V43B-1364
Mantle Dynamics beneath East Asia Constrained by Sr, Nd, Pb and Hf Isotopic Systematics of Ultramafic Xenoliths and their Host Basalts from Hannuoba, North China
We have determined the Sr, Nd, Pb and Hf isotopic compositions of clinopyroxene separated from mantle-derived ultramafic xenoliths (six spinel peridotites, two composite Cr-diopside pyroxenites, and one discrete Al-augite pyroxenite) hosted by Cenozoic alkali basalts at Hannuoba, North China, in order to understand the nature of the mantle source for this intraplate volcanism, and the petrologic history of the mantle lithosphere beneath North China Block, a crustal segment of the Sino-Korean Craton. Measured Sr, Nd, Pb and Hf isotopic compositions in the clinopyroxene grains separated from spinel peridotite and Cr-diopside pyroxenite (87Sr/86Sr = 0.70265 to 0.70485; 206Pb/204Pb = 17.75 to 19.15; eNd = 0 to +11; eHf = +10 to +38) display mixing hyperbolas between mantle compositional end members DMM and EMII on the Sr- Pb and Nd-Pb isotope correlation diagrams. This is distinctly different from the host basalt data which show a mixture of DMM and EMI components on the diagrams. We interpret this to reflect infiltration by metasomatic agents, possibly silicate melts, having an EMII-like isotopic signature, which enriched a precursor time-integrated depleted mantle. An Al-augite pyroxenite, also hosted by these basalts, is characterized by highly enriched Sr, Nd, and Hf isotopic compositions (87Sr/86Sr = 0.70733; eNd = -16; eHf = -18) with only moderately radiogenic Pb that has a 206Pb/204Pb value of 18.23. All of these data plot outside (1) the fields for oceanic basalts, and (2) the mixing arrays defined on the isotopic correlation diagrams by peridotites/Cr-diopside pyroxenite with their metasomatic agents, and by the host basalt. These observations suggest that (1) Al-augite pyroxenite is not cogenetic with the Cr-diopside pyroxenite, (2) parental melts of the pyroxenites are not likely to be the source for the metasomatism, and (3) the thermo/mechanically reactivated pyroxenite and/or spinel peridotite, is not likely to be the source for host basalt magmatism. The Cenozoic intraplate volcanism, therefore, must have originated in the asthenosphere. We observe that the relatively little-metasomatized Hannuoba peridotites define a Lu-Hf isochron of 2587 +/- 86 Ma (2s). This value is, within error, indistinguishable from the Sm-Nd isochron age of the overlying granulite terrain. We suggest, therefore, that the Lu-Hf system can be used to constrain the timing of lithospheric mantle differentiation. Preservation of the Neoarchean mantle lithosphere beneath Hannuoba, despite the protracted tectono-magmatic reactivation during the Mesozoic and Cenozoic in this area, suggests that complete removal of the lithospheric mantle beneath East Asia by wholesale delamination is unlikely.
V43B-1365
Does the mantle control the maximum thickness of cratons?
The stability and longevity of cratons depends on their ability to resist deforming forces induced by the flowing and evolving mantle. Previous studies point to the combination of buoyancy, viscosity and finite strength to provide the cratons with sufficient strength to maintain coherency atop a convecting mantle. This combination of parameters determines a minimum thickness that cratons must exceed in order to maintain stability and longevity. However, is there also a maximum thickness above which a craton is rendered unstable? More specifically, is there a critical craton thickness that would enhance its susceptibility to destruction by basal tractions that will shear and thin the craton? To address this question we employ an analytical approach relating the viscosity structure of the craton to the velocity of mantle flow within the asthenosphere. If the craton°s viscosity structure is determined by the interplay between a chemical and thermal boundary layer, then there exists a buffer zone with temperature dependent viscosity between the rigid chemical layer and the convecting mantle. This thermal boundary layer protects the chemically-distinct craton from the asthenospheric shear, but it cannot do so if it is too weak or too thin. Furthermore, if the net thickness of the chemical and thermal boundary layer increases, then the mantle-induced basal shear tractions on the combined structure will increase. This negative feedback prevents the cratonic lithosphere from growing thicker than some maximum value that depends on the viscosity structure of the thermal boundary layer. We present initial estimates of the maximum thickness of the thermal buffer zone, which in turn controls the maximum thickness of cratonic chemical lithosphere and is required to maintain craton stability in the face of destabilizing mantle flow. This could be a potential explanation for the geochemical observation that cratonic xenoliths arise from depths no greater than 250 km.
