Plagioclase ultraphyric basalts (PUBs) are a class of mid‐ocean ridge (MOR) lavas found in a variety of ocean floor environments, are characterized by abundant (15–40 volume %) plagioclase megacrysts and a diverse trace element and isotopic signature. Paradoxically, we never see lavas erupted on the seafloor that are in equilibrium with these PUB megacrysts. Based on petrographic evidence, melt inclusion composition, and new data on depth of entrapment calculated from CO2contents in plagioclase‐hosted inclusions, many of the megacrysts formed at upper mantle pressures (∼3–7 kbars). To constrain the composition of the parent magmas of the plagioclase megacrysts, we conducted a series of experiments at 5 and 10 kbars using mid‐ocean ridge basalts glasses as starting materials. The experimental results were consistent with the presence of a pseudoazeotrope in the anorthitic segment of the plagioclase + basalt pseudobinary. This has the effect of dropping the anorthitic end of the feldspar loop, lowering the solidus for upper mantle conditions, and driving evolving magmas toward higher Ca. As magmas rise and pressure drops, the pseudoazeotrope disappears, and the feldspar loop at the high‐An end rises, causing those magmas to undergo decompression crystallization of plagioclase and resorption of olivine. Therefore, the conditions which generated the magmas from which the megacrysts form disappear as the magmas rise and magmas evolve toward lower Ca, Mg (as we normally assume during plagioclase + olivine crystallization). In effect, the phase equilibria conditions that allow for the generation of such liquids also prevent them from being erupted as lavas.
Mineral/melt and intermineral Ge/Si exchange coefficients for nine common rock‐forming silicate minerals were determined by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA‐ICP‐MS). Ge/Si mineral/melt exchange coefficients were found to vary by up to a factor of 10. In mafic and ultramafic systems, Ge/Si mineral/melt exchange coefficients are less than 1 for plagioclase (0.48) and olivine (0.72), close to 1 for clinopyroxene (1.17) and orthopyroxene (1.07), and greater than 1 for garnet (2.69). In felsic and silicic systems, the Ge/Si mineral/melt exchange coefficient is less than 1 for quartz (0.23), plagioclase (0.67), and potassium feldspar (0.67) but much greater than 1 for biotite (4.80) and hornblende (3.95). We show that early, olivine‐dominated fractionation of primitive basalts does not fractionate Ge/Si significantly, but subsequent cotectic crystallization of plagioclase and pyroxene can increase the Ge/Si ratio from 6 × 10−6to 7 × 10−6. We show that the only way to decrease Ge/Si during magmatic differentiation is by crystallization of hornblende or biotite (though biotite is typically a late crystallizing phase), consistent with hornblende being a major fractionating phase in hydrous intermediate magmas. The high compatibility of Ge in hornblende makes this element, in conjunction with Si, a potentially useful approach for distinguishing between hornblende and garnet in the source regions of intermediate magmas. The high compatibility of Ge in micas suggests that Ge/Si systematics may also be useful in understanding the origin of ultrapotassic magmas, which are often thought to derive from phlogopite‐rich sources.
more » « less- NSF-PAR ID:
- 10458981
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 20
- Issue:
- 10
- ISSN:
- 1525-2027
- Page Range / eLocation ID:
- p. 4472-4486
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
more » « less
Abstract Cerro Machín, a volcano located in the northern segment of the Andes, is considered one of the most dangerous volcanoes in Colombia with an explosive record that involves at least five plinian events. Prior studies focused on the last dome-building eruption have suggested the presence of a water-rich mid-crustal magma reservoir. However, no direct volatile measurements have been published and little work has been completed on the explosive products of the volcano. Here, we study the largest known eruption of Cerro Machín volcano which occurred 3600 years BP producing dacitic pyroclastic fall deposits that can be traced up to 40 km from the vent. Lapilli pumice clasts have a mineral assemblage of plagioclase, amphibole, quartz, and biotite phenocrysts, with accessory olivine, Fe–Ti oxides, and apatite. The occurrence of Fo89–92 olivine rimmed by high Mg# amphibole and the established high-water contents in the magma imply the presence of magma near or at water saturation at pressures > ~ 500 MPa. Measurements of up to 10.7 wt % H2O in melt inclusions hosted in plagioclase and quartz in the 3600 years BP eruption products support the idea that Cerro Machín is a remarkably water-rich volcanic system. Moreover, this is supported by measurements of ~103 to 161 ppm H2O in plagioclase phenocrysts. The application of two parameterizations of water partitioning between plagioclase and silicate melt allows us to use our water in plagioclase measurements to estimate equilibrium melt water contents of 5 ± 1 wt % to 11 ± 2 wt % H2O, which are in good agreement with the water contents we measured in melt inclusions. Results of amphibole geobarometry are consistent with a magma reservoir stored in the mid-to-lower crust at a modal pressure of 700 ± 250 MPa, corresponding to a depth of ~25 km. Minor crystallization in the shallow crust is also recorded by amphibole barometry and calculated entrapment pressures in melt inclusions. Amphibole is present as unzoned and zoned crystals. Two populations of unzoned amphibole crystals are present, the most abundant indicate crystallization conditions of 853 ± 26°C (1 se; standard error), and the less abundant crystallized at an average temperature of 944 ± 24°C (1 se). Approximately 18% of the amphibole crystals are normally or reversely zoned, providing evidence for a minor recharge event that could have been the trigger mechanism for the explosive eruption. Plagioclase crystals also show normal and reverse zoning. The moderate Ni concentrations (<1600 μg/g) in the high-Fo olivine xenocrysts suggest that Cerro Machín primary magmas are generated by inefficient interaction of mantle peridotite with a high-silica melt produced by slab melting of basaltic material. Some sediment input is also suggested by the high Pb/Th (>2.2) and Th/La (0.3–0.4) ratios. Whole rock chemistry reveals heavy rare earth element (HREE) depletion and Sr enrichment that likely formed during the crystallization of garnet and amphibole in the upper part of the mantle or lower portion of the crust, promoting the formation of water-rich dacitic magma that was then injected into the middle-to-lower crust. Textural and compositional differences in the crystal cargo that erupted during dome-building and plinian events support the idea that large volumes of magma recharge lead to effusive eruptions, while only small recharge events are needed to trigger plinian eruptions at Cerro Machín.
