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Academic Staff


Dr C. Johan Lissenberg

My research focuses on the generation and evolution of oceanic lithosphere and ophiolites. I approach this mainly from a petrological perspective, combining field data with the study of the compositions and textures of rock samples.

Projects

Lower crustal processes in fast-spread oceanic crust; Hess Deep

During my postdoc at Cardiff University, I will be studying the petrology of gabbroic rocks recovered from Hess Deep (equatorial Pacific). These rocks, sampled during the NERC-funded cruise JC-21 (Jan-Feb 2008), are the most comprehensive sample set to date of lower crust formed at a fast-spreading mid-ocean ridge, and thus present an exciting opportunity to study crustal generation in this setting. In collaboration with Dr. Chris MacLeod and Postgraduate student Kerry Howard, I will be addressing several questions: By what mechanism does the lower crust form? How is melt transported through the lower crust? What record is there of interaction between melts and gabbroic rocks? How does fast-spread lower crust compare to slow-spread lower crust and to its widely used analogue, the Oman ophiolite?

Focused crustal accretion along the Mid-Atlantic Ridge

During my Postdoc at Woods Hole Oceanographic Institution, I studied lower crustal samples from the Kane Megamullion (23°N, Mid-Atlantic Ridge). The Kane Megamullion exposes the plutonic basement of nearly an entire ancient second-order ridge segment as a result of detachment faulting. This provides an opportunity to constrain the along-axis variations in lower crustal accretion along slow-spreading ridges. Results show that the lower crust is very primitive in the ancient segment center, but evolved near the segment end. This provides the first petrological evidence that melt delivery from the mantle, and thus crustal accretion, is focused at segment centers. It also shows that there is a petrological decoupling between the lower and upper crust, with a strong along-axis compositional gradient in the lower - but not in the upper – crust.

Melt-rock reaction in lower oceanic crust and its relationship with MORB chemistry

A second project I did while I was in Woods Hole concerns chemical interaction between lower crustal rocks and melt rising through it. The transport of melt through the lower oceanic crust is a poorly understood phenomenon. However, it may have a large effect on the compositions of both lower crustal cumulates and the most abundant magma on Earth, mid-ocean ridge basalts (MORB). In collaboration with Henry Dick, I studied gabbroic rocks from the Kane Megamullion, and found evidence that melt was transported in diffuse cm-wide channels. Mineral compositions and textures indicate that melt flow in these channels is reactive. We modeled this reaction to constrain its effect on melt composition, and compared the results with MORB data. Results suggests that MORB may owe part of their compositional variation, previously attributed to fractional crystallization at elevated pressures in the mantle, to reactive transport in the lower crust.

Melt focusing and redistribution along slow-spreading mid-ocean ridges

During my postdoc at IPGP in Paris, when I worked with Catherine Mével, I developed a theoretical petrological model for crustal accretion at slow-spreading ridges. The model included variable amounts of melt focusing in the mantle and redistribution in the crust. By using geological constraints from two Mid-Atlantic Ridge segment, we were able to constrain the magnitude of focusing and redistribution. In addition, the model predicts the thickness and composition of different units of the oceanic crust, thus providing a reference model for (gabbroic) suites recovered from the oceans.

The role of sills in oceanic crustal accretion; the Annieopsquotch ophiolite (Newfoundland)

During my Ph.D. I studied the origin and evolution of the Annieopsquotch ophiolite, which occurs along the Iapetus suture zone in the Newfoundland Appalachians. In particular, I looked at the mode of accretion of the lower crust of the ophiolite to constrain if sills play an important role; this had been proposed, but remained strongly debated. My work, supervised by Cees van Staal and Jean Bédard, showed that, up to a level ±400 m below the sheeted dikes, the lower crust is comprised of sills, thus showing unambiguously that sills can play an important role in crustal accretion.