Authigenic growths of heavy minerals, crystallised under relatively low temperature diagenetic conditions have been recognised in a number of sedimentary sequence. A range of microbeam techniques have revealed trace element geochemical variations on several scales in authigenic titanites and apatites. These variations occur, within individual cement grains and patches, within a single sedimentary sequence, and between different sedimentary sequences. Mixed fluvial-aeolian, Permian, sediments at the Cock of Arran (Isle of Arran, Scotland) contain pore filling titanite cements. The titanites show a range of zoning patterns, dominated by compositional sector zones. The sector zones are defined by large (order of magnitude) differences in rare earth element (REE) and high field strength element (HFSE) concentrations between titanite grown at different crystal faces. The mechanism responsible for sector zone formation is considered to relate to differences in the surface structure of titanite at different crystal faces. Faces of the forms { 100 } and { 001 } are likely to have surface configurations more amenable for impurity element incorporation than faces of the form { 161 }. This evidence for disequlibirium titanite growth, and the large differences which must exist between effective partition coefficients at different titanite crystal faces, makes models of equilibrium trace element partitioning effectively meaningless. Minor growth zones are also present and relate to subtle differences in the REE composition of the titanite. These variations have been used to construct a model of titanite-fluid REE partitioning which suggests that the HREE have higher effective titanite-fluid partition coefficients than the LREE. In the continental Statfjord Formation (Jurassic, North Viking Graben) authigenic apatite occurs as overgrowths on variably corroded detrital cores. The authigenic apatite is chemically very distinct from the detrital apatite and contains high concentrations of Sr, REE, F, and probable concentrations of C.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:245987 |
| Date | January 1996 |
| Creators | Bouch, Jonathan E. E. |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk/R?func=search-advanced-go&find_code1=WSN&request1=AAIU089767 |
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