-
more » « less
Abstract Rhyolitic tuffs range widely in their crystal contents from nearly aphyric to crystal-rich, and their crystal cargoes inform concepts of upper crustal magma reservoirs. The Earthquake Flat pyroclastics (Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand) are 10 km3of rhyolitic tuffs with abundant (~ 40 vol.%) plagioclase and quartz, minor biotite, hornblende, and orthopyroxene, and accessory Fe-Ti oxides, apatite, and zircon, set in high-silica rhyolitic glass. Major minerals form large, euhedral phenocrysts and abundant glomerocrysts with few disequilibrium textures excepting some faintly resorbed quartz. Plagioclase phenocrysts have thick rims of nearly constant composition near An30, and hornblende is weakly zoned or unzoned. The abundant and texturally complex mineral assemblage contrasts with the nearby (~ 25 km), nearly synchronous, but more voluminous and crystal-moderate rhyolite tuffs from Rotoiti caldera. New H2O-saturated phase-equilibria results on the erupted Earthquake Flat melt (glass) determine its co-saturation with the partial phenocryst assemblage of plagioclase, quartz, biotite, and Fe-Ti oxides at: 140 MPa, 755 ºC. These closely approximate the conditions of the pre-eruptive magma body assuming it was saturated with nearly pure H2O and at an
f O2of ~ Ni–NiO. Absence of hornblende and orthopyroxene from the synthesized assemblages may result from those minerals being in a peritectic reaction relation with melt to produce biotite, so they would not grow from the liquid used as starting material. Experimental results on Rotoiti rhyolite (Nicholls et al. 1992) show that the two bodies resided at similar pressures, temperatures, andf O2s. Lower crystal abundance of the Rotoiti tuffs may result from slight compositional differences. We interpret that the Earthquake Flat pyroclastics were sourced from the crystal-rich periphery of a mushy reservoir system with the Rotoiti occupying a more melt-rich central location. Uncertain is whether this was a single intrusion zoned continuously in crystallinity, or discrete adjacent intrusions, but our results illustrate and quantify complexities of magma storage across relatively short distances. -
more » « less
When magmas erupt at the surface, they may have undergone many changes since their inception. While olivine drives some of these changes through crystallization and fractionation, it also records the magma evolution via mineral chemistry and by trapping mineral and melt inclusions. Olivine is an effective recorder of intensive parameters, such as temperature and melt composition, and provides an outstanding petrological tool for constraining dynamic processes, such as ascent, mixing, and cooling. Olivine sheds light on magmatic puzzles that involve both mafic and more evolved magmas, with protracted and complex magmatic histories that often obscure earlier and deeper processes. This contribution summarizes the current state of how olivine helps reconstruct source-to-surface magma assembly through its chemistry, inclusions, and textures.
-
more » « less
Abstract During the differentiation of arc magmas, fractionating liquids follow a series of cotectics, where the co‐crystallization of multiple minerals control the melt compositional trajectories, commonly referred to as liquid lines of descent (LLD). These cotectics are sensitive to intensive properties, including fractionation pressure and melt H2O concentration, and changes in these variables produce systematic differences in the LLDs of arc lavas. Based on a compilation of experimental studies, we develop two major element proxies that exploit differences in LLDs to constrain the fractionation conditions of arc magmas. Near‐primary fractionating magmas evolve along the olivine‐clinopyroxene cotectic, which is pressure‐sensitive. We use this sensitivity to develop a proxy for early fractionation pressure based on the normative mineral compositions of melts with 8 ± 1 wt.% MgO. Fractionation in more evolved magmas is controlled by the clinopyroxene‐plagioclase cotectic, which is strongly sensitive to magmatic H2O contents. We use this relationship to develop an H2O proxy that is calibrated to the normative mineral components of melts with 2–4 wt.% MgO. These two proxies provide new tools for estimating the variations in pressure and temperature between magmatic systems. We applied these proxies to compiled major element data and phenocryst assemblages from modern volcanic arcs and show that in island arcs early fractionation is relatively shallow and magmas are dominantly H2O‐poor, while continental arcs are characterized by more hydrous and deeper early fractionation. These differences likely reflect variations in the relative contributions of decompression and flux melting in combination with distinct upper plate controls on arc melt generation.
