The 320 kyr Pb isotope evolution of Mauna Kea lavas recorded in the HSDP‐2 drill core

Abstract

We analyzed Pb isotopic compositions of 50 samples from the HSDP‐2 drill hole, covering the time interval 180 to 550 kyr B.P. in the stratigraphic record of Mauna Kea. All analyses were corrected for instrumental bias using a triple‐spike technique. The aims of this study are to document temporal changes in sources contributing to Mauna Kea and to investigate how these may relate to the chemical structure of the Hawaiian plume. Lead isotopic compositions of the lavas have 206Pb/204Pb ratios ranging from 18.41 to 18.63, 207Pb/204Pb from 15.47 to 15.49, and 208Pb/204Pb from 37.97 to 38.22. In 207Pb/204Pb‐206Pb/204Pb space, the samples display a broad linear array, while three distinct arrays are found in 208Pb/204Pb‐206Pb/204Pb space. These arrays can clearly be distinguished by their 208Pb/204Pb ratios and are referred to as “Kea‐lo8,” “Kea‐mid8,” and “Kea‐hi8.” The 206Pb/204Pb isotope ratios exhibit rapid shifts by ∼0.2 over 100 m depth intervals, and jumps from one Pb isotope array to another and back in less than ∼100 m depth. Despite these rapid Pb isotope fluctuations, a particular Pb isotope array dominates over periods of several tens to hundreds of kiloyears. We interpret the Pb isotope arrays found in HSDP‐2 in terms of mixing of end‐members lying along the radiogenic and unradiogenic extensions of the arrays. At the radiogenic extension the three HSDP‐2 arrays converge to a common end‐member. The lower extensions of the arrays diverge in three directions, each with different 208Pb/204Pb ratios. This topology suggests that the HSDP‐2 arrays were produced by mixing of at least four end‐members. The origin of these end‐members was investigated using Monte Carlo simulations of a Pb isotope evolution model. The simulations suggest that the common radiogenic end‐member of the three Pb isotope arrays contains material with elevated μ values and has a relatively young age (<1.5 Ga). Such a signature can be plausibly interpreted in terms of the presence of recycled oceanic crust in the source. The HSDP‐2 Kea‐lo8, Kea‐mid8, and Kea‐hi8 Pb isotope arrays dominate over different time periods and can be related to the displacement of Mauna Kea relative to the plume center over time. The Kea‐lo8 array is present between ∼180 and 370 ka and samples more peripheral parts of the plume, while the Kea‐mid8 and Kea‐hi8 arrays occur in the deeper parts of the core (∼370 to 550 kyr ago), when Mauna Kea was closer to the plume center. Over the time intervals when each array dominates, we derive corresponding “lengths” of materials in the source by integrating the estimated upwelling velocity across the plume. These calculations suggest Pb isotope heterogeneities of at least several tens of kilometers in vertical length within the Hawaiian plume. The Pb isotope arrays may correspond to relatively small‐scale heterogeneities derived from the D″ layer in the lower mantle.