Fractional melting and source heterogeneity cause a wide range of melt compositions to be generated in the mantle. Part of this range is preserved in the composition of olivine-hosted melt inclusions. These inclusions form as olivine crystals grow in magma bodies, and isolate pockets of melt from processes that occur later in the evolution of the batch of magma. We have made a detailed study of the composition of olivine-hosted melt inclusions from Iceland and elsewhere and found that there is a general relationship between the degree of trace element variation in the melt inclusions and the major element composition of the host olivine crystals. This relationship is likely to arise during coupled mixing and crystallisation of melt in lower crustal magma bodies. Mixing takes place in three stages: 1) Juxtaposition of melts of different composition 2) Stirring and stretching to reduce the lengthscale of compositional heterogeneity 3) Compositional homogenisation at the molceular scale by diffusion.
Melt inclusions |
 Stretching structures preserved in banded obsidian (H. Tuffen) |
 Stretching of lava in Hawaiian channel (E. Passmore) |
Related Papers
Maclennan, J., D. McKenzie, K. Gronvöld, N. Shimizu, J. M. Eiler, and N. Kitchen, Melt mixing and crystallization under Theistareykir, northeast Iceland, Geochem. Geophys. Geosyst., 4(11), 8624, doi:10.1029/2003GC000558, 2003.[abstract] [online article]
Maclennan, J., D. McKenzie, F. Hilton, K. Gronvöld and N. Shimizu, Geochemical variability in a single flow from northern Iceland, J. Geophys. Res., 108(B1), 2007, doi10.1029/2000JB000142, 2003.[abstract] [online article]
The oceanic crust is generated by the solidification of mantle melts at mid-ocean ridges. A host of geophysical, geological and petrological observations both from active ridges and from fossil ridegs (such as the Oman ophiolite) have been used to develop models of the oceanic crustal accretion process. We have developed thermal models that include petrological processes (such as fractional crystallisation) and can reproduce that compositional stratification observed in the crust. Our models resolve a supposed inconsistency between geophysical observations from the East Pacific Rise and geological/petrological observations from Oman. We have shown that it is both possible for extensive crystallisation to occur in the lower crust beneath the ridge (sheeted sills model of Kelemen and co-workers) and for the seismically imaged lower crust at active ridges to contain small melt fractions (<10%). We also found that many of the available observations are best fit if hydrothermal circulation cools the oceanic crust to the Moho within a few kilometres of the ridge axis. New observations are being collected to test these theories both by seafloor sampling of the active East Pacific Rise and ocean drilling into 15 million year old Pacific crust in the Guatemala Basin.
 Lower
crustal gabbros from the Oman Ophiolite. |
 C7 drillbit used for sampling Pacific ocean crust by IODP |
 Results of a thermal model. |
Related Papers
Maclennan, J., T. Hulme, and S. C. Singh, Cooling of the lower oceanic crust, Geology, 33, 357-360, 2005.[abstract] [online article]
Maclennan, J., T. Hulme, and S. C. Singh, Thermal models of oceanic crustal accretion: Linking geophysical, geological and petrological observations, Geochem. Geophys. Geosyst., 5, Q02F25, doi:10.1029/2003GC000605, 2004.[abstract] [online article]
Wilson, D.S. and 52 others, Drilling to gabbro in intact ocean crust, Science, 312, 1016-1020, 2006. [abstract] [online article]
Cannat, M., J. Cann and J. Maclennan, Some hard rock constraints on the supply of heat to mid-ocean ridges, In: Mid-ocean ridges: Hydrothermal Interactions between the Lithosphere and Oceans, edited by C.R. German, J. Lin and L.M Parson, Geophys. Monogr. Ser.,vol. 148, AGU, Washington, D.C., 2004.[abstract][online article]
I have been exploring the links between mantle flow, melt generation, surface uplift and climate change for the past 10 years. Recently, we have shown that mantle-generated uplift of the North Atlantic region about 55 million years ago may have been a key process in the origins of the Paleocene-Eocene Thermal Maximum, a rapid climate change event that is commonly used as an analogue to present-day global warming. The uplift causes destabilisation of gas hydrates stored at pressure in the seafloor and leads to the release of methane and/or carbon dioxide to the atmosphere. We have also used a combination of field mapping, geochemistry and physical modelling to understand the relationship between deglaciation and volcanism in Iceland. The removal of a ~2km thick ice sheet from Iceland 12 kyr (12 thousand years) ago resulted in rapid decompression of the mantle melting region and a 50-fold increase in magma production rates. This increase in magma production is reflected by a short-lived 30-100 fold increase in volcanic eruption rates after the retreat of the glaciers. The timing of the deglaciation and burst in eruption rates indicates that the upflow rate of magma under Iceland >50 metres per year.
