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Physics and Chemistry of Mantle Plumes

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Physics and Chemistry of Mantle Plumes Eos, Vol. 72, No. 21, May 21, 1991 tween the creation of heterogeneities and the Physics and Chemistry efficiency of convection in homogenizing the mantle on different scales (Richter et at., 1982; Hoffman and McKenzie, 1985]. One of Mantle Plumes explanation for the persistence of heteroge- neities is that they are continuously forming, PAGES 236-237 so there is always a steady-state concentra­ tion of them in the mantle. Identifiable mechanisms for the introduction of heteroge­ Donald J. DePaolo, Edward M. Stolper, and Donald M. Thomas neities include subduction of oceanic crust, including sediment derived from continents, subduction of continental lithosphere, and Hot spot volcanic chains are a funda­ structure of oceanic volcanos, and a number creation of "depleted zones" in the mantle in mental feature of the Earth's crust, but their of other geophysical and geochemical prob­ regions from which magma has recently origins are still poorly understood [ Okal and lems. Drill coring is an effective means of been extracted. An alternative view is that Batiza, 1987]. The Hawaiian-Emperor vol­ addressing these problems because the tar­ large parts of the mantle are permanently canic chain, which dominates the topogra­ gets-the lava accumulations of the large separated from each other, the prevailing phy of the central Pacific ocean floor, is the Hawaiian shield volcanos-are nearly hori­ model being that the mantle is chemically best developed and most intensely studied zontal subsurface features. The information layered. At one time, these models were of the known hot spot tracks. It continues to accessible through drilling is unlikely to be­ considered in opposition to one another (Zindler et at., 1982], but they have since be one of the world's most important field come available otherwise; hence it appears laboratories for the study of igneous pro­ necessary to employ drilling to effectively been recognized to be complementary (Zin­ dler et at., 1984]. cesses, plate movements, mantle convection, address the scientific problems. structure, geochemical evolution, and the This article describes the primary scien­ Within this general framework, the ques­ properties of the lithosphere. tific basis for the proposed project-elucida­ tion of the nature of hot spots plays a central role. A common line of argument follows Despite continued effort, fundamental tion of the physics and chemistry of the Ha­ from the observation that hot spots seem to questions regarding the composition, struc­ waiian mantle plume on the time scale of a ture, and evolution of Hawaiian volcanos single volcano (about 106 years). be fixed, or at least slowly moving in rela- and their magma sources remain unan­ swered. This is largely due to the fact that b only lavas representing the late stages in the evolution of the volcanos can be sampled at Origin and Chemistry of Mantle the surface. Most of the internal structure of Plumes the volcanos and evidence of their growth The plate tectonic revolution and the history and geochemical evolution are hid­ concurrent exploration of other solar system den from view. The most deeply eroded vol­ bodies caused a profound change in the way canos are exposed only to depths of a kilo­ Earth evolution and structure are viewed. In meter or so, whereas the volcanos rise some many ways, the shifts in the basic paradigms 5-15 km above the old ocean floor (Moore, revolve around the central theme that con­ LOWER MANTLE More primitive ? 1987]. vection in the Earth's interior is probably the More homogeneous ? In late 1986, in recognition of the scien­ single most important process determining tific value of observing the deep interiors of the evolution of the Earth. Study of plate tec­ Hawaiian volcanos, a group of Earth scien­ tonics led to the realization that large-scale tists submitted a proposal to the NSF-spon­ movements are taking place in the Earth's sored drilling consortium DOSECC (Deep mantle, which brought into focus the need Observation and Sampling of the Earth's to understand the properties of mantle mate­ Continental Crust) to drill and core a vol­ rials, convection itself, and the relation of cano to retrieve and study a more complete mantle dynamics to seismic structure, the stratigraphic sequence of its lavas. Following geopotential fields, the figure of the Earth, the reorganization of the drilling programs, and the evolution of continents. this proposal was submitted to the Continen­ Since direct study of the mantle is in tal Lithosphere Program of the National Sci­ most cases impossible, indirect means must Fig. 1. Earth models and their association ence Foundation in 1988 and again in 1989. be employed. Such means include global with the issues of the sources and dynamics The proposed Hawaiian Scientific Drilling heat flow studies, studies of the geoid, and of mantle