es" after their first discovery on Adak Island in the Aleutians. Paradoxically, of the 20 or so occurrences of adakitic magmas around the world, only 5 can be explained by the subduction of very young oceanic crust. The rest are associated with the subduction of older (10 - 45 million year old) crust, which according to published numerical models should not reach sufficiently high temperatures at 100 km depth to allow melting. An unusual type of subduction, "flat subduction" (where the downgoing plate flattens its angle of descent and "underslides" the upper, typically continental, plate sub-horizontally for several hundred kilometers), can produce a special set of temperature and pressure conditions allowing the direct melting of hydrous (water bearing) minerals in the downgoing oceanic plate. Ten "flat slab" zones have been identified worldwide. Eight of these are associated with modern or young (< 3 million year old) adakites. The thermal structure of "flat subduction" has never been modeled numerically and remains an important open question with a direct bearing on the generation of great interplate subduction earthquakes. The importance of the thermal structure of subduction zones and its impact on volcanism and earthquakes was briefly discussed in a "New and Views" article in the journal Nature
(vol. 403, p. 31-34, 6 January, 2000) by Stephen Kirby of the USGS.
Alternative origin of aliphatic polymer in kerogen
B.A. Stankiewicz et al., p. 559
The authors discuss new insights into the origin of kerogen. Kerogen is insoluble organic matter, commonly composed of mostly aliphatic hydrocarbons, that occurs in ancient sedimentary rocks as a result of the accumulation and transformation of biomass into geologically long-lived material. Kerogen has been identified as a largely untapped hydrocarbon source. Here, the authors conclude that arthropod exoskeletons can make a significant contribution to kerogen deposits. Because arthropods are perhaps
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Contact: Ann Cairns
acairns@geosociety.org
303-447-2020 x 156
Geological Society of America
31-May-2000
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