Mantle Melting Beneath Mid-Ocean Ridges

or collective redistirbution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of approval portionthe ofwith any articlepermitted only photocopy by is of machine, reposting, this means or collective or other redistirbution T SPECIAL ISSUE FEATURE article hasis been published in Oceanography MANTLE MELTING BENEATH journal of 20, Number 1, a quarterly , Volume MID-OCEAN RIDGES BY CHARLES H. LANGMUIR AND DONALD W. FORSYTH T The plate-tectonic revolution was initially “kinematic”— pressures equivalent to thousands of atmospheres in the inte- 2007 by e Oceanography Copyright Society. a description of plate motions across Earth’s surface. Plate rior. As mantle ascends beneath the mid-ocean ridge, less and tectonics is now recognized as the surface manifestation of a less rock lies above it, so large pressure changes occur, which greater process—circulation of the solid earth. Magma ascends leads to melting. The melt is less dense than the solid, and rises to the surface at mid-ocean-ridge spreading centers to cool and to the surface to form the oceanic crust. form oceanic crust, which millions of years later returns to the Figure 1 shows how rising mantle crosses the “solidus” (the mantle at subduction zones. Formation of oceanic crust is the transition from complete solid to partial melt) and melts pro- greatest contribution of fl ow from our planet’s interior, as two- gressively towards the surface. Note that because the mantle is T thirds of the earth is resurfaced about every 100 million years. a solid consisting of many different molecules, it does not melt reproduction, systemmatic Republication, article use for research. and this copy in teaching to e Oceanography granted rights All reserved. Society. is Permission T Partial melting of the mantle at spreading centers is the mecha- entirely at a single temperature, but progressively over a range or e Oceanography [email protected] Send Society. to: correspondence all nism by which this fl ow takes place, and thus is fundamental to of temperatures—from 0 percent melting at the solidus to understanding solid-earth circulation. 100 percent melting several hundred degrees higher at the liq- Melting is a primary means by which the earth cools: sea- uidus. Thus, partial melting is possible. fl oor spreading brings hot mantle from depth to the colder sur- Several lines of evidence provide information about this face. Because we normally think that melting occurs through melting process. New maps generated over the past 25 years heating (e.g., putting a slab of butter in a frying pan), it may show the variations in shape and depth of thousands of volca- seem paradoxical to say the earth melts while cooling down. noes distributed along the ridge. These maps have enabled sci- The explanation for this paradox is that melting temperatures entists to sample the volcanic rocks—ocean-ridge basalt—that are dependent on pressure as well as temperature. Just as in- make up the surface pavement of the oceanic crust. About creased temperature excites atoms so they free themselves from 80 percent of the 60,000-kilometer-long mid-ocean ridge their ordered, solid, crystalline state, so increased pressure has been mapped and sampled at least to 100-km spacing T squeezes atoms, making it more diffi cult for them to transi- (see www.petdb.org). At the same time, experimental studies e Oceanography PO Society, Box 1931, Rockville, MD 20849-1931, USA. tion from solid to liquid. Thus, temperature and pressure exert have led to quantitative models of how melt composition and opposite effects on melting, and melting can occur by decreas- amount vary with temperature and pressure (e.g., Jacques and ing pressure at a given temperature as well as by increasing Green, 1980; Kinzler and Grove, 1992; Baker and Stolper, 1994; temperature at a given pressure. The reason melting by pres- Pickering-Witter and Johnston, 2000). And seismic studies, sure release seems foreign to common experience is because which are able to probe Earth’s interior directly, provide infor- human life on Earth is lived in an environment of almost con- mation about the “melting regime” beneath the ridge axis. This stant pressure caused by the weight only of the atmosphere. article synthesizes some of these developments, and outlines a The solid earth, however, is subject to huge changes in pressure, set of major questions for future research. because the weight of hundreds of kilometers of rock exerts 78 Oceanography Vol. 20, No. 1 AXIS 0 15% 4 Ocean Crust 5% 30% 3 50 Solidus 20% 2 pth (km) De 10% 1 100 Solidus Cold Ascending Mantle Hot Ascending Mantle Temperature Temperature 4 3 3 Pr 2 Pr 2 essu essu Ma Ma re re nt nt le So le Solidu 1 1 lidu As Ma s 30% s 30% cending nt 20% 20% le As 10% Ma 