| Abundances of F,Cl S and P in Volcanic Magmas and Their Evolution,Wudalianchi |
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| 摘 要： |
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| 关 键 词： | 火山岩岩浆 氟 氯 硫 富集 五大连池 |
Abundances of F,Cl, S and P in volcanic magmas and their evolution,Wudalianchi |
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| Authors: | Xia Linqi Liu Wenfeng Tang Nan ’an |
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| Institution: | (1) Xi ’an Institute of Geology and Mineral Resources, Ministry of Geology and Mineral Resources, 710054 Xi ’an |
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| Abstract: | F, Cl, S and P were determined, using electron microprobe, in magmatic inclusions trapped within minerals and glass mesostasis
from Wudalianchi volcanic rocks. The initial volcanic magma from Wudalianchi corresponds to the basanitic magma crystallized
near the surface ( pressure < 91 Mpa ). The potential H2O content of this magma is in the range 2 — 4 wt. %. The initial composition of volcanic magmas varies regularly from early
to late volcanic events. From the Middle Pleistocene to the recent eruptions (1719 – 1721 yr.), the basicity of volcanic magma
tends to increase, as reflected by an increase in MgO and CaO contents and by a progressive decrease in SiO2 and K2O contents. Meanwhile. from early (Q2 ) to late (Q3) episodic eruptions of the Middle Pleistocene, the initial concentrations of chlorine in volcanic magmas range from 1430
– 1930 ppm to 1700 ppm and decrease to 700 — 970 ppm for the first episodic eruption during the Holocene (Q
4
1
). The chlorine concentrations of volcanic magmas of recent eruption (Q
4
2
) are increased again to 2600 – 2870 ppm. A parallel evolution trend for phosphorus and chlorine concentrations in magmas
has been certified: 1500 – 5970 ppm (Q2)→ 3500 – 4210 ppm (Q3)→ 1100– 3500 ppm (Q
4
1
)→ 6800– 7900 ppm (Q
4
2
). The fluorine contents of volcanic magmas, from early to late volcanic events, show the same trend: 770 – 2470 ppm → 200–700
ppm → 700 – 800 ppm.
During the crystallization-evolution of volcanic magmas, fluorine and phosphorus tend to be enriched in residual magmas as
a result of crystal-melt differentiation. for example. the fluorine contents reach 5000– 6800 ppm and the phosphorus contents,
2.93wt.% in residual magmas. An appreciable amount of chlorine may be lost from water rich volcanic magmas prior to eruption
as a result of degassing. Apparently, water serves as a gas carrier for the chlorine. The chlorine contents of residual magmas
may decrease to 100 – 300 ppm.
The volcanic magmas from Wudalianchi are poor in sulfur, normally ranging from 200 to 400ppm. On account of the behavior of
sulfur in magmas and the strontium and oxygen isotopic analyses ((87Sr /86Sr)i=0.70503– 0.70589; δ18O = + 5.50 – + 6.89 ‰ ), it can be considered that the basanitic magmas in the Wudalianchi volcanic area came from the upper
mantle and have not yet been contaminated probably by continental crust materials. |
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