David Hilton, Ph.D.
scripps institution of oceanography
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Through the utilization of sub-aerial and submarine hydrothermal fluids
and spatially-related lavas, I have worked on two of the Earth's major
hotspots: Iceland and Hawaii. My thesis work in Iceland was amongst the
first to show agreement in helium isotope ratios (3He/4He) between fluids
and basalts indicating the utility of either medium for obtaining information
on mantle source characteristics. Further studies in Iceland have found
amongst the highest mantle 3He/4He ratios worldwide - a topic of considerable
bearing on the state of the initial Earth and the subsequent interaction
and degassing histories of the lower and upper mantles. Work on Hawaii
has been directed at Loihi Seamount and Kilauea Volcano and the relationships
between helium and carbon dioxide. I was a participant on the first Alvin
dives on Loihi Seamount (with Harmon Craig in 1987) and have published
evidence of temporal variations in the CO2/3He ratio of hydrothermal fluids
at Loihi. I was involved with further dives on Loihi Seamount (in 1996
and 1998 with Gary McMurtry of the University of Hawaii)
following the most intense period of seismicity yet recorded for any Hawaiian volcano
(in the summer of 1996). Dramatic variations in the CO2/3He ratio of newly-formed
vents at the summit of Loihi are interpreted to record changes in the
chemistry of magma supplying volatiles to the vents - a conclusion unobtainable
by other means. Related studies on Kilauea Volcano have been directed
at understanding fractionation processes between helium and CO2, and in
the re-construction of the mantle source helium characteristics of an
subaerial hotspot volcano.
Other hotspot-related research has involved samples from Heard Island
(lavas) and the Galapagos Islands (fumaroles).
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The Heard Island work questioned
the notion that so-called 'low-3He' hotspots are a feature diagnostic
of deep recycled material in the mantle. I suggested that shallow-level
crustal contamination related to magma chamber degassing was the source
of the low 3He isotopic signatures - not only at Heard Island but at other
'low-3He' hotspots worldwide. This topic is one of continuing research
and controversy (and the subject of an AGU Chapman Conference in November
1996). The Galapagos work is directed at characterizing the He-C systematics
of a hotspot source for comparison both with Kilauea and Loihi Seamount
and the much greater database available on mid-ocean ridge basalts.