|
Ultrabasic springs within the Franciscan Complex of the California Coast Range have been intensely investigated by geochemists and geobiologists. Springs located near the town of Cazadero 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 sample collected at the Cedars. 87Sr/86Sr isotopic ratios in the serpentinizing springs ranged from 0.70926 to 0.70955; the Mg2+-HCO3- type stream water had an isotopic ratio of 0.70848. The 87Sr/86Sr in the travertines ranged from 0.70931 to 0.70966. One serpentinite sample had an isotopic ratio of 0.70896. The measured 87Sr/86Sr ratios of 0.7094 are 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. Sr isotopes may be fractionated during serpentinization reflecting the site of origin of serpentinization. The Franciscan complex is believed to represent a site of island arc collision; 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. If this is the case, it may be possible to use 87Sr/86Sr to determine the location of initiation of serpentinization (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. |
|
|
