Full text - Dipartimento di Scienze della Terra - Sapienza

270 M.R. GHIARA, C. PEITI and P. MoRBIDELLI
TABLE Sa
Half-anhydrous Earth's standard cell contents (on the basis of 80 o.>..,ygens) of bulk rock; the
samples are ordered according to t1e increasing of the analcimization process.
F 11 F9 F5 F4
F7
F8 F6 F3 F2 F1
Si 26.33 25.81 25.86 25.74 25.98 25.70 25.92 25.54 25.56 25.58
Ti 0.32 0.32 0.32 0.32 0.32 0.32 0.32 0.32 0.32 0.32
Al 9.66 9.76 9.78 9.84 9.93 10.12 9.96 10.07 10.02 10.13
Fe 3 :t- 2.51 2.90 2.89 2.94 2.63 2.86 2.70 2.86 2.87 2.78
Mg 1.55 1.72 1.67 1.58 1.53 1.73 1.47 1.58 1.64
Ca 2.87 2.98 2.93 3.01 2.71 2.97 2.77 3.03 2.98 2.89
Na 2.85 3.40 3.44 3.54 4.50 4.04 4.92 4.74 5.31 5.70
K 4.46 3.60 3.51 3.42 3.17 2.58
2.59
1.54
2.52 2.16 1.96
Mn 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06
p 0.12 0.18 0.17 0.21 0.17 0.16 0.19 0.21 0.18 0.20
Na/K 0.64 0.94 0.98 1.03
1.43
1.56 1.89 1.89 2.44 2.94
TABLE Sb
Atomic differences observed in the main elements involved during the analcimization process, with
respect to an unaltered sample ( F 11 ).
Si-Si Al-Al Mg-Mg
Ca-Ca Na-Na K-K Na/K
F 11
F9 -0.52 0.10 0.17
F5 -0.41 0.12 0.12
F4 -0.60 0.18 0.03
F7 -0.35 0.27 -0.02
F8 -0.63 0.46 -0.18
F6 -0.42 0.30 -0.08
F3 -0.80 0.41 0.03
F2 -0.78 0.36 0.09
F 1 -0.75 0.47 -0.01
0.64
0.11 0.55 -0.86 0.94
0.06 0.59 -0.95 0.98
0.14 0.69 -1.04 1.03
-0.16 1.65 -1.29 1.43
0.10 1.20 -1.88 1.56
-0.10 2.07 -1.86 1.89
0.16 1.89 -1.94 1.89
0.11 2.46 -2.30 2.44
0.02 2.86 -2.50 2.94
Na/K atomic ratio
1979; Middelburg et al., 1988). In the Fondo
Riccio rock-suite Ti-value of F11 sample have
been used to normalize all the altered samples.
Moreover, assuming that alteration process
takes place without appreciable change in
volume and in order to have a better
comparison between Fondo Riccio rocks, all
the major elements have been normalized using
half of Earth's anhydrous standard cell (Barth,
19S2) (Table Sa). This procedure allows the
comparison of values unaffected by molecular
weight and the reproduction of a more realistic
feature of chemical variations occurring in the
analcimization processes.
The results of a quantitative comparison
between unaltered sample (F11) and altered
samples are given for each major element in
Table Sb and plotted in figure 8.
From the overall data set the following
geochemical balance appears:
1) Si and K decrease whereas AI and Na
increase clearly suggesting an open system
during devitrification and ensuing alteration;
2) Ca and Mg, however scattered, can be