V43B-1366
Continental Assembly and Anisotropy Beneath the CANOE Array
The Canadian Northwest Experiment (CANOE) is an array of nearly sixty broadband seismometers reaching from the Slave Craton in the Northwest Territories (NWT), across a series of Proterozoic orogens and the Canadian Rockies in the NWT, northern British Columbia, and southern Yukon, and across the Churchill Province south to Edmonton, Alberta. The array traverses a wide variety of continental settings, allowing the study of mantle variability associated with the formation of continental cratons and continental assembly over a time span of nearly 4 Ga. The close spacing of instruments in the CANOE array provides a detailed view of the mantle and lithosphere across these transitions. We examine splitting of the shear phases SKS, SKKS, and sSKS to study anisotropy beneath the region. The dataset consists of ~~70 teleseismic events of either magnitude > 5.6 and depth > 500 km or magnitude > 6.4 with depth < 500 km. All earthquakes were recorded at CANOE or nearby Canadian National Seismic Network stations between May 2003 and September 2005. Splitting times derived from multi-event station averages average ~1.4 s, and fast directions are coherent yet suggestive of strong variability of mantle anisotropy across the region. Stations on the craton show a dominant NE-SW fast direction that is roughly consistent with mantle flow dominated by plate motion. At the Cordillera boundary, fast directions flip abruptly to NW-SE, and continuing west across the Cordillera the fast directions rotate from NW-SE to roughly E-W before returning to NW-SE near the edge of the continent. These patterns are suggestive of dominant transpressional deformation through the lithosphere during continental accretion. Within the craton, there is an anomalous cluster of stations with N-S fast directions; these stations sit astride an apparent ancient suture zone (subducted slab?) detected through previous scattered-wave and seismic reflection studies. We will explore the possible relationship between this slab-like feature and the anomalous anisotropy. In addition to describing the general patterns of anisotropy beneath the region, we also investigate variations in the fast directions and delay times suggestive of complexity in the region. A number of stations exhibit "null" behavior even in the multi-event average analysis, and individual event solutions are highly variable. Critical factors to be evaluated include back azimuth, the phase of interest, and frequency content.
V43B-1367
Significance of E. Paleozoic Paleo-Tethyan Ophiolites in the Balkan Terrane and the Greater Caucasus for the Cadomian-Hercynian Continental Growth of Southern Europe
New geochronological & geochemical data from dismembered E. Paleozoic ophiolites in the Balkan Terrane (Serbia) & the Greater Caucasus (Russian Federation) link these remnants of the Paleo-Tethyan oceanic crust to a series of events associated with the Cadomian and Hercynian evolution of S. Europe. The Balkan-South Carpathian ophiolite belt (BSCOB) WSW of the Moesian platform is part of the South European suture zone (SESZ) & includes the Deli Jovan massif (DJM) composed of ultramafic mantle restites, ultrabasic & basic cumulates, gabbros, diabasic dikes and basalts. The analyzed gabbros of DJM are high-Al (19-24.5 wt.%) cumulate gabbros-troctolites (Ol82-89+Cpx79-87+Pl77-91) originated from shallow-depth crystallization of low-K (<0.3 wt.