A melt inclusion is a small parcel or "blobs" of melt(s) that is entrapped by crystals growing
in
magma
Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
and eventually forming
igneous rocks
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or ...
. In many respects it is analogous to a
fluid inclusion within magmatic hydrothermal systems. Melt inclusions tend to be microscopic in size and can be analyzed for volatile contents that are used to interpret trapping pressures of the melt at depth.
Characteristics
Melt inclusions are generally small - most are less than 80
micrometre
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...
s across (a micrometre is one thousandth of a millimeter, or about 0.00004 inches).
They may contain a number of different constituents, including glass (which represents melt that has been quenched by rapid cooling), small crystals and a separate vapour-rich bubble.
They occur in the crystals that can be found in igneous rocks, such as for example
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
,
feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
,
olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
,
pyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
,
nepheline
Nepheline, also called nephelite (), is a rock-forming mineral in the feldspathoid groupa silica-undersaturated aluminosilicate, Na3 K Al4 Si4 O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites ...
,
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...
,
perovskite
Perovskite (pronunciation: ) is a calcium titanium oxide mineral composed of calcium titanate (chemical formula ). Its name is also applied to the class of compounds which have the same type of crystal structure as (XIIA2+VIB4+X2−3), known as ...
and
apatite
Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common e ...
. Melt inclusions can be found in both
volcanic
A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often found where tectonic plates a ...
and
pluton
In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...
ic rocks. In addition, melt inclusions can contain immiscible (non-miscible) melt phases and their study is an exceptional way to find direct evidence for presence of two or more melts at entrapment.
Analysis
Although they are small, melt inclusions can provide an abundance of useful information. Using microscopic observations and a range of chemical
microanalysis
Microanalysis is the chemical identification and quantitative analysis of very small amounts of chemical substances (generally less than 10 mg or 1 ml) or very small surfaces of material (generally less than 1 cm2). One of the pioneer ...
techniques
geochemist
Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the ...
s and
igneous petrologists can obtain a range of unique information from melt inclusions. There are various techniques used in analyzing melt inclusion H
2O and CO
2 contents, major, minor and trace elements including double-sided
FTIR micro transmittance, single-sided FTIR micro reflectance,
Raman spectroscopy,
microthermometry, Secondary Ion Mass Spectroscopy (
SIMS
Sims, sims or SIMS may refer to:
Games
* ''The Sims'', a life simulation video game series
** ''The Sims'' (video game), the first installment, released in 2000
** ''The Sims 2'', the second installment, released in 2004
** '' The Sims 3'', th ...
), Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (
LA-ICPMS), Scanning Electron Microscopy (
SEM) and electron microprobe analysis (
EMPA
The Swiss Federal Laboratories for Materials Science and Technology (Empa, German acronym for ''Eidgenössische Materialprüfungs- und Forschungsanstalt'') is an interdisciplinary Swiss research institute for applied materials sciences and tech ...
). If there is a vapor bubble present within the melt inclusion, analysis of the vapor bubble must be taken into consideration when reconstructing the total volatile budget of the melt inclusion.
Microthermometry
Microthermometry is the process of reheating a melt inclusion to its original melt temperature and then rapidly quenching to form a homogenous glass phase free of daughter minerals or vapor bubbles that may have been originally contained within the melt inclusion.
Microscope-mounted high temperature stage heating
Stage heating is the process of heating a melt inclusion on a microscope-mounted stage and flowing either helium gas (Vernadsky stage)
or argon gas (Linkam TS1400XY) over the stage and then rapidly quenching the melt inclusion after it has reached its original melt temperature to form a homogenous glass phase. Use of a heating stage allows for observation of changing phases of the melt inclusion as it is reheated to its original melt temperature.
One atmosphere vertical furnaces
This process allows for reheating of one or more melt inclusions in a furnace held at a constant pressure of one
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
to their original melt temperatures and then rapidly quenching in water to produce a homogenous glass phase.
Fourier transform infrared spectroscopy (FTIR)
This analytical method requires the use of an
infrared laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...
focused on a spot on the glass phase of the melt inclusion to determine an absorption (or extinction) coefficient for either H
2O and CO
2 associated with wavelengths for each species depending on the parent lithology that contained the melt inclusion.
