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Geochemistry

Molten rocks in motion

We only see the Earth's mantle in action when it melts and rises to the surface, most spectacularly during a volcanic eruption. The 70,000-km-long, global network of mid-ocean ridges is the site of the most abundant and steady supply of magma (and after cooling, new crust) on Earth. Detailed investigations during the past two decades have greatly expanded our view of volcanic and tectonic processes at ocean ridges, but we still have a limited understanding of how magma is delivered to, and concentrated in, the relatively narrow zones of volcanic and hydrothermal activity along spreading ridges.

On page 282 of this issue, Spiegelman and Reynolds1 attempt for the first time to distinguish between different dynamic models of mantle melting beneath ocean ridges by comparing them with geochemical data2,3 from basalts (volcanic rock) that erupted in the northern section of the fast-spreading East Pacific Rise. There is not yet a consensus on the style of mantle melting, but Spiegelman and Reynolds have taken a first step towards trying to reconcile theory with observation.

Chemically, samples of mid-ocean ridge basalts from the oceanic crust have provided indirect information about the composition of the mantle and the chemical and physical processes that can modify magma, but have so far given little insight into the dynamics of melting or upwelling of mantle material4,5. How and where the mantle melts, how magma interacts with the solid mantle matrix on its way to the sea floor, how the chemistry of basaltic magma varies in space and time and how the upper oceanic crust is formed, are all questions that have yet to be answered.

Two fundamentally different theories have been proposed to explain the volcanic activity associated with spreading ridges6,7. One model assumes that magma flow is dynamic or ‘active’ (it buoyantly rises and helps drive plates apart) and the other assumes that magma rises passively (it rises in response to the plates being pulled apart). In the first case, magma is concentrated in a narrow zone directly below the ridge axis, whereas in the other magma converges from a wide area below the ridge towards this narrow zone (Fig. 1, overleaf). In both models magma is concentrated beneath the ridge crest, but tests to determine which model is correct have been wanting.

Figure 1: Cross-section through a mid-ocean ridge.
figure1

Chemical differences between volcanic rocks found at the ridge axis and at off-axis locations support a passive model of mantle flow, as discussed by Spiegelman and Reynolds1. In passive flow, upwelling of 'liquid' magma (solid lines) and solid mantle (dotted lines) is in response to (rather than the cause of) plate motion. The magma is concentrated at the ridge crest but converges from a wide area beneath the ridge.

Geophysical studies8 and results from the recent MELT experiment9 — the most comprehensive seismic and electromagnetic study across the southern East Pacific Rise — have shed new light on our view of the upper mantle where basalts form. Both methods used can detect the presence of magma because seismic velocities change when they pass through molten rocks and electrical conductivity is much higher in magma than in solid rocks. In the MELT area, seismic data9 indicate that small amounts of magma (<2%) are asymmetrically distributed around the ridge axis, in a region extending much farther to the west (up to 350 km) than to the east, and to depths possibly as great as 130 km. The electromagnetic evidence10 is consistent with this finding but also suggests that magma may be present to even greater depths and that the amount of magma is much lower in the east than the west. These results support models for passive magma flow, because the magma appears to converge on the ridge axis from such a wide area in the mantle. But is this consistent with what volcanic and geochemical studies have found along other segments of the East Pacific Rise?

Recent seafloor studies and radioactive dating of basalts on the East Pacific Rise indicate that volcanic eruptions occur both ‘on-axis’ — that is, within a narrow zone along the ridge summit — as well as ‘off-axis’ in the crestal plateau, in a region up to 5 km away11,12,13 (Fig. 2). Some of the basalts found off-axis are geochemically distinct from ‘normal’ basalts recovered closer to the ridge axis. In an upcoming paper, Reynolds and Langmuir3 show that off-axis basalts in the 12°N region of the East Pacific Rise are more depleted in certain elements such as Na, P, Ti and Zr, compared with basalts recovered from the ridge axis. Given what we know (or think we know) about melting and magma flow below ridges, it is difficult to explain such a spatial variation in composition.

Figure 2
figure2

Examples of off-axis pillow lavas that erupted along fissures a few kilometres from the axis of the East Pacific Rise at 9°32′ N.

We may now have an answer. The melt models discussed by Spiegelman and Reynolds1 show that off- and on-axis magmas will differ in composition depending on whether the flow is passive or active in the mantle. The geochemical characteristics of basalts from 12°N are consistent with passive flow, whereby small amounts of magma formed over a wide region in the mantle converge on the volcanic zone. These findings are significant because they are consistent with results from the MELT experiment. They suggest that basalt chemistry can be used as an independent tool to study processes associated with ridge dynamics that are difficult to test with geophysical techniques and geodynamic models alone.

But some problems remain. First, it is not easy to say with certainty which basalts actually erupted off-axis, and which erupted at the ridge axis and were subsequently transported to an off-axis location by spreading. Second, the 12°N area is one of the few areas of the East Pacific Rise where enriched basalts are located on-axis and depleted types are found off-axis. The opposite is true in other parts of the northern East Pacific Rise12,13. How do these areas fit into the picture? Can different segments only a few hundred kilometres apart have different mantle melting styles? Is the distribution of basalt types perhaps related to ocean spreading cycles (for example, active ridges versus those that are starved of magma)? Finally, geophysical data such as those obtained during the MELT experiment do not exist for the northern East Pacific Rise, so it is unclear if those results apply here.

Spiegelman and Reynolds' attempt to pin down a conceptual model for mantle melting is but a first step towards a more complete understanding of melting processes in the upper mantle and their role in creating ocean crust. Despite some widely spaced sampling of basalts along the East Pacific Rise, few areas of the global mid-ocean ridge system have been sampled in detail. So how applicable this model is to other ocean ridges, or even to other fast-spreading segments of the East Pacific Rise, remains to be tested. Correlating the composition of surface basalts and their distribution on the sea floor with models of magma generation in the mantle is a challenge that must be met by marine geologists, geophysicists and geochemists in the coming decades. Fine-scale mapping and sampling of more tectonically diverse segments of the global ridge system will allow us to form a better picture of the mantle processes that ultimately determine the volcanic and tectonic activity that creates the oceanic crust.

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Perfit, M. Molten rocks in motion. Nature 402, 245–247 (1999). https://doi.org/10.1038/46189

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