International Union of Geological Sciences
Subcommission on the Systematics of Igneous Rocks (2001)
I. SEDIMENTARY (RegolithMixture of unconsolidated rocky fragments, soil, dust and other fine granular particles blanketing the surface of a body lacking an atmosphere. Regolith is the product of "gardening" by repeated meteorite impacts, and thermal processes (such as repeated heating and cooling cycles). Click on Term to Read More)
II. METAMORPHICRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. Click on Term to Read More
A. Shocked (monomict and polymict) rocks B. Impact (melt) breccias and glasses
III. IGNEOUS
A. Mare-type 1. Crystalline
a. Ti-poor basaltBasalt is the most common extrusive igneous rock on the terrestrial planets. For example, more than 90% of all volcanic rock on Earth is basalt. The term basalt is applied to most low viscosity dark silicate lavas, regardless of composition. Basalt is a mafic, extrusive and fine grained igneous rock Click on Term to Read More (<1.5 wt% TiO2) [formerly VLT]
b. Medium Ti basalt (1.5–6 wt% TiO2) [formerly low-Ti]
c. Ti-rich basalt (>6 wt% TiO2) [formerly high-Ti] 2. Glassy
a. Green
b. Yellow
c. Orange
d. Red
e. Black B. Highland-type 1. Coarse-grained (>3 mm) [and transitional types]
a. AnorthositeA phaneritic, intrusive igneous rock made with a modal composition (i.e. volume%) > 90% plagioclase feldspar of undefined composition (anorthitic to albitic, or combination thereof), and a small mafic component between 0 - 10% such as pyroxene, ilmenite, magnetite, and olivine 1. The name anorthosite is derived from the calcium-rich Click on Term to Read More
b. NoriteIgneous rock composed of 90% plagioclase, 95% orthopyroxene (low-Ca pyroxene) and less than 10% olivine. Norite is most commonly found in the lunar (highlands) meteorites but has also been found in about a dozen diogenites, a few shergottites, and a very small number of other achondrite types. Gabbro is very Click on Term to Read More
c. GabbroWork in progress Coarse-grained igneous rock of basaltic composition that formed at depth and is 90% plagioclase. clinopyroxene, https://www.sandatlas.org/gabbro/ The most important mineral groups that make up this rock type are plagioclase and pyroxene. Plagioclase usually predominates over pyroxene. Plagioclase is sodium-calcium feldspar. It contains more calcium than sodium in gabbro. If there is Click on Term to Read More
d. TroctoliteTroctolite is an intrusive igneous rock consisting of plagioclase feldspar and olivine. It is a member of gabbroic rocks family. It is compositionally similar to gabbro. The main difference is that it does not contain pyroxene or contains very little while it is a major mineral in gabbro. It can Click on Term to Read More
e. Dunite 2. Fine-grained (<3 mm)
a. Felsite
b. KREEPLunar igneous rock rich in potassium (K), rare-earth elements (REE), phosphorus (P), thorium, and other incompatible elements. These elements are not incorporated into common rock-forming minerals during magma crystallization, and become enriched in the residual magma and the rocks that ultimately crystallize from it. Click on Term to Read More basalt 3. Fragmental (see II. above if metamorphic)
‘Instead of the formation of the vugs through an igneous process, i.e., by the rapid eruption of a high-volatile-containing magmaMolten silicate (rock) beneath the surface of a planetary body or moon. When it reaches the surface, magma is called lava. Click on Term to Read More, these vugs and druses are more consistent with a pneumatolytic formation process. The vugs were originally solid spheres, possibly composed of CaS or nitrides, and covered by anorthiteRare compositional variety of plagioclase and the calcium end-member of the plagioclase feldspar mineral series with the formula CaAl2Si2O8. Anorthite is found in mafic igneous rocks such as anorthosite. Anorthite is abundant on the Moon and in lunar meteorites. However, anorthite is very rare on Earth since it weathers rapidly Click on Term to Read More and olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion Click on Term to Read More rims. The cores were subsequently lost through ‘metasomatism’ processes, while the calcium was utilized in the formation of kirschsteinite. Many of the vugs are now filled with glass.’
pneu-ma-to-lyt’-ic (adj) pneu-ma-tol’-y-sis (n)
A process of rock alteration or mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More formation brought about by the action of gases emitted from solidifying magma.
A metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. Click on Term to Read More process caused by hot vapors or superheated liquids under pressure.
Rock alteration that is caused by gases widely thought to be related genetically to magma.
Cooling of the molten magma began to produce a residual phase, in which the volatileSubstances which have a tendency to enter the gas phase relatively easily (by evaporation, addition of heat, etc.). constituents became increasingly concentrated. High pressure within this residue caused its infiltration into cracks and fissures of the local pre-existing rock, in which chemical and thermal metamorphism occurred. This ‘pegmatitic’ phase proceeded through the temperature range of 700–500°C.
