Research in Progress
Abstracts of work in press, submitted, or almost submitted
Robin T. Holcomb, Bruce K. Nelson, Peter W. Reiners, and Nuni-Lyn Sawyer (submitted)
Overlapping volcanoes: The origin of Hilo Ridge, Hawai'i
The submarine Hilo Ridge has been interpreted as a part of Mauna Kea volcano but is crossed at ~1100 m depth by a submerged shoreline terrace composed of basalts that are isotopically distinct from those of Mauna Kea and similar to those of Kohala volcano. This terrace evidently is a product of Kohala instead of Mauna Kea. Almost all of Hilo Ridge below the terrace therefore must predate the principal growth of Mauna Kea, which has superficially isolated the ridge from its Kohala source by overlapping its proximal segment. The Mauna Kea section penetrated by the HawaiTi Scientific Drilling Project is predicted to be thinner than expected previously, owing to the overlap. Similar overlaps are suspected among other volcanoes and may cause significant changes in the understanding of Hawaiian volcanism.
Nuni-Lyn Sawyer and Bruce K. Nelson (submitted)
Systematic Geochemical and Eruptive Relations in the Late Stage Evolution of Volcanoes from the Hawaiian Plume - with Case Studies of Waianae and East Molokai Volcanoes
A significant geochemical event in the evolution of Hawaiian volcanoes is the decline in magmatic flux from shield to post-shield volcanism as each edifice is carried away from the mantle-plume source. This eruptive decline is accompanied by a shift from tholeiitic to alkalic basalt volcanism, although a full spectrum exists in the extent to which any particular volcano undergoes the transition. Evidence shows that systematic geochemical relations exist among volcanoes for this late-shield stage. Compositional correlations in subaerial tholeiitic basalt among Pb, Sr, and Nd isotope ratios and SiO2, TiO2, FeO, CaO, and Na2O content indicate that, for any particular volcano, the proportion of the two identified endmember isotopic components of the Hawaiian shield-building stage, KOO and KEA, are related to what extent eruptions subsequently shift from tholeiitic to alkalic basalt. The spectrum is described by a model in which KOO-dominated lavas result from melting of a mantle plume with a steep marginal temperature gradient, which accounts for large degree melt production followed by abrupt termination as the volcano moves off the plume. KEA- dominated lavas result from melting of a plume that has developed a long lateral temperature gradient, where large degree melts are followed by continued melt production of increasingly smaller degree. Detailed geochemical data for lavas of Waianae and East Molokai Volcanoes show evolutionary characteristics consistent with the overall model and provide further insight into mantle component characteristics and mixing. The Waianae tholeiite source is an intermediate mix of KEA and KOO. Otherwise apparently well-mixed in the source, the components are systematically sampled by late-shield lavas, where compositions vary stratigraphically from increased KEA to increased KOO. The appearance at Waianae of endmember KOO, previously identified only at Koolau, Lanai, and Kahoolawe, expands the identified isotopic range of Hawaiian lavas and defines KOO isotopic heterogeneity. The East Molokai tholeiite source is KEA-dominated. Late-shield compositions shift toward the isotopically-deleted Post-Erosional (PE) component, which is shown to be characteristic of KEA-dominated volcanoes. Mixing models, accounting for both Sr and Pb isotopic variations, describe PE involvement as metasomatism of the plume margin by small degree melts of a MORB source.
David A. Butterfield, Bruce K. Nelson, C. Geoffrey Wheat, Michael J. Mottl, and Kevin K. Roe (submitted)
Evidence for volcanogenic Sr in mid-ocean ridge-flank hydrothermal systems and implications for the global oceanic Sr isotope balance: Geochimica Cosmochimica Acta.
Previous models and calculations of the global mass balance for Sr in the oceans have shown that the input of unradiogenic basaltic Sr from on-axis mid-ocean ridge (MOR) hydrothermal systems is much less than needed to balance the input of radiogenic Sr delivered to the oceans by rivers. The implication is that either the oceans are far from steady state with respect to Sr isotope balance (and that the 87Sr/86Sr ratio of seawater is increasing at unprecedented rates) or that there is a significant missing source of unradiogenic Sr. It has long been recognized that off-axis hydrothermal fluxes might significantly affect the mass and isotopic balance of Sr (and other elements) in the oceans, but nearly all previous work has concluded that the 87Sr/86Sr ratio of pore fluids in ridge- flank hydrothermal areas is virtually indistinguishable from the seawater ratio or is dominated by authigenic carbonates. In contrast, we report here the 87Sr/86Sr of warm springs, sediment pore fluids and basement reservoir fluid with a clear basaltic signature from the eastern flank of the Juan de Fuca ridge. These results and similar results recently reported by Elderfield et al. (1999) indicate that low-temperature ridge-flank hydrothermal circulation has an important effect on the Sr isotope balance in the oceans. If published values for the other major sources of Sr input to the oceans (rivers and axial hydrothermal flux) are accurate, then the rate of increase of 87Sr/86Sr in seawater (~0.000054 per million years) can be accommodated if ridge flanks on a global scale deliver fluids to the ocean with _(87Sr/86Sr)/heat ratios one third to one half of the ratio found in the present study. The Ca/Sr ratio and 87Sr/86Sr of the JFR flank fluids in this study overlap with fluid properties inferred from some calcite veins in the upper oceanic crust. Based on Sr and O isotope signatures of calcite veins, the average _(87Sr/86Sr)/heat of low-temperature fluids is in the range required to balance the oceanic Sr isotope budget. Fluids venting from ODP Hole 1026B on the Juan de Fuca east flank have elevated levels of Mo, are depleted in U, and have relatively stable Sr isotope and major element composition for the 3 years following drilling. If a significant portion of basaltic basement fluids vent directly to the oceans, then ridge flanks are a significant source of Mo to the oceans. Even if fluids from unsedimented ridge flanks have an average _(87Sr/86Sr)/heat only one third as large as the fluids from the east flank of the Juan de Fuca ridge, the global flux of unradiogenic Sr from ridge flanks is a significant part of the oceanic Sr isotope balance.
Stephanie de Villiers and Bruce K. Nelson(1999)
Detection of low-temperature hydrothermal fluxes by seawater Mg and Ca anomalies
Geochemical fluxes into and out of the ocean control its chemical composition. Measurements of the magnesium (Mg) content of seawater, an assumed "conservative" element in the ocean, reveal mid-depth Mg depletions in the vicinity of the East Pacific Rise. The magnitude of the anomalies suggests that fluxes associated with the low-temperature circulation of seawater through axial mid-ocean ridge systems are much larger than the high-temperature axial component. A higher total axial hydrothermal flux provides a mechanism that simultaneously satisfies the mass balance requirement of several major seawater constituents.