%) tholeiitic basaltic melt (T=1050°-1160°). Dike rocks are composed of fine-grained, Cpx-Pl microphyric, high-Mg tholeiitic microgabbro-diabase showing MORB affinities [(La/Sm)n=0.18-1.31; (La/Yb)n=0.22-1.35; Th/La=0.073-0.19; Zr/Y=1.60-2.39] derived from depleted mantle source(s). New isotopic ages range between 406±24 Ma and 399.7±5.2 Ma [Sm-Nd isochron age of 406±24 Ma, MSWD=0.71; U-Pb isotope dilution zircon age of 399.7±5.2 Ma; U-Pb SHRIMP zircon age of 405.0±2.6 Ma]. Ophiolitic rocks of the BSCOB continue E into the Paleozoic metamorphic basement of the Istanbul-Zonguldak Unit (IZU) in NW Turkey & the Crystalline Core and the Fore Range Zones of the Greater Caucasus. The metaophiolites in the Greater Caucasus include residual peridotites, ultrabasic & basic cumulates, gabbros, dike complexes, and volcanic rocks with limestones, ranging in age from 490 to 416 Ma. The volcanic sequences in these ophiolites include low-silica (SiO2=43.94-48.10 wt.%), high-Ti (TiO2=2.0-3.3 wt.%) and high phosphorus tholeiitic basalts showing slightly enriched T-MORB affinities [(La/Sm)n=2.98±0.57; (La/Yb)n=5.72±2.05; (Tb/Yb)n=1.42±0.62; and (Yb)MORB=0.97±0.35], and high-silica (SiO2=47.54-53.94 wt.%), low-Ti (TiO2=0.46-1.09 wt.%) and low-K tholeiitic basalts, basaltic andesites & andesites, showing suprasubduction zone affinities [(Tb/Yb)n=1.04±0.34; (Yb)MORB=0.62±0.28; and negative Nb and Zr anomalies]. The Early Paleozoic ophiolitic rocks within the SESZ along the southern edge of the European Platform (including the Moesian and Scythian platforms) developed during the evolution of the Paleo-Tethyan back-arc basin via seafloor spreading and arc-trench-subduction rollback processes behind the northward drifting peri-Gondwanan terranes (i.e. Transcaucasian massif in Georgia; IZU, Menderes and Armutlu massifs in Turkey; Florina terrane in Greece). They were accreted to the Eurasian continental margin during the Hercynian orogeny. The Cadomian peri- Gondwanan terranes & the SESZ ophiolites subsequently became part of the basement of a Late Paleozoic magmatic arc above a Paleo-Tethyan subduction zone along the southern margin of Europe.
V43B-1368
Lithospheric Structure Across the Cordillera-Craton Transition in NW Canada from Surface Waves Traversing the CANOE Array
The Canadian Northwest Experiment (CANOE) is a nearly sixty-station broadband PASSCAL array extending from the Slave Craton in the Canadian NWT, across the Canadian Rockies in northern British Columbia and Yukon and south to Edmonton, Alberta. The array crosses a series of compressive orogens that span 4 Ga of geologic time and are undisrupted by later periods of extension or extensive volcanism, and thus provide an ideal platform for studying the growth of continental cratons through accretion. There are two competing hypothesis for the nature of the cordillera-craton transition in this region: (1) Terranes accreted in the cordillera during the Mesozoic are lithospheric-scale features. This suggests substantial growth of the North American continent since the Proterozoic, and a craton boundary near the deformation front. (2) Terranes are thin-skin thrust features overlying Proterozoic North American lithosphere. This suggests little continental growth since the Proterozoic, and a craton boundary near the Pacific coast. Surface waves traversing the CANOE array provide an excellent data set to evaluate these hypotheses. We have developed and applied a new multi-station cross-correlation procedure to precisely measure the frequency-dependent travel times and amplitudes of Love and Rayleigh waveforms traversing the array. The data are derived from recordings of regional and teleseismic earthquakes recorded between June 2004 and Sept 2005. Using accurate three-dimensional sensitivity kernels for both amplitude and phase, these data are inverted for 3D tomographic images of anisotropic shear- velocities in the crust and upper- mantle across the transition from craton to cordillera. Preliminary results suggest a sharp lateral gradient in both shear velocity and shear anisotropy in the lithospheric mantle that is located very close to the current deformation front, consistent with hypothesis (1). The surface-wave observations will be integrated with receiver function, shear-wave splitting, and body-wave tomography data, proving a comprehensive portrait of lithospheric structure across the transition.
