The two parallel chains of Hawaiian volcanoes ('Loa' and 'Kea') are known to have statistically different but overlapping radiogenic isotope characteristics. This has been explained by a model of a concentrically zoned mantle plume, where the Kea chain preferentially samples a more peripheral portion of the plume. Using high-precision lead isotope data for both centrally and peripherally located volcanoes, we show here that the two trends have very little compositional overlap and instead reveal bilateral, non-concentric plume zones, probably derived from the plume source in the mantle. On a smaller scale, along the Kea chain, there are isotopic differences between the youngest lavas from the Mauna Kea and Kilauea volcanoes, but the 550-thousand-year-old Mauna Kea lavas are isotopically identical to Kilauea lavas, consistent with Mauna Kea's position relative to the plume, which was then similar to that of present-day Kilauea. We therefore conclude that narrow (less than 50 kilometres wide) compositional streaks, as well as the larger-scale bilateral zonation, are vertically continuous over tens to hundreds of kilometres within the plume.

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.

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Terranes composed of arc-related igneous and sedimentary rocks were accreted onto the continental margin of southwestern North America from 1.9 to 1.6 Ga. They formed the Yavapai-Mazatzal province in Arizona, of which the so called 'Pinal Schist' and associated rocks in southeastern Arizona form an integral part. 148 U-Pb analyses on magmatic and detrital zircons were performed on 21 samples to constrain timing of geologic events in Paleoproterozoic time in the area of southeastern Arizona. 25 Sm-Nd whole rock analyses were made to infer sources of sedimentary and igneous rocks. Our results allow us to delineate a suture zone between the Pinal tectonostratigraphic unit (Pinal block), which consists primarily of basinal metaturbiditic rocks, and a southeastern allochthonous juvenile volcano-sedimentary domain. The location of the suture is constrained by lithologic contrast across the boundary, an increase in age of detrital zircons, a marked decrease of ⑀ Nd (t), and an intervening subduction complex (Swift and Force, this issue). We propose the name Cochise block for the southeastern part, because it represents a faultbound, allochthonous tectonic unit. In the Cochise block, we document volcanic activity from about 1630 to 1647 Ma. Quartzitic sedimentary rocks were largely derived from almost contemporaneous volcanic rocks and show detrital zircon ages from 1630 to 1729 Ma, with the majority (85 percent) between 1630 to 1674 Ma. Quartzites and intermediate to felsic metavolcanic rocks show a tight cluster of ⑀ Nd (t)-values (2.9-3.8) close to the depleted mantle model and therefore represent juvenile additions to the crust. Post-tectonic metabasalts with ⑀ Nd (1500) ‫؍‬ 4.5 are interpreted as being derived from a depleted upper mantle source around 1.5 Ga. In the Pinal block, detrital zircons yield values from 1678 to 1731 Ma. ⑀ Nd (t)values of metapelites and wackes range from ؊0.2 to 2.3. Values decrease from southeast to northwest, indicating progressive input of more evolved material toward the craton. Our data suggest that sedimentation within the Pinal block was proximal to the continental margin and occurred later than 1.68 Ga and ended before the intrusion of granitoids at 1.65 Ga. We show that the North American crust grew rapidly and progressively by addition of juvenile but evolved metavolcanic arc rocks and derived quartz-rich metasedimentary rocks as documented in the Cochise block. Timespans between depleted mantle model age and zircon crystallization on the order of 50 my attest to rapid crustal processing during this significant time of crustal growth.