Raman spectroscopy
This analysis is similar to FTIR in using a focused laser on the glass phase of the melt inclusion or a vapor bubble that may be contained in the melt inclusion to identify wavelengths associated with the Raman vibrating bands of volatiles, such as H
2O and CO
2. Raman spectroscopy can also be used to determine the density of CO
2 contained in a vapor bubble if present at a high enough concentration within a melt inclusion.
Secondary Ion Mass Spectrometry (SIMS)
This analytical technique is used to determine volatile as well as trace element concentrations by aiming an ion beam (
16O
− or
133Cs
+) at the melt inclusion to produce secondary ions that can be measured by a mass spectrometer.
Laser Ablation-Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS)
This analytical technique can determine major and trace elements, however, with LA-ICPMS, the melt inclusion and any accompanying materials within the melt inclusion are ionized, thus destroying the melt inclusion, and then analyzed with a mass spectrometer.
Scanning Electron Microscopy (SEM)
Scanning electron microscopy is a useful tool to employ before any of the above analyses that may result in loss of the original material since it can be used to check for daughter minerals or vapor bubbles and help determine the best technique that should be chosen for melt inclusion analysis.
Electron Microprobe Analysis (EPMA)
Electron microprobe analysis is ubiquitous in the analysis of major and minor elements in melt inclusions and provide oxide concentrations used in determining parental magma types of the melt inclusions and phenocryst hosts.
Vapor Bubbles
The presence of a vapor bubble adds an additional component for analysis given that the vapor bubble could contain a significant proportion of the H
2O and CO
2 originally in the melt sampled by the melt inclusion.
If the vapor bubble is composed primarily of CO
2, Raman spectroscopy can be used to determine the density of CO
2 present.
Interpretation
Volatile Concentrations
Melt inclusions can be used to determine the composition, compositional evolution and volatile components
of magmas that existed in the history of magma systems. This is because melt inclusions act as a tiny pressure vessel that isolates and preserves the ambient melt surrounding the crystal before they are modified by later processes, such as post-entrapment crystallization.
Given that melt inclusions form at varying pressures (P) and temperatures (T), they can also provide important information about the entrapping conditions (P-T) at depth and their volatile contents (H
2O, CO
2, S, Cl and F) that drive volcanic eruptions.
Major, minor and trace element concentrations
Major and minor element concentrations are generally determined using EPMA and common element compositions include Si, Ti, Al, Cr, Fe, Mn, Mg, Ca, Ni, Na, K, P, Cl, F and S. Knowledge of the oxide concentrations related to these major and minor elements can help to determine the composition of the parental magma, the melt inclusion and the phenocryst hosts.
Trace element concentrations can be measured by SIMS analysis with resolution in some cases as low as 1 ppm. LA-ICPMS analyses can also be used to determine trace element concentrations, however lower resolution compared to SIMS does not provide determination of concentrations as low as 1 ppm.
History
Henry Clifton Sorby
Henry Clifton Sorby (10 May 1826 – 9 March 1908) was an English microscopist and geologist. His major contribution was the development of techniques for studying iron and steel with microscopes. This paved the way for the mass production of st ...
, in 1858, was the first to document microscopic melt inclusions in crystals. The study of melt inclusions has been driven more recently by the development of sophisticated chemical analysis techniques. Scientists from the former Soviet Union lead the study of melt inclusions in the decades after
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
, and developed methods for heating melt inclusions under a microscope, so changes could be directly observed. A.T. Anderson explored analysis of melt inclusions from basaltic magmas from
Kilauea Volcano in Hawaii to determine initial volatile concentrations of magma at depth.
See also
*
Inclusion (mineral)
In mineralogy, an inclusion is any material that is trapped inside a mineral during its formation. In gemology, an inclusion is a characteristic enclosed within a gemstone, or reaching its surface from the interior.
According to Hutton's law ...
*
Fluid Inclusion
References
Further reading
*
*
*
External links
The melt inclusion page(Jake Lowenstern, USGS)
(Phil Brown, University of Wisconsin–Madison)
{{DEFAULTSORT:Melt Inclusions
Igneous petrology
Mineralogy
Geochemistry