Primitive Lower Mantle in the Hawaiian Plume? Evidence from Correlated Os-Pb-Sr-Nd Isotopes in Picrites

Marc Norman1, Vickie Bennett2 and Tezer Esat2

1GEMOC, School of Earth Sciences, Macquarie University, North Ryde NSW 2109 Australia.

2Research School of Earth Sciences, Australian National University, Canberra ACT 0200 Australia

Abstract - PGE abundances and Os isotopic compositions provide important constraints on processes of planetary differentiation, including core formation, melt extraction, and crustal recycling. We have determined Re and Os abundances and Os isotopic compositions for a suite of picritic basalts from the Hawaiian plume to investigate the nature and composition of the deep mantle of the Earth. These data may provide a window through the late accretionary veneer which established near-chondritic relative abundances of PGE in the upper mantle, and constrain models for the origin and early evolution of the Earth. Sr, Nd, and Pb isotopic compositions of Hawaiian tholeiites indicate mixing of two mantle sources [1, 2]. One of these components is isotopically similar to depleted mantle, and the other is similar to that predicted for the primitive mantle. Os isotopic compositions are well correlated with the other isotopic systems, with 187Os/186Os Å 1.09 in the depleted component, and 187Os/186Os Å 1.21 in the "primitive" component, requiring a long-term history with super-chondritic Re/Os. Although crustal recycling can introduce high Re/Os components into the mantle, the trends of the picrite data are better matched by mixing of two mantle components. The upper mantle of the Earth apparently has had near-chondritic Re/Os since at least 3.8 Ga, suggesting long-term isolation of the upper and lower mantle.

Introduction - The Hawaiian plume is the largest, hottest, longest-lived mantle plume currently active on the Earth [3,4]. Tomography indicates a source for the Hawaiian plume in the lower mantle, below the 670 km transition zone [5]. Sr, Nd, Pb, and noble gas isotopic compositions of Hawaiian basalts show that at least two components with distinct long-term histories are present within the plume [1,2,4,6] and fluid dynamics experiments [7] suggest it is reasonable to consider these components as potentially reflecting upper and lower mantle compositions. To investigate the nature of these mantle components, a suite of picrites from 7 major volcanic centres (Koolau, Hualalai, Kohala, Mauna Kea, Mauna Loa, Kilauea, and Loihi), representing 3 million years of plume activity, were analyzed for major element, trace element, and Sr-Nd-Pb-Os isotopic compositions. Picrites are ideal for such a study as they are the best candidates for near-primary magmas. Hawaiian picrites apparently erupt preferentially from deeper in the volcanic plumbing than more typical subaerial tholeiites [3, 8], and have largely escaped the complexities introduced by the higher level magma chambers [9]. Picrites, therefore, should provide a clearer picture of source variations within the Hawaiian plume.

Results - The excellent correlations between the isotopic and trace element compositions of the picrites are consistent with mixing of two source components (Fig. 1). One component in the Hawaiian plume has Sr, Nd, and Pb isotopic compositions similar, but not identical to, those of Pacific MORB, and this component is most reasonably interpreted as predominantly depleted upper mantle [1,2] (Fig. 2). Depleted compositions are best expressed at Kilauea and Loihi. The second component has Sr, Nd, and Pb isotopic compositions close to those predicted for the primitive mantle, and predominates at Koolau [1,2], Lanai [10] and Kahoolawe [11] (Fig. 2).

Os isotopic compositions of the picrites are well-correlated with those of Sr, Nd, and especially Pb (Fig. 3), with 187Os/186Os ranging from 1.09 in lavas from Kilauea to 1.21 in samples from Koolau. Although crustal recycling can produce elevated Os isotopic compositions in the mantle, the Os-Pb picrite arrays are better fit by mixtures of two mantle components with subequal Pb/Os (Fig. 3). 187Os/186Os values of Hawaiian picrites require a time-integrated super-chondritic Re/Os for the "primitive" lower mantle component of these picrites, at least 60% higher than chondritic if the fractionation occurred 4.5 billion years ago. In contrast, the compositions of abyssal peridotites and mantle xenoliths show that the upper mantle has evolved with a near-chondritic Re/Os (187Os/186Os = 1.06 today), consistent with the relative abundances of other PGE in the upper mantle [12], and near-chondritic Re/Os apparently has been a feature of the upper mantle of the Earth since at least 3.8 Ga [13]. If the "primitive" component of the Hawaiian plume does represent the lower mantle, the isotopic compositions of Hawaiian basalts imply long-term isolation of the upper and lower mantle.

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[12] Capobianco et al. (1993) JGR 98, 5433-5443.

[13] Bennett et al. (1995) Eos Spring AGU, and in prep.

Figures available from Marc Norman.