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great many present sites include the oceanic regions dominated by mantle-derived
basalts. The extensive subduction zones are characterised by rocks of the andesite
family, and there are many random hot spots or mantle plumes, which cause
flooding
of ocean
ooded the
surface of the Arhean crust, whereas Table 1.1 shows, highly potassic silica-under-
saturated K-rich molten rocks are relatively more recent. A very intriguing question
comes to our mind, how these convecting patterns have changed over the 4.5 billion-
year history of the earth. Since the classic work of the great geochemists of the early
part of this century, the elements have been classi
floors and continents. Highly ultrama
c komatiitic lava
ows
ed according to their main sites of
concentration. When one examines the classic ideas, reviewed by Mason (1966), it is
clear that potassium is a typical element of the continental crust (average 1.9 %),
whereas ocean ridge tholeiites average about 0.26 %. Why are these mantle-derived
ultrapotassic rocks with high K 2 O/Na 2 O ratios so interesting? First, they come from
the mantle at high temperatures. They tend to contain anomalous concentrations of
many incompatible elements such as K, Rb, Sr, U,F, P, etc., along with compatible
Ni, Co, and Cr indicating a mixture of crust and mantle. For key isotopic indicators
like Sr 86 /Sr 87 ,O 16 /O 18 , they show great variability. They thus appear to be a result of
a geosphere mixing process. In addition, most of these rocks are young (for compi-
lation of ages of K-rich silica-undersaturated igneous rocks of various localities
(Table 1.1 , with few ancient examples). Is this real or the result of a sampling
problem? There is not much well-preserved Archaean crust in the world, and they are
not easy to recognise! Over the past decades, there has been a steady debate on the
extent of new surface geosphere materials being carried back to the mantle. One needs
only to examine the composition of some sea
floor spilites with over 4 wt%K 2 O, 3 wt
%CO 2 , which are subducted; and modern observations leave no doubt that, as Gilluly
(1971) suggested, sediments must be subducted on a large scale (see Fyfe 1992).
More and more, there is evidence that the upper mantle (and perhaps even the lower
mantle) is not homogeneous but rather like a fruitcake (Gupta and Fyfe 2003) and
that there are thermal anomalies in the mantle resulting from deep mantle plumes.
After deep subduction, the modi
ed mantle is partially fused, releasing these unusual
K-rich silica-undersaturated rocks. Do these unusual rocks formed due to
flushing out
from the mantle of left over subduction materials? An elegant example of this mantle
complexity has recently been provided by Pilot et al. (1998, pp. 393, 678), who have
reported zircons of ages 330
1600 Ma in mid-Atlantic ridge gabbros! Thus, the
ultrapotassic igneous rocks perhaps provide evidence on mixing processes, thermal
evolution and variation of these processes over time (cf komatiites). We need more
details on the total chemistry and isotopic variation of these rocks.
Oceanic crustal samples including sediments might have been subjected to
recycling into the mantle (Jia et al. 2002; Avanzinelli et al. 2009) in order to explain
high Sr isotopic ratio and extreme potassium enrichment. Thus, metasomatism
could develop directly and continuously from subducted potassium-bearing crust
from shallow levels to a maximum depth of 300 km. The analogy between mantle
processes and chromatographic fractionation in the laboratory has been used by
Navon and Stolper (1987) to explain large-scale mantle metasomatism. The ques-
tion is whether such a method related to mantle metasomatism by the process of
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