Geology Reference
In-Depth Information
those of oceanic-island basalts, which de
ne a linear array of positive slope,
field of MORB to the high 207 Pb/ 204 Pb and
206 Pb/ 204 Pb ratios of ocean island basalt of St. Helena and Tubuaii.
The position of the MORB-OIB array to the right of the geochron (Fig. 5.8 b) has
been attributed to the progressive loss of Pb from the mantle to the core. However,
if the lamproites of Western Australia and Gaussberg were generated from low
U/Pb source components within the subcontinental lithosphere, the existence of
such a reservoir could also compensate for a general increase of U/Pb ratio in the
OIB mantle source rock (Nelson et al. 1986). Lamproites from Smoky Butte have
the most extreme nonradiogenic Pb isotopic compositions recorded for ultrapotassic
rocks. Their isotopic characteristics are similar to those of old granulite facies rocks
from the lower crust. However, on the basis of their trace element characteristics, it
is extremely unlikely that crustal contamination could have modi
extending from the unradiogenic Pb
ed the isotopic
characteristics of these magmas. Thus, their isotopic composition suggests their
mantle source.
5.4 Minor Element Geo-chemistry of Potassium
Rich
-
Silica-De
cient Volcanic Rocks from Italy
Hawkesworth and Volmer (1979) divided the Roman petrographic province into
three separate zones: 1) The northern province of Tuscany, where the rocks are
predominantly calc-alkalic, 2) the second zone, which includes Vulsini, Vico and
Sabatini, and is characterized by hybrid magma of mantle origin, and 3) the
southern zone comprising primitive potassic suite. Rogers et al. (1985) investigated
the trace element content of the potassic rock of Vulsini. Their chondrite-norma-
lised REE patterns are summarized in Fig. 5.9 a
c. These
figures show that in leucite
-
basanite, Mg
leucitite and tephritic leucitite, the REE concentration shows a con-
siderable variation with La content, which is very high, and 120
-
420 times the
value of chondrites. The La N /Yb N ratio of these rocks is equal to 22.5. The rocks
also have consistent small negative Eu anomaly. The abundance of Th, U, Pb, and
Nb vary by a factor of 3, as CaO content decreases from 14 to 3 wt%. The content
of light REE increases by a factor of 12 over the same range of CaO and this
enrichment took place before CaO is reduced to 5 wt%. According to the fractional
crystallization model of Holm et al. (1982), apatite began to fractionate
-
first from a
highly potassic magma. With removal of 5
8 vol % apatite, there was paucity of
P 2 O 5 and this had great effect on the relative abundance of REE. They thought that
the partitioning of the REE between apatite and silicate melts vary systematically
with atomic number and the middle REEs (Sm, Eu and Gd) were more easily
accommodated than either the light or heavy rare earth elements. Thus, when
perovskite starts fractionating at nearly 8 wt% CaO, the middle REEs are extracted
from the melt at greater rate than other elements resulting in the increase of La/Sm
ratio. This further results in the cross-cutting REE profiles in 87 Sr/ 86 Sr isotopic ratio
-
 
Search WWH ::




Custom Search