Igneous Rocks
55
The oxygen isotope fractionations among coexisting minerals in volcanic and plutonic igneous rocks are compared in Figs. 22 and 23, using
data from TAYLOR and EpSTEIN (1962b, 1963), GARLICK (1966), and TAYLOR (1968a).
At first we observe that the 180rO fractionations, reported in this
case as Ll-values, are in general larger in metamorphic rocks than in
plutonic rocks. Secondly, we note that the Ll-values are clearly smaller in
rhyolite, dacite, and basalt than in granite, tonalite, and gabbro. We may
generally conclude that volcanic and plutonic igneous rocks have distinctly different 180rO ratios. Figs.22, 23, and 24 further indicate an
approach to isotopic equilibrium in the very different mineral assemblages. However, complete isotopic equilibrium was obviously not attained in all the rocks, because otherwise all points would plot on the
straight lines.
Regardless of the complicating factors, many of the Ll-values shown
in Figs. 22, 23 and 24 may reflect temperatures of final equilibration or
"freezing-in" of oxygen isotopes in the respective assemblages.
Table 9 shows some temperatures of "formation". If we take into
account the uncertainties in the experimentally determined calibration
curves and errors due to retrograde effects, especially of the feldspars and
biotites, then the calculated temperature values or at least the relative
differences between the values agree relatively well with other calculated
temperature values based on other petrological criteria.
In several igneous rocks it is clear that coexisting quartz and feldspar
and/or coexisting feldspar and pyroxene are out of isotopic equilibrium.
'/
7fKyanite Schis/.
-
/
I
I
6fPlutonic
o I
-
Granitic
/ 0
~ 5fRacks 0 ,0
~8
-
(;
0/ 0
' ? 4rIe Horse Mesa Rhyolite
-
!:!
e Dantes View Rhyolite
0
o 3fIHypersthene Granite
-
"
e La{sen Dacite
e
I
if- I
-
I
oV I
I
I
I
I
I
o
1
2
3
4
5
6
7
!::. Feldspar- Biotite
Fig. 23. Relationship between Ll-values of quartz-biotite and feldspar-biotite for
quartz-feldspar-biotite assemblages in igneous rocks and a kyanite-zone schist
studied by TAYLOR et al. (1963). (From TAYLOR, 1968a)
55
The oxygen isotope fractionations among coexisting minerals in volcanic and plutonic igneous rocks are compared in Figs. 22 and 23, using
data from TAYLOR and EpSTEIN (1962b, 1963), GARLICK (1966), and TAYLOR (1968a).
At first we observe that the 180rO fractionations, reported in this
case as Ll-values, are in general larger in metamorphic rocks than in
plutonic rocks. Secondly, we note that the Ll-values are clearly smaller in
rhyolite, dacite, and basalt than in granite, tonalite, and gabbro. We may
generally conclude that volcanic and plutonic igneous rocks have distinctly different 180rO ratios. Figs.22, 23, and 24 further indicate an
approach to isotopic equilibrium in the very different mineral assemblages. However, complete isotopic equilibrium was obviously not attained in all the rocks, because otherwise all points would plot on the
straight lines.
Regardless of the complicating factors, many of the Ll-values shown
in Figs. 22, 23 and 24 may reflect temperatures of final equilibration or
"freezing-in" of oxygen isotopes in the respective assemblages.
Table 9 shows some temperatures of "formation". If we take into
account the uncertainties in the experimentally determined calibration
curves and errors due to retrograde effects, especially of the feldspars and
biotites, then the calculated temperature values or at least the relative
differences between the values agree relatively well with other calculated
temperature values based on other petrological criteria.
In several igneous rocks it is clear that coexisting quartz and feldspar
and/or coexisting feldspar and pyroxene are out of isotopic equilibrium.
'/
7fKyanite Schis/.
-
/
I
I
6fPlutonic
o I
-
Granitic
/ 0
~ 5fRacks 0 ,0
~8
-
(;
0/ 0
' ? 4rIe Horse Mesa Rhyolite
-
!:!
e Dantes View Rhyolite
0
o 3fIHypersthene Granite
-
"
e La{sen Dacite
e
I
if- I
-
I
oV I
I
I
I
I
I
o
1
2
3
4
5
6
7
!::. Feldspar- Biotite
Fig. 23. Relationship between Ll-values of quartz-biotite and feldspar-biotite for
quartz-feldspar-biotite assemblages in igneous rocks and a kyanite-zone schist
studied by TAYLOR et al. (1963). (From TAYLOR, 1968a)