Publication Date:
2017-04-04
Description:
The separate effects of pressure (10 4 and 1.0 GPa), water, CO2, oxygen fugacity and calcium doping on the liquid line of
descent of a primitive leucite-basanite magma (SiO2¼ 47.06 wt%, MgO¼ 12.76wt%andMg#¼ 75.1) fromthe Montefiascone Volcanic
Complex (Vulsini volcanoes, central Italy) were experimentally investigated in the 1350–1160 C temperature range. Results indicate that
low-pressure liquidus temperatures are 1280 C and that the high-pressure Tliquidus is 1350 C under anhydrous conditions; the latter is
lowered to 1275 C by the addition of 3 wt% water. Cr-spinel is always the liquidus phase. At comparable fO2 values, high and low
pressure runs produced the same phase assemblage (spinel þ olivine þ clinopyroxene) up to 50 % crystallization, although olivine was
partially or totally replaced by phlogopite in hydrous experiments. An increase in oxygen fugacity and the addition of CaO determine an
increase in both the degree of melt crystallization and the stability field of clinopyroxene. These determine contrasting effects on the
composition of residual liquids: the former increases SiO2 content, whereas the latter induces the desilication of melts. The replacement of
olivine by phlogopite, induced by increasing amounts of water, leads to the production of glass with lower potassium contents.
Comparison of the natural and experimental melts shows that many of major and trace element variations exhibited by high-K
primitive (i.e., highMg/Mg þ Fe) magmas at Montefiascone, are consistent with their derivation from a single parental leucite-basanite
melt by fractional crystallization of different proportions of mineral phases, plus carbonate assimilation. The changes in phases stability
and melt composition caused by carbonate assimilation may also have fundamental implications for the origin of the calcic highmagnesium
leucitites and melilitites. In particular, the complex metasomatic interactions that can develop at the interface between
potassic magmas and carbonate wall rocks, may lead to melting of calcite. This low-viscosity melt readily mixes with the surrounding
magma inducing the crystallization of Ca-Tschermak-rich pyroxene and hercynitic spinel, affecting significantly the SiO2, CaO and
alumina composition of the resulting hybrid melt.
A key finding of our study is that magmas such as the studied leucite-basanite may be considered parental to the wide spectrum of
mafic high-K compositions in the Roman Province, which have been traditionally considered as representing near primary magmas
reflecting distinct mantle source compositions and/or processes.
Description:
Published
Description:
763-782
Description:
2.3. TTC - Laboratori di chimica e fisica delle rocce
Description:
JCR Journal
Description:
reserved
Keywords:
ultrapotassic magma
;
experimental petrology
;
limestone assimilation
;
Montefiascone Volcano
;
Roman Province
;
04. Solid Earth::04.04. Geology::04.04.05. Mineralogy and petrology
Repository Name:
Istituto Nazionale di Geofisica e Vulcanologia (INGV)
Type:
article
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