Isotopic evolution of Mauna Loa and the chemical structure of the Hawaiian plume

Abstract

New He isotopic data from the HSDP pilot hole core, lava accumulation rate models, and data from the literature are used to develop a 200,000 year isotopic record for the lava erupted from the Mauna Loa volcano. This record, coupled with an analogous record from Mauna Kea from the Hawaii Scientific Drilling Project (HSDP) pilot hole project and other literature data from the GEOROC database, are used to construct a “map” of lava isotopic compositions for the island of Hawaii. The isotopic map is converted to a map of the He and Nd isotopic compositions of melts from the mantle plume, which can be compared with a published melt supply map derived from geodynamic modeling. The resulting map of the plume indicates that values of helium 3He/4He > 20 Ra are confined to the core of the plume (radius ≈ 20–25 km) and correspond to potential temperatures >1565°C, suggesting the He isotopic signal is derived from deep in the mantle. The 3He/4He map has closed contours down to 10 Ra; the contours are teardrop‐shaped and elongated in the general direction of plate motion. The closed contours indicate that most of the plume He signal is lost during the early stages of melting, which is consistent with helium behaving as a strongly incompatible element (KHe ≤ 0.001). The εNd contours (and by inference the contours for Sr, Pb, Hf, and Os) do not all close on the scale of the island of Hawaii but instead partially follow material flow lines within the plume beneath the lithosphere. The plume signal for Nd extends circa 100 km in the direction of plate motion, which is consistent with the moderately incompatible behavior of Nd (KNd ≈ 0.02). Downstream from the plume core epicenter, plume Nd occurs with asthenospheric He; this could be mistaken for an additional plume component, whereas it may be only a manifestation of differing incompatibility. Data from Mauna Loa suggest the presence of a low‐3He/4He plume component that has low εNd and high 87Sr/86Sr. The plume map suggests that this component may be a blob (circa 20 km scale), located between Mauna Loa and Hualalai and separated from the main plume core by a zone of more asthenosphere‐like material. The HSDP data preclude a proposed model where this material represents a ring of entrained material from the lower mantle. The orientation of the elongation of contours on the plume He and Nd isotope maps (∼N45°W) does not match the modern plate motion as measured by GPS (N65°W) nor does it match the trend of the ridge axis between Maui and Loihi (N30°W). The geochemical evidence, as well as the locations and growth histories of the Hawaiian volcanoes, suggest that the plume, as well as the Pacific plate, has been moving at a velocity of several centimeters per year over the past 1 to 2 million years.

Combined U–Th/He and 40Ar/39Ar geochronology of post-shield lavas from the Mauna Kea and Kohala volcanoes, Hawaii

“…One way to do this is to map the source of the lava flow, the individual vents, relative to the location of maximum melt supply at the time of eruption (DEPAOLO et al, 2001). The petrology and geochemistry of the lavas can then provide information about the section of the plume it is sampling: the source material via radiogenic isotopes and melting dynamics via U-series isotopes (see e.g.…”

“…Multiple chemical components (HART et al, 1992;RODEN et al, 1994), radial (DEPAOLO et al, 2001) and asymmetric (ABOUCHAMI et al, 2005) zonation of the mantle plume source have been invoked to explain the chemical heterogeneity found in the volcanoes. The ability to characterize the temporal evolution of the volcanoes, and to tie the lava geochemistry to the structure of the mantle plume, is critically dependent on accurate dating of the lavas.…”

Deep Drilling into a Mantle Plume Volcano: The Hawaii Scientific Drilling Project

“…Inside this circle melt fractions are relatively large, and the magma compositions produced are tholeiitic; outside, the magma production is minimal, and the magma compositions are alkalic. Ribe and Christensen, 1999;DePaolo et al, 2001a;Bryce et al, 2005). Systematic variability in Hawaiian lavas with depth (age) in the drillcore can be attributed to structure in the plume, and one of the interesting results is that there is such structure even though melting within the plume represents only the innermost third or so of the plume radius (Fig.…”

“…Figures 4a and 7 show the island geography roughly 600,000 years ago as reconstructed from the HSDP age data and the models of . To date, we have had difficulty in reconciling the growth history of the volcanoes with what is known of the Pacific plate velocity over the plume, both its speed and direction (DePaolo et al, 2001a). Based on the ages of the older islands and seamounts in the Hawaiian chain, the long-term inferred velocity of the plate is about 8.6 cm yr -1 .…”

Probing the Earth’s Deep Interior Through Geochemistry

“…Based on results from the shallower pilot hole, DePaolo et al (2001) proposed a steady-state concentrically zoned Hawaiian plume, with a lower mantle-derived plume core with high 3 He/ 4 He and enriched radiogenic isotopic signatures surrounded by a rim of less primitive He and less enriched radiogenic isotopic signatures. In the model, individual volcanoes sample a 50-60 km wide strip as they pass over this plume and much of the temporal and spatial variations in the composition of the volcanoes result from this motion over the plume.…”