K, U and Th in mid-ocean ridge basalt glasses and heat production, K/U and K/Rb in the mantle

Jochum, Klaus Peter; Hofmann, Albrecht W.; Ito, Emi; Seufert, H.; White, William M.

doi:10.1038/306431a0

Cited by 388 publications

(182 citation statements)

References 40 publications

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“…The contribution of the Earth's core is between 3 and 7 TW [Buffett et al, 1996]. Assuming the DMM abundances are a factor 1/2.6 smaller than the Bulk Silicate Earth (BSE) abundances according to Jochum et al [1983], then a whole mantle DMM would generate only 7.2 TW [Porcelli and Ballentine, 2002]. The contribution of secular cooling is between 21.8 TW and 17.8 TW.…”

Section: Chemical Differentiationmentioning

confidence: 99%

Mantle convection and evolution with growing continents

Walzer

Hendel

2008

J. Geophys. Res.

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[1] We present a three-dimensional spherical shell numerical model for chemical differentiation and redistribution of incompatible elements in a convective Earth's mantle heated mostly from within by U, Th, and K and slightly from below. The evolution-model equations guarantee conservation of mass, momentum, energy, angular momentum, and four sums of the number of atoms of the pairs 238 U-206 Pb, 235 U-207 Pb, 232 Th-208 Pb, and 40 K-40 Ar. The pressure-and temperature-dependent viscosity is supplemented by a viscoplastic yield stress, s y . The lithospheric viscosity is partly imposed to mimic its increase by dehydration of oceanic lithosphere and other effects. Also, the asthenosphere is generated not only by the distribution of temperature and melting temperature, but essentially by the profiles of water solubility and water abundance. Therefore we introduced a radial viscosity profile factor describing that behavior. However, the focus of this paper is the episodic growth of continents and oceanic plateaus. As a complement, the differentiation generates the depleted MORB mantle (DMM) which predominates immediately beneath the lithosphere. Our continents are not artificially imposed on the surface of the spherical shell, but instead they evolve by the interplay between chemical differentiation and convection/mixing. No restrictions are imposed regarding number, size, form, and distribution of continents. However, oceanic plateaus that impinge upon a continent have to be united with it. This mimics the accretion of terranes. The numerical results show an episodic growth of the total mass of the continents and display a plausible time history for the laterally averaged surface heat flow, qob, and the Rayleigh number, Ra. We use our model to explore a moderate region of Ra-s y parameter space. We find Earth-like continent distributions in a central part of the Ra-s y space we explored. We identified a Ra-s y region where the calculated total continental volume is very close to the observed value; another Ra-s y region where the Urey number, Ur, is close to the accepted value; a third Ra-s y area where surface heat flow is very close to the present-day observed mean global heat flow using typical abundances of the heat-producing elements. It is remarkable that these different acceptable Ra-s y regions share a common overlap area, where Earth-like behavior is simultaneously fulfilled.Although the convective flow patterns and the chemical differentiation of oceanic plateaus are coupled, the evolution of time-dependent Rayleigh number, Ra t , is relatively well predictable and the stochastic parts of the Ra t (t) curves are small. Regarding the time distribution of juvenile growth rates of the total mass of the continents, predictions are possible only in the first epoch of the evolution, presumed that the initial conditions are given. Later on, the distribution of the continental growth episodes is increasingly stochastic. Independent of the varying individual runs, our model shows that the total mass of the presen...

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Section: Chemical Differentiationmentioning

confidence: 99%

Mantle convection and evolution with growing continents

Walzer

Hendel

2008

J. Geophys. Res.

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“…The present-day UCTh concentration in the MORB-source or 'convecting' mantle, however, is reasonably well known (e.g. Jochum et al 1983). An estimate of the 3 He concentration in this mantle reservoir can be made from the present-day 3 He flux into the ocean (Lupton & Craig 1975) and the average oceanic crust production rate (Parsons 1981).…”

Section: Helium: the Case For An Accretionary Volatile Reservoir In Tmentioning

confidence: 99%

“…Comparing the solar 40 Ar/ 36 ArZ3!10 K4 (Anders & Grevesse 1989) with the terrestrial atmosphere value of 295 clearly shows the important input of 40 Ar into the atmosphere derived from the decay of 40 K from the solid Earth. If we take bulk silicate Earth (BSE) estimates of the K concentration based on the K/Uw12 500 ratio found in basalts (Jochum et al 1983), then some 45-50 per cent of the planet's 40 Ar is now found in the Earth's atmosphere (e.g. Hart et al 1979;Allègre et al 1996).…”

Section: Argon: a Note On Mantle Degassingmentioning

confidence: 99%

What CO ₂ well gases tell us about the origin of noble gases in the mantle and their relationship to the atmosphere

Ballentine

Holland²

2008

Phil. Trans. R. Soc. A.