V43B-1369
Constraints on Lithospheric Enrichment and Crustal Contamination in the Central Rio Grande Rift: Evidence From Basaltic Melt Inclusions
Many basalts erupted during the early stages of rifting throughout the southwestern United States have an enrichment in water-soluble elements leading to high ratios of water-soluble to insoluble elements, e.g. Ba/Nb, K/Nb, and Sr/Nd, often found in volcanic arcs. Prior studies have suggested that these basalts with "arc-like" trace element signatures originate from melting of a mantle source that has experienced hydrous metasomatism as a result of shallow subduction of the Farallon Plate beneath western North America. Rio Grande Rift extension and volcanism correlates with the eastern extent of flat slab subduction of the Farallon. We have analyzed olivine- and orthopyroxene-hosed melt inclusions from Rio Grande Rift (New Mexico, U.S.A.) basalts for major-, trace-, and volatile-species concentrations. Melt inclusions from lavas with an "arc-like" signature (e.g. elevated Ba/Nb or Sr/Nd) should be also enriched in volatile species such as water and Cl. In melt inclusions from Rio Grande Rift tholeiites and alkaline basalts Cl/K and Cl/Nb ratios broadly correlate with Ba/Nb and Sr/Nd. In melt inclusions with higher Ba/Nb and Sr/Nd ratios, Cl/Nb and Cl/K ratios are also slightly elevated. Similarly, tholeiitic basalts with lower Ba/Nb and Sr/Nd possess low Cl/Nb and Cl/K ratios, more consistent with ratios observed in fresh MORB. Low Cl/Nb ratios relative to melt inclusions from modern arc and backarc basalts from the Casacade range in western North America are consistent with minor volatile enrichment occurring in an extreme backarc position. However, partial melting and assimilation of continental crust may produce qualitatively similar trends. Whole rock 208Pb/204Pb from Rio Grande Rift lavas negatively correlates with inclusion Cl/Nb, Ba/Nb and Sr/Nd ratios. Basement rocks in the study area generally have low radiogenic Pb and high Ba/Nb, consistent with a model of crustal contamination for the Rio Grand Rift basalts. However, low Sr/Nd of basement rocks precludes a model for simple assimilation of local basement. Further investigation is required to distinguish between lithospheric and crustal sources for trace element and volatile enrichment.
V43B-1370
P and S body wave travel-time tomography of northwestern Canada
Northwestern Canada is a unique collage of different geologic domains that accreted together at different stages in the planet's history. In this region the rock record spans close to 4.0 Ga, from Early Archean to the present day, over a distance of three thousand kilometers. This mosaic represents the most nearly complete and continuous sampling of geological time on the surface of the Earth. The IRIS-PASSCAL/{\sc Lithoprobe}-sponsored Canada NOrthwest Experiment (CANOE) experiment was designed to address fundamental questions on the variation in structure and geometry of the lithosphere from early Archean to present. Nearly 60 broadband, three-component instruments were deployed during a period of over two years between June 2003 to September 2005 along aY-shaped array with over 1000km aperture. The three legs of the array radiate away from Fort Nelson, the array center, to termini at Yellowknife, Whitehorse and Edmonton. In this poster, we present P and S body-wave travel-time tomography images of upper mantle velocity structure beneath the array using 675 earthquakes of magnitude 5 and above in the epicentral range of 30\circ to 100\circ recorded by the CANOE array.
V43B-1371
Formation of Garnet Orthopyroxenites and Mobility of Siderophile and Chalcophile Elements in the Subcontinental Lithospheric Mantle During Metasomatism by Asthenospheric Mantle- derived Melt Below the Southern South America
Garnet-bearing orthopyroxenite is common as discrete mantle xenoliths and veinlets in peridotite xenoliths brought to the surface by the Quaternary Pali Aike basalts, the southernmost Patagonian plateau basalt field in South America. Orthopyroxenites commonly contain Ti-rich minerals and relict grains of Ol or rare Cpx as inclusions in secondary Opx (>85 % vol). The secondary Opx contains high TiO2 (0.20-0.59 wt%), moderate Al2O3 (2.8-5.1 wt%) and low Mg# (0.84-0.89) compared with Opx in garnet-bearing peridotites. This suggests that secondary Opx formed at the expense of Ol during metasomatism by a Ti-rich evolved melt. Secondary Opx formed from Ol through reaction with slab-melt or fluid has been documented in sub-arc mantle peridotites. In contrast with such Opx in subarc mantle samples, secondary Opx in the Pali Aike orthopyroxenites contains high Ti and Al and low Mg. High Ti and low Mg in our samples reflect the evolved nature of the metasomatizing melt that originated from the underlying asthenospheric mantle. This type of orthopyroxenite may be common elsewhere in the SCLM affected by asthenospheric upwelling. The orthopyroxenites contain similar bulk-rock concentrations of Cr, Ni and PGE as do the peridotites, suggesting that these metals were essentially immobile during this type of metasomatism, and that the metasomatizing melt did not introduce these elements into the mantle. Instead, the metasomatizing melt contributed alkalis, Ti, Si, Cu, and S to the orthopyroxenites. The evolved metasomatizing melt was saturated with S and introduced immiscible sulphide liquid containing Cu and S to the orthopyroxenites. The contents of PGE are independent of S and they are most likely present in alloys, silicate and oxide minerals.
