Clinopyroxene–liquid thermometers and barometers specific to alkaline differentiated magmas

Abstract

We present new thermometers and barometers based on clinopyroxene-liquid equilibria specific to 37 alkaline differentiated magmas. The new models were calibrated through regression analyses of 38 experimental datasets obtained by merging phase equilibria experiments from literature with new 39 experiments performed by using trachytic and phonolitic starting compositions. The regression 40 strategy was twofold: i) we have tested previous thermometric and barometric equations and 41 recalibrated these models using the new datasets; ii) we have calibrated a new thermometer and a 42 new barometer including only regression parameters that closely describe the compositional 43 variability of the datasets. The new models yield more precise estimates than previous 44 thermometers and barometers when used to predict temperatures and pressures of alkaline 45 differentiated magmas. We have tested the reliability of the new equations by using clinopyroxene-46 liquid pairs from trachytes and phonolites erupted during major explosive eruptions at the Phlegrean 47 Fields and Mt. Vesuvius (central Italy). The test yielded crystallization conditions comparable to 48 those determined by means of melt and fluid inclusion analyses and phase equilibria studies; this 49 validates the use of the proposed models for precise estimates of crystallization temperatures and 50 pressures in differentiated alkaline magmas. Because these magmas feed some of the most 51 voluminous, explosive, and threatening volcanic eruptions in the world, a better understanding of 52 the environmental conditions of their reservoirs is mandatory and this is now possible with the new 53 models provided here. 54 55 56 table 2 Click here to download table: Table 2.doc

An improved clinopyroxene-based hygrometer for Etnean magmas and implications for eruption triggering mechanisms

“…To assess the extent of this limitation, we have tested Equation 2 using experimental clinopyroxenes in equilibrium with trachy-phonolitic melts (P = 50-300 MPa, T = 725-950 °C, H 2 O = 5-9 wt%, and f O 2 = NNO+1-NNO+2; data from Fabbrizio andCarroll 2008 andMasotta et al 2013; Supplementary 1 Data Table 5S) and calc-alkaline melts from basalt to andesite (P = 0.1-200 MPa, T = 965-1075 °C, H 2 O = 0-6.7 wt%, and f O 2 = QFM-NNO+1.5; data from Sisson and Grove 1993a, 1993band Mollo et al 2012; Supplementary 1 Data Table 5S). In these experiments, crystals of clinopyroxene + K-feldspar and crystals of clinopyroxene + plagioclase coprecipitated from the trachy-phonolitic liquids and calk-alkaline liquids, respectively.…”

Peralkaline felsic magmatism at the Nemrut volcano, Turkey: impact of volcanism on the evolution of Lake Van (Anatolia) IV

“…Recalculated clinopyroxene analyses for samples V-70 (2B-5H-5cc) and V-111 (2E-22A-1, 78-83 cm) lack a jadeite (Jd) component and therefore cannot be used to estimate T or P. For these models, it was assumed that the melt water content equalled the difference of the oxide total from 100 wt% and pressure was set at 1 kbar (column 1a) and 2 kbar (column 1b). Estimated errors are ±45 °C for Putirka (2008), ±31.4 °C for the recalibrated Putirka model (Masotta et al 2013) and ±18.2 °C and ±1.15 kbar for the new models (Talk2012 and Palk2012) of Masotta et al (2013).…”

“…Otherwise, average matrix glass and pyroxene compositions were used. The results are based on the models of Putirka (2008) and Masotta et al (2013), some of which include recalibrations of the Putirka models for differentiated alkaline (including peralkaline) rocks. Recalculated clinopyroxene analyses for samples V-70 (2B-5H-5cc) and V-111 (2E-22A-1, 78-83 cm) lack a jadeite (Jd) component and therefore cannot be used to estimate T or P. For these models, it was assumed that the melt water content equalled the difference of the oxide total from 100 wt% and pressure was set at 1 kbar (column 1a) and 2 kbar (column 1b).…”

“…The temperature estimates from the Masotta et al (2013) White et al (2005) for peralkaline rhyolites of similar peralkalinity index (<1.3) from Pantelleria, Italy, but higher than those established for comenditic rhyolites of the Olkaria complex, Kenya (740-660 °C; Scaillet and Macdonald 2001). In a compilation of T-fO 2 data for peralkaline trachytes and rhyolites, Macdonald (2012) showed that there is no simple relationship between temperature and either SiO 2 content or peralkalinity and suggested that it reflected the role of water in depressing crystallisation temperatures.…”

Geochemical Characterization, Geochronology, and Geodynamic Implications of Grenville Rare Earths Bearing Syenites, Haut-Saint-Maurice, QC, Canada

“…Crystallization temperatures and pressures were calculated for the Bouvreuil, Lortie, Dix Milles, Châteauvert, Rheaume, and Wemotaci syenites using the [25] method. This method uses the clinopyroxene (using the electron microprobe) versus liquid (estimated from whole rock geochemistry) to determine the pressure and temperature of formation of the intrusions.…”