Geochemistry, Mineralogy, Volcanology Research Spotlight

A Significantly Hotter Mantle Beneath Iceland

Estimates of crystallization temperatures from four eruptions in northern Iceland offer improved constraints on the mantle's temperature beneath this anomalous divergent plate boundary.

Source: Geochemistry, Geophysics, Geosystems

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Variations in the temperature of the mantle drive its convective circulation, a process that links the deep mantle with the atmosphere and oceans through volcanic and tectonic activity. Because of this connection, effective models of Earth’s evolution must incorporate the planet’s thermal history, for which a crucial constraint is the mantle’s current temperature.

Researchers look at chemistry of Iceland’s newly erupted lava to analyze the temperature of the mantle below.
A false-color backscatter electron image of an olivine crystal from Borgarhraun, a lava field in northern Iceland. The crystal contains a spinel inclusion, set in a fine-grained crystalline groundmass. The chemistry of these crystals records the temperatures at which they crystallized. The image is approximately 1.5 millimeters wide. Credit: S. Matthews

Because the mantle’s temperature cannot be measured directly, scientists have devised a number of creative methods to derive this information, but these have produced widely varying results. Now Matthews et al. offer new constraints on this parameter beneath Iceland, one of the few places on Earth where a divergent plate boundary is subaerially exposed because of an anomalously large amount of melting occurring beneath the island.

Using a recently developed mineral thermometry technique, the researchers found that lava flows from four different eruptions along Iceland’s Northern Volcanic Zone crystallized at substantially higher temperatures (maximum 1399°C) than average mid-ocean ridge samples that have experienced little melting (maximum 1270°C). Next, the team developed a thermal model of mantle melting and used it, along with other observations such as the local thickness of the crust, to quantify the uncertainties in deriving mantle temperatures from their data.

Researchers look at chemistry of Iceland’s newly erupted lava to analyze the temperature of the mantle below.
An analysis of fresh lavas from Iceland indicates the mantle below the island is much hotter than beneath other locations on divergent plate boundaries. Credit: Terri Cook and Lon Abbott

Their results indicate that the mantle below Iceland is at least 140°C hotter than that beneath average mid-ocean ridges. This outcome should shed light on the factors that control the extent of melting beneath Iceland, including the ongoing debate about whether the voluminous melting is due to a deep mantle plume and, if so, whether changes in its magma production reflect variations in the plume’s temperature. (Geochemistry, Geophysics, Geosystems, doi:10.1002/2016GC006497, 2016)

—Terri Cook, Freelance Writer

Citation: Cook, T. (2016), A significantly hotter mantle beneath Iceland, Eos, 97, doi:10.1029/2016EO062987. Published on 18 November 2016.
© 2016. The authors. CC BY-NC-ND 3.0
  • Glenn Rivers

    Is there also evidence that the mantle under Iceland is wetter than normal ?

    • Simon Matthews

      A number of different mechanisms have been suggested in the past for explaining the anomalous melt production in Iceland. A subset of these mechanisms are those involving a much more ‘fusible’ mantle, i.e. a mantle that produces more melt at a given temperature. This may be due to a difference in composition, e.g. a pyroxenitic mantle rather than lherzolitic, or due to high concentrations of water. Individual techniques for extracting mantle temperature from proxies such as crustal thickness often suffer from trade-offs between mantle temperature and mantle fusibility. The approach we use in this study is to fit a number of observations whilst allowing mantle fusibility and temperature to vary. Though we consider a variable pyroxenite fraction, rather than variable water content, our model would behave in a comparable way for either contribution to fusibility. We find that we cannot simultaneously fit both crustal thickness and crystallisation temperature observations by changing only the fusibility of the mantle. A very fusible mantle, i.e. a mantle with a high pyroxenite or water content, whilst able to fit Iceland’s abnormally thick crust, would result in crystallisation temperatures that are much cooler than those we observe. We, therefore, do not find a requirement for the Icelandic being any wetter than ‘ambient’ mantle.