V43B-1372
Geochemical Evidence of a Metasomatized Mantle Source Beneath the West Antarctic Rift System
We analyzed alkaline samples dredged from seamounts in the western Ross Sea, Antarctica, to determine their trace element concentrations and Pb, Sr, Nd and Hf isotopic compositions aimed at characterizing source components and melting processes. These samples yield very narrow ranges in isotopic ratios (206Pb/204Pb = 19.279125-19.98534, 207Pb/204Pb = 15.475681 – 15.629633, 87Sr/86Sr = 0.702885 – 0.703524, 143Nd/144Nd = 0.512883 – 0.513010 176Hf/177Hf = 0.282977 – 0.283061), suggesting a homogenous mantle source. This is consistent with previous studies of subaerial volcanism from the West Antarctic Rift System (WARS) and the associated diffuse alkaline magmatic province of the southwest Pacific. The large geographic extent of this homogeneous Cenozoic volcanism supports a passive rifting model and resultant decompressional melting (e.g., Wörner, 1999), rather than the mantle plume model supported by high average elevations for the Antarctic continent and the HIMU isotopic signature observed throughout the WARS (e.g., Behrendt, 1999). Large low-velocity seismic anomalies have been observed at 50-200 km beneath the WARS (Finn et al., 2005) and may represent a "wet" mantle modified by subduction along the paleo-Pacific margin of Gondwana. During this 400-Myr subduction history, infiltration of subduction related melts from the downgoing slab must have metasomatized the mantle, with concomitant partial melting generating pyroxenite dikes and veins in the overlying mantle wedge. The lower solidus of these pyroxenites allows generation of higher melt volumes relative to a dry peridotite source under the same conditions. Similarities between major element concentrations (e.g., high TiO2, Na 2O) and trace element ratios (e.g., Nb/Th) of these new samples from the Ross Sea with experimental data for garnet- bearing pyroxenite melts (Kogiso et al., 2003, 2004) further support a metasomatized mantle source for WARS magmatism.
V43B-1373
Physical Controls on the Thickness of Stable Continental Lithosphere
Formation of cratonic mantle lithosphere is achieved through melt depletion generating a residue that is less dense and more viscous than the underlying convecting mantle. These characteristics act to stabilize cratonic roots against convective overturn. Stability, however, also depends on thickness, which provides independent constraints on the sequence of events that lead to stable roots. The stability problem of intrinsically buoyant lithosphere which is cooled from above has been studied with both laboratory experiments and numerical simulations in 2-D and 3-D involving materials with either constant or temperature-dependent viscosity. Stability of thick lithosphere depends weakly on the viscosity contrast and strongly on the density contrast between root material and the underlying mantle. The threshold thickness for a stable root increases with intrinsic buoyancy, which may account for the differences that are observed between Archean and Phanerozoic continental roots. Stable thickness values may a priori be achieved by thickening an initially thin root or thinning of an initially thick root. Both processes may involve melting, however, which generates root material with a different density. Thus, stablilization involves changes of both thickness and density and is best discussed in (thickness, buoyancy) space. Instability of a root leads to both downwellings and upwellings, and hence to partial melting and depletion in the latter. This acts to increase simultaneously buoyancy and thickness. Mixing processes are also significant. At low viscosity contrast, instability leads to mixing with the underlying fluid and to a thinner layer. At large viscosity contrast, material underlying the root gets entrained into the root, leading to a thicker layer. Different scenarii for the stabilization of a continental root that are consistent with dynamical constraints will be presented. They can be separated by the sequence of events (i.e. thickening or thinning, increasing or decreasing density contrast) as well as by the characteristic times and dimensions of successive instabilities.
