Insight into the East African lithosphere: hafnium isotope composition of clinopyroxene from Mega peridotite xenoliths (Ethiopia)

Mantle xenoliths from the Sidamo region, in the southern tip of the Main Ethiopian Rift, have been extensively studied [1-5] and have provided many insights into the chemical and thermal history of this active rift region. In this study, we focus on a collection of spinel-peridotite xenoliths from Mega previously characterized in terms of major and trace elements in both bulk rocks and constituent minerals and Sr-Nd isotopic compositions of separated clinopyroxene (cpx) [5]. The new Hf isotopic analyses carried out on mantle cpx by multi-collector inductively-coupled plasma mass spectrometry show extremely heterogeneous 176Hf/177Hf varying from slightly sub-chondritic (0.282737) to extremely radiogenic compositions (0.313198 in sample MA35, a cpx-poor lithology). 176Hf/177Hf correlates positively with Lu/Hf, providing an apparent time-integrated ingrowth of 1.96 Ga across the suite, close to the CHUR model age of the most radiogenic sample (1.99 Ga). Interestingly, 176Hf/177Hf also correlates with the modal abundance of cpx, with the more enriched Hf isotopic compositions (corresponding to the lower Lu/Hf ratios) recorded in the most refractory harzburgites. These observations, taken together, indicate that melt extraction alone cannot explain the process that led to these signatures. We therefore propose a multi-step model that includes metasomatism, which affected preferentially the more refractory olivine-rich (more permeable) mantle domains. The anomalously radiogenic Hf isotopic composition of MA35 (and also MA19, another cpx-poor peridotite) is particularly visible when expressed with the ΔεHf parameter, which is a measure of the displacement of the Hf isotopic signature from the mantle array as defined by oceanic basalts (ΔεHf = εHf - 1.36*εNd+3, after [6], revised from [7]). Extremely radiogenic εHf (up to +1076 in sample MA35) accompanied by relatively normal mantle-like Nd isotopic compositions, are reflected by positive ΔεHf of over +1000, clearly indicating the decoupling of the Lu-Hf and Sm-Nd isotopic systems. One explanation for these observations may be found in a two-step process of (1) ancient melt extraction in the presence of residual garnet, which fractionated Lu from Hf [7,8], followed by (2) subsequent ascent of this mantle domain to depths shallower than the spinel-garnet transition [9,10]. A second explanation, provided by [11], is that diffusional loss of Nd is faster than Hf at mantle temperatures, leading to the development of what appears to be anomalously high εHf for a given εNd value. References: [1] Bedini et al. (1997): E.P.S.L.,153, 67-83. [2] Bedini & Bodinier (1999): Geochim. Cosmochim. Acta, 63, 3883-3900. [3] Reisberg et al. (2004): Chem. Geol., 208, 119-140. [4] Orlando et al. (2006): Ofioliti, 31, 71-87. [5] Beccaluva et al., GSA Special Paper “Volcanism and evolution of the African Lithosphere, in press”. [6] Vervoort et al. (1999): E.P.S.L., 168, 79-99. [7] Johnson & Beard (1993): Nature, 362, 441-444. [8] Salters & White (1998): Chem. Geol. 145, 447-460. [9] Bianchini et al. (2007): Lithos, 94, 25-45. [10] Downes et al. (2003): Chem. Geol. 200, 71-87. [11] Bedini et al. (2004): E.P.S.L., 223, 99-111.