high-Sr BEM
RDEM AEM
5. C ONSTRAINTS ON CRYSTALLIZATION CONDITIONS
The uniform mineral assemblage and mineral chemistry observed in all dacitic stages of Taapaca indicate constant pre-eruptive crystallization conditions. Amphibole is a basic mineral phase that enables estimation of intensive crystallization parameters (P-T-ƒO2) as well water contents in Taapaca magmas. However, its complex mineral chemistry and substitution mechanisms lead to large compositional variability, which make the estimation of pre-eruptive physical parameter difficult.
A validity of different geothermometer and geobarometer, based on amphibole compositions (e.g.
Johnson & Rutherford, 1989; Blundy & Holland, 1990; Holland & Blundy, 1994; Ernst & Liu, 1998; Féménias et al., 2005; Ridolfi et al., 2010; Ridolfi & Renzulli, 2011) has been tested with experimental studies carried out directly on the Taapaca dacite (Botcharnikov et al., in prep.; Blum-Oeste, 2014). These empirical and experimental results can be compared with studies carried out on volcanics, compositionally comparable to the Taapaca dacites (e.g. Mount St Helens: Rutherford &
Devine, 1988; Fish Canyon: Johnson & Rutherford, 1989; Pinatubo: Scaillet & Evans, 1999; Unzen:
Sato et al., 1999, 2005; Soufriere Hill: Rutherford & Devine, 2003).
C
OMPOSITIONS OF NATURAL AND EXPERIMENTAL AMPHIBOLE5.1.
Experimental amphibole compositions are presented for comparison with natural Taapaca amphibole compositions using an Al2O3 vs. TiO2 plot (Figure 8), adapted from the study of Ernst &
Liu (1998). According to this study, conducted on MORB as a starting material (49-52 SiO2 wt%), Al2O3 contents of experimental Ca-amphibole increase with both P and T; TiO2 contents increase almost exclusively with increasing T. Therefore, both oxides give a first view into P-T relations in Taapaca magmas from amphibole composition. Figure 8 includes experimental studies performed on dacite as starting materials at temperature range from 725 to 1033°C and pressure 1.5 to 3.9 kbar.
The experimental studies of Botcharnikov et al. (in prep.), and Blum-Oeste (2014) were carried out on a representative crystal-rich dacite sample TAP 87-002 from stage IV. This sample has an average silica content of 65 wt% SiO2, and mineral assemblage typical for stages II to IV.
Experimental conditions were chosen according to crystallization conditions obtained previously from natural amphiboles (Banaszak, 2007 and this study). The geothermobarometry data indicate the most likely temperature and pressure conditions for Taapaca dacite at 700-850°C and 850-1000°C at 1-3 kbar (presented in next sections). The ƒO2 was varied from NNO to NNO+1. Because the whole rock composition of Taapaca mafic enclaves correspond to the basaltic andesite lavas from Parinacota volcano, amphibole compositions from crystallization experiments with natural basaltic andesite from Parinacota volcano, sample PAR 11 (Botcharnikov et al., in prep.), conducted at 900-1000°C and 3 kbar are included in this study.
A comparison of the natural Taapaca and experimental amphiboles reveals three essential findings that play a crucial role in evaluating pre-eruptive crystallization conditions and verification of GTOB methods applicable for Taapaca dacite.
1) Two amphibole populations found in Taapaca dacites, the Mg-Hbl and Mg-Hst crystallized from different magma compositions. The results of the experimental approach of Botcharnikov et al.
(in prep.) confirm the observations from natural systems. Referring to Al and Ti contents in Ca-amphibole, Féménias et al. (2006) describe decreasing Ti contents in amphiboles with increasing differentiation. Ridolfi et al. (2010) emphasize the role of decreasing alumina/silica ratios in calc-alkaline magmas leading to decreasing Al2O3 contents in amphibole with increasing SiO2 contents of magma from which the amphiboles crystallized. The Mg-Hst in Taapaca experiments crystallized only from basaltic andesite starting material. Mg-Hbl crystallized only form dacite starting material.
Other experimental studies, e.g. Johnson & Rutherford (1989) and Rutherford & Devine (2008) did not synthetize Mg-Hst from dacite starting material, even at temperatures of 920-1033°C or pressures of 8 kbar. Although the amphiboles from the latter studies reach Al2O3 contents comparable with Mg-Hst from Parinacota basaltic andesite experiments (~10-13 wt%), they are characterized by significantly lower TiO2 contents (Figure 8).
2) Crystallization conditions of Mg-Hbl can be restricted to relatively low temperatures according to the experimental range of 725-780°C, because the best compositional fit to the natural Mg-Hbl is represented by amphibole synthesized at T<760°C (Johnson & Rutherford, 1989; Scaillet & Evans,
1999; Botcharnikov et al., in prep.). It is worth to noting, that an increase in experimental P or T conditions does not reproduce the Mg-Hst compositions from the dacitic starting material. This observation is based on the experimental studies presented in Figure 8.
