科研之友
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摘要
Magma differentiation processes that take place in conduits delivering basaltic magmas to the Earth%26apos;s surface are still poorly understood. To get new insights into this problem we have studied in detail the whole-rock geochemistry of a section across a dolerite dyke (21 m thick) and three sections across its narrow apophysis (69 cm, 29 cm and 17 cm thick) from the Russian Fennoscandian Shield. The dyke is fresh, aphyric and shows no evidence of in situ contamination by crustal rocks. Chemical zonation in the dyke changes systematically with the thickness of the sections. In particular, rocks become more primitive inwards (MgO, Mg number and normative An increase and P2O5 decreases) in the main dyke while rocks become more evolved inwards in the apophysis. In addition, the chemical zonation of the apophysis is anomalous since compatible and incompatible elements behave in a manner inconsistent with fractional crystallization of basaltic magma. To explain the chemical zonation, we introduce a novel concept that attributes compositional trends in mafic dykes to competition between two processes: (a) the filling of dykes with magmas that become increasingly more evolved with time as a result of fractionation in the deeper parts of conduits and (b) in situ crystal growth on dyke walls accompanied by effective removal of evolved melt by through-flowing magma. The processes have opposite effects on rock geochemistry, with the first making dykes more evolved and the second more primitive inwards. The key idea is that dykes become more evolved inwards when quenching of inflowing magmas controls the distribution of all components, and become more primitive inwards when in situ crystal growth governs the distribution of all components. Anomalous compositional trends develop when these two processes control the distribution of different groups of chemical components. The chemical zonation in the studied dyke can be attributed to a change from rocks mainly produced by quenching of progressively more evolved magmas (in the apophysis) to those predominantly formed by in situ crystal growth (in the main dyke). This happens in response to a decreasing degree of undercooling as sections of the magma conduit become thicker. We believe that the competitive operation of these two petrogenetic processes is likely a general feature of magma differentiation and crystallization in basaltic conduits.
- 出版日期2012-9-1