V43B-1374
Rebuilding the Mojave Mantle Lithosphere by Underplating of Farallon Slab
Several lines of evidence indicate that much of the Laramide and post-Laramie extension of the Mojave-Salinia lithosphere occurred atop the flat Farallon oceanic slab, possibly as it began to rollback to a steeper subduction trajectory. Simple mass balance that ignores erosion suggests that the ~200% extension should have been accompanied by about the same amplitude of thinning. That is, for a pre-extension lithosphere thickness of 120-150 km, similar to that of the neighboring stable Colorado Plateau, the average thickness of the Mojave- Salinia lithosphere resulted by extension should not exceed 40-50 km. Thus, the present day 70¬-90 km thickness of the lithosphere beneath the Mojave Desert cannot be explained by extension alone. Using spinel peridotite xenoliths from Dish Hill, here we test two potential causes that could be responsible for the thickness of the Mojave mantle: a) preservation of a large segment of Farallon slab attached to the bottom of the attenuated continental lithosphere, and b) upwelling and melt depletion of asthenosphere as response to removal of the Farallon slab. Xenolith whole rock chemistry, clinopyroxene trace element and Nd isotope data in combination with two-pyroxene geothermometry indicate important compositional layering of the mantle beneath Dish Hill. The shallowest levels represented by harzburgites with εNd = -13 to -6.4 are interpreted as relics of the attenuated continetal lithospheric mantle. In contrast, the deeper xenoliths range from relatively fertile lherzolites to harzburgites, are moderately LREE depleted, consistently enriched in radiogenic Nd (ε¬Nd = +5.7 to +16.1), and define two superposed, downwards depleted mantle slices. Mineral compositions of harzburgites indicate 12-15% depletion that are consistent with melting at mid-ocean ridges, but preclude deep in-situ melting of decompressing asthenosphere beneath the Mojave. The composition of the superposed mantle slices strongly suggest the presence of Farallon mantle lithosphere in which duplex structures developed during its underthrusting. Altogether, xenolith data support the idea that much of the Mojave lithosphere consists of underplated oceanic slab. If so, this may explain the relative strength of the Mojave lithosphere in comparison with the Basin and Range Province that was strongly thinned during most of the Tertiary, after it has lost the underlying flat slab, if it ever existed in that region.
V43B-1375
Hafnium Isotope Composition of Archean Zircons from Xenoliths of the Snake River Plain, Idaho
The composition, structure, spatial extent, and history of Archean crust buried beneath the Snake River Plain (SRP) are important for assessing the role of the lithosphere in regional igneous and tectonic activity. We report the U-Pb age and Hf isotope composition of Archean zircons from xenoliths entrained in Snake River Plain basalts. The xenoliths come from three localities on the SRP: Square Mountain (SM), Craters of the Moon National Monument (COM) and the Spencer-Kilgore (SK) volcanic field. Cathodoluminescence imaging and previous age dating of the zircons show that many are complexly zoned and for this reason a majority of the Hf isotope data was determined via laser ablation MC-ICPMS. We used a New Wave UP-213 Nd-YAG laser interfaced with a Thermo-Finnigan Neptune MC-ICPMS and Element II HR-ICPMS for Hf isotope determinations U-Pb age dating, respectively. Previous U-Pb age dating has shown that the zircons vary from having simple age systematics (e.g., SM xenolith DM103 has a single zircon age population of ~ 2.58 Ga) to highly complex (e.g., COM xenoliths have zircons with ages populations from 2.7 to 3.2 Ga). All these zircons are Archean in age, although some have young low-U overgrowths jacketing their Archean cores which yield 206Pb/238U ages of ~19-25 Ma. There is no evidence in any of the xenoliths for zircon growth between late Archean and these young ages which attests to the stability of the lithosphere during this span of time. The Hf isotopic compositions of the Archean zircon grains are extremely unradiogenic consistent with their old age. For example, a SM xenolith (DM103) has present-day εHf values of -61 to -58 (initial εHf values at 2.56 Ga of -5 to -2), comparable with data determined by conventional solution-based methods. Another zircon from a COM xenolith (COM22) has an Archean core (2.7 Ga) and a large overgrowth with an age of ~20 Ma. Remarkably the present day εHf values of the core and overgrowth are identical within analytical uncertainties, -76 and -73, respectively. This indicates the young overgrowth consists entirely of recycled Archean crust with no detectable involvement of mantle derived Hf. The low εHf values imply little material exchange between Archean crust and SRP melts (typical εHf between -10 and +10). The ages (~20 Ma) and the lack of a mantle component of the zircon overgrowths seem to preclude that young zircon growth was produced during SRP magmatism. However, the recent transfer of heat into the Archean basement may provide important clues concerning the processes operative in the lithosphere prior to and perhaps during SRP magmatism.
