DISTRIBUTION OF BORON, LITHIUM AND BERYLLIUM IN OCEAN ISLAND BASALTS FROM FRENCH-POLYNESIA - IMPLICATIONS FOR THE B BE AND LI/BE RATIOS AS TRACERS OF SUBDUCTED COMPONENTS/
J. Dostal et al., DISTRIBUTION OF BORON, LITHIUM AND BERYLLIUM IN OCEAN ISLAND BASALTS FROM FRENCH-POLYNESIA - IMPLICATIONS FOR THE B BE AND LI/BE RATIOS AS TRACERS OF SUBDUCTED COMPONENTS/, Mineralogical Magazine, 60(401), 1996, pp. 563-580
The study focuses on the distribution of B, Be, Li, rare earth element
s (REE), high-field-strength elements (HFSE), Tn, U and Pb in fresh an
d hydrothermally altered ocean island basalts (GIB) from French Polyne
sia, and evaluates B/Be and Li/Be ratios as potential tracers of subdu
cted components in the mantle. Hydrothermal solutions affecting the ro
cks during cooling were derived from meteoric water, sea water and mag
matic fluids. The concentrations of REE, HFSE, Th and Be in the OIB we
re not affected by secondary processes except during advanced stages o
f subaerial hydrothermal alteration where saponite was a dominant seco
ndary phase. This alteration modified the contents of these elements,
changed REE patterns and produced a positive Ce anomaly. The subaerial
and submarine hydrothermal alteration (T similar to 70-100 degrees C)
may change U concentrations in GIB, whereas Pb was only marginally re
distributed during alteration. Boron was enriched during submarine and
subaerial hydrothermal alteration but was not noticeably affected in
basalts altered by magmatic fluids at T > 200 degrees C. Like B, the m
obility of Li during the alteration varies with fluid temperature. Lit
hium became enriched in the basalts during advanced stages of lower T
hydrothermal alteration (<100 degrees C). However, this element was pa
rtly removed from the rocks during higher T alteration (>200 degrees C
) by magmatic fluids. Boron, Be and Li behave as incompatible trace el
ements in basaltic magmas. Beryllium content in primitive mantle is es
timated to be 0.07 ppm. Fresh Polynesian OIB have low abundances of B
and Li and low B/Be (2-5) and Li/Be (2.5-5) ratios compared with volca
nic are rocks, marine sediments and altered oceanic crust. Various OIB
including even those which have HIMU- and EM-affinities have similar
overlapping B/Be and Li/Be ratios. Both B and Li are probably stripped
from a lithospheric slab during subduction-related metamorphism and a
re, thus, not involved in deep mantle recycling. The mantle-normalized
trace element abundances of MORE and OIB usually display patterns cha
racterized by negative B, Pb and Li anomalies. The patterns of contine
ntal crust and crustal rocks have distinct positive anomalies for thes
e elements whereas continental basaltic rocks have variable relative a
bundances of B, Pb and Li. The anomalies of these elements in basalts
can be useful in discriminating their tectonic setting and constrainin
g the mantle source regions of basalts.