The Earth is a dynamic, evolving system whose processes we only partly understand. How do plate tectonics at the surface interact with the Earth's solid, slowly convecting mantle? How do those processes generate new crust through the production and emplacement of magma, especially at divergent mid-ocean ridge boundaries? Where, how, and how rapidly is that magma generated and transported? How do those divergent boundaries initiate as continental rifts? In my scholarly work, I principally use geochemistry, particularly the chemical and isotopic makeup of volcanic materials, as a tool to explore these questions.
In addition to major and trace element and traditional radiogenic isotope compositions, much of my geochemical work uses short-lived isotopes from the uranium-series decay chains to better constrain the timing of igneous processes (melt generation, extraction, and transport). This timing information, when coupled with isotopic and trace element data that reveal the chemical nature of the mantle rocks that melt to make magma, provide key insights into the factors that control the dynamic production of new crust on Earth. Those factors include the rates of melting and melt transport, the depth at which melting initiates within the mantle, the overall percentage of melting of the original source rock, what kind of rock that source rock must have been, and how much the magma must interact with the surrounding solid rock matrix during transport to the surface.
The North Atlantic Mid-Ocean Ridge System
Much of my ongoing research focuses on the mid-ocean ridges in the North Atlantic, with a series of studies of the geochemistry of fresh lavas from these ridges in order to better characterize the igneous processes there. These ridges diverge at slow spreading rates (1-2 cm/year), including the slowest spreading recorded on Earth at the eastern end of the Gakkel Ridge, making the North Atlantic an end member region. The melt generation that occurs in the mantle beneath mid-ocean ridges must likewise occur very slowly in the Arctic; in places the underlying mantle rocks upwell so slowly they likely have time to cool, stalling melting as deep as 25 km beneath the ridge axis. Generation of new ocean crust by emplacement of fresh magma is expected to differ in many ways on very slow-spreading ridges, compared to their more normal global counterparts.
The North Atlantic ridges are also an excellent location in which to test the influence of hotspot volcanism (volcanic activity apparently independent of plate tectonics, located at nearly-stationary “hotspot” locations around the world) on crustal generation processes at mid-ocean ridges. The large Iceland hotspot lies at the southern end of the Kolbeinsey Ridge and greatly influences both the geochemical composition of the magma produced and the sheer quantity of melt generation along the ridge. This excess melt generation thickens the crust, and the ridge axis along the Southern Kolbeinsey Ridge is located at particularly shallow depths that slope gently away from Iceland.
Our isotopic analyses have confirmed that both the Icelandic hotspot and the effects of cooling due to ultraslow spreading likely influence the generation of new crust along the adjacent mid-ocean ridges by changing local or regional mantle temperature. However, we found that other factors are also important, such as the presence of subducted eclogitic rocks or sub-continental lithospheric veins in various parts of the Arctic mantle. This is well supported by previous isotopic studies, which demonstrated that the nature of the mantle source varies considerably in the North Atlantic.
With collaborators, I have conducted an additional detailed case study of the geographic area surrounding Jan Mayen Island, a second, smaller hotspot north of Iceland. Jan Mayen is a volcanically active island situated just south of the Jan Mayen Fracture Zone, a major fracture zone offsetting the Mid-Atlantic Ridge system. Jan Mayen Island is also located at the northern extent of the Jan Mayen Platform, a bathymetric high though to be a small microcontinent of subcontinental material rifted from the Greenland coast during a relocation of the mid-ocean ridge 25 million years ago. The Jan Mayen Fracture Zone also joins the Northern Kolbeinsey Ridge segment in the southwest, which includes the very shallow Eggvin Bank, with the Southern Mohns Ridge in the northeast of the field area. Both segments are anomalously shallow compared to their adjacent ridge segments. Recent work (including ours) has found large volcanic edifices on both segments, with a particularly large and active volcano straddling the Northern Kolbeinsey Ridge axial valley wall. The two ridge segments produce basaltic lavas with dramatically different isotopic compositions across the fracture zone, despite their close proximity. These factors altogether create a highly unusual set of circumstances in a very narrow geographic zone: abrupt changes in mantle rock composition across a fracture zone; hotspot-ridge interactions near a fracture zone; and the unusual positioning of the hotspot at the terminus of an otherwise non-volcanically-active submarine microcontinent. Work is presently underway, but in recent conference proceedings we have made the case that there are at least three distinct solid mantle sources contributing to active volcanism around Jan Mayen.
Continental Rift Zones
I am also interested in the origins of divergent boundaries as continental rift zones, and how the nature of magmatic activity in those rift zones evolves towards the melt generation and emplacement we observe at mature ocean boundaries. Similar to the isotopic evidence for the influence of subcontinental lithospheric mantle in the relatively young Greenland basin of the North Atlantic, many rift zone magmas host similar signatures, suggesting that melting of the lithospheric mantle is important during rifting events. A preliminary study of the isotopic makeup of Central Atlantic rift magmas emplaced in the Eastern North American province is presently underway. These Central Atlantic Magmatic Province (CAMP) magmas are Triassic-Jurassic in age and mark the early stages of the opening of the Atlantic ocean basin, during one of the most major phases of rifting of the Pangea supercontinent.
I have additional interests in the more exotic magmas found in many rift zones, particularly carbonatitic magmas. The origins of carbonatites are highly disputed, and it seems likely that different carbonatites could be generated by different mechanisms. Due to their unusual nature as carbonate magmas, however, they can be easily misidentified in the field. A small pilot study is underway to explore the use of geochemistry in identifying carbonatites previously assumed to be non-igneous (e.g., hydrothermal) in origin.
Current and Former Students:
I frequently collaborate with scientists at University of Wyoming, Woods Hole Oceanographic Institution, Ecole Normale Supérieure de Lyon, GEOMAR in Kiel, Germany, University of Bristol, University of Iowa, Carnegie Institution of Washington, and University of Bergen.
Tri-College students who are interested in exploring igneous and/or geochemical research should contact me about possible projects!!