath an extensively thinned (or absent) mantle lithosphere. Shallow melting is consistent with
geobarometry and isotopic compositions of local mantle xenoliths. Lithospheric thinning was caused by thermal
erosion during Mesozoic subduction, and/or simple shear or foundering during Cenozoic extension. In contrast,
melting beneath the central B&R occurs beneath thick mantle lithosphere, and requires mantle potential temperatures
200°C hotter than normal (melting paths from 140-100 km). The excess temperature beneath the central B&R is
consistent with active upwelling of hot mantle in this region. USArray would provide an excellent opportunity to
compare the seismic response of this region with the melting profile predicted from petrologic data.
References:
DePaolo, D. J., and E. E. Daley, Neodymium isotopes in basalts of the southwest basin and range and lithospheric
thinning during continental extension, Chem. Geol., 169, 157-185, 2000.
Fram, M. S., and C.E. Lesher, Generation and polybaric differentiation of east Greenland early Tertiary flood basalts,
J. Petrol., 38, 231-275, 1997.
Gaetani, G.A., and T.L. Grove, The influence of water on mantle melting, Contrib. Mineral. Petrol., 131, 323-346,
1998.
Hirose, K., Partial melt compositions of carbonated peridotite at 3 GPa and role of CO2 in alkali-basalt magma
generation, Geophys. Res. Lett., 24, 2837-2840, 1997.
Hirschmann, M. M., M. S. Ghiorso, and E. M. Stolper, Calculation of peridotite partial melting from thermodynamic
models of minerals and melts; II, Isobaric variations in melts near the solidus and owing to variable source
composition, J. Petrol., 40, 297-313, 1999.
Kinzler, R.J., Melting of mantle peridotite at pressures approaching the spinel to garnet transition: Application to midocean ridge basalt petrogenesis, J. Geophys. Res., 102, 853-874, 1997.
Langmuir, C. H., E. M. Klein, and T. Plank, Petrological systematics of mid-ocean ridge basalts: constraints on melt
generation beneath ocean ridges, in Mantle Flow and Melt Generation at Mid-Ocean Ridges, Geophys. Mon.,
71,.183-280, 1992.
McKenzie, D., and M. J. Bickle, The volume and composition of melt generated by extension of lithosphere, J. Petrol.,
29, 625-679, 1988.
Rogers, N. W., C. J. Hawkesworth, and D. S. Ormerod, Late Cenozoic basaltic magmatism in the Western Great Basin,
J. Geophys. Res., 100, 10287-10301, 1995.
Wang, K., Plank, T., Walker, J.D. and Smith, E.I. (2001) A mantle melting profile across the Basin and Range, SW
USA. J. Geophys. Res., in press.
E. Sierra
Death
Valley
Mojave
Reveille &
L. Mead
Transition
Zone, Colorado Plateau
0
0
Pressure (GPa)
East Pacific
Rise
crust
20
1.0
40
mantle
lithosphere
60
2.0
Z
1435°C
80
1450°C
Depth (km)
Sp
100
3.0
Gar
1600°C
120
4.0
1700°C
140
asthenosphere
0-10Ma
160
0
100
200
300
400
500
Snake River
Basalts
5.0
600
Distance (km)
Figure Caption: Melting profile across the Basin and Range (from Wang, et al., JGR, in press). Arrow represents
melting column calculated for each volcanic field, based on the most primitive FeO and Na2O compositions. The
bottom of the arrow marks the onset of melting at the solidus, and is a function of mantle temperature, while the top of
the arrow marks the end of decompression melting, presumably due to the change in rheology near the lithosphere/
asthenosphere boundary. The thin line extension to the arrows include depth estimates and errors using Fe8.0. Crustal
thickness was compiled from Das and Nolet (1998). Shaded lithosphere is based on lithosphere thickness (Lm+Moho)
estimates in Jones, et al. (1996), which were averaged and smoothed for the Sierras, Central and Southern Basin and
Range, and western edge of the Colorado Plateau. Estimates for the Northern Basin and Range were not used, due to
negative Lm values; thus there is a lack of control from 300-500 km. Shape of the lithosphere is consistent with the
conceptual model in Wernicke (1992). Alternative boundary for the base of the lithosphere (marked with a Z) based on
P-wave residuals from Zandt et al. (1995), although this model does not take into account mantle T or compositional
effects on seismic velocity. Spinel/garnet transition in peridotite after Klemme and O'Niell (2000) and Robinson and
Wood (1998). Ruled bars are for melting columns with average eNd of > + 0.5. Melting depths for the Snake River
basalts shown for reference (average of data in Lum, et al., 1989). East Pacific Rise shows analogous melting
calculation for primitive MORB near the Tamayo Fracture Zone (Bender, et al., 1984), assuming melt pooling from a
triangular shaped melting regime (Plank and Langmuir, 1992). Temperatures given are solidus temperatures of
adiabatically ascending mantle. Pressure axis calculated assuming 35 km crust with 2.85 g/cc density overlying mantle
of 3.25 g/cc density.