V43B-1376
Upper Crustal Abundances of Trace Elements: A revision and update
We report new estimates of abundances of rarely analyzed elements (Be, B, Ge, As, Mo, Cd, In, Sn, Sb, Te, W, Tl, Bi) in the upper continental crust based on precise analysis by ICP-MS of well-characterized upper crustal samples (shales, pelites, loess, graywackes, granitoids and their composites) from Australia, China, Europe, New Zealand and North American. Abundances of Li, Sc, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Nb, Cs, Er, Tm, Yb, Lu, Ta, and U are also given, which show variations of >50% in previous estimates. Our new esitmate is mainly based on significant (r>0.78) inter-element correlations observed in the clastic sediments and sedimentary rocks we studied and from previous studies, which yield the following elemental ratios in the upper continental crust: La/Nb=2.68, La/Ta=33.73, Nb/Ta=12.70, Zr/Hf=36.23, Al2O3/Ga=8280, Ga/In=280, Th/Sn=43.38, Th/Rb=0.109, Rb/Cs=19.20, Rb/Tl=0.548, Th/Be=5.42, Rb/Be=50.24, Rb/Ge=72.07, Rb/W=66.40, Be/Bi=8.68, W/Bi=6.24, La/U=11.72, Fe2O3/Sc=4070, Fe2O3/Co=4030, Fe2O3/V=558, Fe2O3/Cr=804, Fe2O3/Ni=1880, Fe2O3/Cu=2180, Fe2O3/Zn=794, V/Sc=7.65, V/Co=6.97, Co/Cr=0.202, Co/Ni=0.433, Ni/Cr=0.458, In/Li=0.00168, B/Te=1758, Er/Tm=6.31, Er/Yb=0.986 and Er/Lu=6.46. Given the well-known upper crustal abundances of La (31 ppm), Th (10.5 ppm), Al2O3 (15.40%) and Fe2O3 (5.92%), these ratios lead to upper crustal abundances of Nb=11.6 ppm, Ta=0.92 ppm, Ga=18.34 ppm, In=0.066 ppm, Sn=2.28 ppm, Rb=94 ppm, Cs=4.92 ppm, Tl=0.53 ppm, Be=1.92 ppm, Ge=1.31 ppm, W=1.42 ppm, Bi=0.23 ppm, U=2.65 ppm, Sc=13.9 ppm, Co=14.7 ppm, V=106 ppm, Cu=27.2 ppm, Zn=74.6 ppm, Ni=34.0 ppm, Cr=72.8 ppm, Li=39 ppm, Te=0.019 ppm. No significant correlations exist between As, Sb, Mo, Cd, B and Te and other elements in the clastic sediments and sedimentary rocks. We thus calculate abundances of these elements by assuming the upper continental crust consists of 65% granitoid rocks plus 35% clastic sedimentary rocks. The validity of this assumption is supported by back- calculated upper crustal SiO2, Al2O3, La and Th abundances, which are identical to the values of Rudnick and Gao (2003). We thus estimate upper crustal abundances of As=3.41 ppm, Sb=0.46 ppm, Mo=0.57 ppm, Cd=0.062 ppm, B=31.5 ppm and Te=0.018 ppm. We also suggest a ~20% increase in Tm, Yb and Lu abundances (i.e., 0.365 ppm Tm, 2.34 ppm Yb and 0.356 ppm Lu) compared to those of Rudnick and Gao (2003).