Os isotope evidence for a differentiated plume head reservoir for the Ontong Java Nui source

Abstract:

Previous Os isotopic investigations of lavas from the Ontong Java Plateau¹ observed that geographically widely dispersed samples of differing chemistries preserved an isochron of 123±8 Ma with an initial ¹⁸⁷Os/¹⁸⁸Os = 0.1289±0.0095. Samples from the Manihiki Plateau, itself a portion of the greater Ontong Java Nui (OJN) magmatic event, preserve a far greater range in Os isotopic signatures than previously reported for the OJP alone.

In contrast to the OJP data which points towards a near-chondritic, primitive mantle source for both Kroenke and Kwambaita lavas, the low Ti Manihiki samples preserve ¹⁸⁷Os/¹⁸⁸Os_(i) ranging from 0.1056-0.1714. High Ti Manihiki samples preserve ¹⁸⁷Os/¹⁸⁸Os_(i) = 0.1094-0.1288. Such strongly subchondritic signatures require some component of recycled material in the mantle source, possibly SCLM (T_RD low Ti samples ~3.1Ga; and ~2.3-2.6Ga for the high Ti samples). Higher initial Os isotope ratios could indicate the presence of metasomatised lithosphere and/or lower crust.

The low Ti samples from Manihiki have been interpreted as the result of a two stage melting process, analogous to boninites², the depleted source of which has itself been metasomatised by a HIMU component entrained within the plume head.

Collectively the Ontong Java and Manihiki samples could conceivably contain mantle sourced from both an undifferentiated, near-chondritic source, as well as ancient, unradiogenic recycled sources. Thus the greater OJN province samples a heterogeneous source containing both primitive and recycled components. It is probable that greater degress of partial melting beneath Ontong Java homogenised these heterogeneities, whereas more complex, multi stage melting processes near the plume margin at Manihiki allowed sampling of the inherent heterogeneities within the plume head.

 1: Parkinson et al., 2002, GCA 66(15A) A580.

2: Golowin et al., in prep.