Abstracts of Recent Papers
Kent C. Condie, Professor of Geochemistry

1. High field strength element ratios in Archean basalts: a window to evolving sources of mantle plumes?
Lithos, 79 (2005): 491–504

Kent C. Condie
    In terms of high field strength element ratios Nb/Th, Zr/Nb, Nb/Y and Zr/Y, most basalts from non-arc type Archean greenstones are similar to oceanic plateau basalts, suggestive of mantle plume sources. A large number of these basalts have ratios similar to primitive mantle composition. Perhaps the Archean mantle was less fractionated than at present and bprimitive mantleQ comprised much of the deep mantle and made a significant contribution to mantle plumes. The near absence of Archean greenstone basalts similar to NMORB in composition is also consistent with a relatively unfractionated mantle in which a shallow depleted source (DM) was volumetrically insignificant. The element ratios in basalts also indicate the existence of recycled components (HIMU, EM1, EM2) in the mantle by the Late Archean. This suggests that oceanic lithosphere was recycled into the deep mantle and became incorporated in some mantle plumes by the Late Archean. High field strength element ratios also indicate an important contribution of continental crust or/and subcontinental lithosphere to some non-arc Archean greenstone basalts. This implies that at least thin continental lithosphere was relatively widespread in the Archean.

2. U–Pb isotopic ages and Hf isotopic composition of single zircons: The search for juvenile Precambrian continental crust
Precambrian Research, 139 (2005): 42–100

Kent C. Condie, Eloise Beyer, Elena Belousova, W.L. Griffin, Suzanne Y. O’Reilly
    U–Pb isotopic ages and Hf isotope compositions by laser microprobe ICPMS and MC-ICPMS can be useful in identifying detrital zircons derived from juvenile continental sources. Hf isotopes in detrital zircons from modern river deposits in Brazil, Australia, India and the Ukraine show evidence for production of juvenile crust at about 2.5 Ga. However, if detrital zircon populations in the 2.4–2.2 Ga time window are representative of the proportion of juvenile crust in their primary sources, they yield little evidence for significant volumes of juvenile crust of this age.
   Hf isotopic compositions of detrital zircons from the Ukraine and eastern Australia also record production of juvenile continental crust between 1.65 and 1.40 Ga. Zircons from granitoids in south-central Laurentia and in western Brazil have EHf(T) values that fall near the depleted mantle growth curve recording production of juvenile continental crust in these regions between 1.5 and 1.3 Ga.
   Although Hf isotope compositions of detrital zircons can be useful in identifying detrital zircons derived from juvenile continental sources, the results do not necessarily equate to the volume of juvenile continental crust produced during specific time intervals.

3. TTGs and adakites: are they both slab melts?
Lithos, 80 (2005): 33– 44

Kent C. Condie
    Although both high-Al TTG (tonalite–trondhjemite–granodiorite) and adakite show strongly fractionated REE and incompatible element patterns, TTGs have lower Sr, Mg, Ni, Cr, and Nb/Ta than most adakites. These compositional differences cannot be easily related by shallow fractional crystallization. While adakites are probably slab melts, TTGs may be produced by partial melting of hydrous mafic rocks in the lower crust in arc systems or in the Archean, perhaps in the root zones of oceanic plateaus. It is important to emphasize that geochemical data can be used to help constrain tectonic settings, but it cannot be used alone to reconstruct ancient tectonic settings.
   Depletion in heavy REE and low Nb/Ta ratios in high-Al TTGs require both garnet and low-Mg amphibole in the restite, whereas moderate to high Sr values allow little, if any, plagioclase in the restite. To meet these requirements requires melting in the hornblende eclogite stability field between 40- and 80-km deep and between 700 and 800 8C.
    Some high-Al TTGs produced at 2.7 Ga and perhaps again at about 1.9 Ga show unusually high La/Yb, Sr, Cr, and Ni.
These TTGs may reflect catastrophic mantle overturn events at 2.7 and 1.9 Ga, during which a large number of mantle plumes bombarded the base of the lithosphere, producing thick oceanic plateaus that partially melted at depth.

4. Archean Geodynamics: Similar to or Different From Modern Geodynamics?, Archean Geodynamics and Environments
AGU Geophysical Monograph Series 164, (2006), 10.1029/164GM05

Kent C. Condie, Keith Benn
    There is a wealth of geologic, geochemical, structural, volcanologic, and sedimentologic data that are consistent with Archean plate tectonics, especially after 3.0 Ga. Neither the eruption of submarine basalts onto thinned continental crust nor the existence of ductile or viscous diapirism precludes the existence of plate tectonics during the Archean. Some “missing indicators” of plate tectonics are found in Archean terranes (probable oceanic crust, melange, possible passive margin sequences, boninite), whereas the absence of others (such as blueschists) can be explained by a higher Archean mantle geotherm. Bimodal magmatism is not limited to the Archean but occurs in several modern tectonic settings. The relative abundance of komatiites in the Archean reflects hotter Archean mantle and possibly widespread mantle plume activity. Any viable model for Archean geodynamics must accommodate the following 10 constraints: During the Archean, the mantle was hotter than it is today; there are two styles of crustal deformation in the Archean; komatiite is proportionally more abundant in Archean greenstones than in younger greenstones; tonalite–trondhjemite–granodiorite depleted in heavy rare earth elements is more widespread in the Archean than afterwards; thick lithosphere underlies many Archean cratons; portions of the mantle were strongly depleted in large ion lithophile elements during the Archean; many Archean greenstones comprise arc-like rock assemblages; a significant proportion of Archean greenstones contain volcanic rocks with geochemical characteristics similar to modern plume-derived basalts; paleomagnetic data indicate that apparent polar wandering occurred during the Archean; and a large volume of continental crust was produced about 2.7 Ga.

