Meteorite zircon constraints on the bulk Lu−Hf isotope composition and early differentiation of the Earth

Abstract

Knowledge of planetary differentiation is crucial for understanding the chemical and thermal evolution of terrestrial planets. The 176 Hf radioactive decay system has been widely used to constrain the timescales and mechanisms of silicate differentiation on Earth, but the data interpretation requires accurate estimation of Hf isotope evolution of the bulk Earth. Because both Lu and Hf are refractory lithophile elements, the isotope evolution can be potentially extrapolated from the present-day 176 Hf radioactive decay system is a powerful tool to study planetary silicate differentiation and offers a unique means of examining the origin of the Earth's earliest crust (1-12). To allow for the correct interpretation of Lu−Hf data, however, the Hf isotope growth curve of the bulk Earth must be firmly established. On the basis of the refractory lithophile nature of both Lu and Hf, the Lu−Hf isotope composition of chondrites has been proposed as a reference for the composition of the bulk Earth and its silicate portion (13-15). However, the present-day 176

The 176Lu-176Hf systematics of ALM-A: A sample of the recent Almahata Sitta meteorite fall.

“…The purest mineral fractions and all washes provide a crystallisation age for ALM-A of 4569 ± 24 Ma. The 176 Hf/ 177 Hf i of the ALM-A isochron, 0.279796 ± 0.000011, is identical to 1) the value of 0.279794 ± 0.000011 derived from the average composition of unequilibrated chondrites (Bouvier et al, 2008) calculated back to the start of the Solar System using λ 176 Lu = 1.867 ×10 -11 yr -1 and 2) the value of 0.279781 ± 0.000018 measured in eucrite zircon by Iizuka et al (2015). These estimates are all clearly higher than that of the Sahara 99555 regression (0.279685 ± 0.000019; Bizzarro et al, 2012).…”

Early formation of the Moon 4.51 billion years ago

“…Also, SIMS U-Pb ages were determined at the very surface of the zircon (~1-μm-deep, 40-μm-diameter pit), whereas LA-MC-ICPMS used a ~200 times larger analytical volume, therefore leaving open the possibility of inaccuracies in calculating ε 176 Hf( t ) by not measuring Lu-Hf and U-Pb isotopes in the same volume of zircon ( 25 ). Finally, the initial value and the evolutionary trajectory of 176 Hf/ 177 Hf of the solar system (CHUR) has been recently revised by direct measurement of zircon grains from the eucrite Agoult ( 26 ) that crystallized within the first ~12 My of the solar system. This work has significantly improved the precision and accuracy with which CHUR is known and therefore allows the determination of Lu/Hf model ages with high confidence.…”

“…ε 176 Hf CHUR is evaluated by the difference between initial ε 176 Hf( t ) and the Hf isotopic composition of CHUR at time t ( 26 ). Taylor et al ( 20 ) reported the data before and after neutron capture (NC) correction.…”

Protocrustal evolution of the Nuvvuagittuq Supracrustal Belt as determined by high precision zircon Lu–Hf and U–Pb isotope data

“…The first is which Lu/Hf ratios are to be considered representative for the source of the rocks analysed and the second is which initial Hf-isotopic composition to use for the chondritic uniform reservoir (CHUR; or bulk silicate Earth: BSE; Bouvier et al, 2008;Bizzaro et al, 2012;Iizuka et al, 2015). Amelin et al (1999) and subsequently Kemp et al (2010) suggested a 176 Lu/ 177 Hf of ca.…”

“…The choice of CHUR Lu-Hf parameters also changes the calculated model ages and even though Bizzaro et al (2012) recently proposed a new, lower CHUR initial Hf-isotopic composition, most workers still use the parameters of Bouvier et al (2008) which has recently been corroborated by Iizuka et al (2015) which obtained a CHUR initial of 176 Hf/ 177 Hf of 0.282793 (±0.000018), overlapping that of Bouvier et al (2008) of 0.282785 (±0.000011) and that in the work of Blichert-Toft and Albarède (1997; 0.282772 ± 0.000029). The choice of these parameters affects the 176 Hf/ 177 Hf (t) , ε Hf(t) , and model ages, and is thus a highly important consideration to make when comparing Lu-Hf model ages with model ages from other isotope systems (e.g.…”