As the residual molten magma progressively cooled through 500°C, and crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. Click on Term to Read More proceeded, the magma became more highly enriched in the volatile constituents, while pressures continued to increase. These evolved solutions, containing gas and steam, penetrated deeply into the surrounding country rock, resulting in the formation of new minerals from existing ones—a process called ‘pneumatolysis’. When the metamorphic agent consists primarily of fluids and/or ions, the process is described as a metasomatic process; the rock having undergone ‘metasomatism’.
As temperatures further declined through 400°C, hydrothermal metamorphism was initiated. During this phase, hot, watery solutions altered existing anhydrous minerals into hydrated minerals—the process of ‘hydrothermal’ alteration.
Meteorites are among the oldest objects we know about – formed about 4.5 billion years ago. But how do scientists know this? This article describes the principles and methods used to make that determination.
There are well-known methods of finding the ages of some natural objects. Trees undergo spurts in growth in the spring and summer months while becoming somewhat dormant in the fallMeteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If Click on Term to Read More and winter months. When a tree is cut down, these periods are exhibited in a cross section of the trunk in the form of rings. Simply counting the number of rings will give one a fairly good idea of the age of the tree. Periods of heavy rain and lots of sunshine will make larger gaps of growth in the rings, while periods of drought might make it difficult to count individual rings.
When determining the ages of very old objects, the only suitable clocks we have found involve the measurement of decay products of radioactive isotopes.
Isotopes are atoms of the same elementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of Click on Term to Read More with different amounts of neutrons. Some isotopes are stable, whereas others are radioactive and decay into other components called daughter isotopes. For example, hydrogenLightest and most common element in the universe (~92% by atoms; ~75% by mass). Hydrogen's isotopes are: • 1H (99.9885 %) • 2H (0.0115 %), also called deuterium. • 3H, also called Tritium, is a radioactive (t½ = 12.32 y) by-product of atmospheric thermonuclear tests in Earth's hydrosphere and atmosphere. Click on Term to Read More has two stable isotopes 1H (ordinary hydrogen), 2H (deuteriumAlso called heavy hydrogen, deuterium is an isotope of hydrogen (D, or 2H) whose nucleus contains one proton and one neutron. As a trace element formed during the nucleosynthesis epoch of the Big Bang, deuterium is an important indicator of the baryon density in the universe. The larger the density, the Click on Term to Read More), and one radioactive isotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: 1H, 2H (deuterium), and 3H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus. Click on Term to Read More3H (tritium). The superscript denotes the atomic weightMass of a neutral atom of a nuclide - also called "atomic weight." The atomic weight of an element is the weighted average of each isotope. Click on Term to Read More of the isotope (the number of protons and neutrons).
Radioactive isotopes decay according to a power law, and the typical unit given for this is called the half-lifePeriod of time required for 50% (½) of the atoms of a radioactive nuclide in a sample to decay. After two half-lives, 25% ( ½ x ½ = 1/4) of the original radioactive nuclide will remain. After three half-lives, 12.5% ( ½ x ½ x ½ = 1/8) of the Click on Term to Read More of the isotope. When a given quantity of an isotope is created (in a supernovae, for example), after the half-life has expired, 50% of the parent isotope will have decomposed into daughter isotopes. After the second half-life has elapsed, yet another 50% of the remaining parent isotope will decay into daughter isotopes, and so on. For all practical purposes, the original isotope is considered extinct after 6 half-life intervals.
Some of the isotopes and their daughters are shown in the following table (from Dodd):
The isotopes above the line in that figure are now extinct, since there are no means of replenishing the parent isotope in the Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids..
Note that there are vast ranges of time exhibited in the decay rates, allowing a suitable measure if one knows or guesses the approximate age.
The clock most suitable for meteorites is the decay of Rubidium (87Rb) into Strontium (87Sr), which has a half-life of about 49 billion years. The manner in which the age is determined is based on calculating ratios of these isotopes, as the following calculation will show:
We know if there is some 87Rb present in the meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and Click on Term to Read More, that there will also be the decay product 87Sr. However, there will also be some unknown amount of 87Sr that was in the meteorite when it formed. We can state mathematically, that the amount of 87Sr present now, must have come from the amount that was there originally, plus any decay product from 87Rb:
87Srnow = 87Sroriginal + (87Rboriginal – 87Rbnow)
The term in parenthesis, the amount of 87Rb that decayed into 87Sr can be related by the radioactive decayProcess in which an isotope's nucleus changes ('decays') to produce another isotope. The original atom is called the 'parent' and the resulting atom, the 'daughter'. There are three modes of radioactive decay: • Emission of a particle (He nucleus) that decreases the atomic number (Z) by 2 and the atomic Click on Term to Read More law:
87Rboriginal = 87Rbnow * (elt) where, e is the base of the natural logarithm, l is the rate of radioactive decay, and t is the elapsed time.
