John Maclennan

Drilling to gabbro in intact ocean crust

Sampling an intact sequence of oceanic crust through lavas, dikes, and gabbros is necessary to advance the understanding of the formation and evolution of crust formed at mid-ocean ridges, but it has been an elusive goal of scientific ocean drilling for decades. Recent drilling in the eastern Pacific Ocean in Hole 1256D reached gabbro within seismic layer 2, 1157 meters into crust formed at a superfast spreading rate. The gabbros are the crystallized melt lenses that formed beneath a mid-ocean ridge. The depth at which gabbro was reached confirms predictions extrapolated from seismic experiments at modern mid-ocean ridges: Melt lenses occur at shallower depths at faster spreading rates. The gabbros intrude metamorphosed sheeted dikes and have compositions similar to the overlying lavas, precluding formation of the cumulate lower oceanic crust from melt lenses so far penetrated by Hole 1256D.

Plume-driven upwelling under central Iceland

A suite of 70 basaltic samples from the Herdubreid region of the Northern Volcanic Zone (NVZ) in central Iceland has been analysed for major and trace element compositions. The average light rare earth element concentration of these basalts is more than a factor of 2 higher than that of basalts from the Theistareykir volcanic system near the northern end of the NVZ. Seismic surveys of the NVZ have shown that the crustal thickness increases from ∼20 km near Theistareykir to 32–40 km in central Iceland. The observed REE composition and crustal thickness of the Theistareykir area can be reproduced by a melting model where mantle with a potential temperature of ∼1480°C upwells under the spreading ridge and the mantle upwelling is driven by plate separation alone. However, plate-driven upwelling models cannot simultaneously reproduce the composition of the Herdubreid region lava and the observed crustal thickness. Forward and inverse techniques show that plate-driven models that match the crustal thickness underestimate the La concentration by more than a factor of 2, and models that reproduce the compositions underestimate the crustal thickness by a factor of 4–5. Therefore one of the assumptions involved in the plate-driven upwelling models is not appropriate for central Iceland. A new set of models was developed in which mantle upwelling rates are allowed to differ from those of plate-driven upwelling in order to investigate the role of plume-driven mantle upwelling. The lava composition and crustal thickness of the Herdubreid region can be reproduced by models where the upwelling rates near the base of the melting region (>100 km depth) are ∼10 times higher than those expected from plate-driven upwelling alone and the mantle potential temperature is 1480–1520°C. About half of the melt generation under central Iceland results from plume-driven upwelling, with the remainder caused by plate-driven upwelling of hot material. This result is in agreement with numerical models of ridge-centred plumes.

V‐shaped ridges around Iceland: Implications for spatial and temporal patterns of mantle convection

[1] V-shaped lineations in the bathymetry and in the free-air gravity field surrounding Iceland result from crustal thickness variations caused by temporal variations in melt production rate at the Mid-Atlantic Ridge. We have studied the record of V-shaped ridges in the basins surrounding Iceland by plotting the shortwavelength component of the gravity field in terms of age versus distance from Iceland. The V-shaped ridge gravity signal is obscured by crustal segmentation and by sediment more than 1–2 km thick. The best V-shaped ridge record is found in the unsegmented part of the Irminger Basin, where Oligocene-Recent Vshaped ridges occur with a primary periodicity of 5–6 Myr and a secondary periodicity of 2–3 Myr. Vshaped ridge records from the Iceland Basin and from east of the Kolbeinsey Ridge to the north of Iceland correlate with the record from the Irminger Basin but are less complete. A record of uplift of the GreenlandIceland-Faroes Ridge based on paleoceanographic data is correlated with the gravity record of V-shaped ridges. There is less decisive evidence for V-shaped ridges in crust of Eocene age. The observation that Vshaped ridges propagate up to 1000 km from Iceland is compatible with a model in which the Iceland Plume head spreads out from the plume stalk below a depth of � 100 km, as suggested by geochemical arguments and studies of mantle rheology. Time-dependent flow in the plume head probably results from time-dependent flow up the plume stalk from deep below Iceland. These pulses may have triggered jumps in location of the spreading axis observed in the Icelandic geological record.