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Study of commercially produced volcanic CO 2 gas associated with the Colorado Plateau, USA, has revealed substantial new information about the noble gas isotopic composition and elemental abundance pattern of the mantle. Combined with published data from midocean ridge basalts, it is now clear that the convecting mantle has a maximum 20 Ne/ 22 Ne isotopic composition, indistinguishable from that attributed to solar wind-implanted (SWI) neon in meteorites. This is distinct from the higher 20 Ne/ 22 Ne isotopic value expected for solar nebula gases. The non-radiogenic xenon isotopic composition of the well gases shows that 20 per cent of the mantle Xe is 'solar-like' in origin, but cannot resolve the small isotopic difference between the trapped meteorite 'Q'-component and solar Xe. The mantle primordial 20 Ne/ 132 Xe is approximately 1400 and is comparable with the upper end of that observed in meteorites. Previous work using the terrestrial 129 I-129 Xe mass balance demands that almost 99 per cent of the Xe (and therefore other noble gases) has been lost from the accreting solids and that Pu-I closure age models have shown this to have occurred in the first ca 100 Ma of the Earth's history. The highest concentrations of Q-Xe and solar wind-implanted (SWI)-Ne measured in meteorites allow for this loss and these high-abundance samples have a Ne/Xe ratio range compatible with the 'recycled-aircorrected' terrestrial mantle. These observations do not support models in which the terrestrial mantle acquired its volatiles from the primary capture of solar nebula gases and, in turn, strongly suggest that the primary terrestrial atmosphere, before isotopic fractionation, is most probably derived from degassed trapped volatiles in accreting material.By contrast, the non-radiogenic argon, krypton and 80 per cent of the xenon in the convecting mantle have the same isotopic composition and elemental abundance pattern as that found in seawater with a small sedimentary Kr and Xe admix. These mantle heavy noble gases are dominated by recycling of air dissolved in seawater back into the mantle. Numerical simulations suggest that plumes sampling the core-mantle boundary would be enriched in seawater-derived noble gases compared with the convecting mantle, and therefore have substantially lower 40 Ar/ 36 Ar. This is compatible with observation. The subduction process is not a complete barrier to volatile return to the mantle.

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“…These K/U values bracket thermal half-lives of 2.25 to 1.5 b.y., respectively. Low values of K/U are suggested from the generally low ratios of all terrestrial rocks (and in particular from an analysis of very fresh MORB glasses; Jochum et al [1983]. This suggests that the thermal half-life is close to 2 b.y.…”

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confidence: 99%

Mixing in numerical models of mantle convection incorporating plate kinematics

Gurnis¹,

Davies²

1986

J. Geophys. Res.

128

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Abstract.The process by which subducted lithosphere is mixed by mantle convection is investigated in numerical calculations.The results show that the observed isotopic heterogeneity of mantle sources and their ancient (1-2 b.y.) apparent ages are consistent with convective mixing.Passive tracers, which are introduced below "trenches," are efficiently dispersed, but nonetheless, heterogeneities in tracer density with a large range of length scales are observed to persist for 40 or more transit times (one transit time is the time to travel the fluid depth with the boundary velocity). In particular, there is a strong tendency to form high-density folds of the tracer strings, which persist much longer than simple shearing indicates. The folds persist because there is a strong tendency for material that enters the flow at the margins of cells to be transferred to adjacent cells, where it is "unmixed."When the simulations are scaled to the whole mantle, the tight clumps (folds) of tracers are shown to persist for up to 1-2 b .y. There is also a tendency for large-scale convection cells to remain isolated from recycled material for 1-2 b.y. These results are consistent with the significant chemical heterogeneity of the mantle as revealed by isotopic studies of oceanic basalts. Despite the spatial heterogeneity in tracer density, the average time tracers remain in the box from subduction at trenches to sampling at ridges (i.e., the residence time) is well constrained and within 20% of the mean residence time expected from an analytic model in which tracers are assumed to be sampled randomly. Model ages of the mantle that explicitly incorporate increased convection rates in the past and assume random sampling of heterogeneities bracket the -2 b.y. apparent Pb-Pb and Rb-Sr isochrons of midocean ridge basalts and oceanic island basalts. The conclusion of persistent spatial heterogeneity is different from the conclusions drawn from other studies. The different conclusions result, primarily, from our emphasis on the details of spatial variations as opposed to some average of the mixing, from a difference in flow unsteadiness, and from the different ways tracers have been introduced into the flow.

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K, U and Th in mid-ocean ridge basalt glasses and heat production, K/U and K/Rb in the mantle

Cited by 388 publications

References 40 publications

Mantle convection and evolution with growing continents

Mantle convection and evolution with growing continents

What CO ₂ well gases tell us about the origin of noble gases in the mantle and their relationship to the atmosphere

Mixing in numerical models of mantle convection incorporating plate kinematics

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K, U and Th in mid-ocean ridge basalt glasses and heat production, K/U and K/Rb in the mantle

Cited by 388 publications

References 40 publications

Mantle convection and evolution with growing continents

Mantle convection and evolution with growing continents

What CO 2 well gases tell us about the origin of noble gases in the mantle and their relationship to the atmosphere

Mixing in numerical models of mantle convection incorporating plate kinematics

Contact Info

Product

Resources

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What CO ₂ well gases tell us about the origin of noble gases in the mantle and their relationship to the atmosphere