Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 122
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PÁI.I. l.MSI.AND
TABLE 19
Si02 47.61 46.02
tío2 1.71 2.33
ai2o3 4.68 6.95
Cr203 0.01 0.03
FeO‘ 7.61 7.96
MnO 0.20 0.18
MgO 15.14 13.54
CaO 22.55 22.08
Na20 0.37 0.43
k2o n.d. n.d.
Total 99.88 99.52
Microprobe analyses of the same zone in different sectors in a single titan-salite
crystal in a pl-ol-cpx porphyritic basalt; Jan 15. n.d. = not detected.
In Fig. 77, where the Mg/Fe ratio of the clinopyroxene is plotted against
wt. per cent MgO of the whole rock sample, the range of the composition of
the clinopyroxenes of each sample can be seen. Fhis intra sample variation is
greatest in the ankaramites, which contain both the titan-salite and the most
magnesium rich chromian diopsides. It is relatively great in most of the
basalts, which primarily contain titan-salite, but some chromian diopside as
well. It is generally smallest in the intermediate rocks, which contain either
the titan poor salite or the ferroaugite. In the figure the chromian diopside
plots at high Mg/Fe ratios, while the salites plot at the lower values.
Fig. 77 further shows that the lowest Mg/Fe ratios are almost constant
(between 2 and 3) for the salites of all rock types (except the greenish
clinopyroxene of Jan 66), while the highest Mg/Fe ratios of the chromian
diopsides increase with increasing MgO in the whole rock. Fhe highest Mg/
Fe ratio found in some of the chromian diopsides of the ankaramites is
slightly greater than the high but constant Mg/Fe ratio of the chromian
diopside of the wehrlite xenolith.
b. Chemical variation of the clinopyroxenes
Apart from the four major elements (Si, Ca, Mg & Fe) of the common
pyroxenes, seven other elements were analysed. These are Ti, Al, Cr, Mn,
Na, K and P. All these elements except K and P occur in varying amounts
ranging up to several per cent (expressed as wt. per cent oxides). K and P,
on the other hand, usually show quite low values. For example P failed to
show up in most of the analyses and has therefore been neglected in the
study and is not reported in the analyses. When detected the values are