Updated source for Helium in ocean-island volcanismUniversity of Hawaiʻi at Mānoa
School of Ocean and Earth Sciences and Technology
HONOLULU — New analysis of measured noble-gas contents in lavas from submarine volcanic eruptions is trying to resolve a long-standing controversy about the structure and dynamics of the Earth‘s mantle. The study, published in the prestigious journal Nature, found that Helium in lavas from ocean islands, such as Hawaiʻi, may be derived from a part of the Earth‘s mantle that has retained most of the gases originally incorporated into the Earth during its accretion, 4.5 billion years ago.
"Lavas from submarine volcanic eruptions typically contain gases, predominantly carbon dioxide and water, as well as small amounts of noble gases, such as Helium for example," says lead author Helge Gonnermann, a SOEST Young Investigator at the School of Ocean and Earth Science and Technology, at the University of HawaiʻI at Mānoa. These gases are derived from the part of the Earth‘s mantle that melted to produce the erupted magma. Most magma derived from the Earth‘s mantle erupts along the Earth‘s mid-ocean ridges, producing mid-ocean ridge basalts (MORBs). However, throughout the oceans there are ocean islands, such as Hawaiʻi, whose basalt lavas are commonly called ocean-island basalt (OIB).
The movements of Earth‘s tectonic plates induce flow in the Earth‘s upper mantle and partial melting of those parts of the upper mantle that pass beneath mid-ocean ridges produces MORBs. OIBs, on the other hand, are produced by partial melting of mantle that is thought to rise as thermally buoyant mantle plumes from deeper parts of the mantle. All gases contained in mantle rock will be lost to the basalt magma that is produced during partial melting. Much of theses gases will in turn be lost from the magma upon eruption.
Most 4Helium is produced by the radioactive decay of Uranium and Thorium in approximately equal amounts throughout the Earth‘s mantle, whereas any 3Helium has been present since the accretion of the Earth, it is primordial. The Earth‘s upper mantle, from which MORBs are derived, has been degassed throughout most of Earth‘s history by volcanism along mid-ocean. Thus, much of its primordial 3Helium has been lost, while some of the 4Helium loss has been offset by radiogenic production. Over time, this will have resulted in a steady decrease of the 3Helium/4Helium ratio. Any part of the Earth‘s mantle that has remained isolated from melting would have retained its primordial 3Helium, and its 3Helium/4Helium ratio would have decreased much less. The fact that 3Helium/4Helium is significantly higher in OIBs than in MORBs has therefore been seen as a critical piece of evidence that part of the Earth‘s mantle has remained undegassed throughout Earth‘s history.
It is paradoxical that Helium concentrations in OIBs, thought to be derived from the undegassed part of Earth‘s mantle, are typically much lower than in MORBs, known to be derived from the degassed upper mantle. This is known as the "Helium concentration paradox" and has led to much controversy about the interpretation of 3Helium/4Helium ratios, as well as about the history and structure of the Earth‘s mantle.
In their paper, Gonnermann and Sujoy Mukhopadhyay from Harvard University show that the Helium concentration paradox can self-consistently be explained by degassing of the erupting magma. "The big breakthrough was realizing the importance of CO2 content, which is ultimately at the heart of the Helium concentration paradox," says Gonnermann. "The presence of higher CO2 concentrations in OIBs allows more Helium to be lost from the erupting magma." Careful modeling of the eruptive degassing process demonstrates that a higher CO2 content of OIBs relative to MORBs leads to more extensive degassing of Helium in OIB magmas upon eruption. Thus, because the mantle from which OIBs are derived is relatively undegassed in Helium and also CO2, eruptive degassing produces OIB lavas with lower Helium concentrations than MORB lavas.
"What this study shows us is that there is no Helium concentration paradox and that indeed a part of the Earth's mantle probably still contains much of its primordial noble gas content," adds Gonnermann. "This requires us to come up with a geodynamical model that allows for the isolation and preservation of such a primordial mantle component throughout the course of Earth's history. It will also require reconciliation of seismological observations that suggest that the whole of the Earth's mantle appears to be actively convecting, which would make it difficult to isolate such a primordial mantle."
For more information, contact:
Helge Gonnermann, SOEST Young Investigator, Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawaiʻi at Mānoa, Tel: 808 956 5036, firstname.lastname@example.org
Nature 449, 1037-1040 (25 October 2007), Non-equilibrium degassing and a primordial source for helium in ocean-island volcanism; Helge M. Gonnermann & Sujoy Mukhopadhyay