Costo de la capacitación

Most of us can probably remember being taught in primary school that nature wastes little: animal waste, fallen leaves, and dead logs are recycled by soil organ- isms into nutrients for plants that provide food for animals, etc. The environ- mental movement urges society to become more like nature and waste less and recycle more. Recycling, in that social sense of actions like separating metals, glass, paper, etc, from ‘non-recyclables’, came into vogue in the 1980’s, but it’s origins are earlier and date to the explosion of environmental concern in the 1960’s and early 1970’s – triggered, perhaps, by Rachel Carson’s 1962 book Silent Spring. It turns out that ecosystems are not the only way in which nature recycles. Another kind of recycling movement also caught on in 1980’s and also had its origins in the 1960’s – triggered by the work of Dick Armstong (1968). He wrote, “Contrasting interpretations of existing models of Sr and Pb isotope evolution can be eliminated with a model in which crustal material is recycled through the mantle. In this model the earth’s crust and upper mantle (above approximately 500 km depth) are in a steady-state system, and the volumes and bulk compositions of oceanic crust, continental crust, and mantle have been nearly constant for at least the last 2.5 b.y. and probably for most of the Earth’s history.” The paper is remarkable in a number of respects, not the least of which is that it was published before the papers that same year by Isacks et al. (1968) and Le Pichon (1968) that integrated seafloor spreading and continental drift into the modern theory of plate tectonics. Armstrong was way ahead of everyone else in thinking about the implications of plate tectonics for the chemical evolution of the Earth.

As a graduate student, I was intrigued by Armstrong’s ideas. In trying to interpret my thesis data from the Mid-Atlantic Ridge and the Azores, it seemed to me that neither the Icelandic nor the Azores plume could consist of primitive material or some mixture of primitive material and depleted mantle: their evolu- tion must have been more complex. In addition, using simple melting model calculations, such as equation 2.1, I found that the mantle source of Azores basalts was much like somewhat incompatible element-enriched MORB (what we today might call E-MORB). Armstrong’s idea seemed the explanation: what if the Azores plume consisted, in part, of recycled oceanic crust with perhaps some subducted sediment to provide a bit more incompatible-element enrichment? What if, instead of subducted oceanic crust and sediment being homogenised back into the upper mantle as Armstrong seemed to imply, it remained sequestered somewhere in the deep mantle until its higher heat-producing element concen- trations made it buoyant? I wrote those ideas up in the draft of my thesis. When

Schilling read it, however, he suggested it was too speculative and that I drop it. I was too close to finishing to engage in another battle with my advisor at that point. Furthermore, I was only a grad student: what did I know? So I deleted that from my thesis, but held on to the hand-written draft, intending to write up the idea and publish it later.

Afraid of being ‘scooped’, I discussed the idea with few other people. I am not sure whether Al Hofmann came to the idea independently or through discussions with me. I do remember a telephone conversation with him during my post-doc at USGS in Denver. Al asked if he had told me of his new idea (plumes from recycled crust) that he was writing up for that year’s Carnegie Annual Report. I replied that the idea wasn’t new – I had written it up several years earlier in my thesis and I sent him copies of my handwritten draft. He graciously added my name to a brief report in the Carnegie Yearbook (Hofmann and White, 1980). When I joined him in the newly established Geochemistry Division at the Max Planck Institut für Chemie in Mainz, we continued to work on the idea and the result was the Hofmann and White (1982) paper. By that time, however, we had already been scooped – twice. Although it wasn’t the main thrust of the paper, Hawkesworth et al. (1979) proposed the idea as an explanation for their Sr and Nd isotope data from the Azores, and it was the main thrust of the Chase (1981) paper (Clem Chase was one of the few people that I do remember talking to about the recycling idea; as he was a geophysicist, I thought it was safe).

One of the reasons I had held off writing up the recycling idea was that I wanted more evidence. Sr and Nd isotope data on oceanic island basalts that I had acquired at Carnegie, USGS, and MPI appeared to provide it. In particular, extreme isotopic compositions from Samoa, the Society Islands, Tristan da Cunha, and Kerguelen seemed best explained by the recycling hypothesis; they certainly could not be explained by primitive mantle–depleted mantle mixing, which was then in vogue. At the time, the Sr-Nd isotope mantle array appeared to bifurcate into two trends: one through Tristan and Kerguelen (what we now call EM I), the other through the Society Islands, Samoa, and the Azores (what we

now call EM II). In White and Hofmann (1982), we suggested the latter resulted from sediment-dominated subducted material, while the former resulted from subduction of oceanic crust without sediment.