Variations of Yb and La vs. SiO2 and La/Yb vs. MgO for arc-front and rear-arc volcanoes in the Katmai region. Symbols identified in
HREE and Y are more strongly depleted than HFSE, and this pertains
to mafic and andesitic samples from rear-arc and arc-front alike,
all of which have far lower Yb and Y concentrations than average
NMORB (Fig. 7 , eFig. 9).
In contrast to HFSE and Th (which are enriched at Mount Griggs
relative to the arc front; Fig. 5 , eFig. 9), the Yb and
Y arrays (vs SiO 2 ) for Griggs, the mafic monogenetic
centers, and the volcanic front all overlap extensively (Fig.
7 , eFig. 9).
In the rear-arc suite, the Y- and Yb-deficient andesite and dacite
samples (conspicuous in Fig. 7 , eFig. 9) reflect
fractionation of cpx and amphibole.
LREE abundances are, in contrast to HREE, generally elevated in
rear-arc samples relative to those from the arc front (Fig. 7
), and they are consistently well above the average value of NMORB.
Accordingly, La/Yb ratios are relatively elevated in the rear-arc:
4 to 6 at Mount Griggs, 4 to 8 for mafic monogenetic centers, but
mostly 2 to 4 on the volcanic front (Fig. 7 ).
Lower Cs/La and Cs/Th ratios of most rear-arc samples (Fig.
10 ) thus principally reflects higher rear-arc abundances of La and
Th (Figs. 5 , 7 ) rather than systematic across-arc decline
Despite this, Ba/La and Ba/Th are generally lower in the rear-arc
(Fig. 10 ), owing to relatively elevated La and Th behind the
front (Figs. 5 , 7 )..
Because Yb (and Y) abundances in mafic rocks are similar on the
arc-front and in the rear-arc (Fig. 7 ), the clear La/Yb
separation (Fig. 11 ) reflects relative LREE enrichment of
In view of the low and limited range of HREE (Figs. 7 , 8 ),
the simplest explanation is that La/Yb in the mafic rocks roughly
reflects the mean melt fraction represented by parent magmas,
higher ratios indicating lower-degree melts..
If HFSE, HREE, and Y do not range greatly in the mantle source and
if the modest ranges of these elements in mafic magmas sampled
across the arc (Figs. 6 , 7 , 8 ) largely reflect degree of
melting, then ratios of fluid-transported LILE to such
wedge-dominated elements (e.g., Rb/Zr, Ba/Ta) should decline
across-arc (owing to diminishing slab-fluid input).
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