plumes. The upper mantle is Project (HSDP) is aimed primarily at improv­ seismic studies combined with high-pressure known to be depleted in magmaphile and ing understanding of the structure and dy­ material properties research. These ap­ volatile elements and to be very heteroge­ namics of the mantle, particularly with re­ proaches study the mantle as it is currently. neous. The lower mantle may be more prim­ gard to the origin of mantle plumes and their Geochemical studies of mantle-derived mate­ itive (less depleted) and less heterogeneous. interaction with the lithosphere and shallow rials have a special role in that they provide The lowermost mantle might contain primi­ asthenosphere. The proposed studies would insight both about the existing structure and tive material, mantle material that has re­ also provide unique data on the physics and composition as well as time-dependent infor­ acted with the core, or subducted slab (recy­ chemistry of magma generation, the internal mation, such as residence times and dis­ cled) material. The 670-km seismic persal times. The time dimension is accessi­ discontinuity may be a thermal boundary ble through the study of isotopic variations, layer between two convecting systems, or and the most useful vehicles are basaltic simply a phase-change boundary. Plumes lavas that bring the information to the Earth's Donald J. DePaolo, Center for Isotope Geochemis­ might originate at thermal boundary layers try, Department of Geology and Geophysics, Uni­ surface from source regions in the mantle. at (a) the 670-km level, (b) the core-mantle versity of California, Earth Sciences Division, Over the past 2 decades, isotopic studies boundary, or (c) within the lower mantle. Lawrence Berkeley Laboratory, Berkeley, CA 94720; of oceanic basalts have shown that the man­ The erupted lava compositions are affected Edward M. Stolper, Division of Geological and tle contains diverse materials [e.g., Zindler not only by the source of the plume, but also Planetary Sciences, California Institute of Technol­ by the amount of entrainment (in upper and ogy, Mail Code 170-25, Pasadena, CA 91125; Don­ and Hart, 1986]. Convection should eventu­ ald M. Thomas, Hawaii Institute of Geophysics, ally make heterogeneities disappear, so their lower mantle) and the amount of assimila­ University of Hawaii, Honolulu, HI 96822. existence is a measure of the balance be- tion of lithosphere. This page may be freely copied. Eos, Vol. 72, No. 21, May 21, 1991 tion to the plates, suggesting a deep source pretation of seismic velocities in the deep lavas, and then to transitional tholeiitic la­ (Morgan, 1971]. In addition, some of the Earth, are thus affected. If the lower parts of vas, in the sense that the isotopic composi­ geochemical properties of lavas associated the mantle are not well degassed, it impacts tions look less like those of MORB and more with hot spots suggest that the mantle mate­ theories of the origin of the atmosphere and like those of primitive mantle material (eNd rial from which they are derived is different oceans (e.g., Hart eta/., 1979]. If only the = 0). This has been interpreted as evidence and more primitive (that is, more similar to upper mantle has been involved in the for­ for two major mantle sources for the lavas. the original bulk composition of the Earth) mation of the continents, it affects the way One source, the one with the MORB-like sig­ than what is typical of the upper mantle as we view the composition of the continents nature, is interpreted to be the oceanic litho­ indicated by MORB (mid-ocean ridge ba­ and oceans and how their compositions sphere. The other source is presumably a salts) (e.g., Schilling, 1973; Chen and Frey, have changed with time (DePaolo, 1980]. deeply-sourced mantle plume. There is a 1985]. This mantle material is also different The interiors of Hawaiian volcanos may suggestion that farther down in the lava se­ in such a way that it may not be explainable carry particularly diagnostic information for quence, in the "middle" of the main "tholei­ in terms of recycled oceanic or continental assessing the ultimate origin and nature of itic" stage, the lavas may have isotopic com­ crust (Kurz eta/., 1982; Hofmann and White, mantle plumes. This statement stems from positions that are different from the MORB­ 1982]. There has followed the hypothesis the observation that erosion exposes lavas type values. It is noteworthy that the study of that hot spots represent "plumes" of chemi­ that represent only the end stages of the vol­ Chen and Frey ( 1985], which was an impor­ cally and isotopically special material from canos' lifetime, and that there are trends in tant advance from previous work, was done deep in the mantle. composition in these late-stage lavas that on drill-core material. If plumes are chemically and isotopically suggest that the interiors of the volcanos are Two other observations are important. different from normal upper mantle material, different.
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