3) Compositions of Mg-Hbl and Mg-Hst can be reproduced from different magma composition at the same pressure conditions ranging between 2 and 3 kbar. Therefore, the crystallization pressures, in spite of their different Al-contents may overlap for both amphibole populations.
Figure 8. Al2O3 and TiO2 contents of natural Taapaca and experimental amphibole generated from dacite starting material in a pressure range of 1.3-3.9 kbar and different temperatures from studies of: Blum-Oeste (2014) and Botcharnikov et al. (in prep.) for Taapaca; Johnson & Rutherford, 1989, (J&R89) for Fish Canyon; Scaillet & Evans, 1999, (S&E99) for Pinatubo; Sato et al. (2005) for Unzen; Rutherford & Devine, 2003 (R&D2003) for Soufriere Hill; Rutherford & Devine, 2008 (R&D2008) for Mount St Helens. Only amphibole generated at low temperatures (<760 °C) match the trend of magnesiohornblende (Mg-Hbl) found in Taapaca rocks. Due to the results of Botcharnikov et al. (in prep.), high-Al-Ti magnesiohastingsite (Mg-Hst) could be reproduced only from basaltic andesite starting material of Parinacota volcano at 900-1000°C at the same pressure values as Mg-Hbl.
A
L-
IN-H
ORNBLENDE GEOBAROMETRY5.2.
Al-in-Hornblende (Al-in-Hbl) barometer of Johnson & Rutherford (1989) was applied exclusively to the Taapaca magnesiohornblende. Three criteria allow an application of the Al-in Hbl (J&R89) barometer to the Taapaca Mg-Hbl. First, consistent experimental and natural whole rock compositions; however, the results of PVA study presented in Chapter 2 reveal more silica-rich and MgO-poor rhyodacitic composition (RDEM end-member), than the composition used in the experimental studies, from which most probably Taapaca Mg-Hbl crystallized. Secondly, an identical mineral assemblage obtained by the calibration experiments of J&R89 and this found in natural Taapaca samples. Third, nearly equal natural and experimental low-Al amphibole compositions - experimental amphiboles synthesized at 2-3 kbar and T<760°C correspond most
5 Sato et al. 2005 800-850°C, 2-3 kbar, NNO Sato et al. 2005 850°C, 1.85 kbar, NNO+4 R&D2003 810-870°C, 1.3-2.5 kbar
closely to the composition of Taapaca Mg-Hbl (Figure 8). Slightly higher MgO contents in the experimental amphibole (~12.5-14.5 wt% MgO, not presented here) in comparison to natural Mg-Hbl (~11.3-13.5 wt% MgO) may reflect: 1) higher log ƒO2 compared with the natural conditions or 2) compositional differences between the experimental dacite and rhyodacitic RDEM end-member from which Taapaca Mg-Hbl originate.
Following numerous publications presenting different formulations of AlTOT-in Hbl barometers (Hammarstrom & Zen, 1986; Hollister et al., 1987; Johnson & Rutherford, 1989), Blundy & Holland (1990) showed a strong dependence of [4]Al contents in Ca-amphibole on temperature. For that reason, the authors concluded that AlTOT-in Hbl barometers yield a considerable uncertainty. Figure 9 shows variations of [4]Al vs. [6]Al in experimental amphiboles of Botcharnikov et al. (in prep.), Blum-Oeste (2014), and Stechern et al. (in prep.), in terms of temperature and pressure. The amphiboles of Stechern et al. (in prep.) comprise tschermakite(10) synthetized from mafic andesite composition from Lascar volcano. They show AlTOT similar to Mg-Hst and similar (Na+K)A<0.5 and Ti contents to Mg-Hbl.
The tetrahedral [4]Al shows clearly strong T-dependence (Figure 9a) and lacking correlation with P. The octahedral [6]Al reveals higher contents in amphibole synthetized at both, low temperature experiments at 300 MPa (725-775°C, blue points, Figure 9b) and higher temperatures at 150-200 MPa (825-900°C, green points) in comparison to amphiboles synthetized at 200 MPa and intermediate temperature range of 775-850°C. This observation suggests an influence of other intensive variables on [6]Al in two presented experimental studies carried out on the same starting whole rock composition. Although the Mg-Hbl and tschermakite show a positive [6]Al-pressure correlation from the experiments of Botcharnikov et al. (in prep.) and Stechern et al. (in prep.), the amphiboles of Blum-Oeste (2014) do not fall along this trend. These observations point out that AlTOT in Hbl is not only governed by P and T. [6]Al depends also on ƒO2 (Simakin et al., 2009);
additionally, Sisson & Grove (1993) found out, that Al/Si in synthetized amphibole correlates linearly with Al/Si in the melt. Therefore, the AlTOT-in Hbl barometry includes a substantial error in pressure estimations for natural systems.