By substituting that in the original equation we get:
87Srnow = 87Sroriginal + 87Rbnow * (elt – 1)
Along with 87Sr, 86Sr also occurs in meteorites, but it is not a decay product and its amount does not change over time, so we can divide this constant in the above equation without changing the equality:
Note that this is the equation of a line in the form
y = mx + b where, m, the slope, is (elt – 1) and b, the y intercept, is the original strontium isotope ratio.
Two of these quantities can be measured: 87Srnow / 86Sr and 87Rbnow / 86Sr. By taking samples from various parts of a meteorite and plotting these results, the data will fall on a straight line whose slope characterizes the age of the meteorite. These lines are called isochrons, an example for the meteorite Tieschitz (fall, 1878, Czechoslovakia, unequilibrated H3) is shown in the following figure (from McSween):
How are these Measured?
Scientists use a mass spectrometerInstrument used to measure the masses of molecules and atoms by volatilizing and then ionizing them. The ions are then separated magnetically according to their mass-to-charge ratio in order to determine the chemical or isotopic composition of a substance. Click on Term to Read More to obtain these ratios. A small portion of a meteorite is vaporized in the device forming ions. These ions are accelerated in an electric field through collimating slits and subject to a magnetic field which causes the ions to follow a curved path. The ions are deflected according to their mass. By adjustment of the strength of the magnetic field and suitable placement of an ionAtom with a net electrical charge because it has lost, or gained, one or more electrons relative to the number possessed by a neutral atom of the same element. A positively charged ion (cation) has fewer electrons than a neutral atom; a negatively charged ion (anion) has more. Click on Term to Read More collector, the different isotopes can be measured with precision. Complications
There are some things that affect these measurements. Thermal processes that may occur during meteorite impact in the lifetime of the specimen can reset some of the atomic clocks, mixing components and releasing important gases such as 129Xe and 40Ar.
In practice, several isotope systems and several samples are used to determine the ages. Meteorites that are mostly unaltered (petrological type 3) serve as the best samples. Epilog: Jan. 24, 1999
I received private communications from scientists about this paper, which was based mainly on work done in the 1980’s. Nowadays, 146Sm – 142Nd with a mean life of 1.49 x 108 years is also used, along with other methods to date meteorites.
In one note, from Dr. Bogard at NASA, it was mentioned to me that:
‘You refer to extinct nuclides 14C, 26Al, and 129I. Only the latter two ‘extinct’ nuclides are used in dating. The use of 14C in meteorite dating is solely based on its production by cosmic raysHigh-energy subatomic particles mainly originating outside the Solar System that continuously bombard the Earth from all directions. They represent one of the few direct samples of matter from outside our solar system and travel through space at nearly the speed of light. These charged particles – positively charged protons or Click on Term to Read More (and for terrestrial samples, with its production in the atmosphere). 26Al and some other nuclides not mentioned are also used in this way. Thus, although ‘extinct’, these nuclides are present in meteorites, but produced by a more recent process.
‘The idea that Rb-Sr is the most used chronometer for meteorites is largely based on work done 10-30 years ago. Increasingly, the other techniques are used, such that probably no one technique dominates for meteorite dating. Rb-Sr is a good example for explaining the process, however.
‘Near the end you imply that low petrologic typeMeasure of the degree of aqueous alteration (Types 1 and 2) and thermal metamorphism (Types 3-6) experienced by a chondritic meteorite. Type 3 chondrites are further subdivided into 3.0 through 3.9 subtypes.chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More are the most easily dated. Actually, meteorites that formed by melting, e.g., the various types of achondrites, usually give more precise ages. Type-3 chondrites can contain phases with slightly different ages, and some phases have been slightly altered by parent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. Click on Term to Read More processes.’
References
Meteorites and Their Parent Planets, Harry Y. McSween, Cambridge University Press, 1987.
Thunderstones and Shooting Stars, Robert T. Dodd, Harvard University Press, 1986.
Created on 25 Sep 1998 by Jim Hurley
For additional information on radiometric dating, read the PSRD article by Alexander N. Krott: Dating the Earliest Solids in our Solar System , Sep 2002.
Another PSRD article, Using Aluminum-26 as a Clock for Early Solar System Events , Sep 2002, by Ernst Zinner, examines how Al-26 can be used as a fine-scale chronometer for early solar systemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with events.
A detailed description of radiometric dating using the isochron method can be found on the website of Chris Stassen.
