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the mantle. Menzies et al. (1987) have recorded effects of such silicate melts and
hydrous
fluids in mantle xenoliths.
Experiments on partitioning of REE in H 2 O-rich
fluid on peridotite minerals at
pressure ranging between 1.0 and 3.0 GPa have been conducted by Mysen (1983).
On the basis of these data
cients could be determined. In
this determination Ce/Sm and Se/Tm ratio is assumed to be equal to 1. Wendlandt
and Harrison (1979, cited by Eggler 1987) also studied REE partitioning between
coexisting CO 2 -rich
fluid
melt partition coef
fluid and two immiscible melts (one silica-rich and the other
carbonate) in the system K 2 O
Al 2 O 3 -
SiO 2 -
CO 2 . In the investigation by both the
-
groups,
fluid compositions were calculated by mass balance from analyses of
crystals and melts. Their data have been summarized by Eggler (1987), which show
that at 1.0 GPa, the REE is compatible with H 2 O and with CO 2 at any pressure.
Fluid-melt partitioning data show LREE enrichment. Analyses of the C-1 carbo-
naceous chondrites show the presence of 5
10 wt% of total C, H and S.
-
15.7 Crust
Mantle Mixing
-
It has been estimated by Richter et al. (1992) that Sr
fluxes of present day rivers
worldwide is as high as 0.710, but those contributed by the trans-Himalayan rivers
(the Ganges, the Brahmaputra and Tsangpo) during early Miocene are on an
average usually high (0.7236). Thus, ocean sediments should be extremely enriched
in this ratio; and subduction of ocean sediments should result in the increase in this
ratio in the mantle wedge above the lithosphere.
The presence of
high 3 He
low 4 He
in lavas from Herd islands (Indian
Ocean) has been reported by Hilton et al. (1995). According to them whereas the
high 3 He lavas provide unambiguous evidence for deep-seated plume in their ori-
gin, they also thought that the low 3 He/ 4 He ratios in other lavas result from shallow-
level contamination by radiogenic helium before eruption.
Fairly (1995) reported rapid cycling of subducted sediments into the Samoan
mantle plume. He found that harzburgite xenoliths from the Samoan hot spot carry
extraordinarily enriched Sr, Nd, and Pb isotopic ratios derived from recycled sed-
iments. He described gas composition and micro-thermometric properties of
and
fluid
inclusions, which are dominantly pure CO 2 and were apparently formed at the base
of the crust. They have high 3 He/ 4 He ratios (12 times the atmospheric ratio)
inconsistent with the radiogenic values expected for recycled sediments. Moreover,
C/ 3 He ratios of inclusions have higher values of
10 9 , which are indistin-
guishable from typical mantle values and far lower than those in sediments (>10 11 ).
The metasomatic
3
×
fluid is probably a product of mixing with Samoan hot spot of
volatiles high in 3 He/ 4 He ratios in the plume melt and recycled sediments. The
retention of high 3 He/ 4 He requires very rapid cycling of sedimentary component
through the mantle (probably < 10 7 years) than billions of years envisaged. The
time scale indicates involvement of materials recently returned to the mantle at
the nearby Tonga trench, and pelagic sediments near the trench have approximately
 
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