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Suzanne Y. O'reilly, GEMOC National controversy over the physical nature of the lithosphere and asthenosphere and the boundary zone between them. The subcontinental lithospheric mantle is isolated from the convecting mantle and thus tends to resist homogenization over time. At the lithosphere-asthenosphere boundary, the temperatures of the lithosphere and the uppermost asthenosphere coincide, and the greater buoyancy and viscosity of the lithosphere are important in maintaining its mechanical integrity. Suzanne Y. O’Reilly, GEMOC National mantle roots are highly buoyant; they This emphasizes a thermal and Key Centre, Department of Earth cannot be delaminated but require rheological distinction between and Planetary Sciences, Macquarie mechanical disaggregation (lithospheric lithosphere and asthenosphere that also University, Sydney, NSW 2109, Australia thinning and/or rifting) and infiltration coincides well with seismic observations of upwelling fertile material to be and with the geochemical signatures William L. Griffin, GEMOC National Key destroyed or transformed. In contrast, that are used in this paper to help define Centre, Department of Earth and Phanerozoic subcontinental lithospheric the location and character of the Planetary Sciences, Macquarie mantle is denser than the asthenosphere lithosphere-asthenosphere boundary. University, Sydney, NSW 2109, Australia, for observed thicknesses (~100 km) and Four-dimensional lithosphere and CSIRO Exploration and Mining, can “delaminate” under stress. The mapping is a methodology that P.O. Box 136, North Ryde, NSW 1670, contrasting properties of different integrates petrological, geochemical, Australia mantle domains require lateral contrasts geophysical, and tectonic information to in composition, density, thickness, and map the composition of the sub- Yvette H. Poudjom Djomani, GEMOC seismic response in the present-day continental lithospheric mantle (Fig. 1) National Key Centre, Department of subcontinental lithospheric mantle. They and the location of its important Earth and Planetary Sciences, Macquarie also suggest a secular evolution in boundaries through time (O’Reilly and Earth’s geodynamics from Archean to Griffin, 1996). Volcanic rocks (basalts, University, Sydney, NSW 2109, Australia Proterozoic time, and an increased lamproites, kimberlites) carry fragments importance for lithosphere-delamination of the subcontinental lithospheric Paul Morgan, GEMOC National Key processes in Phanerozoic orogens. mantle to the surface as xenoliths and Centre, Department of Earth and xenocrysts (e.g., garnet, chromite, and Planetary Sciences, Macquarie LITHOSPHERE DOMAINS IN TIME diamond). Xenoliths can be used to University, Sydney, NSW 2109, Australia, AND SPACE recognize mantle rock types and and Department of Geology, Northern Knowledge of the architecture and processes, and to measure physical Arizona University, Flagstaff, AZ 86011- evolution of the mantle portion of the properties (e.g., elastic, electric, 4099, USA continental plates—the subcontinental magnetic properties, density, and heat lithospheric mantle—is critical to production). Volcanic episodes of ABSTRACT understanding the large-scale processes different ages in one region can provide The lithospheric mantle beneath responsible for the development of this information for different time slices, continents is often the same age as the Earth’s continents. Plate tectonicists use corresponding to the age of the host superjacent crust, but remains less well a mechanical definition (rigid plates) for volcanism. Geophysical data can be understood. Analysis based on a large the lithosphere, contrasting it with the used to extend the geological database of xenoliths and xenocrysts less rigid asthenosphere. Geochemists information laterally by matching geo- shows that mantle domains that consider the subcontinental lithospheric physical signatures with mapped stabilized during different geologic mantle to be a chemically depleted subcontinental lithospheric mantle eons have distinctly different mean reservoir that is the residue of partial sections. compositions. There is a secular melting of Earth’s asthenosphere. This methodology can provide some evolution from depleted Mg-rich low- Seismologists define it using velocities important constraints on fundamental density Archean mantle to more fertile, and extrapolated densities and consider questions about Earth’s geological denser Phanerozoic mantle; the most its base to coincide with the top of a evolution. These include the significant differences are between the low-velocity zone in tectonically young compositional structure of subcontinen- Archean and Proterozoic mantle. The regions, and others use heat flow and tal lithospheric mantle formed at compositional variations