Supplemental Student Support


Joint Program Graduate: Matthew Jackson

Graduate Research

With generous field work support from the Deep Ocean Exploration Institute, my thesis research focused on the geochemistry of lavas from the Samoan hotspot, and provided some important clues about mantle dynamics and the origins of mantle heterogeneity. In Chapter 3 (Jackson et al., Nature, 2007) of the thesis, remarkably enriched Samoan lavas (87Sr/86Sr up to 0.7214) hosting unambiguous recycled sediment signatures are reported. Such recycled sediment signatures are unprecedented, and indicate that, in spite of the tendency for the convective motions of the mantle to greatly attenuate subducted signatures, there must be regions of the mantle that escape chaotic mantle convection. This observation is important, as recycled sediment signatures are incredibly rare in hotspot lavas, in spite of the large quantities of sediment observed to enter the mantle at subduction zones.

Chapter 2 (Jackson et al., Earth Planet. Sci. Lett., 2007) argues that heterogeneities can persist in the mantle at the largest length scales for (geologically) long time periods in the (deep?) mantle’s ancient high 3He/4He reservoir. In this chapter, I report 3He/4He ratios in Samoan lavas that are as high as Hawaii, Iceland and the Galapagos, and find that these new Samoan high 3He/4He lavas have much less depleted 143Nd/144Nd and 87Sr/86Sr signatures. In fact, the highest 3He/4He sample from each southern hemisphere high 3He/4He hotspot is less isotopically-depleted (generally higher 87Sr/86Sr and lower 143Nd/144Nd) than their counterparts in the northern hemisphere. The observation of a large-scale isotopic enrichment in the southern hemisphere high 3He/4He mantle compared to the northern hemisphere high 3He/4He mantle is similar to the DUPAL anomaly, a globe-encircling feature of isotopic enrichment observed primarily in southern hemisphere ocean island basalts. The origin and preservation of such large-scale (hemispheric) mantle heterogeneities remain a mystery. Compared to the hemispheric-scale heterogeneities reported in Chapter 2, Chapter 1 (Jackson and Hart, 2006) reports significant 87Sr/86Sr heterogeneities in melt inclusions hosted in olivines from a single sample. This indicates that the mantle is isotopically heterogeneous at length scales smaller than the melting zones beneath hotspot volcanoes.