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).
Viewing this image requires a subscription. If you are a subscriber, please log in.
This image is from the article titled "Rear-arc vs. arc-front volcanoes in the Katmai reach of the Alaska Peninsula: a critical appraisal of across-arc compositional variation"
(from Contributions to Mineralogy and Petrology), which is copyrighted by Springer-Verlag. For more information on the
copyright for this image, please refer to the full image caption and to the
The image is being made available for non-commercial purposes for subscribers to SpringerImages. For more information on what you are allowed to do with this image, please see our copyright policy.
To request permissions to use any copyrighted material, please visit the source document.
Report a copyright concern regarding this image.
Log in or register to save your favorite images and download them as high-quality PowerPoint or PDF files.
Log in or register to save your search criteria.
© Springer, part of Springer Science+Business Media.
Remote Address: 188.8.131.52 Server: 21