Geologic Evolution of North America: Geologic features suggest that the continent has grown and differentiated through geologic time

The oldest decipherable rock complexes within continents (more than 2.5 billion years old) are largely basaltic volcanics and graywacke. Recent and modern analogs are the island arcs formed along and adjacent to the unstable interface of continental and oceanic crusts. The major interfacial reactions (orogenies) incorporate pre-existing sial, oceanic crust, and mantle into crust of a more continental type. Incipient stages of continental evolution, more than 3 billion years ago, remain obscure. They may involve either a cataclysmic granite-forming event or a succession of volcanic-sedimentary and granite-forming cycles. Intermediate and recent stages of continental evolution, as indicated by data for North America, involve accretion of numerous crustal interfaces with fragments of adjacent continental crust and their partial melting, reinjection, elevation, unroofing, and stabilization. Areas of relict provinces defined by ages of granites suggest that continental growth is approximately linear. But the advanced differentiation found in many provinces and the known overlaps permit wide deviation from linearity in the direction of a more explosive early or intermediate growth.     

40K-(40)Ar constraints on recycling continental crust into the mantle

Extraction of potassium into magmas and outgassing of argon during melting constrain the relative amounts of potassium in the crust with respect to those of argon in the atmosphere. No more than 30% of the modern mass of the continents was subducted back into the mantle during Earth's history. It is estimated that 50 to 70% of the subducted sediments are reincorporated into the deep continental crust. A consequence of the limited exchange between the continental crust and the upper mantle is that the chemistry of the upper mantle is driven by exchange of material with the deep mantle.     

Niobium/Uranium Evidence for Early Formation of the Continental Crust

Niobium/uranium ratios in greenstone-belt basalts and gabbros indicate that parts of the Late Archean mantle beneath Western Australia underwent a level of melt extraction, resulting in formation of the continental crust, comparable to that seen in the present mantle. The implication is either that (i) the amount of continental crust that formed before 2.7 x 10(9) years ago was much greater than generally thought or (ii) crustal growth occurred by severe depletion of small volumes of the mantle rather than by moderate depletion of a large volume of mantle.     

Rutile-bearing refractory eclogites: missing link between continents and depleted mantle

A mass imbalance exists in Earth for Nb, Ta, and possibly Ti: continental crust and depleted mantle both have subchondritic Nb/Ta, Nb/La, and Ti/Zr, which requires the existence of an additional reservoir with superchondritic ratios, such as refractory eclogite produced by slab melting. Trace element compositions of minerals in xenolithic eclogites derived from cratonic lithospheric mantle show that rutile dominates the budget of Nb and Ta in the eclogites and imparts a superchondritic Nb/Ta, Nb/La, and Ti/Zr to the whole rocks. About 1 to 6 percent by weight of eclogite is required to solve the mass imbalance in the silicate Earth, and this reservoir must have an Nb concentration >/= 2 parts per million, Nb/La >/= 1.2, and Nb/Ta between 19 and 37-values that overlap those of the xenolithic eclogites. As the mass of eclogite in the continental lithosphere is significantly lower than this, much of this material may reside in the lower mantle, perhaps as deep as the core-mantle boundary.     

Hotspots, basalts, and the evolution of the mantle

The trace element concentration patterns of continental and ocean island basalts and of mid-ocean ridge basalts are complementary. The relative sizes of the source regions for these fundamentally different basalt types can be estimated from the trace element enrichment-depletion patterns. Their combined volume occupies most of the mantle above the 670 kilometer discontinuity. The source regions separated as a result of early mantle differentiation and crystal fractionation from the resulting melt. The mid-ocean ridge basalts source evolved from an eclogite cumulate that lost its late-stage enriched fluids at various times to the shallower mantle and continental crust. The mid-ocean ridge basalts source is rich in garnet and clinopyroxene, whereas the continental and ocean island basalt source is a garnet peridotite that has experienced secondary enrichment. These relationships are consistent with the evolution of a terrestrial magma ocean.     

