Detall

The history of the Earth wirtten in helium

Notícia | 28-03-2007

Stephen Parman, Dept. of Earth Sciencies.
Univ. of Durham, United Kingdom

Dimecres, 28 de març de 2007, Sala de conferències de l'Institut Jaume Almera (CSIC), a les 12h.

The continental crust (CC) is a first order feature of the Earth, and one that distinguishes it from the other terrestrial planets. Precisely when it formed is a long-standing debate, with models ranging from steady, smooth growth, to pulsed growth, to early formation with little subsequent growth. In all of the models, the CC is thought to ultimately be derived from mantle melting. If so, the history of CC growth may also be recorded in the depletion of the mantle. Potentially, this mantle depletion history could be compared to the CC growth history to help distinguish between the competing growth models.
Recent experimental data suggest that He is more compatible than its parent isotopes U and Th. If so, high the 3He/4He ratios in OIB may record ancient, depleted mantle components, rather than primordial mantle as they are traditionally interpreted. In this talk I will show that the He isotopic spectrum of OIBs has distinct peaks that can be correlated from island to island (thus they are not sampling artifacts). Significantly, these global He isotopic peaks correlate with published peaks in CC zircon U-Pb and Hf model ages at 1.2, 1.9, 2.7 and 3.3 Ga. Even more significantly, the correlation projects to a modern 3He/4He value of 8 Ra (the dominant value for MORB) and an initial value of 120 Ra at 4.55 Ga (the expected initial Earth ratio based upon the atmosphere of Jupiter). The independent prediction of these two values is strong evidence that the correlation between the He and zircon age peaks is not spurious. The correlation of CC growth pulses with mantle depletion ages suggest


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