Episodic Crustal Growth Recorded in the Depletion History of the Mantle

Details Episodic Crustal Growth Recorded in the Depletion History of the Mantle Episodic Crustal Growth Recorded in the Depletion History of the Mantle

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.v22c..04p&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #V22C-04

Other

1009 Geochemical Modeling (3610, 8410), 1025 Composition Of The Mantle, 1040 Radiogenic Isotope Geochemistry, 1115 Radioisotope Geochronology, 9623 Archean

Scientific paper

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 change in volume. In all of the models, the CC is assumed to form from mantle-derived melts. If so, the history of CC growth may be recorded in the radiogenic isotope composition of the mantle. Potentially, this mantle depletion history could be compared to the CC growth history to help distinguish between the competing 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. Indeed, recent studies of OIB and flood basalts have demonstrated that the highest 3He/4He magmas have depleted Sr, Nd and Pb isotopes similar to MORB, inconsistent with a primordial source. Here I 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 highly with published peaks in CC zircon U-Pb and Hf model ages. Even more interesting, the correlation projects to a modern 3He/4He value of 8 Ra (the value for most 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 evolution line suggests that convection is whole-mantle and that the dominant peak of 8 Ra in MORB is the average value for the whole mantle, while the high 3He/4He values in OIB record ancient melting events. The correlation of CC growth pulses with mantle depletion ages suggests that both were produced by the same large melting events, and that crustal growth was episodic.

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