Depletion ages and factors of MORB mantle sources

Abstract

Sr-Nd-Hf-Pb isotopes show that the depleted MORB mantle (DMM) is not homogeneous. The heterogeneity is attributed to different ages of depletion and/or various degrees of depletion for a given domain of DMM, as well as multiple depletion events, metasomatism, and mixing between DMM and other mantle components. A mid-ocean ridge basalt, in principle, should contain information about the depletion history of its mantle sources. Here we develop a model to extract the model depletion age and the composition of a MORB mantle source prior to MORB production using Sr-Nd isotopes or Sr-Hf isotopes in a MORB. The complexities of multiple depletion events, mixing, metasomatism, and enrichment are not addressed in this contribution. The model is based on two assumptions: (i) Isotope evolution in a MORB mantle follows a two-stage evolution model, the first stage in the primitive mantle from the beginning of the solar system to the time of mantle depletion at age Td, and the second stage in the depleted mantle from age Td to the present day. That is, there is only one single depletion event. The depletion age and degree of depletion of a given mantle source are to be determined. (ii) The trace element composition of a depleted mantle source corresponding to the given MORB can be related to a reference DMM by a log-linear relation with the compatibility index CoI (Zhang, 2014). Applying the two assumptions to the available and large MORB database (Gale et al., 2013), we calculate the global distribution of sub-ridge mantle age and composition. The results show: (i) Mean or reference MORB mantle composition of Salters and Stracke (2004) is close to the average depleted MORB mantle composition, whereas that of Workman and Hart (2005) is significantly more depleted than the average depleted MORB mantle. (ii) Model ages for sub-ridge mantle depletion are mostly between 0.8 to 3.0 Ga. (iii) There are large-scale patterns in depletion ages for sub-ridge mantle regions. For example, beneath Mid-Atlantic Ridge, mantle depletion ages are young (0.8 to 2.1 Ga) north of 30°N, older (1.6 to 4.5 Ga) between 25°N to 35°S), and mixed (0.6-4.4 Ga) south of 35°S. The Pacific sub-ridge mantle has a narrow range of model depletion ages of 1.6 to 3.0 Ga, with a mean of 2.3 Ga. Indian sub-ridge mantle has a younger mean depletion age of 1.7 Ga. These large-scale patterns reveal history of mantle depletion, mantle convection, and possible mixing between older and younger depleted mantles.