produce magnetotelluric data to define it as a different times, the lateral variability of differences in the density and elastic thermal boundary layer. These various subcontinental lithospheric mantle properties of lithospheric mantle of definitions may coincide (or not) and composition and its effects on tectonics, different age. Archean and Proterozoic give rise to a persistent and fascinating and the extent to which lithospheric 4 APRIL 2001, GSA TODAY mantle can be recycled into the because input parameters such as maximum depth of mafic granulite convecting mantle or irreversibly thermal conductivity and heat xenoliths and can be used to estimate differentiated from it. production are poorly constrained and the depth of the crust-mantle boundary variable, both with depth and laterally, (O’Reilly and Griffin, 1996). We approx- TOOLS DEVELOPED FOR in the crust. imate the depth to the geochemical FOUR-DIMENSIONAL lithosphere-asthenosphere boundary by LITHOSPHERE MAPPING The Crust-Mantle Boundary and the maximum depth from which low-Y Lithosphere-Asthenosphere Boundary (<10 ppm) garnets, characteristic of Paleogeotherms Once a geotherm is inferred, the depleted lithosphere (Griffin et al., 1999; Heat drives all Earth processes and many xenolith samples for which references therein), are derived; it the thermal state of the lithosphere temperature (T ) can be calculated are typically coincides with temperatures of affects its thickness and density projected to the geotherm to estimate 1250–1300 °C (Fig. 3). Deeper garnets (Morgan, 1984; Lachenbruch and their depth of origin. In the resulting have high Y + Ti + Zr, interpreted as the Morgan, 1990). Geotherms are a plot of sections, the minimum depth of signature of asthenosphere-related temperature variation with depth at a abundant ultramafic rocks (mantle metasomatism. The depth of the given time and place. Empirical peridotite) commonly coincides with the lithosphere-asthenosphere boundary paleogeotherms can be constructed using temperatures and pressures calculated from mineral assemblages in mantle xenoliths and can provide a framework for mapping the geochemical structure of the subcontinental lithospheric mantle. Unfortunately, xenoliths from which pressures (depths) of origin can be calculated (e.g., with coexisting orthopyroxene and garnet) are limited in their geographic distribution. Nevertheless, single-element SN thermometers and barometers (e.g., Ni Mirny Udachny Kuoika 0 0 and Cr in garnet; Ryan et al., 1996) TERRANES CMB Spinel MAGAN 50 DALDYN 50 BIREKTE based on element partitioning between Lherzolite MARKHA MARKHA HAPSCHAN Zone Garnet Lherzolite garnet and mantle olivine and pyroxene 100 th 100 Graphite ± Eclogite 0ûIso Zone 0 Billyakh Shear 0 Diamond Kotuykan Shear 1 ias allow the derivation of paleogeotherms Jur-Tr 150 >30% Harzburgite 150 from the more abundant suites of garnet Depth (km) 200 200 xenocrysts. Asthenosphere 0 100 200 km We have constructed, or compiled ARCHEAN PROTEROZOIC from published data, paleogeotherms 50 for more than 300 localities worldwide. 100 These paleogeotherms represent the temperature variation with depth at the 150 time of volcanic eruption; they are Depth (km) typically low beneath cratonic areas 200 Low Ca harzburgite with Archean crust, higher beneath Pro- 250 terozoic cratons, and still higher beneath 0 200 400 600 800 (km) 50 Phanerozoic mobile belts (Fig. 2). In areas of active basaltic volcanism, 100 geotherms are generally high and strongly convex, consistent with 150 advective heat transport by magmas and Depth (km) 200 underplating of basaltic rocks in the TiO2 (0-1 wt%) upper part of the subcontinental 250 lithospheric mantle (O’Reilly and Griffin, 0 200 400 600 800 (km) 1985; O’Reilly et al., 1997). These Figure 1. Example of lithosphere mapping across eastern Siberia, using xenoliths and empirical geotherms are preferred over xenocrysts (after Griffin et al., 1998a). Top view shows crustal terranes. Second view shows models for the thermal state of the lithosphere sections mapped from xenoliths and xenocrysts in kimberlites (stars), delineating lithosphere that are based on the rock type distribution, the lithosphere-asthenosphere boundary and the 1000 °C isotherm downward extrapolation of surface heat (dashed). Next view shows distribution of low Ca-harzburgite, confined to Archean terranes. flow (e.g., Pollack and Chapman, 1977) Lower view shows lithosphere-asthenosphere boundary reflected in Ti contents of garnets (higher in the asthenosphere). CMB—crust-mantle boundary. GSA TODAY, APRIL 2001 5 beneath the Lac de Gras area (Fig. 4). The upper part of the subcontinental lithospheric mantle (to 140–150 km) consists of extremely depleted harzburgite, while the lower part (150–220 km) is significantly less depleted,
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