Geology Reference
In-Depth Information
In order to explain variable as well as high
87
Sr/
86
Sr and
18
O/
16
O ratios in
potassium-rich volcanic rocks of Vulsinian lavas, Turi and Taylor (1976, also see
Ferrara et al. 1986) concluded that the primary magma was contaminated by high
18
O and
87
Sr crustal materials.
According to Holm et al. the two samples of Turi and Taylor with high
18
O
values (+11), are in fact quartz-normative trachytic lavas, which are members of the
silica-saturated group, and do not belong to a undersaturated variety. The
δ
18
O
δ
values of the primary Vulsinian lava may be 2
3 % higher than typical values for
primary mantle-derived magmas. According to Holm et al. (1982), it is dif
-
cult to
18
O in the mantle, other than mixing of normal
18
O primary mantle
explain high
δ
δ
18
O content as suggested by Muelenbachs and Kushiro
(1974). Holm et al. (1982) concluded that such a
melt with a
fluid of high
δ
fluid could have been derived by
interaction with or melting of materials which were in equilibrium with surface
water at low temperatures, which is with sediments or meta-sediments. They further
considered that such a
fluid was derived from sediments and/or hydrated oceanic
basalt of the subducted crustal slab.
They further observed that the
87
Sr/
86
Sr ratio is nearly constant in the entire
SiO
2
-undersaturated series with the values falling in the range of 0.7105
-
0.7108.
The olivine
leucite melilitite of San Venanzo, which may be a part of the Vulsini
volcanic province has similar values: 0.71037 and some silica-saturated ones have
the value between 0.7098 and 0.7100. The available
87
Sr/
86
Sr ratio of the silica-
saturated and under-saturated lavas cannot be distinguished (Holm et al. 1982), and
they suggested a narrow range of
-
87
Sr/
86
Sr ratio (0.7104
0.7108) for the entire
suite. They found that their
87
Sr/
86
Sr values and those of Vollmer (1976) are not
intermediate between silica-saturated high
87
Sr/
86
Sr ratio of Tuscan lavas and low
87
Sr/
86
Sr ratio of K-rich lavas of Roman district. Their observation thus, contradicts
the conclusion of Turi and Taylor (1976). Holm et al. thought that the lavas of
Vulisini district have a primitive composition and they represent rather uniform
magmatism extending from Alban Hills and northward.
-
15.2 Subtraction of Eclogite from a Picrite Magma
Experimental studies at high pressures (up to 3 GPa) on a nepheline-normative and
a hypersthenes-normative picritic lava were made by Gupta and Yagi (1979). In
both the cases above the solidus, omphacitic pyroxene together with pyrope-rich
garnet (similar to eclogite fraction) crystallized just above the solidus from the both
types of picritic liquid, but when the composition of the melt minus the eclogite
fraction was determined they were not ultra potassic in nature as suggested by
O
'
Hara and Yoder (1967).
Holmes (1932) observed the presence of eclogite xenoliths included in K-rich
lavas at Ruwenzori (Africa). Holmes and Paneth (1936) however, observed that
eclogite xenoliths included in Ruwenzori lavas were much older in age than these
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