Lake Pavin History, geology, biogeochemistry, and sedimentology of a deep meromictic maar lake

139

Fig. 7.4

Calculation of bulk D values in logarithmic diagrams. (

a ) Example of logarithmic dia-

grams. Symbols as in Figs.

7.1

and

7.2

. Lines are regression lines of slopes

Δ^ i

with

Δ

~1–Di. i

Comparison between Chaîne des Puys (stricto sensu) and Pavin group magmas evidence a step-wise differentiation process, each step being characterized by approximately constant D

values. i

( b

) Bulk D values of some incompatible elements, comparison with amphibole and clinopyroxene/

melt partition coeffi

cients. Zr, Hf, Nb, Ta and REE partition coeffi

cients calculated for Basalt-

Hawaïte and Hawaïte-Benmoreite differentiation steps in Pavin group (Dg Pav) and Chaîne des

Puys (stricto sensu; Dg CP) differentiation series. Mineral/melt partition coeffi

cients for clinopy-

roxene (d CPx) and amphibole (d Amph) in alkaline series are from Lemarchand et al. (

1987

). For

Basalt-Hawaïite differentiation step, mineral/melt partition coeffi

cients vary in a relatively large

range: extreme values are represented). Notice the large differences in bulk D values of LREE in basalt-hawaïite differentiation steps in Pavin and Chaîne des Puys series. The mineral/melt parti-tion coeffi

cients of amphibole and clinopyroxene are generally close for all elements (‘parallel D

values patterns’), except Nb and Ta which are much more effi

ciently partitioned into amphibole

2.8 2.6 2.4 2.2 2.0

0.4

0.6

0.8

1.0

1.2

1.4

log Zr

log Th

3.2 3.0 2.8 2.6 2.4

0.4

0.6

0.8

1.0

1.2

1.4

log Ba

log Th

1.2 1.0 0.8 0.6 0.4 0.2

0.4

0.6

0.8

1.0

1.2

1.4

log Ta

log Th

2.5 2.0 1.5 1.0 0.5 0.0 -0.5 -1.0

0.4

0.6

0.8

1.0

1.2

1.4

log Co

log Th

7 Magmatic Evolution of Pavin’s Group of Volcanoes: Petrology, Geochemistry...