Magmatic water

{{short description|Aqueous phase of minerals that have been dissolved by magma}} thumb|Mammoth Hot Springs

'''Magmatic water''', also known as '''juvenile water''', is an aqueous phase in equilibrium with minerals that have been dissolved by magma deep within the Earth's crust and is released to the atmosphere during a volcanic eruption. It plays a key role in assessing the crystallization of igneous rocks, particularly silicates, as well as the rheology and evolution of magma chambers. Magma is composed of minerals, crystals and volatiles in varying relative natural abundance.<ref name=":52">{{Cite web |last=Nelson |first=Stephen |date=September 2015 |title=Volcanoes, Magma, and Volcanic Eruptions |url=https://www.tulane.edu/~sanelson/Natural_Disasters/volcan&magma.htm |access-date=March 1, 2021 |website=Tulane University EENS 3050}}</ref> Magmatic differentiation varies significantly based on various factors, most notably the presence of water.<ref name=":12">{{Cite journal |last1=Petrelli |first1=M. |last2=El Omari |first2=K. |last3=Spina |first3=L. |last4=Le Guer |first4=Y. |last5=La Spina |first5=G. |last6=Perugini |first6=D. |date=2018-02-22 |title=Timescales of water accumulation in magmas and implications for short warning times of explosive eruptions |journal=Nature Communications |language=en |volume=9 |issue=1 |pages=770 |doi=10.1038/s41467-018-02987-6 |pmid=29472525 |pmc=5823946 |bibcode=2018NatCo...9..770P |issn=2041-1723 |doi-access=free}}</ref> An abundance of volatiles within magma chambers decreases viscosity and leads to the formation of minerals bearing halogens, including chloride and hydroxide groups. In addition, the relative abundance of volatiles varies within basaltic, andesitic, and rhyolitic magma chambers, leading to some volcanoes being exceedingly more explosive than others. Magmatic water is practically insoluble in silicate melts but has demonstrated the highest solubility within rhyolitic melts. An abundance of magmatic water has been shown to lead to high-grade deformation, as a result of altering the composition of hydrogen isotope biogeochemistry (''δ''²H) and stable oxygen isotope ratios (''δ''¹⁸O) within host rocks.

== Composition == thumb|Mineralogic composition of rhyolitic, andesitic, and basaltic magmas Magma exists in three main forms that vary in composition.<ref name=":22">{{Cite web |date=2019-04-05 |title=Magma |url=http://www.nationalgeographic.org/encyclopedia/magma/ |access-date=2021-02-26 |website=National Geographic Society |language=en}}</ref> When magma crystallizes within the crust, it forms an extrusive igneous rock. Dependent on the composition of the magma, it may form either rhyolite, andesite, or basalt.<ref name=":22" /> Volatiles, particularly water and carbon dioxide, significantly impact the behavior of each form of magma differently.<ref name=":02">{{Cite web |title=What does magmatic water mean? |url=https://www.definitions.net/definition/magmatic+water |access-date=2021-02-21 |website=www.definitions.net |language=en}}</ref>^,<ref name=":12"/> Magma with a high concentration of volatiles has a significant reduction in temperature of up to hundreds of degrees, which reduces its inherent viscosity.<ref name=":32">{{Cite web |title=Igneous rock – Assimilation |url=https://www.britannica.com/science/igneous-rock |access-date=2021-02-27 |website=Encyclopedia Britannica |language=en}}</ref> The behavior of magma is also altered by varying mineralogic compositions, which is noted in the figure. For instance, magmatic water leads to the crystallization of several minerals abundant in hydroxyl- or halogenated groups, including garnets. Analyses of these minerals can be used to analyze the conditions of formation in the interior of rocky planets.<ref name=":32" />^,<ref name=":42">{{Cite journal |last=Bell |first=David |title=Hydroxyl in mantle minerals |url=https://thesis.library.caltech.edu/1334/1/Bell_dr_1993.pdf |journal=California Institute of Technology Pasadena, California}}</ref>

=== Volatiles === Volatiles are present in nearly all magma in different concentrations. Examples of volatiles within magma include water, carbon dioxide, and halogen gases.<ref name=":52"/> High pressures allow these volatiles to stay relatively stable within solution.<ref name=":52" /> However, over time, as the magmatic pressure decreases, volatiles will rise out of solution in the gaseous phase, further decreasing the magmatic pressure.<ref name=":52" /> These pressure differences cause drastic differences in the volume of a magma.<ref name=":52" /> Pressure difference causes some forms of volcanoes to be highly explosive and others to be effusive.<ref name=":52" />

