Geochemical Arguments Favoring an Hawaiian Plume J. Michael Rhodes University of Massachusetts Dominique Weis University of British Columbia Michael O. Garcia University of Hawaii Marc Norman Australian National University

I don’t intend to dwell on the obvious:- 1.Ocean Island Basalt (OIB) mantle is less depleted and more diverse than Mid Ocean Ridge Basalt (MORB) mantle. 2.This diversity is widely attributed to subducted crustal components. 3.There has to be a mechanism to return these subducted components to shallow depths of melting (~ 130 – 90 km in the case of Hawaii).

These cartoons illustrate the point that the Hawaiian plume is thought to be concentrically zoned in both temperature and composition. If so, in the last ka Mauna Loa and Mauna Kea will have traversed about km of the plume. Over this time period we might expect to see changes in magma composition reflecting changes in melting, melt supply and changes in source components.

Volcanic Growth Stages, (Stearns, 1946) The submarine pre-shield stage (Loihi) is characterized by low melting and magma supply. Eruption of alkalic basalts followed by tholeiites reflecting initiation of volcanism at the margins of the plume. Shield building stage reflects increased magma supply. Eruption of tholeiites and picrites as the volcano traverses the axial zone of the plume Post-shield stage is characterized by a return to low magma supply, eruption of alkalic lavas as the volcano nears the margins of the plume.

Results from the Hawaii Scientific Drilling Project confirm high magma supply rates, eruption of tholeiites and picrites, between 600 and 400 ka. Followed by decline in eruption rates between ka and onset of post- shield volcanism and eruption of alkalic basalts. Note. Model growth curve of DePaolo & Stolper (1996) was based on a simple geometric model of a thermally zoned plume, prior to dating!

Evolution of Hawaiian volcanoes from an alkalic pre-shield stage, through a tholeiitic shield stage, to an alkalic post shield stage is consistent with movement of the Pacific plate over a thermally zoned melting anomaly. Hualalai Loihi 94 km The distance from Loihi to Hualalai (94 km) provides a constraint on its dimensions.

SiO 2 TiO 2 and CaO differ at a given value of MgO between Hawaiian volcanoes. This is presumably a consequence of differences in melting and melt segregation processes in different parts of the plume. Given thermal gradients we might expect to see changes in these values as a volcano transits the Hawaiian plume.

SiO 2 in basalts (normalized to 17% MgO) is dependent on depth of melt segregation and on the extent of melting. Marked decrease in SiO 2 (17) after 320 ka reflects a decline in melting and melt production as the volcano enters the post-shield stage. Increase in incompatible trace data (e.g. Nb/Y) supports the interpretation. This is the case for Mauna Kea

In contrast Mauna Loa shows no obvious change in SiO 2 (17) or Nb/Y in about 400 ka. This implies that melting conditions have remained relatively uniform as Mauna Loa transits about 30 to 40 km of the Hawaiian plume. Not so for Mauna Loa!

Magma production and evolution of Hawaiian volcanoes is frequently presented like this But perhaps the Mauna Loa data is telling us it should really be like this with a wide, hot, central core.

How hot is the plume relative to ambient mantle? Compare Mauna Loa with MORB Maximum Fo in olivine in both magmas is close to Fo 91.3 T Mauna Loa = 1547 o C T MORB = 1401 o C Difference = 146 o C You can play around with olivine compositions and K D, but the results are the same – a hotter Mauna Loa magma relative to MORB, implying significant differences in mantle potential temperatures.

He Isotopes Both volcanoes exhibit a decline in 3 He/ 4 He with decreasing age Interpreted as a decline in an undegassed (primitive?) mantle component as the volcanoes approaches the plume margin. High (>14.5) 3 He/ 4 He at Mauna Kea are spikes of “Loihi-like” lavas inter-layered with “normal” lavas. Kurtz et al. (2004) interpret this as evidence for an asymmetric plume.

Sr – Pb Isotopes There is a progressive increase in a Kea (or Loihi?) component in Mauna Loa lavas as they age (0 to > 400 ka). Mauna Kea and Kilauea lavas are similar (0 to ~ 600 ka). Consistent with a zoned plume in which Mauna Loa is closer to the axis and Mauna Kea and Kilauea are closer to the margins.

Pb – Pb Isotopes High-precision Pb data from Abouchami et al. (2000, 2005) and unpublished data of Weis. Distinct bilateral asymmetry in the Pb data between Loa and Kea trends. Older Mauna Kea (> 320 ka) overlaps with Kilauea – long-lived (~ 400 ka) heterogeneities sampled by the two volcanoes. Mauna Loa lavas become progressively more like Loihi (not Kea!) lavas with increasing age (~ 100 to 400 ka). Hualalai submarine tholeiites overlap with <100 ka Mauna Loa lavas.

Distinct bilateral asymmetry in the plume (not concentric). Mauna Kea would have been close to where Kilauea is today 500 – 600 ka ago. Implies long-lived, vertically stretched source components. Mauna Loa was closer to Loihi at 400 ka, consistent with greater proportion of Loihi components in Mauna Loa lavas at that time. Implications for a Zoned Plume?

Summary Evolution of Hawaiian volcanoes is consistent with a thermally zoned plume. This in itself requires that the mantle source is hotter than the surrounding mantle. Temperatures of Hawaiian primary magmas are hotter than MORB primary magmas. He isotopes are consistent with an undegassed (primitive?) mantle component in the plume center. Distribution, however, is asymmetric. Most isotopic data (Sr, Nd, Hf) can be reconciled with a concentrically zoned plume resulting from entrainment. Pb isotopic data require bilateral asymmetry in the plume with long-lived vertical heterogeneities. These inferences are consistent with (but derived independently!) recent plume models (Farnetani and Samuel, 2004).