HR: 0800h
AN: V51B-1481    [Abstracts]
TI: The Origin of 87Sr/86Sr in Cold Springs and Travertines of the Franciscan Complex near Cazadero, California
AU: * Marks, N
EM: marks@geology.ucdavis.edu
AF: University of California, Davis, Geology Department, One Shields Avenue, Davis, Ca 95616 United States
AU: Schiffman, P
EM: schiffman@geology.ucdavis.edu
AF: University of California, Davis, Geology Department, One Shields Avenue, Davis, Ca 95616 United States
AU: Yin, Q
EM: yin@geology.ucdavis.edu
AF: University of California, Davis, Geology Department, One Shields Avenue, Davis, Ca 95616 United States
AU: Zierenberg, R
EM: zierenberg@geology.ucdavis.edu
AF: University of California, Davis, Geology Department, One Shields Avenue, Davis, Ca 95616 United States
AB: Ultrabasic springs within the Franciscan Complex of the California Coast Range have been intensely investigated by geochemists and geobiologists. Springs located in Sonoma County in an area historically known as The Cedars are of particular interest to scientists exploring Martian analogues (Johnson et al. 2004) or investigating serpentinization processes (Barnes and O'Neil, 1969; Barnes et al. 1972). Laser ablation and solution phase multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) were used to obtain 87Sr/86Sr isotope ratios in fluid, travertine and serpentinite samples collected at the Cedars. 87Sr/86Sr isotopic ratios in the serpentinizing springs range from 0.70926 to 0.70955; the Mg2+-HCO3- type stream water has an isotopic ratio of 0.70848. The 87Sr/86Sr ratio in the travertines ranges from 0.70931 to 0.70966. The mean 87Sr/86Sr ratio of the travertine (0.7094) is far more radiogenic than typical mantle values of 0.703 to 0.705, indicating that the peridotite is an unlikely source of the radiogenic Sr. Similarly, the measured ratio is much higher than the expected Sr isotope ratio of seawater that might be trapped in Jurassic Franciscan Sediments or oceanic crust. Strontium leached from Franciscan sediments themselves should reflect a Sierran or Klamath source with expected values in the range of 0.705 to 0.706. Indeed the measured isotope ratios even exceed modern seawater values. The observed radiogenic values suggest the presence of older, potassium (and rubidium)-rich rocks within the fluid flow path. Alternatively, the presence of clay minerals that readily substitute Sr for Ca may well account for the radiogenic strontium signal. It is possible that the serpentinization observed at The Cedars initiated along a ridge flank and the Sr isotopic chemistry reflects the site of initiation. The radiogenic strontium in these springs may result from fluid interaction with seafloor sediments deposited along the flank of a slow spreading ridge. If this is the case, it may be possible to use 87Sr/86Sr to determine the location of serpentinization initiation (Gruau et al, 1998). Such a revelation might provide insight into the geochemical processes associated with mid-ocean ridge flank serpentinization at sites like Lost City (Kelley et al., 2005). The implications of this correlation could provide information about the preservation of geochemical systems through obduction of ophiolitic sequences and provide insights into ridge flank hydrothermal and serpentinizing processes.
DE: 1040 Radiogenic isotope geochemistry
DE: 3042 Ophiolites (8140)
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 3653 Fluid flow
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005