12. RARE EARTH ELEMENT COMPOSITION OF ANHYDRITE: IMPLICATIONS FOR DEPOSITION AND MOBILITY WITHIN THE ACTIVE TAG HYDROTHERMAL MOUND1Susan E. Humphris2 |
ABSTRACTRare earth elements (REEs) of 39 anhydrite samples from different textural settings and from three different areas on the active TAG hydrothermal mound have been analyzed to examine the geochemical evolution of the circulating fluid. All of the chondrite-normalized REE patterns of anhydrite are characterized by positive Eu anomalies that range from 2 to 18 and varying levels of light rare earth (LREE) enrichment with NdN/YbN values from 5 to 25. No systematic variations in the REE patterns are observed with depth or with textural setting. In the TAG-1 area, the samples fall into two groups when normalized to the composition of the end-member black smoker hydrothermal fluid. The first group shows relatively flat REE patterns (NdFL/YbFL of 0.8-2.8) with only a small or no Eu anomaly ranging from 0.8 to 1.5. If it is assumed that the Group I anhydrites were derived from mixing of seawater with an end-member black smoker hydrothermal fluid, then this implies that, during the precipitation of this type of anhydrite from the circulating fluid, all of the REEs, including Eu, are taken up in proportions that reflect their relative distribution in the fluid. Consideration of the redox conditions of the hydrothermal fluid during mixing with either cold or conductively heated seawater indicates that both divalent and trivalent Eu may be present. Hence, a possible explanation for the flat REE patterns relative to hydrothermal fluid is that the trivalent Eu was not discriminated against during precipitation, and hence there was no fractionation of Eu relative to the other REEs. Mixing of a hydrothermal fluid with seawater would result in a decrease in temperature and an increase in fO2 in the mix relative to the hydrothermal fluid, both of which would result in more Eu being present as the trivalent ion. An alternative possibility that cannot be ruled out is that the Group I anhydrites precipitated from mixing of seawater with a hydrothermal fluid of different composition. The second group is characterized by a distinct negative Eu anomaly ranging from 0.2 to 0.7 relative to the end-member hydrothermal fluid. This implies that during the precipitation of this anhydrite group, Eu is excluded relative to the other REEs, resulting in an increase in the concentration of Eu in the fluid while the concentrations of other REEs in the fluid decrease. There is an inverse correlation between Eu anomaly and the absolute REE concentrations in anhydrite suggesting that the Eu anomaly can be used as an indicator of the degree of evolution of the hydrothermal fluid. In addition, an increase in the LREE enrichment as the fluid evolves is also observed, which may be related to the greater stability of LREE chloride complexes in high temperature, low pH fluids. In the TAG-2 area, the Eu anomalies in the anhydrite are more strongly developed than in the TAG-1 area, suggesting that they have formed from hydrothermal fluids that had previously mixed with seawater and precipitated anhydrite to a greater degree than those in the TAG-1 area. The chondrite-normalized pattern of the one anhydrite sample from the TAG-5 area shows some distinct differences from the other samples analyzed. The absolute REE concentrations in this sample, with the exception of Eu, are the highest found in this study and the Eu anomaly, although still positive, is extremely small. Possible mechanisms to explain this REE pattern in these samples include mixing between a hydrothermal fluid that has undergone conductive cooling prior to mixing, or addition of REEs to the hydrothermal fluid prior to mixing with seawater by dissolution of anhydrite. The REE data provide evidence for the entrainment of seawater and mixing with hydrothermal fluids down to at least 58.68 mbsf. The mixing process is complex and chaotic at all scales, and shows no systematic variations either laterally or vertically on the mound scale, or even within an individual vein. |
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