ORPHAN SR-87 IN ABYSSAL PERIDOTITES - DADDY WAS A GRANITE

The Sr-87/Sr-86 ratios in some bulk abyssal and alpine peridotites are too high to be binary mixtures of depleted mantle and seawater compon ents. The apparent excess, or ''orphan,'' Sr-87 appears to be separate d from its radioactive parent. Such observations were widely held to b e analytical artifacts. Study of several occurrences of orphan Sr-87 s hows that the orphan component in abyssal peridotite is located in the alteration products of olivine and enstatite in the peridotite. The o rphan Sr-87 is most likely introduced by infiltration of low-temperatu re (<200-degrees-C) seawater bearing suspended detrital particulates. These particulates include grains of detrital clay that are partly der ived from continental (that is, granitic) sources and thus are highly radiogenic. Orphan Sr-87 and other radiogenic isotopes may provide a t racer for low-temperature seawater penetrating into the oceanic crust.