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Ionov et al. (1937) observed that mica is the dominant host mineral for Rb and
Ba. This phase also has high but variable Rb/Sr (0.13
60) and Ba/Sr ratios.
Amphibole and mica are major hosts for Nb in the peridotitic upper mantle and their
presence strongly affects its Zr/Nb ratio. Accessory wadeite can be a source of Zr,
and K and priderite may supply K, Ba, and Ti in the mantle in an alumina-de
-
cient
environment. Ionov et al. further observed that apatite is a major host for Th, U, Cl,
Br, LREE and Sr, and clinopyroxene contains signi
cant amount of Sr, Zr, Y, REE.
The presence of both these minerals in fertile lherzolites with disseminated
amphibole and mica can account for signi
cant enrichments of Th, U, Pb.
Edgar and Pizzolato (1995) observed the presence of a F-rich phase in their
experiments on the system hydroxy-
fluor phlogopite and hydroxy
fluor richterite at
2.0 GPa and 900
C. This amphibole has a F content of 4.2 wt%. Such a
phase may be present in the mantle, and enhance F-enrichment in the melt.
1,000
°
-
15.6 Production of Fertile Source Rocks by Mantle
Metasomatism
The low REE content of komatiites suggests that primitive mantle had concentra-
tion of these elements similar to those of chondrites. Modern tholeiitic basalts have
the REE contents 10
30 times the abundance of chondrites, and the alkali basalts
have even higher REE enrichment than the tholeiites. Ultrapotassic rocks show high
enrichment in LREE/HREE ratio. There seems to be a general agreement that on the
basis of currently known partition coef
-
cients, such an enrichment cannot easily be
accounted for using simple partial melting models from depleted mantle source
regions with low
87
Sr/
86
Sr and high
143
Nd/
144
Nd ratios. Geochemists argue that
there has been recent enrichment within the mantle by metasomatic events during
last 0.2 billion years.
Lloyd (1987) studied spinel lherzolites and alkali pyroxenite xenoliths from
K-rich lavas of west Eifel (Germany) and Southern Uganda. The K-rich ma
c rocks
from both the Eifel and S. Ugandan lavas are LILE-enriched. Lloyd (1987) showed
that concentration of K and associated LILE elements in these rocks would require
enrichment of K by nearly a factor of 150 and more than 200 in the mantle below
Eifel and Uganda, respectively. The regional K-enrichment is most satisfactorily
explained in terms of pervasive K-metasomatism in the underlying mantle. Selv-
edges, veins and interstial crystals of K-richterite and phlogopite (Erlank et al.
1987; Mitchell et al. 1987; Tanton and McKenzie 1994) in lherzolites are con-
sidered to be evidence for the in
ux of magmas and
fluids into the mantle
(Kempton 1987).
The metasomatic agents, which are responsible for in
ux into the mantle include
a variety of melts or
fluids either rich in H
2
OorCO
2
(Roden et al. 1984, cited by
Kempton 1987). Incompatible trace element enrichment in anhydrous lherzolite
may be attributed to fluxing of incompatible element-laden with CO
2
-rich fluid into
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