The composition of a high-Mg basalt (HMB) from the Capo Marargiu Volcanic District (CMVD, Sardinia, Italy) was used for a phase relation study at high pressure and hydrous conditions. The CMVD is a calc-alkaline complex of basaltic to intermediate hypabyssal (dikes and sills) and volcanic rocks (lava dome and flows, pyroclastic deposits) emplaced during the Oligo-Miocene orogenic magmatism of Sardinia. The HMB (10 wt.% MgO) occur as magmatic enclaves hosted in a basaltic-andesitic dome (BA). Minerals within the enclaves are clinopyroxene (Cpx), plagioclase (Plg), amphibole (Amph), olivine (Ol) and oxides (Ox), and those of the BA host are Plg + Cpx + Ox + low-Ca pyroxene. Thermobarometric estimates indicate that Cpx and Amph from the enclaves crystallized at P = 500-800 MPa and T = 1010-1100°C. In order to investigate the stability of Amph in the HMB magma we conducted preliminary experiments at 800 MPa and in a range of temperatures of 1050-1110°C. Au75Pd25 capsules were loaded with ~100 mg of a powdered HMB (sample CM42) and 2 wt.% H2O (low water experiments, LWE) or 5 wt.% H2O (high water experiments, HWE). The experiments were run in the piston-cylinder apparatus installed at the HP-HT Laboratory of Sapienza University of Rome and held at the target P-T conditions for 3 h. The stable phases in the LWE are Ol + orthopyroxene (Opx) + Cpx + Plg + Ox and a hydrous basaltic andesite (~ 4 wt.%) as residual glass (Gl). The crystallization degree is higher than 50 vol.%. Run products of HWE is Gl + Ol + Amph + Cpx + Plg + Ox. The crystallinity is lower than that of the LWE H2Osaturated basalt (~ 7 wt.% H2O), as testified by the presence of vesicles. This preliminary study shows that Opx is stable in a HMB at P = 800 MPa and T = 1050-1110°C and H2O ≤ 2 wt.% (i.e., LWE), whereas its crystallization is hindered when magma water content is increased to ~5 wt.%, providing conditions suitable for Amph saturation (i.e., HWE). This modal discrepancy is ultimately reflected in the composition of residual melt, being more evolved in the low-water experiment and more primitive in the hydrous run. We conclude that (i) the P–T–H2O conditions of the HWE more realistically represent those of natural enclave crystallization, consistently with thermobarometric estimates, and (ii) at a global scale, the occurrence of amphibolebearing cumulates in exhumed lower crustal section testify to magmatic differentiation in a water-rich environment nearby the Moho. Importantly, the flux of H2O from this “wet and hot deep zone” (Perinelli et al., 2017) would play a critical role in the eruptive style of the overlying volcanoes.

High-Mg basalts from Capo Marargiu (Sardinia, Italy): experimental constraints on amphibole stability in a primitive calc-alkaline magma / Bonechi, B.; Tecchiato, V.; Perinelli, C.; Gaeta, M.. - In: ITALIAN JOURNAL OF GEOSCIENCES. - ISSN 2038-1719. - ELETTRONICO. - 1:(2017), pp. 470-470. (Intervento presentato al convegno Congresso congiunto SIMP-SGI-SOGEI-AIV "Geosciences: A tool in a changing world" tenutosi a Pisa nel Settembre 2017).

High-Mg basalts from Capo Marargiu (Sardinia, Italy): experimental constraints on amphibole stability in a primitive calc-alkaline magma

Bonechi B.
;
Tecchiato V.;Perinelli C.;Gaeta M.
2017

Abstract

The composition of a high-Mg basalt (HMB) from the Capo Marargiu Volcanic District (CMVD, Sardinia, Italy) was used for a phase relation study at high pressure and hydrous conditions. The CMVD is a calc-alkaline complex of basaltic to intermediate hypabyssal (dikes and sills) and volcanic rocks (lava dome and flows, pyroclastic deposits) emplaced during the Oligo-Miocene orogenic magmatism of Sardinia. The HMB (10 wt.% MgO) occur as magmatic enclaves hosted in a basaltic-andesitic dome (BA). Minerals within the enclaves are clinopyroxene (Cpx), plagioclase (Plg), amphibole (Amph), olivine (Ol) and oxides (Ox), and those of the BA host are Plg + Cpx + Ox + low-Ca pyroxene. Thermobarometric estimates indicate that Cpx and Amph from the enclaves crystallized at P = 500-800 MPa and T = 1010-1100°C. In order to investigate the stability of Amph in the HMB magma we conducted preliminary experiments at 800 MPa and in a range of temperatures of 1050-1110°C. Au75Pd25 capsules were loaded with ~100 mg of a powdered HMB (sample CM42) and 2 wt.% H2O (low water experiments, LWE) or 5 wt.% H2O (high water experiments, HWE). The experiments were run in the piston-cylinder apparatus installed at the HP-HT Laboratory of Sapienza University of Rome and held at the target P-T conditions for 3 h. The stable phases in the LWE are Ol + orthopyroxene (Opx) + Cpx + Plg + Ox and a hydrous basaltic andesite (~ 4 wt.%) as residual glass (Gl). The crystallization degree is higher than 50 vol.%. Run products of HWE is Gl + Ol + Amph + Cpx + Plg + Ox. The crystallinity is lower than that of the LWE H2Osaturated basalt (~ 7 wt.% H2O), as testified by the presence of vesicles. This preliminary study shows that Opx is stable in a HMB at P = 800 MPa and T = 1050-1110°C and H2O ≤ 2 wt.% (i.e., LWE), whereas its crystallization is hindered when magma water content is increased to ~5 wt.%, providing conditions suitable for Amph saturation (i.e., HWE). This modal discrepancy is ultimately reflected in the composition of residual melt, being more evolved in the low-water experiment and more primitive in the hydrous run. We conclude that (i) the P–T–H2O conditions of the HWE more realistically represent those of natural enclave crystallization, consistently with thermobarometric estimates, and (ii) at a global scale, the occurrence of amphibolebearing cumulates in exhumed lower crustal section testify to magmatic differentiation in a water-rich environment nearby the Moho. Importantly, the flux of H2O from this “wet and hot deep zone” (Perinelli et al., 2017) would play a critical role in the eruptive style of the overlying volcanoes.
2017
Congresso congiunto SIMP-SGI-SOGEI-AIV "Geosciences: A tool in a changing world"
04 Pubblicazione in atti di convegno::04d Abstract in atti di convegno
High-Mg basalts from Capo Marargiu (Sardinia, Italy): experimental constraints on amphibole stability in a primitive calc-alkaline magma / Bonechi, B.; Tecchiato, V.; Perinelli, C.; Gaeta, M.. - In: ITALIAN JOURNAL OF GEOSCIENCES. - ISSN 2038-1719. - ELETTRONICO. - 1:(2017), pp. 470-470. (Intervento presentato al convegno Congresso congiunto SIMP-SGI-SOGEI-AIV "Geosciences: A tool in a changing world" tenutosi a Pisa nel Settembre 2017).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11573/1147273
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