Small-scale convective instability and upper mantle viscosity under california

Thermal calculations and convection analysis, constrained by seismic tomography results, suggest that a small-scale convective instability developed in the upper 200 kilometers of the mantle under California after the upwelling and cooling of asthenosphere into the slab window associated with the formation of the San Andreas transform boundary. The upper bound for the upper mantle viscosity in the slab window, 5 x 10(19) pascal seconds, is similar to independent estimates for the asthenosphere beneath young oceanic and tectonically active continental regions. These model calculations suggest that many tectonically active continental regions characterized by low upper mantle seismic velocities may be affected by time-dependent small-scale convection that can generate localized areas of uplift and subsidence.     

Argon-lead isotopic correlation in mid-atlantic ridge basalts

Step-heating analyses for Mid-Atlantic Ridge glass samples show that maximum 40Ar/36Ar values correlate with 206,207,208Pb/204Pb. These correlations hold for the whole Atlantic Ocean and therefore are unlikely to result from shallow-level contamination processes. Instead, they are taken as mixing hyperbolae between the degassed-depleted upper mantle and a recycled component characterized by high 206Pb/204Pb ratios (19 to 21) and low 40Ar/36Ar ratios (300 to 1000). These relations imply that argon may also be a tracer of mantle recycling.     

Production of isotopic variability in continental basalts by cryptic crustal contamination

Regional variations in the Nd, Sr, and Pb isotopic compositions of Neogene basalts from the western United States are commonly interpreted to originate in the subcontinental mantle. In southern California, isotopic variability is restricted to lavas that lack mantle-derived xenoliths; xenolith-bearing basalts have uniform isotopic compositions similar to those of ocean-island basalts (OIBs). Combined with available geochemical data, these observations suggest that isotopic variability at these volcanoes results from subtle crustal contamination, locally by mafic crust, of primitive OIB-like magma. Recognition of such cryptic contamination may help to reconcile local discrepancies between tectonic and isotopic views of the subcontinental mantle.     

Lithospheric Contributions to Arc Magmatism: Isotope Variations Along Strike in Volcanoes of Honshu, Japan

Major chemical exchange between the crust and mantle occurs in subduction zone environments, profoundly affecting the chemical evolution of Earth. The relative contributions of the subducting slab, mantle wedge, and arc lithosphere to the generation of island arc magmas, and ultimately new continental crust, are controversial. Isotopic data for lavas from a transect of volcanoes in a single arc segment of northern Honshu, Japan, have distinct variations coincident with changes in crustal lithology. These data imply that the relatively thin crustal lithosphere is an active geochemical filter for all traversing magmas and is responsible for significant modification of primary mantle melts.     

Mantle oxidation state and its relationship to tectonic environment and fluid speciation

The earth's mantle is degassed along mid-ocean ridges, while rehydration and possibly recarbonaton occurs at subduction zones. These processes and the speciation of C-H-O fluids in the mantle are related to the oxidation state of mantle peridotite. Peridotite xenoliths from continental localities exhibit an oxygen fugacity (fo(2)) range from -1.5 to +1.5 log units relative to the FMQ (fayalite-magnetite-quartz) buffer. The lowest values are from zones of continental extension. Highly oxidized xenoliths (fo(2) greater than FMQ) come from regions of recent or acive subduction (for example, Ichinomegata, Japan), are commonly amphibole-bearing, and show trace element and isotopic evidence of fluid-rock interaction. Peridotites from ocean ridges are reduced and have an averae fo(2) of about -0.9 log units relative to FMQ, virtually coincident with values obtained from mid-ocean ridge basalt (MORB) glasses. These data are further evidence of the genetic link between MORB liquids and residual peridotite and indicate that the asthenosphere, although reducing, has CO(2) and H(2)O as its major fluid species. Incorporation of oxidized material from subduction zones into the continental lithosphere produces xenoliths that have both asthenospheric and subduction signatures. Fluids in the lithosphere are also dominated by CO(2) and H(2)O, and native C is generally unstable. Although the occurrence of native C (diamond) in deep-seated garnetiferous xenoliths and kimberlites does not require reducing conditions, calculations indicate that high Fe(3+) contents are stabilized in the garnet structure and that fo(2) deareases with increasing depth.     

