Geochemistry of Kauai shield‐stage lavas: Implications for the chemical evolution of the Hawaiian plume

Abstract

[1] We measured He, Sr, Nd, Pb, and Os isotope ratios and major and trace element concentrations in stratigraphically and paleomagnetically controlled shield-stage lavas from Kauai, Hawaii. The range of 3 He/ 4 He ratios (17-28 R A ) from Kauai is similar to that reported from Loihi and thus challenges the prevailing notion that high 3 He/ 4 He ratios are restricted to the preshield stage of Hawaiian magmatism. 3 He/ 4 He ratios vary erratically with stratigraphic position, and chronostratigraphic control from paleomagnetic data indicates very rapid changes in the 3 He/ 4 He ratios (up to 8 R A in $10 2 years). These variations in helium isotopic ratios are correlated with variations in radiogenic isotope ratios, suggesting rapid changes in melt composition supplying the magma reservoir. A three-component mixing model, previously proposed for Hawaiian shield lavas, does not adequately explain the isotopic data in Kauai shield lavas. The addition of a depleted-mantle (DM) component with the isotopic characteristics similar to posterosional basalts explains the isotopic variability in Kauai shield lavas. The DM component is most apparent in lavas from the Kauai shield and is present in varying proportion in other Hawaiian shield volcanoes. Shield lavas from Kauai sample a high 3 He/ 4 He end-member (Loihi component), but while lavas from western Kauai have a larger contribution from the Kea component (high 206 Pb/ 204 Pb, anomalously low 207 Pb/ 204 Pb relative to 206 Pb/ 204 Pb), lavas from eastern Kauai have a larger proportion of an enriched (Koolau) component. The systematic isotopic differences between eastern and western Kauai reflect a gradual migration of the locus of volcanism from west to east, or alternatively east and west Kauai are two distinct shield volcanoes. In the latter case, the two shield volcanoes have maintained distinct magma supply sources and plumbing systems. Our new geochemical data from Kauai are consistent with the existence of a single high 3 He/ 4 He reservoir in the Hawaiian plume and suggest that the proportion of the different mantle components in the plume have changed significantly in the past 5 Myr. The long-term evolution of the Hawaiian plume and the temporal variability recorded in Kauai lavas require more complex geochemical heterogeneities than suggested by radially zoned plume models. These complexities may arise from heterogeneities in the thermal boundary layer and through variable entrainment of ambient mantle by the upwelling plume.

Tungsten in Hawaiian picrites: A compositional model for the sources of Hawaiian lavas

“…Conversely, the Loihi component, which may represent a relatively undepleted deep mantle reservoir, is defined by more radiogenic Sr and Os, and less-radiogenic Nd, Hf and Pb isotopes relative to the Kea component. This endmember is also distinguished by high 3 He/ 4 He ratios (Kurz et al, 1983;Mukhopadhyay et al, 2003). The Koolau (Makapuu) component, which is not characterized by any of the samples analyzed here, occupies the isotopic extreme with the highest 87 Sr/ 86 Sr and 187 Os/ 188 Os, coupled with lowest 143 Nd/ 144 Nd, 176 Hf/ 177 Hf and Pb isotopic ratios seen in Hawaiian lavas (West et al, 1987;Roden et al, 1994;Hauri, 1996;Lassiter and Hauri, 1998;Blichert-Toft and Albarede, 1999;.…”

Rapid helium isotopic variability in Mauna Kea shield lavas from the Hawaiian Scientific Drilling Project

“…Even though the resolution of the timescale is not adequate at the shortest timescale (i.e., at the 2-3 Ka resolution), there is clearly coherence that reflects the rapid geochemical variability in the submarine HSDP2 Mauna Kea lavas, and demonstrates that helium is strongly coupled to the other elements. Kurz and Kammer, 1991;Lassiter and Hauri, 1998;Mukhopadhyay et al, 2003]. The possible components include recycled materials (such as lithosphere, oceanic crust, and altered oceanic crust, and sediments), depleted ambient mantle, and a possible undegassed mantle reservoir.…”

“…Helium isotopes are important tracers of mantle components because the early Earth had very high 3 He/ 4 He ratios (i.e., greater than the present-day sun, or >100 times atmospheric, Ra) and, by inference, high He/(Th + U) ratios. Hawaiian volcanoes have some of the highest oceanic 3 He/ 4 He ratios, and also the largest documented variations within single volcanoes [e.g., Kurz et al, 1983Kurz et al, , 1995Mukhopadhyay et al, 2003]. High 3 He/ 4 He ratios in the present-day Earth would indicate preservation of mantle reservoirs via high time-integrated He/(Th + U) ratios in the mantle.…”

Postshield stage transitional volcanism on Mahukona Volcano, Hawaii

“…This was a primary rationale for assigning these lavas to the preshield stage, since such high He R/R A values were only known at that time for alkalic lavas from Loihi Seamount (e.g., Kurz et al 1983). High He R/R A are, however, now known from shield stage lavas from Mauna Loa (to R/R A = 20.0; Kurz et al 1995), Haleakala (to R/R A =22.4; Hanyu et al 2007), Mauna Kea (to R/R A =24.8; Kurz et al 2004), Kauai (to R/R A =28.3; Mukhopadhyay et al 2003), Koko Seamount (to R/R A =24.1; Keller et al 2004), and Suiko Seamount (to R/R A =23.5; Keller et al 2004). We conclude that He R/R A >20 can characterize the preshield, shield, or early postshield growth stages of Hawaiian volcanoes.…”