Apri11998 NOTICIAS DE GALÁP AGOS 23 of the Archipelago) by prohibiting potentially dangerous Anderson, D. 1995b. Ecological and population dynamics of fishing techniques within this area. This is not only for the Waved Albatross: preliminary report to the Charles Darwin protection of single species, but for the we11being of the Research Station, June 23,1995. Unpublished report, depos- ecosystem. ited in the Charles Darwin Research Station, Puerto Ayora, Galápagos. 1would like to thank David Parer and Elizabeth Parer- Gales, R. 1993. Co-operative mechanisms for the conservation Cook for giving me the opportunity to travel with them. of albatross. Australian Nature Conservation Agency and Also to the crew of the Samba for the use of their keen eyes Australian Antarctic Foundation, Tasmanian Government and especia11y to David Day. Printer, Hobart. My appreciation is also extended to Dr. Mike Harris Harris, M. P. 1973. The biology ofthe WavedAlbatross Diomedea and Dr. Sarah Wanless for their help in preparing this irrorata of Hood Island, Galapagos. Ibis 115: 483-510. manuscript. de la Mare, W. K. and K. R. Kerry. 1994. Population dynamics of the Wandering Albatross, Diomedeaexulans, on Macquarie LITERA TURE CITED Island and the effects of mortali ty fram longline fishing. Polar Biology 14: 231-241. Anderson, D. 1995a. Census of Waved Albatrass, 1994: final Godfrey Merlen, Puerto Ayara, GaIápagos. report to the Charles Darwin Research Station, June 19,1995. lReprinted with permission of the Pacific Seabird Group, this Unpublished repart, deposited in the Charles Darwin Re- article appeared in Pacific Seabirds 23 (2): 10-12, Fall1996. search Station, Puerto Ayara, Galápagos. , GALAPAGOS PLUMOLOGY By: Karen Harpp and Dennis Geist INTRODUCTION THE GALAP AGOS HOTSPOT Although the Galápagos Islands are best known as Although the Galápagos were proposed as resulting one of the world's greatest naturallaboratories for bio- from a mantle plume early in the development of the theory logical studies, they have also lent considerable insight (Morgan 1972), there are two problems with the simple into the dynamics of the earth' s interior. Modern accounts plume model when it is applied to Galápagos. First, at of the geologic origin of the Galápagos Islands, including more conventional hotspots such as Hawaii and popular descriptions (e.g., Boyce 1994, Jackson 1990), at- Ye11owstone, only one to four volcanoes at the young tribute their formation to a "hotspot" or "mantle plume." "upstream" (in terms of plate motion) end of the chain are The term "hotspot" refers to localities where volcanoes active. In contrast, nearly a11of the Galápagos islands occur in the middle of one of the earth's great tectonic have erupted in the recent geologic past, regardless plates; hotspots are unusual, beca use over 90% of the whether they sit at the easternmost, or oldest, end of the world's volcanic activity occurs at plate boundaries. It is chain, or at the westernmost, or youngest, end of the ar- thought that hotspots result from "mantle plumes," con- chipelago. duits of hot, plastic (but not molten) rocks that ascend from deep within the earth's mantle (Figure 1). As these Extinct Hotspot Active Hotspot plumes of hot rock rise to depths of about 100 km from Mid-Ocean Ridge Volcano Volcano earth' s surface, they begin to melt. When the melted frac- tion of the rock reaches several percent, it segregates from the rock (like wa ter being squeezed from a damp sponge), eventua11y erupting to form volcanoes. N o one knows the depth to the roots of mantle plumes, but most geologists +- Mantle Depleted believe they come from a layer in the earth' s mantle either Currents Upper Mantle at 650 km or from the bottom of the mantle at 2700 km depth. Recent work on Galápagos lavas has identified Enriched Hot, some problems with the simple mantle-plume theory, but Mantle Plastic Plume Rock has taught us much about the origin, composition, and behavior of hotspots. Figure 1. Cartoon of a simple plume rising from the lower mantle. The plume is a plastic solid which begins to melt as it nears the bottom of the overlying plate. 24 NOTICIAS DE GALÁP AGOS No. 59 Second, the compositions of Galápagos magmas are Bíllions of years ago unusual for hotspots. Generally, basalts erupted from A B hotspotvolcanoes are compositionally distinct from those erupted in other tectonie environments, and each volcano Continental Crust evolves in a compositionally similarway. Galápagos lavas Melt + Sr2+ exhibit a wider range of chemical compositions than is 'Rb >- usual, and compositional differences are more related to the volcanoes' location than to their stage of development. 