Crustal accretion under northern Iceland

Successful models of oceanic crustal accretion should be consistent with both geochemical and geophysical observations from active mid-ocean ridges as well as those from ophiolite sections. The major element concentrations of a set of basalt and picrite samples from the Krafla and Theistareykir volcanic systems of the Northern Volcanic Zone of Iceland show two distinct trends. The compositional variation in samples with MgO>9.5 wt% can be explained by addition/removal of a wehrlitic cumulate, while the major element variability in samples with 5–9.5 wt% MgO is dominated by gabbro removal. The results of thermobarometry based on the composition of the samples and the crystals found within them show that crystallisation took place at a range of temperatures (1160–1350°C) and pressures (<0.3–0.9 GPa) in the crust and uppermost mantle under Krafla and Theistareykir. The geochemical results are consistent with crustal accretion models where crystallisation takes place over a range of depths in the crust and uppermost mantle (<10–30 km). The geochemical observations allow estimates of the composition, mineralogy, pressure and temperature of material in the crust and shallow mantle under northern Iceland to be made and these estimates can be used to predict the seismic velocity of the material at the ridge axis. These P-wave velocity estimates are in agreement with the results of a seismic survey of the ridge axis at Krafla. The presence of temperatures of over 1000°C and magma chambers at depths greater than 10 km in the Icelandic crust cannot be ruled out using the available geophysical data from the Icelandic rift zones. Therefore both the geochemical and geophysical observations are consistent with models where crustal accretion takes place at a range of depths under northern Iceland.

Melt inclusions track pre-eruption storage and dehydration of magmas at Etna

At Mount Etna, Italy, vigorous gas-rich eruptions in A.D. 2001, 2002, and 2003 were followed by gas-poor eruptions in 2004, 2006, and 2007. Analyses of volatile (CO2, H2O, S, Cl, F), semivolatile (Cu), and involatile (Nb, La) elements trapped in olivine-hosted melt inclusions from these latest eruptions reveal the effects of the sustained interaction between a percolating gas phase and the stored magma. Melt inclusion compositions indicate that magmas erupted from 2004 to 2007 were residual from the 2001–2003 eruptions, and show significant evolution in the volatile content of the melt. These melt inclusion observations, and variations in the C/S of volcanic gases, can be accounted for if melts reequilibrated with CO2-rich gases during storage and prior to entrapment as melt inclusions. Sustained gas percolation caused loss of water and enhancement of CO2 in the evolving melt and may strongly influence the behavior of Cu, which potentially partitions into the gas phase. Vapor-melt interactions during magma storage are important controls on magma evolution at persistently degassing volcanoes.

Cooling of the lower oceanic crust

Thermal models of mid-ocean ridges that balance the influx of heat from magmatic sources with the removal of heat by conduction and hydrothermal circulation allow quantification of cooling of young oceanic crust. These models reproduce key observations relating to crustal accretion and hydrothermal cooling at fast-spreading ridges. The rate of cooling is constrained both by the bathymetry of ridge axes and by olivine compositions from ophiolite gabbros. Successful models involve extensive hydrothermal cooling of the lower crust within 20 km of the ridge, with ~50–70 kW of hydrothermal cooling for every 1 m of ridge axis at crustal ages of <0.1–0.4 Ma. These timates can be used to refine global models of geochemical and thermal fluxes close to spreading ridges.

Compositional trends of Icelandic basalts: Implications for short–length scale lithological heterogeneity in mantle plumes

Lithological variations in the mantle source regions under mid-ocean ridges and ocean islands have been proposed to play a key role in controlling melt generation and basalt composition. Here we combine compositional observations from Icelandic basalts and modelling of melting of a bi-lithologic peridotite-pyroxenite mantle to demonstrate that, while short-lengthscale major element variation is present in the mantle under Iceland, source heterogeneity does not make an important contribution to excess melt production. By identifying the major element characteristics of endmember Icelandic melts, we find enriched melts to be characterised by low SiO2 and CaO, but high FeO. We quantitatively compare endmember compositions to experimental partial melts generated from a range of lithologies, pressures and melt fractions. This comparison indicates that a single source composition cannot account for all the major element variation; depleted Icelandic melts can be produced by depleted peridotite melting, but the major element composition of enriched melts is best matched by melting of mantle sources that have been refertilised by the addition of up to 40% mid-ocean ridge basalt. The enriched source beneath Iceland is more fusible than the source of depleted melts, and as such will be over-represented in accumulated melts compared with its abundance in the source. Modelling of peridotite-pyroxenite melting, combined with our observational constraints on the composition of the Icelandic mantle, indicates that crustal thickness variations in the North Atlantic must be primarily due to mantle temperature and flow field variations.

