Jökull - 01.01.2016, Blaðsíða 77

Grain characteristics of silicic Katla tephra layers
Changes in chemical composition with time
Thirteen silicic tephra layers from Katla volcano have
been chemically analyzed or the majority of the sili-
cic tephra layers. The average values of the major el-
ement composition of the SILK layers YN, UN, MN,
LN, N4, N3, N2, N1, A1, A8, A9 (Larsen et al., 2001)
A11 and A12 (Newton, 1999 and previously unpub-
lished analyses) are shown in Table 5.
The chemical composition of the tephra from the
SILK layers reveals discrete changes between indi-
vidual layers, as indicated by colors and arrows in
Table 5. The compositional changes of SILK-YN, -
UN, -MN, -LN, -N4, -N3, -N2, -N1, -A1, -A8 and
-A9 have been treated by Larsen et al. (2001) where
two groups are apparent (Figure 7). SILK-UN, SILK-
N3, SILK-N2 and SILK-A8 tephra layers have some-
what lower SiO2, higher MgO and CaO and gener-
ally higher TiO2 than SILK-YN, SILK-MN, SILK-
LN, SILK-N4, SILK-N1, SILK-A1 and SILK-A9.
TiO2, MgO and CaO abundances are considerably
lower and SiO2 is higher in the two oldest tephra lay-
ers, SILK-A11 and SILK-A12, than in all the other
SILK layers (Table 5). This difference between the
tephra layers is presented in Figure 7, where CaO
is plotted against FeO. The two oldest tephra layers
form the third group on the plot. Tephra samples an-
alyzed for grain size and grain shape characteristics
came from the three „groups“ (see Table 3).
There appears to be potential correlation between
grain sizes and chemical composition of the 11 SILK
tephra layers analyzed for grain size (Tables 3 and
5). When considering the largest Φ (Table 3) the
tephra layers with lower SiO2 and higher MgO, CaO
and TiO2 tend to be coarser grained than the others.
Although samples from only five SILK tephra layers
have been analyzed for grain shape so far, there seems
to be no correlation between chemical composition
and the grain shape parameters. This is best demon-
strated by very significant difference in grain shape
parameters between SILK-A11 and -A12 (Table 4),
although their chemical composition is very similar
(Figure 6).
Table 5. Average chemical analyses (wt %) performed on the SILK layers YN, UN, MN, LN, N4, N3, N2, N1,
A1, A8 and A9. Layers UN, N3, N2 and A8 show higher TiO2 , MgO and CaO and lower SiO2 concentration
than the other 8 layers (red colored numbers). A11 and A12 show higher SiO2 and lower TiO2 , MgO and CaO
abundances compare to the other 11 layers (purple colored numbers) (Newton, 1999; Larsen et al., 2001 and
unpublished data). Grain size4 and grain shape* analysed material. – Meðaltalsgildi (wt%) efnagreininga á
SILK lögunum YN, UN, MN, LN, N4, N3, N2, N1, A1, A8 og A9. UN, N3, N2 og A8 hafa hærra hlutfall TiO2,
MgO og CaO og lægra hlutfall SiO2 en hin átta lögin (rauðar tölur). A11 og A12 hafa hærra hlutfall SiO2 og
lægra hlutfall TiO2 , MgO og CaO en hin 11 lögin (fjólubláar tölur) (Newton, 1991; Larsen og fl., 2001 og
óútgefin gögn). Kornastærðar4 og kornalögunar* greint efni.
JÖKULL No. 66, 2016 77