1 Some of these complexities may arise from the fact Billions of years pass... that the Galápagos hotspot lies only about 100 km from the Galápagos Spreading Center, an active mid-ocean Radioactive decay of 37Rb to 37Sr occurs ridge. Great volumes of magma are produced beneath Present day 1 mid-ocean ridges, and in the islands these appear to con- C D taminate the hotspot magmas produced by the Galápagos plume. Furthermore, the plates are strongly fractured, Hi her 87Sr/86Sr thin, and weak near mid-ocean ridges, which may permit magmas to ascend through the plate and erupt in places where they would not normally reach the surface. This contribution describes recent studies using isoto- pie ratios of the elements lead, strontium, and neodymium, which have shed some light on the structure of the Figure 2. Schematic diagram showing how the 87Sr/86Sr ratios Galápagos hotspot and the dynamics of mantle plumes. of the crust and the depleted mantle have evolved and changed with time. GEOCHEMISTRY OF PLUMES Before considering the details of the isotopic ratios of The continental crust began to form about 4 billion Galápagos lavas, we shall describe how they are used as years ago, and, as time passed, some of the radioactive a diagnostic tool in studies of volcanic systems. Some 87Rbisotope decayed, producing increasing amounts of naturally-occurring elements are radioactive, which 87Sr. Meanwhile, in the part of the mantle depleted of means that their nuclei decay spontaneously into nuclei melt, where there was les s 87Rb,les s 87Srwas produced of other elements. The isotopes we focus on have half- during the same time (Figure 2C). When abundances of lives (the time for half of the nuclei to decay) ofbillions of isotopes are determined, the concentration of the daugh- years. The original radioactive element is called the "par- ter isotope (87Sr)is measured with respect to another ent" isotope, and the element resulting from the decay isotope of the same elementwhose abundance in the earth transformation is referred to as the "daughter," or "radio- has not been affected by radioactivity (e.g., 86Sr).Thus, as genic," isotope. For example, naturally-occurring a result of the melting process and the passage of billions rubidium of atomic mass 87 (87Rb)is radioactive and de- of years, the old continental crust has a distinctly higher cays to produce strontium of atomic mass 87 (87Sr)with a 87Sr/86Sr ratio than does the depleted mantle (Figure 2D). half-life of 50 billion years. A very important aspect of such isotopic systems is Isotopes such as 87Srand 87Rbare useful in the stud y of that present-day geologie processes, such as weathering, young magmas such as those of the Galápagos hotspot melting, and solidification, are incapable of changing the because of processes that have taken place over the course ratios of isotopes of the same element (for example, 87Sr/ of geologic time. This concept is best illustrated by track- 86Sr). This is because natural chemical and physical pro- ing the behavior of a typical parent-daughter isotope pair cesses do not differentiate between isotopes of the these through the early part of earth history. The elements Rb heavyelements. Moreover, in young magmatic systems, and Sr, for example, were partly segregated from each the time for decay of the radioactive parent isotopes is other when the continental crust formed from magma very long relative to the ages of the erupted material. In made by partial melting of the earth's mantle billions of other words, although a partial melt has a higher 87Rb/ years ago. When rock begins to melt, larger ion s migrate 86Srratio than the rock it lea ves behind, the melt and rock preferentially into the liquid fraction, with the smaller have identieal 87Sr/86Srratios, untilmanymillions ofyears ions remaining behind in the residual solid. Because Rb+ have passed. is a larger ion than Sr2+,more Rb+partitions into the melt Because oceanic basalts derive directly from the earth' s than Sr2+(Figure 2A). It is simplest to consider the varia- mantle, the isotopic ratios of the basalts faithfully record tions of the Rb/Sr ratio during melting; the melt that goes the isotopic ratios of the mantle, thus serving as a "finger- on to form the crust will have higher Rb /Sr relative to the print" of their mantle sources. Several other isotopie source material, and the rock left behind in the mantle systems are similar to the Rb-Sr system, including the will have relatively lower Rb/Sr (Figure 2B). samarium-neodymium (Sm-Nd) and uranium-thorium- April 1998 NOTICIAS DE GALÁP AGOS 25 lead (U-Th-Pb) systems; for these we measure present ample, lavas fram Santa Fe are nearly 3 million years old. day values of 143Nd/lHNd, 206Pbj204Pb,2°7pbj204Pb,and Because the plate on which the Galápagos Islands lie is 20sPb j204Pb. moving about 5 cm/yr to the east, then at the time that Lavas erupted from mid-ocean ridges such as the Santa Fe was active, it was located about 150 km to the Galápagos Spreading Center have isotopic fingerprints west of where it is now, approximately where Fernandina id en tical to the melt-depleted mantle.