Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 298

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PALL IMSLAND
Fig. 129. Analysed Y content of the Jan Mayen rocks plotted against the Y content,
that should be in the rocks according to the fractionation model calcula-
tions. Individual points are connected and numbered. The numbered
groups correspond to the rocks of the tables of fractionation in previous
chapters. Thus nr. 1 shows the result of the fractionation within the
ankaramites (Table 40) and so on. 3 = Mg poor ankaramites to Mg rich
basalts (42), 4=ankaramitic basalts (43), 5=Mg rich basalts of low K20
(44), 6=Mg rich basalts of high K20 (45), 7 = Mg poor basalts (46), 8=Mg
poor basalts to basaltic tristanites (47), 9=basaltic tristanites to tristanites
(48), 10=Nord-Jan tristanites to trachytes (49), ll=Sör-Jan tristanites to
trachytes (50). Y is here treated as a truly incompatible element, with all
crystal-magma distribution coefficients like 0.
elements the distribution coefficient of the element between crystal and
magma (Ka/L, a indicating the mineral phase) is zero for all minerals. D.s,
the bulk distribution coefficient (Ds=WaKa/L+W^K^/L+ . . . , W
being the weight fraction of the mineral, a, (5 etc., fractionated), is thus zero
as well. Then, CL/Ci= 1/F’ (CL=concentration of the element in the residual
liquid, C;=concentration of the element in the original liquid and F’=
fraction of the liquid left after fractionation). The incompatible trace
element concentration enrichment produced by the crystal removal is thus
calculated and the calculated content can be compared to the one analysed
in the rocks considered. In Fig. 129 the analysed and calculated values ofY
are shown plotted against each other. In the case of rock groups 1— 6,
(corresponding to those of the fractionation calculation Tables nr. 40—45
respectively) there is a broad agreement between these values. The slope
and position of the curves is close to the 1 to 1 line. These are the
ankaramites and less evolved basalts. Rock groups 7—11 (corresponding to
Tables 46—50), representing evolved basalts and intermediate rocks, show a
clear deviation from the 1 to 1 ratio, the calculated Y values increasing
greatly at fairly constant analyed Y values. Exactly the same pattern is