Short-lived chemical heterogeneities in the archean mantle with implications for mantle convection

The neodymium isotope and samarium-neodymium systematics of 2.7-billion-year-old mantle-derived magmas indicate that the lifetime of chemical heterogeneities was much shorter in the Archean mantle than in the modern mantle. Isotopic evidence is compatible with a Rayleigh number 100 times larger and convection 10 times faster in the Late Archean compared with the present-day mantle. Modern plate tectonics thus may be an improbable analog for the Archean. Chemical heterogeneities in the mantle may originate upon magma migration and mineralogical phase changes rather than by recycling of oceanic and continental crust.     

Inner core differential motion confirmed by earthquake waveform doublets

We analyzed 18 high-quality waveform doublets with time separations of up to 35 years in the South Sandwich Islands region, for which the seismic signals have traversed the inner core as PKP(DF). The doublets show a consistent temporal change of travel times at up to 58 stations in and near Alaska, and they show a dissimilarity of PKP(DF) coda. Using waveform doublets avoids artifacts of earthquake mislocations and contamination from small-scale heterogeneities. Our results confirm that Earth's inner core is rotating faster than the mantle and crust at about 0.3 degrees to 0.5 degrees per year.     

Earthquakes beneath the Himalayas and Tibet: evidence for strong lithospheric mantle

Eleven intracontinental earthquakes, with magnitudes ranging from 4.9 to 6, occurred in the mantle beneath the western Himalayan syntaxis, the western Kunlun Mountains, and southern Tibet (near Xigaze) between 1963 and 1999. High-resolution seismic waveforms show that some focal depths exceeded 100 kilometers, indicating that these earthquakes occurred in the mantle portion of the lithosphere, even though the crust has been thickened there. The occurrence of earthquakes in the mantle beneath continental regions where the subduction of oceanic lithosphere ceased tens of millions years ago indicates that the mantle lithosphere is sufficiently strong to accumulate elastic strain.     

A model for plate tectonic evolution of mantle layers

In plate tectonic theory, lithosphere that descends into the mantle has a largely derivative composition, because it is produced as a refractory residue by partial melting, and cannot be resorbed readily by the parent mantle. We suggest that lithosphere sinks through the asthenosphere, or outer mantle, and accumulates progressively beneath to form an accretionary mesosphere, or inner mantle. According to this model, there is an irreversible physicochemical evolution of the mantle and its layers. We make the key assumption that the rate at which mass has been transferred from the lithosphere to the mesosphere is proportional to the rate of radiogenic heat production. Calculations of mass transfer with time demonstrate that the entire mass of the present mesosphere could have been produced in geologically reasonable times (3 x 10(9) to 4.5 x 10(9) years). The model is consistent with the generation of the continental crust during the last 3 x 1O(9) years and predicts an end to plate tectonic behavior within the next 10(9) years.     

Heterogeneous Hadean hafnium: evidence of continental crust at 4.4 to 4.5 ga

The long-favored paradigm for the development of continental crust is one of progressive growth beginning at approximately 4 billion years ago (Ga). To test this hypothesis, we measured initial 176Hf/177Hf values of 4.01- to 4.37-Ga detrital zircons from Jack Hills, Western Australia. epsilonHf (deviations of 176Hf/177Hf from bulk Earth in parts per 10(4)) values show large positive and negative deviations from those of the bulk Earth. Negative values indicate the development of a Lu/Hf reservoir that is consistent with the formation of continental crust (Lu/Hf approximately 0.01), perhaps as early as 4.5 Ga. Positive epsilon(Hf) deviations require early and likely widespread depletion of the upper mantle. These results support the view that continental crust had formed by 4.4 to 4.5 Ga and was rapidly recycled into the mantle.     