Subgroup classification after Weyrauch et al., MAPS, vol. 53, #3, pp. 394–415 (2018) ‘Chemical variations of sulfides and metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More in enstatiteA mineral that is composed of Mg-rich pyroxene, MgSiO3. It is the magnesium endmember of the pyroxene silicate mineral series - enstatite (MgSiO3) to ferrosilite (FeSiO3). Click on Term to Read More chondrites—Introduction of a new classificationPositively charged ion. Click on Term to Read More scheme’
ENSTATITE CHONDRITESChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More (4 subgroups plus ungroupedModifying term used to describe meteorites that are mineralogically and/or chemically unique and defy classification into the group or sub-group they most closely resemble. Some examples include Ungrouped Achondrite (achondrite-ung), Ungrouped Chondrite (chondrite-ung), Ungrouped Iron (iron-ung), and Ungrouped Carbonaceous (C-ung). Click on Term to Read More/anomalous members, based on mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More and chemical data [see below])
ELa
ELa3 (e.g. AhS MS-189, MAC 88136 [3.8/3.9], MAC 02747 [3/4], QUE 94594)
EH3 (e.g. Galim (b) [IMB], Hadley RilleLong narrow depression on the surface of the Moon; also called "sinuous rilles". Lunar rilles usually flow away from small pit structures and probably mark lava channels or collapsed lava tubes that formed during mare volcanism. In some cases, the lunar flows may have melted their way downwards into older Click on Term to Read More [IM])
EH4 (e.g. Bethune [4/5], Dhofar 1015, LAP 031220, Y-791810)
EH7/impact melt phase (e.g.Itqiy [Meta-EH-anom or partial melt residue], NWA 2526 [similar to Itqiy], NWA 7324 [MR], NWA 10237 [MR], QUE 94204 [7], Y-82189 [IM], Y-8414 [IM])
UNGROUPED E CHONDRITES
E-ung (e.g. LAP 031220 [4], LEW 87223 [3-anom], NWA 974 [6], PCA 91020 [3-anom; poss. rel. to LEW 87223], QUE 94204 [7], Y-793225 [6-anom])
Weyrauch et al. (2018) analyzed the mineral and chemical data from 80 enstatite chondrites representing both EH and EL groups and spanning the full range of petrologic types for each group. They found that a bimodality exists in each of these groups with respect to both the Cr content in troiliteBrass colored non-magnetic Fe sulfide, FeS, found in a variety of meteorites. and the Fe concentration in niningerite and alabanditeMagnesium sulfide found in aubrites and EL chondrites. Its formula is MnS. Click on Term to Read More (endmembers of the [Mn,Mg,Fe] solid solutionCompositional variation resulting from the substitution of one ion or ionic compound for another ion or ionic compound in an isostructural material. This results in a mineral structure with specific atomic sites occupied by two or more ions or ionic groups in variable proportions. Solid solutions can be complete (with series present in EH and EL groups, respectively). In addition, both the presence or absence of daubréelite and the content of Ni in kamaciteMore common than taenite, both taenite and kamacite are Ni-Fe alloys found in iron meteorites. Kamacite, α-(Fe,Ni), contains 4-7.5 wt% Ni, and forms large body-centered cubic crystals that appear like broad bands or beam-like structures on the etched surface of a meteorite; its name is derived from the Greek word Click on Term to Read More were demonstrated to be consistent factors for the resolution of four distinct E chondriteChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More groups: EHa, EHb, ELa, and ELb (see table below).
ENSTATITE CHONDRITEType of meteorite high in the mineral enstatite and also referred to as E-chondrites. Although they contain substantial amounts of Fe, it is in the form of Ni-Fe metal or sulfide rather than as oxides in silicates. Their highly reduced nature indicates that they formed in an area of the Click on Term to Read More SUBGROUPS Weyrauch et al., 2018
EHa
EHb
ELa
ELb
Troilite
Cr <2 wt%
Cr >2 wt%
Cr <2 wt%
Cr >2 wt%
(Mn,Mg,Fe)S
Fe <20 wt%
Fe >20 wt%
Fe <20 wt%
Fe >20 wt%
Daubréelite
Abundant
Missing
Abundant
Missing
Kamacite
Ni <6.5 wt%
Ni >6.5 wt%
Ni <6.5 wt%
Ni >6.5 wt%
A few other E chondrites with intermediate mineralogy have also been identified, including LAP 031220 (EH4), QUE 94204 (EH7), Y-793225 (E-an), LEW 87223 (E-an), and PCA 91020 (possibly related to LEW 87223). Studies have determined that these meteorites were not derived from the EH or EL source through any metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. Click on Term to Read More processes, and some or all of them could represent separate E chondrite asteroids.
