We demonstrate finite structures formed as a consequence of the “reactive infiltration instability” (Chadam et al., 1986) in a series of laboratory and numerical experiments with growth of solution channels parallel to the fluid flow direction. Regions with initially high porosity have high ratios of fluid volume to soluble solid surface area and exhibit more rapid fluid flow at constant pressure, so that dissolution reactions in these regions produce a relatively rapid increase in porosity. As channels grow, large ones entrain flow laterally inward and extend rapidly. As a result, small channels are starved and disappear. The growth of large channels is an exponential function of time, as predicted by linear stability analysis for growth of infinitesimal perturbations in porosity. Our experiments demonstrate channel growth in the presence of an initial solution front and without an initial solution front where there is a gradient in the solubility of the solid matrix. In the gradient case, diffuse flow is unstable everywhere, channels can form and grow at any point, and channels may extend over the length scale of the gradient. As a consequence of the gradient results, we suggest that the reactive infiltration instability is important in the Earth's mantle, where partial melts in the mantle ascend adiabatically. Mantle peridotite becomes increasingly soluble as the melts decompress. Dissolution reactions between melts and peridotite will produce an increase in liquid mass and lead to formation of porous channels composed of dunite (>95% olivine). Replacive dunite is commonly observed in the mantle section of ophiolites. Focused flow of poly baric partial melts of ascending peridotite within dunite channels may explain the observed chemical disequilibrium between shallow, oceanic mantle peridotites and mid‐oceanic ridge basalts (MORB). This hypothesis represents an important alternative to MORB extraction in fractures, since fractures may not form in weak, viscously deforming asthenospheric mantle. We also briefly consider the effects of crystallization, rather than dissolution reactions, on the morphology of porous flow via a second set of experiments where fluid becomes supersaturated in a solid phase. Formation of short‐lived conduits parallel to the flow direction occurs rapidly, and then each conduit is eventually choked by interior crystallization; fluid flow then passes through the most permeable portion of the walls to form a new conduit. On long time scales and length scales, transient formation and destruction of conduits will result in random and diffuse flow. Where liquid cools as it rises through mantle tectosphere on a conductive geotherm, it will become saturated in pyroxene as well as olivine and decrease in mass. This process may produce a series of walled conduits, as in our experiments. Development of a low‐porosity cap overlying high porosity conduits may create hydrostatic overpressure sufficient to cause fracture and magma transport to the surface in dikes.

letters to nature NATURE | VOL 389 | 2 OCTOBER 1997 479 not corroborated this high value [16][17][18] . Quinlan and Beaumont 16 matched the stratigraphy of the Appalachian basin by using a layered visco-elastic model that had an average EET of 67 km. Although they note that a pure elastic plate model does not adequately reproduce the stratigraphy, elastic plate models in which the EET decreases with curvature would produce offlapping stratigraphic patterns as seen in the observed stratigraphy. Recently, Stewart and Watts 17 re-estimated the EET of several mountain belts by using a variable-rigidity formulation. Converting their estimates to the values of E and u used in this paper gives a range of 50-88 km for the EET. These studies suggest that the high value found by Karner and Watts 1 is an overestimate and that a lower value averaging ϳ65-70 km is a better estimate. If so, our prediction of 60 km is within uncertainties.We have established a parametrization of flexural strength at continents based on the yield stress envelope that successfully predicts the EET of the continental lithosphere at foreland basins and mountain belts. We have also demonstrated the importance of sediment fill as parameter controlling flexural strength at continents. The sediment cover is most likely to control the value of EET in places where the lithosphere's crust is thin compared with an average 35-km-thick continental crust 5 , the age of the lithosphere is close to its thermal equilibrium and for which the sediment cover reaches thicknesses greater than 3-5 km. Accounting for the effect of sediments and crustal thickness should facilitate the evaluation of the flexural strength at other types of basins and continental margins. Ⅺ

No abstract

The Austral archipelago, on the western side of the South Pacific superswell, is composed of several volcanic chains, corresponding to distinct events from 35 Ma to the present, and lies on oceanic crust created between 60 and 85 Ma. In 1982, Turner and Jarrard proposed that the two distinct volcanic stages found on Rurutu Island and dated as 12 Ma and 1 Ma could be due to two different hotspots, but no evidence of any recent aerial or submarine volcanic source has ever been found. In July 1999, expedition ZEPOLYF2 aboard the R/V L'Atalante conducted a geophysical survey of the northern part of the Austral volcanic archipelago. Thirty seamounts were mapped for the first time, including a very shallow one (Ͻ27 m below sea level), located at lat 23؇26.4S, long 150؇43.8W, ϳ120 km southeast of Rurutu. A nepheline-rich scoriaceous basalt sample from pillow lavas dredged on the newly mapped seamount's western flank gave a K-Ar age of 230 ؎ 0.004 ka obtained on pure selected nepheline. We propose that this seamount, already called Arago Seamount after a French Navy ship that discovered its summit in 1993, is the missing hotspot in the Cook-Austral history. This interpretation adds a new hotspot to the already complicated geologic history of this region. We suggest that several hotspots have been active simultaneously on a region of the seafloor that does not exceed 2000 km in diameter and that each of them had a short lifetime (Ͻ20 m.y.). These short-lived and closely spaced hotspots cannot be the result of discrete deep-mantle plumes and are likely due to more local upwelling in the upper mantle strongly influenced by weaknesses in the lithosphere.