=== Mineralogy === An example of a mineral containing hydroxyl groups is garnet. Garnet is an anhydrous mineral commonly analyzed within geological subdisciplines because of its general stability. One study analyzed the presence of garnets within the upper mantle through infrared spectroscopy and showed absorption at approximately 3500&nbsp;cm⁻¹, which is consistent with the presence of hydroxyl groups. These garnets have been shown to vary in composition dependent on its geographic origin.<ref name=":42" /> One particular study in Southern Africa determined concentrations ranging from 1 to 135&nbsp;ppm.<ref name=":42" /> However, this is significantly lower than the hydroxyl content in regions such as the Colorado Plateau. It was also demonstrated that there is an inverse correlation regarding the concentration of OH and Mg + Fe.

=== Basaltic magma === Basaltic magma is the most abundant in iron, magnesium, and calcium but the lowest in silica, potassium, and sodium.<ref name=":52" />^, <ref name=":22" /> The composition of silica within basaltic magma ranges from 45 to 55&nbsp;% mass fraction (wt.%), or mass fraction of a species.<ref name=":52" /> It forms in temperatures ranging from approximately 1830&nbsp;°F to 2200&nbsp;°F.<ref name=":52" />^, <ref name=":22" /> Basaltic magma has the lowest viscosity and volatiles content, yet still may be up to 100,000 times more viscous than water.<ref name=":52" /> Because of its low viscosity, this is the least explosive form of magma. Basaltic magma may found in regions such as Hawaii, known for its shield volcanoes.<ref name=":52" />^, <ref>{{Cite web |last=Watson |first=John |date=May 1997 |title=Eruptive Style: Powerful but Unsually &#91;sic&#93; Benign |url=https://pubs.usgs.gov/gip/hawaii/page26.html |access-date=March 1, 2021 |website=USGS}}</ref>

Basaltic magma forms minerals such as calcium-rich plagioclase feldspar and pyroxene. The water composition of basaltic magma varies dependent on the evolution of the magma chamber. Arc magmas, such as Izarú in Costa Rica, range from 3.2 to 3.5 wt.%.<ref>{{Cite journal |last1=Benjamin |first1=Ezra R. |last2=Plank |first2=Terry |last3=Wade |first3=Jennifer A. |last4=Kelley |first4=Katherine A. |last5=Hauri |first5=Erik H. |last6=Alvarado |first6=Guillermo E. |date=2007-11-15 |title=High water contents in basaltic magmas from Irazú Volcano, Costa Rica |url=https://www.sciencedirect.com/science/article/pii/S0377027307002570 |journal=Journal of Volcanology and Geothermal Research |language=en |volume=168 |issue=1 |pages=68–92 |doi=10.1016/j.jvolgeores.2007.08.008 |bibcode=2007JVGR..168...68B |issn=0377-0273 |url-access=subscription}}</ref>

=== Andesitic magma === Andesitic magma is an intermediate magma and is approximately evenly dispersed regarding iron, magnesium, calcium, sodium, and potassium.<ref name=":52" /><ref name=":22" /> The silica composition of andesitic magma ranges from 55 to 65 wt.%.<ref name=":52" /> It forms in temperatures ranging from approximately 1470&nbsp;°F to 1830&nbsp;°F.<ref name=":52" />^, <ref name=":22" /> Andesitic magma has an intermediate viscosity and volatiles content.<ref name=":52" /> It forms minerals such as plagioclase feldspar, mica, and amphibole.

=== Rhyolitic magma === Rhyolitic magma is felsic and the most abundant in silica, potassium, and sodium but the lowest in iron, magnesium, and calcium.<ref name=":52" /><ref name=":22" /> The silica composition of rhyolitic magma ranges from 65 to 75 wt.%.<ref name=":52" /> It forms in the lowest temperature range, from about 1200&nbsp;°F to 1470&nbsp;°F.<ref name=":52" />^, <ref name=":22" /> Rhyolitic magma has the highest viscosity and gas content.<ref name=":52" /> It produces the most explosive volcanic eruptions, including the catastrophic eruption of Mount Vesuvius.<ref name=":52" /> It forms minerals such as orthoclase feldspar, sodium-rich plagioclase feldspar, quartz, mica, and amphibole.

