Geology Reference
In-Depth Information
Sharda et al. (1999) conducted step-heating analyses for Mid-Atlantic Ridge
glass samples to show that maximum
40
Ar/
36
Ar values correlate with
206
Pb/
207
Pb
and
208
Pb/
204
Pb ratios. These correlations hold for the whole Atlantic Ocean and
therefore, are unlikely to result from shallow-level contamination processes.
Instead, they are taken as mixing between the degassed-depleted upper mantle and a
recycled component characterized by high
206
Pb/
204
Pb ratio (19
21) and low
-
40
Ar/
36
Ar ratio (300
1,000). These relations imply that argon may also be a tracer
-
of mantle recycling.
Petrological experiments on oceanic crust samples characterize recycling of
potassium from mid-oceanic ridge basalts and sediments. Metasomatism could
develop directly and continuously from subducted potassium-bearing crust from
shallow levels to a maximum depth of 300 km.
15.9 Crustal Contamination
There are numerous lines of evidence, which rule out any signi
cant crustal con-
tamination during ascent of a K-rich magma to the surface, and therefore, in
uence
of crustal nitrogen on the lamproites. From the petrological and mineralogical
studies on these Gondwana ultra-potassic rocks, Jia et al. (2003) observed the
following: (1) the dominant primary mineralogy of olivine, phlogopite, Cr-spinel,
and priderite is typical of ultrapotassic mantle rocks, but not typical crust; (2) As a
corollary, no xenocrysts of abundant crustal minerals such as quartz or plagioclase
have been observed; (3) xenoliths of harzburgitic composition in these lavas con-
firm their mantle source; (4) REE patterns are much more fractionated than upper
continental crust (La/Yb
cn
values of 68
191 in ultrapotassic rocks versus 9.4 in
continental crust), and all fractionated HREE data are indicative of residual garnet
all feature an in
-
ection at Sm
Nd, and zero or minor negative Eu anomalies (Eu/
-
Eu* = 0.646
0.982), whereas upper continental crust has a systematic negative Eu
anomaly of 0.661 (Hall 1999) both Mg# of 90
-
75, and Cr, Ni, Ba, Th, LREE
contents are high relative to continental crust (Table
5.2
).
The results are also consistent with previous studies in some of these Gondwana
ultrapotassic suites. According to Middlemost et al. (1988), Rock et al. (1992) and
Paul and Sarkar (1986), the K-rich rocks in the Gondwana lamproites from Rani-
ganj, Jharia, and Bokaro basins are all of mantle origin, given the presence of
mantle-derived nodules and xenoliths of harzburgitic composition, with undetect-
able crustal contamination. Jia et al. also noted their primitive ma
-
c
major element chemistry; enrichment of refractory trace elements like Ni, Cr, Co,
and Sc; high Mg-value of the phlogopite xenocrysts or phenocrysts associated with
them and high concentrations of REE (up to 1,139 ppm) and pronounced REE
fractionation (La/Lu
cn
of 16
c
-
ultrama
560) all rule out crustal contamination.
Strontium isotopic compositions of these rocks also support an enriched mantle
source. The
-
87
Sr/
86
Sr ratios in the Bokaro suite range from 0.70394 to 0.71544
Search WWH ::
Custom Search