Lynne Elkins
Research
The Earth is a dynamic, evolving system whose processes we only partly understand. How does plate tectonics at the surface interact with the Earth's solid, convecting mantle? How do those processes produce brand new crust through volcanic activity, especially at mid-ocean ridges, the divergent ocean boundaries that host 90% of the Earth's volcanism? How have those layers and processes changed over Earth time? And then, how do we as people live alongside and sometimes in volcanic landscapes? How has that shaped world culture and society? How can we be better citizens of this dynamic planet and reduce our own risks?
In my scholarly work, I principally use geochemistry, particularly the chemical and isotopic makeup of volcanic materials, as a tool to explore questions like these. That effort both informs my teaching and student mentorship. One major project I am working on right now is a detailed case study of the geographic area surrounding Jan Mayen Island, located in the Arctic Ocean. 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. The Fracture Zone joins the Northern Kolbeinsey Ridge segment in the west, which includes the very shallow Eggvin Bank, with the Southern Mohns Ridge in the northeast of the field area.
Map of the Arctic Ridge system, showing samples previously worked on from the Kolbeinsey Ridge (light blue data points), the Mohns and Knipovich Ridges (majority of the dark blue data points), and the Gakkel Ridge (single dark blue data point at 90ºE).
A number of things are strange about this area:
- Jan Mayen Island is not situated on a tectonic boundary, yet it is volcanically active;
- The Island is the northern culminating end of the mostly submarine Jan Mayen Platform, a bathymetric feature likely formed by rifting a portion of East Greenland shelf (or at least its underlying mantle) westward about 25 million years ago; the rest of the Platform is non-volcanic;
- Volcanic rocks from the Island have some geochemical signatures that indicate the mantle that melted to produce the magma was ancient subducted crust. This is typical of hotspot volcanoes, and is widely thought to reflect the presence of a rapidly rising plume of mantle rock beneath the hotspot;
- On the other hand, Jan Mayen Island volcanics lack certain indicators of hotspot activity, making it suspicious and the subject of argument for several decades;
- The Kolbeinsey Ridge (located between Iceland and the Jan Mayen Fracture Zone) produces volcanic rocks whose chemistry overall support the presence of normal mantle rocks below, that are melting more than normal for a mid-ocean ridge. The high amounts of melting are widely attributed to the proximity to the Iceland hotspot. This is in good agreement with overall thick crust being formed at the mid-ocean ridge. But despite being the farthest from Iceland, the Northern Kolbeinsey Ridge crust grows thicker again, making one of the shallowest spots anywhere in the Arctic (<50m deep!). No one had mapped this area well, so no one knew why the Eggvin Bank area of the segment was so unusually shallow;
- The Northern Kolbeinsey Ridge lavas again show signs of melting mantle beneath that contains subducted crustal material, unlike the rest of the Kolbeinsey segment;
- Likewise the Mohns Ridge in the north has similar geochemical indicators--but the indicators aren't identical to those in the Kolbeinsey Ridge or Jan Mayen. Recent work by colleagues on the Southern Mohns Ridge segment found a very large volcano producing unusually thick crust in the center of that segment, similar to the Eggvin Bank.
There are many suggested scenarios for why this area is so geochemically and morphologically strange. Perhaps there is a hotspot under this region that, despite being relatively close to Iceland, is geochemically different from Iceland (and does not influence the Central Kolbeinsey Ridge). Perhaps the rifted portion of sub-Greenland mantle produced strange, unusual mantle compositions that melt differently or more easily, making unusual geochemical compositions, shallow crust, and a volcanic island where we wouldn't expect one. Maybe the flow of slowly conducting, solid mantle beneath this region is complex, producing "cool edge effects" that create strange melt regimes deep underground. Maybe the mantle deep under this region harbors extra water, causing more magma production than normal.
To better understand the region, collaborators and I are conducting a focused case study of this field area. We aim to better understand how and why volcanism is happening here. My work in particular focuses on new mapping efforts on the Northern Kolbeinsey Ridge and on geochemical and isotopic analysis of young, unaltered volcanic rocks from all three zones (Kolbeinsey, Jan Mayen, and Mohns).
A major technique in my arsenal is the analysis of uranium-series isotopes. Relationships between this isotopes in both mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) have provided new insights into the melt generation and magma transport processes occurring beneath volcanic centers. By looking at the isotopes in the U-series decay chains, I attempt to better constrain how mantle rocks melt to produce basaltic magmas, through a series of experimental and analytical approaches.
Diagram of 238U decay chain, with intermediate nuclides, their half-lives, and the nature of their radioactive decay.
230Th is a short-lived (half-life of 75,000 years) daughter nuclide produced by the radioactive decay of 238U, a very long-lived isotope (half-life of 4.5 billion years). Unless perturbed by a chemical reaction such as melting, these isotopes exist in a state of secular equilibrium where their measured activities are the same. (The activity of an isotope is its abundance multiplied by its decay constant, which is inversely proportional to its half-life.) Most MORB and OIB have isotopic U-Th signatures such that the ratio of their activities, (230Th/238U) > 1, indicating that there has been a recent perturbation (the melting event), and that this occurred in the presence of the mineral garnet (Gt), or possibly clinopyroxene with small M2 site radii. While a number of upper mantle rocks can contain Gt, the stability of Gt in peridotite (the most important upper mantle lithology) depends on pressure: Gt only occurs in peridotites deeper than ~70 km.
The especially useful part of this is that when melting occurs, the way the isotopes behave in the rocks and the new magma depends on timing: how fast melting occurs, how fast the melt is extracted, how deep it was forming, etc. These and other geochemical tools also help us distinguish between types of rocks in the mantle: were they deep enough to host garnet? did they contain ancient subducted crust that has been reworked into the mantle, or is the source more primitive? More importantly, what does that mean for how the mantle is melting to produce new ocean crust all over the world?