== Water in silicate melts == Precipitation of minerals is affected by water solubility within silicate melts, which typically exists as hydroxyl groups bound to Si⁴⁺ or Group 1 and Group 2 cations in concentrations ranging approximately from 6 to 7 wt.%.<ref name=":6">{{Cite journal |last1=Le Losq |first1=Charles |last2=Mysen |first2=Bjorn O. |last3=Cody |first3=George D. |date=2015-08-14 |title=Water and magmas: insights about the water solution mechanisms in alkali silicate melts from infrared, Raman, and 29Si solid-state NMR spectroscopies |journal=Progress in Earth and Planetary Science |volume=2 |issue=1 |pages=22 |doi=10.1186/s40645-015-0052-7 |bibcode=2015PEPS....2...22L |issn=2197-4284 |doi-access=free |hdl=1885/153807 |hdl-access=free}}</ref>^,<ref name=":7">{{Cite journal |last=Stolper |first=Edward |date=1982-12-01 |title=The speciation of water in silicate melts |url=https://dx.doi.org/10.1016/0016-7037%2882%2990381-7 |journal=Geochimica et Cosmochimica Acta |language=en |volume=46 |issue=12 |pages=2609–2620 |doi=10.1016/0016-7037(82)90381-7 |bibcode=1982GeCoA..46.2609S |issn=0016-7037 |url-access=subscription}}</ref> Specifically, the equilibrium of water and dissolved oxygen yields hydroxides, where the K_eq has been approximated between 0.1 and 0.3.<ref name=":7" />

This inherent solubility is low yet varies greatly depending on the pressure of the system. Rhyolitic magmas have the highest solubility, ranging from approximately 0% at the surface to nearly 10% at 1100&nbsp;°C and 5&nbsp;kbar. Degassing occurs when hydrous magma is uplifted, gradually converting the dissolved water to aqueous phase. This aqueous phase is typically abundant in volatiles, metals (copper, lead, zinc, silver and gold), and group-1 and group-2 cations. Dependent on which cation the hydroxyl is bound to, it significantly impacts the properties of a volcanic eruption, particularly its explosiveness.<ref name=":6" /> During unusually high temperature and pressure conditions exceeding 374&nbsp;°C and 218 bar, water enters a supercritical fluid state and becomes no longer a liquid or a gas.<ref name=":6" />

==Stable isotope data== Isotopic data from various locations within the Mid-Atlantic Ridge indicates the presence of mafic-to-felsic intrusive igneous rocks, including gabbro, diorite, and plagiogranite.<ref name=":8">{{Cite journal |last=Stakes |first=Debra |date=1991 |title=Oxygen and hydrogen isotope compositions of oceanic plutonic rocks: high-temperature deformation and metamorphism of oceanic layer 3 |url=https://www.geochemsoc.org/files/6814/1269/7637/SP-3_077-090_Stakes.pdf |journal=The Geochemical Society, Special Publication |volume=3 |pages=77–90}}</ref> These rocks showed high-grade metamorphism because of the presence of magmatic water, exceeding 600&nbsp;°C. This deformation depleted host rocks of ¹⁸O, leading to further analysis of the ratio of ¹⁸O to ¹⁶O (δ¹⁸O).<ref name=":8" />

Water in equilibrium with igneous melts should bear the same isotopic signature for ¹⁸O and δ²H. However, isotopic studies of magmatic water have demonstrated similarities to meteoric water, indicating circulation of magmatic and meteoric groundwater systems.<ref>{{Cite book |last=Hall |first=Anthony |title=Igneous petrology |date=1987 |publisher=Longman Scientific & Technical |isbn=0-470-20781-7 |location=Harlow, Essex, England |oclc=14098243}}</ref>

Isotopic analyses of fluid inclusions indicate a wide range of δ¹⁸O and δ²H content.<ref>{{Cite book |last=Guilbert |first=John M. |title=The geology of ore deposits |date=1986 |publisher=W. H. Freeman |others=Charles F., Jr. Park, Charles F., Jr. Park |isbn=0-7167-1456-6 |location=New York |oclc=12081840}}</ref> Studies within these environments have shown an abundance of ¹⁸O and depletion in ²H relative to SMOW and meteoric waters. Within ore deposits, fluid inclusion data showed that the presence of δ¹⁸O vs δ²H are within the expected range.

==See also== * Connate fluids

== References == {{Reflist}}

{{DEFAULTSORT:Magmatic Water}} Category:Igneous petrology Category:Geochemistry Category:Water Category:Scientific problems