Start of the Wilson cycle at 3 Ga shown by diamonds from subcontinental mantle

Mineral inclusions encapsulated in diamonds are the oldest, deepest, and most pristine samples of Earth's mantle. They provide age and chemical information over a period of 3.5 billion years--a span that includes continental crustal growth, atmospheric evolution, and the initiation of plate tectonics. We compiled isotopic and bulk chemical data of silicate and sulfide inclusions and found that a compositional change occurred 3.0 billion years ago (Ga). Before 3.2 Ga, only diamonds with peridotitic compositions formed, whereas after 3.0 Ga, eclogitic diamonds became prevalent. We suggest that this resulted from the capture of eclogite and diamond-forming fluids in subcontinental mantle via subduction and continental collision, marking the onset of the Wilson cycle of plate tectonics.     

The Seismic 8degrees Discontinuity and Partial Melting in Continental Mantle

Strong, scattered reflections beyond 8 degrees (8degrees) offset are characteristic features of all high-resolution seismic sections from the continents. The reflections identify a low-velocity zone below approximately 100 kilometers depth beneath generally stratified mantle. This zone may be caused by partial melting, globally initiated at equal depth in the continental mantle. Solid state is again attained at the Lehmann discontinuity in cold, stable areas, whereas the zone extends to near the 400-kilometer discontinuity in hot, tectonically active areas. Thus, the depth to the Lehmann discontinuity may be an indicator of the thermal state of the continental mantle.     

Birth of the Kaapvaal tectosphere 3.08 billion years ago

The crustal remnants of Earth's Archean continents have been shielded from mantle convection by thick roots of ancient mantle lithosphere. The precise time of crust-root coupling (tectosphere birth) is poorly known but is needed to test competing theories of continental plate genesis. Our mapping and geochronology of an impact-generated section through the Mesoarchean crust of the Kaapvaal craton indicates tectosphere birth at 3.08 +/- 0.01 billion years ago, roughly 0.12 billion years after crust assembly. Growth of the southern African mantle root by subduction processes occurred within about 0.2 billion years. The assembly of crust before mantle may be common to the tectosphere.     

Genetic relations of oceanic basalts as indicated by lead isotopes

The isotopic compositions of lead and the concentrations lead, uranium, and thorium in samples of oceanic tholeiite and alkali suites are determined, and the genetic relations of the oceanic basalts are discussed. Lead of the oceanic tholeiites has a varying lead-206: lead-204 ratio between 17.8 and 18.8, while leads of the alkali basalt suites from Easter Island and Guadalupe Island are very radiogenic with lead-206: lead-204 ratios between 19.3 and 20.4 It is concluded that (i) the isotopic composition of lead in oceanic tholeiite suggests that the upper mantle source region of the tholeiite was differentiated from and original mantle material more than 1 billion years ago and that the upper mantle is not homogeneous at the present time, (ii) less than 20 million years was required for the crystal differentiation within the alkali suite from Easter Island, (iii) no crustal contamination was involved in the course of differentiation of rocks from Easter Island; however, some crustal contamination may have affected Guadalupe Island rocks, and (iv) alkali basalt may be produced from the tholeiite in the oceanic region by crystal differentiation. Alternatively the difference in the isotopic composition of lead in oceanic basalts may be produced by partial melting at different depths of a differentiated upper mantle.     

Copper systematics in arc magmas and implications for crust-mantle differentiation

Arc magmas are important building blocks of the continental crust. Because many arc lavas are oxidized, continent formation is thought to be associated with oxidizing conditions. On the basis of copper's (Cu's) affinity for reduced sulfur phases, we tracked the redox state of arc magmas from mantle source to emplacement in the crust. Primary arc and mid-ocean ridge basalts have identical Cu contents, indicating that the redox states of primitive arc magmas are indistinguishable from that of mid-ocean ridge basalts. During magmatic differentiation, the Cu content of most arc magmas decreases markedly because of sulfide segregation. Because a similar depletion in Cu characterizes global continental crust, the formation of sulfide-bearing cumulates under reducing conditions may be a critical step in continent formation.