Figure 9a and d confirms the results of Blundy & Holland (1990) showing general dependence of
[4]Al on temperature and [6]Al on pressure. However, the [4]Al shows correlation with T in a much narrower range in comparison to [6]Al, showing a wide range of contents increasing with P. This observation suggests that amphibole documents higher T-sensibility and may give results that are more reliable for T than for P.
10 Tschermakite is a third compositional group of amphibole occurring in CVZ magmas, therefore the comparison may be useful for
Figure 9. Variations of [4]Al and [6]Al depending on a) and b) temperature and c) and d) pressure in experimental amphibole of Botcharnikov et al. (in prep.), abbr. Betal(in prep.), Blum-Oeste (2014), abbr. B-O(2014), and Stechern et al. (in prep.), abbr. Setal(in prep.), synthetized from Taapaca dacite starting material. Red points: 775-850°C, 200 MPa; Blue points 725-775°C; 300 MPa, green pints 825-900°C, 150-200 MPa. Black points represent high-Al Mg-Hst synthetized from Parinacota basaltic andesite, and yellow triangles represent tschermakite synthetized from Lascar mafic andesite as starting composition, presented for comparison. TAP=Taapaca, PAR=Parinacota, LAS=Lascar.
AlTOT of experimental Mg-Hbl of Botcharnikov et al. (in prep.) presented in Figure 10a, yields the widest range between 1.3 and 1.7 p.f.u. in comparison to the Mg-Hbl synthetized by J&R89 at 2-3 kbar, due to higher temperatures (up to 850°C) used in their experiments. The experimental calibration of J&R89 was carried out at 740-780°C. This temperature range overlaps with temperatures obtained from hornblende-plagioclase thermometer of Holland & Blundy (1994), (section 5.4.2). Furthermore, Taapaca Mg-Hbl and J&R89 synthetic Mg-Hbl show consistent Al2O3 -TiO2 composition for pressure up to 3 kbar (Figure 8). Therefore, the AlTOT-in Hbl barometer calibration of J&R89 can be applied to Taapaca Mg-Hbl, to reconstruct approximately crystallization pressure.
Pressure values obtained from Mg-Hbl (Figure 10b) using the equation presented in Figure 10a, overlap in the range of 1.2-2.8 ±0.5 kbar for Mg-Hbl from the dacite samples and inclusions in sanidine megacrysts, with extreme values of 0.76 and 3.4 kbar. The cores of large Mg-Hbl phenocrysts yield 1.3-3.4 kbar and overlap with pressure values determined at the other crystal parts.
Regarding possible T-fluctuations and the simultaneous dependence of AlTOT-in-Hbl on P, T, ƒO2
conditions as well as magma composition, these results demonstrate an approximation of pressure conditions in the Taapaca rhyodacite reservoir.
Figure 10. a) Crystallization pressure and total Al-contents (AlTOT) in experimental Mg-Hbl of Botcharnikov et al. in prep., (red points) and Johnson & Rutherford (1989), (blue points). The equation represents P-AlTOT correlation based on 13eCNK normalization of amphibole cation-site occupancy. b) Pressure values obtained from Mg-Hbl in Taapaca lavas obtained form AlTOT-in-Hbl geobarometer using calibration of Johnson &
Rutherford (1989) presented in a).
A
MPHIBOLE THERMO-
OXY-
BAROMETER5.3.
Results of P-T-ƒO2-H2Omelt obtained from a thermobarometric formulation of Ridolfi & Renzulli (2011), (further referred as R&R2011), are presented in Figure 11 and Figure 13. In their previous study, Ridolfi et al. (2010) divided the experimental and natural amphibole into “consistent” and
“inconsistent” amphibole using Al-number Al#=[6]Al/AlTOT. Consistent experimental amphiboles (Al#<0.21) represent compositions synthetized in equilibrium with melts which meet the main Al2O3
vs. SiO2 trend of volcanic rocks and glasses, in equilibrium with typical crystallization conditions of calc-alkaline magmas: melt water contents H2Omelt of 3.7 to 8.2 wt% (average 5.9 wt%) and ƒO2 in a range of -1<NNO<+2.7. Inconsistent amphibole (Al#>0.21) refer to crustal (high-P) compositions and experimental pargasites, crystallized in equilibrium with melts characterized by high water contents (>4.5-13 wt%, average 8.3 wt%) and a broad range of ƒO2 (-3<NNO<+4.8). All Taapaca amphiboles show Al#<0.21 and classify as “consistent” (0.04<Al#<0.15 for Mg-Hbl inclusions in sanidine; Al#<0.17 for Mg-Hst from mafic inclusions).
The P-T and T-H2Omelt diagrams include the maximal and lower “thermal stability curve” defined by Ridolfi et al. (2010) for consistent amphiboles. These curves constrain a narrow crystallization conditions range estimated based on experimental amphiboles selected in the mentioned study.
y = 4.2707x - 3.4013