V21D-01 INVITED
Mantle Episodicity From Within and From Above
Compilations of continental crustal age distribution show large peaks at 1.1, 1.9-2.1, 2.7 and 3.5Ga. These peaks have been interpreted as evidence of episodicity in the Earth's mantle-lithosphere system. Paleomagnetic evidence suggests periods of rapid plate motions coinciding with the peaks in crustal age distribution. We review a recent model for episodic tectonics in the Precambrian, where higher mantle temperatures result in lower lithospheric stresses, causing rapid pulses of subduction interspersed with periods of relative quiescence. Plate-driven episodicity arises for hotter mantle temperatures of the early Earth and can explain rapid pulses of plate motion and crustal production. Increased internal temperatures within the mantle of a terrestrial planet can also be driven from above by a runaway greenhouse atmosphere and this serves as another trigger for episodic mantle convection. We review a scaling theory and numerical simulations that explore this mechanism and discuss the implications for Earth-Venus differences.
V21D-02 INVITED
Episodicity of Orogeny Revisited
Although it is well established that orogeny is episodic, the duration, correlation and geographic distribution of orogenic episodes is not well constrained. Using large numbers of concordant U/Pb zircon ages from subduction-related granitoids (> 7000), it is now possible to better constrain these variables. Monte Carlo simulation probabilistic histograms of zircon age spectra remove questionable and spurious age peaks, yet allow resolution of peaks with >10 My duration with the data sets. Orogenic episodes with durations < 20 My, herein called pulses, are generally of regional geographic extent, whereas long-lived events (100-250 My), herein called periods, may be of regional or global extent. Orogenic periods comprise several to many pulses. Most orogenic pulses reflect geographic variations in intensity of subduction or/and plate collisions as for instance recorded around the perimeter of the Pacific basin in the last 100 My. Neither of the widely recognized pulses at 2.7 nor 1.9 Ga is global in extent. Orogenic pulses at 2700 and 2680 Ma occur on four continents each (2700: Superior, Hearne-Rae, Nain, North China; 2680: Yilgarn, Africa, Slave, Wyoming). Likewise, an orogenic pulse at 1880 is found on four continents (Laurentia, Baltica, East Asia, South America), and another pulse at 1860 Ma occurs on three continents (Africa, Siberia, Australia). Some orogenic pulses track lateral continental growth, such as 2730, 2715, and 2700 Ma pulses in the Abitibi greenstone belt, and 850, 800 and 750 Ma pulses in the Arabian-Nubian shield. Major orogenic periods are recognized at 2750-2650, 1900-1650, and 1250-1000 Ma and each of these is associated with supercontinent formation. Orogenic periods at 2600-2500 (China and India) and 2150-2050 Ma (West Africa, Amazonia, Rio de la Plata) may be associated with the formation of small supercontinents. Our results suggest that orogenic periods with intervening gaps may not require sudden and short-lived changes in mantle behavior, but may be associated primarily with the supercontinent cycle, and thus be a characteristic feature of planets with plate tectonics.
V21D-03 INVITED
Preservation of Age Peaks in the Continental Record
Peaks in the distribution of U-Pb crystallization ages, and the ages of rocks that reflect new continental crust, are striking features of the continental crust. In the last 1 Gyr there are marked peaks in the zircon crystallization ages, and no analogous peaks of crust formation. In older rocks there appears to be a better match between the peaks of magmatic activity (zircon ages) and the peaks of crust generation. This contribution explores the extent to which these apparent peaks of magmatic activity reflect the preservation potential of magmatism in different tectonic settings, rather than fundamental pulses of magmatic activity. Peaks of crystallization ages can be linked to the ages of super-continents. These involve subduction-related magmatism, collisional orogens and crustal melting, and subsequently extensional magmatism. The former have poor preservation potential: the half life of Cu porphyry deposits is ~ 100Myr, and the available data indicate that the rates of subduction of continental sediment are similar to the rates at which it is generated at magmatic arcs. Extensional magmatism is unlikely to result in large volumes of zircons, and the rocks may be relatively sensitive to erosion into the oceans. Collisional magmatism is dominated by partial melting of the pre-existing crust, it is granitic, and protected within the super-continent. Thus, the preservation potential, particularly for crystallization ages for zircons, may be greater for late stage events as the super-continents come together, rather than for subduction- and extension-related magmatism. This is consistent with the data for the last billion years (Fig. 1), but earlier in earth history the peaks of ages of crystallisation also match up with peaks of crust generation. The peaks of ages are not dominated by periods of crustal remelting, and so other processes must have dominated, and shaped the geological record. These will be discussed in terms of the cratonization of Archaean crust and the record of back-arc magmatism in the turbidite-granitoid belts of the Proterozoic. Back-arc settings, as seen in the 520-230 Myr Delamerian to New England belts of eastern Australia, offer better preservation potential than magmatic arcs, but lower rates of crust generation.
V21D-04 INVITED
Does the Earth's Mantle pulse? Mixture Models and Model Ages
The possibility that the Earth's mantle produces magma in major episodes has been lead by the observation that crustal rocks of certain ages appear more abundant than others. Recently other lines of evidence in support of episodic melting events have come from He isotopes [1] and the Os isotope analysis of mantle- derived platinum group alloy (PGA) grains [2]. More detailed scrutiny of this possibility is required from a statistical view-point and within the context of the significance of Os model ages in mantle rocks and minerals. One obvious problem is the mis-match in parameters used for Os model age calculations compared with the absolute ages obtained from U-Pb dating of crustal rocks. Another is the potential over-sampling of crustal rocks of specific ages. Here we present a summary of Os isotope evidence from both orogenic and ophiolitic peridotite massifs together with PGA grains to document potential age coincidences. We compare simple and complex Bayesian mixture models to pick out possible "age" populations within large PGA datasets and we evaluate differing methods of obtaining more precise melting ages from mantle rocks and minerals. Os isotope variability in some basaltic magmas is also examined to look for evidence of the sampling of old heterogeneities during mantle melting. [1] S.W. Parman (2006) Helium isotopic evidence for episodic mantle melting and crustal growth. Nature, 446, 900-903. [2] D.G. Pearson, S.W. Parman and G.M. Nowell (2007) A link between large mantle melting events and continent growth seen in Osmium isotopes. Nature, 449, 202-205.