Uplift of Atlantic ~55 Ma and region of hydrate breakdown |
 Icelandic bergs and shrinking glacier |
 Volcanoes related to the wane of the last major Icelandic glaciation |
Related Papers
Maclennan, J. and S.M. Jones, Regional uplift, gas hydrate dissociation and the origins of the Paleocene-Eocene Thermal Maximum, Earth Planet. Sci. Lett., 245, 65-80, 2006. [abstract] [online article]
Maclennan, J., M. Jull, D. McKenzie, L. Slater and K. Gronvöld, The link between volcanism and deglaciation in Iceland, Geochem. Geophys. Geosyst., 3(11), 1062, doi10.1029/2001GC000282, 2002.[abstract] [online article]
Jones, S., N. White and J. Maclennan, V-shaped ridges around Iceland: Implications for spatial and temporal patterns of mantle convection, Geochem. Geophys. Geosyst., 3(10), 1059, doi10.1029/2002GC000361, 2002.[abstract][online article]
Maclennan, J. and B. Lovell, Control of regional sea level by surface uplift and subsidence caused by magmatic underplating of Earth's crust, Geology, 30, 675-678, 2002.[abstract][online article]
The geochemistry of basalt is sensitive to the flow, temperature and composition of the melting mantle. The volume of melt generated and resulting crustal thickness is also controlled by these properties of the mantle. Long-wavelength uplift of the Earth's surface and the resulting erosional and sedimentary response provide further constraints on the nature of the mantle. We have used a wide range of observations to constrain the proporties of the mantle under Iceland and the North Atlantic throughout the last 65 million years. A combination of geochemical and geophysical observations from Iceland can be accounted for if the mantle flow rate in the deepest parts of the melting region (>120 km depth) under central Iceland is 10 times higher than that under northern Iceland. These constraints on the kinematics of mantle flow are in agreement with dynamical models of mantle convection that include mantle plumes. We are collecting further geochemical and geophysical data in order to better test models of the temperature, composition and flow structure of the mantle under Iceland and North Africa.
Part of the Krafla volcano in northern Iceland |
 Swells and basins of Africa |
 An lava flow from Mount Etna (S. Collins) |
Related Papers
Maclennan, J. and S.M. Jones, Regional uplift, gas hydrate dissociation and the origins of the Paleocene-Eocene Thermal Maximum, Earth Planet. Sci. Lett., 245, 65-80, 2006. [abstract] [online article]
Jones, S., N. White and J. Maclennan, V-shaped ridges around Iceland: Implications for spatial and temporal patterns of mantle convection, Geochem. Geophys. Geosyst., 3(10), 1059, doi10.1029/2002GC000361, 2002.[abstract][online article]
Maclennan, J., D. McKenzie and K. Gronvöld, Plume driven upwelling under central Iceland, Earth Planet. Sci. Lett., 194, 67-82, 2001.[abstract][online article]
The physical properties of rocks are controlled by their temperature, pressure and composition. Geophysical observations are, however, often interpreted in terms of temperature veriations alone. My work on the composition of Icelandic basalts made me aware of important compositional variations in the crust that should be incorporated in the interpretation of geophysical results. For example, the solid products of crystallisation of Icelandic basalts have P-wave velocities that vary by up to ~1 km s-1 at fixed temperature. This variation in seismic velocity is equivalent to that generated by a temperature variation of 1500OC or a variation in melt fraction of ~20%. We have recently completed a reinterpretation of the Apollo lunar seismic data in order to better understand the constraints that this data places on the internal compositional structure of the Moon. We find no geophysical requirement in the present lunar dataset for strong compositional layering. Many lunar magma ocean models predict that such layering should be present.
Related Papers
Khan, A., J.A.D Connolly, J. Maclennan and K. Mosegaard, Joint Inversion of Seismic and Gravity Data for Lunar Composition and Thermal State, Geophys. J. Int., 168, 243-258, 2007.[abstract] [online article]
Khan, A., J. Maclennan, S.R. Taylor and J.A.D. Connolly, Are the Earth and the Moon Compositionally Alike? - Inferences on Lunar Composition and Implications for Lunar Origin and Evolution from Geophysical Modeling, J. Geophys. Res., 111, E05005, doi: 10.1029/2005JE002608, 2006. [abstract] [online article]
Maclennan, J., T. Hulme, and S. C. Singh, Cooling of the lower oceanic crust, Geology, 33, 357-360, 2005.[abstract] [online article]
Maclennan, J., T. Hulme, and S. C. Singh, Thermal models of oceanic crustal accretion: Linking geophysical, geological and petrological observations, Geochem. Geophys. Geosyst., 5, Q02F25, doi:10.1029/2003GC000605, 2004.[abstract] [online article]
Maclennan, J., D. McKenzie and K. Gronvöld, Plume driven upwelling under central Iceland, Earth Planet. Sci. Lett., 194, 67-82, 2001.[abstract][online article]
Maclennan, J., D. McKenzie, K. Gronvöld and L. Slater, Crustal accretion under northern Iceland, Earth Planet. Sci. Lett., 191, 295-310, 2001.[abstract][online article]