Melt mixing and crystallization under Theistareykir, northeast Iceland

Analysis of the compositions of crystals and melt inclusions from a suite of 40 gabbroic and wehrlitic nodules in a single eruptive body provides a record of concurrent mixing and crystallization of melts under NE Iceland. The crystals in the nodules have a similar range of compositions to those found as phenocrysts in the flow, and many of the nodules may have been generated by crystallization of a magma with a similar composition to that of the host flow. While plagioclase is only present in nodules where the average forsterite content of olivines is 0.8 GPa and is in agreement with estimates of crystallization pressures for the host basalt. The relationship between the compositional variability of melt inclusions and the forsterite content of the host olivine is revealed by REE analyses of over 120 melt inclusions. The degree of variability in REE concentrations and REE/Yb ratios decreases with falling forsterite content of the host olivine, as expected if melt mixing and fractional crystallization are operating together. The standard deviation of the REEs falls by a factor of ~4 between Fo90 and Fo87. This change in olivine composition can be produced by crystallization of 20% which occurs on cooling of ~50°C. The relative rates of mixing, cooling and crystallization may provide constraints upon the dynamics of magma bodies. The oxygen isotopic composition of olivines from the nodules and phenocrysts is highly variable (δ18O from 3.3–5.2 per mil) and shows little correlation with the forsterite content of the olivine. The full range of oxygen isotope variation is present in olivines with Fo89–90, and the low δ18O signal is associated with melts of high Mg# and La/Yb. Such geochemical relationships cannot be produced by assimilation of low Mg# crustal materials alone, and may reflect oxygen isotopic variation within the mantle source. The geochemistry of the melt inclusions and their host crystals can be accounted for by fractional melting of a mantle source with variable composition, followed by concurrent mixing and crystallization beneath the Moho.

Melting during late-stage rifting in Afar is hot and deep

Investigations of a variety of continental rifts and margins worldwide have revealed that a considerable volume of melt can intrude into the crust during continental breakup, modifying its composition and thermal structure. However, it is unclear whether the cause of voluminous melt production at volcanic rifts is primarily increased mantle temperature or plate thinning. Also disputed is the extent to which plate stretching or thinning is uniform or varies with depth with the entire continental lithospheric mantle potentially being removed before plate rupture. Here we show that the extensive magmatism during rifting along the southern Red Sea rift in Afar, a unique region of sub-aerial transition from continental to oceanic rifting, is driven by deep melting of hotter-than-normal asthenosphere. Petrogenetic modelling shows that melts are predominantly generated at depths greater than 80 kilometres, implying the existence of a thick upper thermo-mechanical boundary layer in a rift system approaching the point of plate rupture. Numerical modelling of rift development shows that when breakup occurs at the slow extension rates observed in Afar, the survival of a thick plate is an inevitable consequence of conductive cooling of the lithosphere, even when the underlying asthenosphere is hot. Sustained magmatic activity during rifting in Afar thus requires persistently high mantle temperatures, which would allow melting at high pressure beneath the thick plate. If extensive plate thinning does occur during breakup it must do so abruptly at a late stage, immediately before the formation of the new ocean basin.

Control of regional sea level by surface uplift and subsidence caused by magmatic underplating of Earth's crust

Magmatic underplating of the crust is a common feature of major basalt provinces. The emplacement of magma within the lithosphere leads to surface uplift, with a magnitude of the order of 10% of the thickness of the underplated material. The composition of associated igneous rock samples suggests that much of the underplated material is made of gabbro. This gabbro is denser than the magma from which it crystallizes, so uplift of Earths surface caused by the emplacement of magma at the crust-mantle boundary must be followed by subsidence as the magma solidifies. This subsidence is equal to approximately half the original uplift and takes place within ∼0.1 m.y. of injection of the magma. The association of magmatic underplating with surface uplift followed by subsidence provides a mechanism for changes of sea level, including previously unexplained episodic highstands of sea level.