Original paper

Liquid-forsterite-anorthite-spinel assemblage at 1 bar in the CMAS system: implications for low-pressure evolution of high-Al and high-Mg magmas

Soulard, Henri; Boivin, Pierre; Libourel, Guy

European Journal of Mineralogy Volume 6 Number 5 (1994), p. 633 - 646

25 references

published: Sep 28, 1994
manuscript accepted: May 13, 1994
manuscript received: Jun 11, 1993

DOI: 10.1127/ejm/6/5/0633

BibTeX file

ArtNo. ESP147050605003, Price: 29.00 €

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Abstract

Abstract New experiments on the assemblages liquid-forsterite-anorthite-spinel (liq-fo-an-sp) and liq-fo-an have been performed at one atmosphere in the system CaO-MgO-Al2O3-SiO2 (CMAS), in order to shed more light on the evolution of high-Mg to high-Al magmas, which often undergo olivine and/or plagioclase and/or spinel crystallisation during early stages of low-pressure differentiation. In CMAS, the prior crystallisation of fo, an, sp, fo+sp or an+sp forces high-Mg to high-Al compositions to converge toward intermediate Mg- and Al-rich magmas with constant MgO/(MgO + AI2O3) ratio (≈ 2/3 in mole). This first step also increases the CaO/Al2O3 ratio of liquids to produce Al-rich basalts in equilibrium with fo-an-sp. The odd univariant assemblage liq + sp = an + fo displays a similar evolution trend to liq-fo-an-di through the three main basaltic fields (alkali-basalts, ol- and qz-tholeiites) but, due to its odd character, liquids directly evolve from liq-fo-an-sp towards pyroxene-bearing parageneses, crystallising forsterite and anorthite (30:70 wt%). Then, increase of normative pyroxene content allows modal ortho- or clinopyroxene to appear along liq-fo-an boundaries. These new data yield to a simple model of parent/daughter relationships between high-Mg to high-Al compositions and basalts (s.L). In this part of the CMAS system, CaO solubility in forsterite seems to be mainly controlled by the degree of silica-saturation of the liquid. Silica-saturation seems to also control MgO solubility in anorthite as well as the CaO/Al2O3 ratio of the liquid

Keywords

CMASbasaltdifferentiationhigh-Al basalthigh-Mg basaltexperimentationliq-fo-an-spthermal divide