10% cend 0% nt le ing 0% Figure 1. Diagrams illustrating the melting mechanisms beneath ocean ridges. At any one pressure, the mantle melts over a temperature range of several hundred degrees. T e boundary between melt absent and melt present is called the mantle solidus. As mantle ascends beneath the ocean ridge, it begins melting as the solidus is crossed, and melts progressively dur- ing further ascent. T us, the mantle melts by pressure decrease rather than by temperature increase. Hot mantle crosses the solidus at greater depths, leading to a larger melting regime, greater extents of melting, and thicker crust than that produced by cold mantle. T e numbers on the bottom diagrams correspond to the pressures where melting stops for the numbered fl ow lines on the upper diagrams. Oceanography March 2007 79 A FIRSTORDER MODEL FOR larger range of pressures, leading to the water. The same principle applies on OCEANRIDGE MELTING greater extents of melting. Here, then, Earth to crust “floating” on the denser The deep mantle is solid, but not brittle. the common-sense intuition holds true: mantle. Oceanic crust is denser and At the high temperatures and pres- hot mantle melts more than cold man- thinner than continental crust, and for sures characteristic of Earth’s interior, tle. The greater quantity of melt from this reason the ocean floor is generally the mantle beneath the plates flows like hot mantle thus produces thicker oce- at a lower elevation than continents. a very viscous liquid at rates of up to anic crust than is produced from colder Variations in the thickness of the oceanic several tens of centimeters per year. As mantle (see Figure 1). All of this can be crust along ocean ridges lead to varia- the rigid plates separate at mid-ocean understood as a consequence of how the tions in the elevation of the ocean floor, ridges, the deeper mantle rises to fill the pressure-temperature diagram relates with thick crust, in areas such as Iceland, “gap” created by spreading. The ascend- to the melting regime created by mantle actually rising above sea level, and very ing mantle crosses its melting point flow driven by plate separation. thin crust lying as much as 5000 meters and begins to melt. The mantle-melting The actual extent of mantle melt- below sea level. region beneath the ridge, the “melting ing can be estimated from the chemical Putting these considerations together, regime,” is roughly triangular in shape compositions of the basalts that rise to the first-order prediction is that hotter (see Figure 1). The total amount of melt the surface and are sampled at ocean mantle leads to lower sodium content, that can be produced by any particular ridges. Elements that are preferentially thicker crust, and shallow water depths, part of the mantle within the melting concentrated into the liquid (that is to and colder mantle to higher sodium con- regime is proportional to how far this say, elements that are incompatible with tents, thinner crust, and greater depths. mantle rises after crossing the solidus. the crystals remaining in the solid man- Observations are in agreement with this The melting regime ranges in extent of tle, called “magmaphile” elements) have prediction, as Figure 2 shows. Quanti- melting, therefore, from zero at the bot- concentrations that are inversely propor- tative models of this simple picture of tom where the mantle begins to melt, tional to the extent of melting. The most mantle melting can to first order suc- to a maximum at the shallowest point abundant element with this behavior is cessfully account for the composition of melting. The remarkable fact is that sodium. High extents of melting lead to and thickness of the oceanic crust, and as the mantle melts more and more, it liquids with low sodium concentrations, the global variations in depths of the is at lower and lower temperatures, so it and low extents of melting to high con- ocean ridges (Klein and Langmuir, 1987; actually melts while cooling down rather centrations, because most of the “incom- Langmuir et al., 1992). Thus, a simple than while heating up. patible” sodium is partitioned into the bathymetric map of the ocean-ridge Because the melts produced in the first small melt fraction. Further melting system could be considered to reflect melting regime are buoyant and fluid, then dilutes the sodium concentration. largely the temperature structure of the they separate from the solid and rise to A physical measure of the extent of underlying mantle. the surface to form the oceanic crust. melting is the amount of crust produced If the mantle is hotter, it starts to melt per increment of spreading, which is COMPLEXITIES OF THE deeper, and therefore can melt over a the crustal thickness.

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