5. Episodic continental growth and supercontinents:  a mantle avalanche connection?
Earth & Planetary Science Letters, 163:  97-108.

Kent C. Condie
    Episodic growth of continental crust and supercontinents at 2.7, 1.9, and 1.2 Ga may be caused by superevents in the mantle as descending slabs pile up at the 660-km seismic discontinuity and then catastrophically sink into the lower mantle. Superevents comprise three or four events, each of 50-80 My duration, and each of which may reflect slab avalanches at different locations and times at the 660-km discontinuity. Superplume events in the late Paleozoic and Mid-Cretaceous may have been caused by minor slab avalanches as the 660 became more permeable to the passage of slabs with time. The total duration of a superevent cycle decreases with time reflecting the cooling of the mantle.

6. The 1.75-Ga Iron King Volcanics in west-central Arizona:  a remnant of an accreted oceanic plateau derived from a mantle plume with a deep depleted component
Lithos, 64: 49-62.

Kent C. Condie, Bonnie A. Frey and  Robert Kerrich
    Two lithologic assemblages are recognized in the 1.75-Ga Iron King Volcanics in west-central Arizona:  an arc assemblage composed of pillow basalts, intermediate and felsic volcanics and associated volcaniclastic sediment, and an oceanic plateau assemblage composed chiefly of pillow basalts and mafic hyaloclastic breccia. During collision of the Iron King oceanic plateau with Laurentia 1.7 Ga, plateau and arc components were tectonically interleaved.
    Iron King arc volcanics have subduction-related geochemical signatures with affinities to continental margin arcs. The Iron King plateau basalts include two groups: enriched and nonenriched in very incompatible elements. The enriched group may reflect smaller degrees of melting in the outer, cooler part of a plume head. Incompatible element distributions indicate that the plateau basalts came from a mantle source with mixed depleted and recycled components and a small contribution of an enriched component. Nb/Y-Zr/Y relationships suggest the depleted component came from the deep mantle, rather than being entrained at shallow depths. Our results suggest that recycled, depleted and enriched components were available in the deep mantle by 1.75 Ga.

7. The supercontinent cycle: are there two patterns of cyclicity?
Journal of African Earth Sciences, 35(2): 179-183.

Kent C. Condie
    Continental rifting and collisional events in the last 1000 My indicate two types of supercontinent cycles: one in which breakup of one supercontinent is followed by formation of another supercontinent, and one in which a new supercontinent forms from long-lived, small supercontinents, which never fragment or incompletely fragment due to insufficient mantle shielding. The small supercontinents may form over linear, disconnected subduction arrays rather than over a region with a high density of closely connected subduction arrays.

8. Accretionary orogens in space and time
Geological Society of America Memoir 200: 145-158.

Kent C. Condie
    Accretionary orogens form along continental margins where oceanic lithosphere is subducted. They are primary sites of juvenile continental crust production and have been active on Earth since the earliest Archean. Orogen lifetimes expressed as accretion intervals range from 50 to over 300 m.y. The short duration of Late Archean accretionary orogens (<70 m.y.) may reflect the short duration of a global mantle plume event at 2.7 and 2.5 Ga. Although there is no simple relationship between the onset or duration of accretionary orogens and the supercontinent cycle, many post-Archean orogens terminate with continent-continent collisions during supercontinent assembly.
    Average terrane lifespan is typically 100–200 m.y. in post–1 Ga orogens, 50–100 m.y. in pre–1 Ga Proterozoic orogens, and 70–700 m.y. in Archean orogens. Accretionary orogens can be grouped into two end members: simple orogens containing chiefly juvenile terranes with lifespans of <100 m.y., and complex orogens with both juvenile accreted components and exotic microcratons, with terrane lifespans of 100 m.y. Terrane lifespan is controlled by (1) terrane tectonic setting, (2) complexity of precollisional terrane history, (3) availability of continental crust on Earth, and (4) plate history of ocean basins adjacent to accretionary orogens.
   Average accretion rates in accretionary orogens are 70 to 150 km3/km/m.y. in Phanerozoic orogens and 100 to 200 km3/km/m.y. in Precambrian orogens. Some orogens at 2.7 Ga have unusually high accretion rates greater than 300 km3/km/m.y., which may reflect a global mantle plume event. Production rates of juvenile crust in accretionary orogens are typically 10%–30% lower than total accretion rates, but can be up to 50% lower in Phanerozoic orogens.