W HAT DOES GOVERN AMPHIBOLE COMPOSITIONS

7.2.1. W

Amphibole is a main mineral phase that allows estimation of the pre-eruptive intensive parameter in Taapaca magmas. The strong compositional variability of amphibole, even within one population arise questions about the reliability of the P-T-ƒO2-H2Omelt results.

7.2.1.1. Magma composition – primary factor influencing amphibole composition The results of this study show that the chemistry of calcic amphibole in calc-alkaline magmas primarily depends on magma composition from which the amphibole crystallized. Spear (1981) and Sisson & Groove (1993) note, that Al content in amphibole is controlled by whole rock composition.

Féménias et al. (2006) link different Ca-amphibole species to the calc-alkaline magmatic series, due to the magmatic differentiation from basalt to rhyolite, in order kaersutite→ Ti-pargasite→

pargasite→ Ti-magnesiohastingsite→ magnesiohastingsite→ edenite→ tschermakite→

magnesiohornblende. Ridolfi et al. (2010) present three main amphibole-species in calc-alkaline magmas for variable magma compositions in terms of SiO2 contents: magnesiohastingsite (Mg-Hst) in a range of 52-64 wt% SiO2, tschermakite ~54-70, and magnesiohornblende (Mg-Hbl) >70. The experimental Mg-Hst and Mg-Hbl, species relevant for this study, selected for the thermo-oxy-barometric formulations by Ridolfi & Renzulli (2011), have been synthesized in equilibrium with basaltic andesites or dacite-rhyolite glasses, respectively.

There are lines of evidence summarized below, which suggest a link between the three main Ca-amphibole species and a type of the three end-member magmas, which occur in the CVZ.

A) Natural amphibole compositions

1) Taapaca high-Sr mafic enclaves and Parinacota lavas, dominated by the BEM-type magma, contain the high-Al-Ti magnesiohastingsite (Mg-Hst). 2) The most silicic Taapaca and Parinacota rocks representing the RDEM-type end-member magmas contain low-Al-Ti magnesiohornblende (Mg-Hbl). 3) High-Al and low-Ti tschermakite are present in rocks of Lascar volcano, representing the prevailing AEM-type end-member. The amphiboles found in the low-Sr mafic enclaves of Taapaca, as well as low-Sr Parinacota lavas, dominated by AEM-type end-member show complete breakdown that prevent the determination of the amphibole composition. This finding suggests that this amphibole may represent tschermakite, which may not be stable during mixing with the hotter and drier BEM type magma, or it undergoes breakdown during magma ascent, due to decreasing pressure.

B) Experimental amphibole compositions

1) The experimental amphiboles synthetized from BEM-type basaltic andesite (Parinacota) by Botcharnikov et al. (in prep.) are Mg-Hst. 2) Mg-Hbl could be reproduced by Botcharnikov et al. (in prep.) from RDEM-dominated Taapaca dacite. 3) The experimental amphiboles synthetized by Stechern et al. (in prep.) from AEM-type basaltic andesite (Lascar) at the same P-T-ƒO2 as at Parinacota experiments, are tschermakite.

C) Differences between single mafic enclaves

The cation-site variations in Taapaca amphiboles (Figure 7) reveal compositional differences in composition of Mg-Hst cores from different mafic enclaves. These differences correlate with other proportions of two mafic end-members involved in the petrogenesis of these mafic enclaves. For instance, Mg-Hst from samples characterized by higher proportions of the high-Al AEM end-member show higher Al^TOT in the amphibole and lower K contents than Mg-Hst from mafic enclave dominated by the shoshonitic BEM end-member.

The observations A, B, and C require a detailed examination in connection with REE patterns of amphibole. The occurrence of these three amphibole species could be used as a fingerprint of the end-member type magmas involved in the petrogenesis. Furthermore, it could be a basis for a verification and improvement of the GTOB-formulations based on the amphibole compositions, which are perhaps more affected by relations between different major elements in the melt, for example Al/Si ratio or Na2O contents mentioned by Sisson & Groove (1993), than the intensive parameter of crystallization. Thus, the relative P-T-ƒO2-H2Omelt variations may rather reflect the differences in the melt compositions and not necessarily the intensive parameter of crystallization.

