The sup 87 Sr/ sup 86 Sr ratios in some bulk abyssal and alpine peridotites are too high to be binary mixtures of depleted mantle and seawater components.The apparent excess, or ``orphan,'' sup 87 Sr appears to be separated from its radioactive parent. Such observations were widely held to be analytical artifacts. Study of several occurrences of orphan87 Sr shows that the orphan component in abyssal peridotite is located in the alteration products of olivine and enstatite in the peridotite. The orphan87 Sr is most likely introduced by infiltration of low-temperature (``less than''200 ``degrees'' C) seawater bearing suspended detrital parti``culates. These particulates include grains of detrital clay that are partly derived from continental (that is, granitic) sources and thus are highly radiogenic. Orphan87 Sr and other radiogenic isotopes may provide a tracer for low-temperature seawater penetrating into the oceanic crust.