7.2.1.2. Intensive parameter - cation substitutions used in the GTOB

It is well known, that compositional differences within one amphibole species are caused by a general ability of introduction of a large number of cations, governed by variable, coupled iso- and heterovalent substitution mechanisms, demanding charge balance (Hawthorne, 1983; Leake et al.

1997). The variations in cation sites occupancy are connected to changes in the intensive magmatic parameters (e.g. Spear, 1981; Blundy & Holland, 1990; Ernst & Liu, 1998; and references therein), which influence the introduction of single components in the amphibole structure, depending on valence and ion radius.

In an isochemical system, changes in amphibole composition caused by changes in intensive parameter involve numerous cations simultaneously. Furthermore, the same cations are sensible for more than one parameter. 1) Ti, Na, K, Al^TOT(^[4]Al) increase, and Si decreases with increasing temperature; Al^TOT(^[6]Al) increases with increasing pressure; Si, Mg, Fe³⁺ and Mn increase and Al^TOT(^[6]Al), Fe²⁺, Ti, Na, K decrease with increasing ƒO2 (Spear 1981; Blundy & Holland, 1990;

Ernst & Liu, 1998; Ridolfi et al., 2010, Ridolfi & Renzulli, 2011 and references therein). The natural geological processes are characterized by simultaneous changes of the intensive parameter of crystallization. Hence, the effects of changes of the physico-chemical parameters on the amphibole composition overlap. The prominent example is the Al^TOT content. Successive studies have shown that Al^TOT depends not only on pressure but also temperature (e.g. Holland & Blundy, 1994), oxygen fugacity (Simakin et al., 2009) as well as magma composition (e.g. Spear, 1981; Sisson & Grove, 1993).

The most frequently discussed substitution vectors defined on the basis of the idealized tremolite formula comprise pressure-dependent Mg-1Si-1[4]

Al^[6]Al tschermakite vector, with participation of Mg²⁺ and tetrahedrally and octahedrally coordinated Al³⁺. This substitution mechanism is a basis for geobarometer formulations based on Al total (Al^TOT) content in amphibole (e.g. Hammarstrom &

Zen, 1986; Johnson & Rutherford, 1989). However, when Al^TOT from different amphibole species is used, it does not produce reliable results. As broadly discussed (e.g. Blundy & Holland, 1990;

Anderson & Smith, 1995; Ridolfi et al. 2008, 2010; Ridolfi & Renzulli, 2011), Al^TOT content in amphibole therefore cannot be directly related to differences in crystallization pressure, because of strong coupled T-, ƒO2- and Al2O3melt-dependence of Al^TOT in amphibole. Temperature and oxygen-fugacity sensitive Ti-tschermakite (Mg-1Si-1TiAl) and ferri-tschermakite (Mg-1Si-1Fe³⁺Al) substitutions involve also Mg and Al, masking the effect of single intensive parameter (P, T and ƒO2) on amphibole chemistry. Additionally, Blundy & Holland (1990), And Holland & Blundy (1994) explain the changes in Al^TOT in amphibole by edenitic exchange (Na^[4]Al□-1Si-1) in reactions participating plagioclase. Well defined correlation between Na^(A) and K^(A) vs. ^[4]Al in the Mg-Hbl of Taapaca (Figure 7g, i) corresponds to the strongly temperature-dependent edenite exchange (Blundy

& Holland, 1990). Therefore, higher crystallization temperatures are inevitably expected from the Mg-Hst defined by (Na+K)A>0.5.

The cation-variation diagrams presented in Figure 7 display a large variability of the cation-site occupancy in the Mg-Hst, reflected as pronounced textural-chemical zoning of the Mg-Hst, in comparison to rather strictly defined trends in relatively homogenous the Mg-Hbl. A considerable chemical zoning in the Mg-Hst (Figure 6c) is a typical feature of Mg-Hst, reported by e.g. Sato et al.

(2005), and Féménias et al. (2006). The Mg-Hst displays therefore an amphibole species that is very

sensible to any changes during crystallization, in any magmatic systems. To establish which factors play a major role, and how they influence the GTOB results, further studies are required.

7.2.2. D