Two Lectures on the Moon and Planetary Geochemistry

Ge151: Spring 2011

What the rocks tell us.

"It's basalt! It's igneous!"

- Exclaimed by C. Frondel upon blowing the dust off the first lunar sample.

In these lectures it is our goal to understand the significance of Frondel's statement and why it summarizes many of the key things that were learned from the lunar samples.

The following sections provide illustrations that supplement a paper entitled "Lunar Rocks" (Encyclopedia of Physical Science and Technology, vol. 8, 2001, pg. 825-838).

An exceedingly short introduction to planetary geochemistry and sample analysis

1. Samples provide information:
• On present bulk and surface composition of a planetary body
• About the types, rates, and chronology of various processes that have affected the planetary body.
2. Minerals, rocks, formations, samples - Definitions????
• Mineral-a phase (in the thermodynamic sense)
• Rock-an aggregate of mineral grains
• Igneous = "fire"rock -- formed from melt
• Sedimentary = "water" rock -- deposited in layers
• Metamorphic = "changed"rock -- recrystallized under elevated pressure and temperature conditions
As defined in Calif. Schools in the 3rd grade.
• Formation-a rock unit, distinctive enough to be spatially mapped. A sedimentary formation will normally have a horizontal top and bottom, prior to any deformation.
• Sample-should be selected to represent a rock unit. Lunar samples were packaged to prevent cross-contamination and biological contamination
3. Knowledge learned from types of sample analysis
• Mineralogy, mineral chemistry, texture, and bulk chemical composition of rocks can be diagnostic of formative processes.
• Trace element chemistry can provide a wide variety of signatures of specific geochemical processes.
• Precise isotopic analyses are used to study a wide variety of chronologic and geochemical problems.
• Long-lived radioactive species (U-Th-Pb, K-Ar, Rb-Sr, Nd-Sm) can be used to measure isotopic ages and to establish an absolute chronology for a planet.
• Stable isotopes (O, Si, C, S, N, H) provide geochemical tracers for many processes.
• Rare gas (He, Ne, Ar, Kr, Xe) isotopic abundance's provide insight into the differentiation history of a planet, its interaction with cosmic radiation, and the evolution of the atmosphere.
• Remnant magnetization provides a clue to the thermal history and the magnetic field history of a planet.
4. Meteorites
• Meteorite study is basic to planetary geochemistry
• 86% chondrites -- consist of chondrules, fragments, & carbonaceous hydrous matrix
  7% basaltic achondrites -- these include Mars meteorites
  6% irons and stony irons
• V. M. Goldschmidt coined terms lithophile, siderophile, and chalcophile to indicate how the elements separate in the meteorites.
5. Geochemical differentiation
• Condensation sequence-primordial gas and dust.
  Condensation of solar gas
• Thermodynamic partitioning between phases -- melt-crystal, melt-melt, melt-gas, crystal-crystal -- provides basic differentiation of planets.
  Thermodynamic partitioning between phases
• Basalt is a result of partial melting in planet interior. Partial melts in complex systems do not have the composition of the bulk solid. Abundant in time and space. Sensitive probe of interior composition
  Basalt
• Some key elemental ratios that point to process:
  Fe/Si-fractionation of metallic iron from silicates
  Al/Si & Ca/Si-fractionation of refractory elements during nebular condensation and agglomeration
  Fe0/Mn0 in silicates-measure of planetary oxidation, i.e., ferrous to ferric
  K/U -moderately volatile element to refractory element, both "incompatible", remaining unchanged in igneous processes.
  K/U ratios of meteorites and planets
  

Lunar rocks and instrumental analysis

1. Petrographic microscope, electron microprobe, SEM, ion probe, mass spectrometer, INAA
   A wide variety of analytical techniques were brought to bear on the study of the lunar samples. For the first time portions of many samples were analysed by different techniques and by a number of different laboratories. It provided validation of the new instrumental techniques and killed off many classical chemical techniques. The instrumental techniques are repeatable and can be applied to very small samples.
   
   Photo of a petrographic microscope.-The microscope can view a thin, polished slice of a rock in plane light or cross-polarized light and in transmitted mode and in reflected mode. As will be seen this permits observations of the texture and mineral composition of the rock.
2. Electromagnetic spectrum.
   The electromagnetic spectrum can be seen as a basis for almost all instrumental analysis. Typically, photons of higher energy are used to bombard a sample and excite photons of lower energy, which characterize the sample.
   
   Schematic of an electron probe microanalyser, usually called an electron microprobe. Electrons are boiled off a filament, accelerated, and focused to a < 1 micron spot on a polished and carbon coated rock slice. The position of the spot can be scanned across the sample. X-rays are excited in the one micron volume, their wavelength (and energy) characteristic of the element and their intensity dependent upon the abundance of the element in the sample. Analysis can be accomplished by a wavelength-dependent crystal spectrometer or an energy-dependent detector. Some of the impacting electrons are backscattered, the number depending on the atomic number of a smooth sample but on the topography of a rough sample. This forms the basis of the SEM-scanning electron microscope.
   An ion probe works on a similar principle, using accelerated ions rather than electrons as the projectile. A mass spectrometer measures isotopic abundance by separately counting particles of different mass as they are accelerated through a magnetic field. The elements of interest may be separated from the sample as a gas or as a chemical that can be ionized on a filament.
   INAA or instrumental neutron activation analysis involves neutron radiation of the sample and then accurate counting of decay particles binned by their energy.

Hawaii lava lake

Observations of the crystallization sequence and the textures in some terrestrial basalts are important keys to understanding these features in lunar basalts. U.S. Geological Survey volcanologists collected a series of quenched samples at different levels below the surface of a lava lake as the basalt melt cooled -- measuring the temperature and making thin sections of the quenched, glassy sample. These thin sections are shown in the next series of slides.

• Lava fountain erupting through the cooling crust of Makaopuhu lava lake, 1965.
• Surface sample, 1120° C, 90 % glass.
  The light brown glass results from quenching the silicate melt; the transparent crystals are pyroxene and olivine.
• 1130° C, 70 % glass.
  The lathlike crystals are plagioclase.
• 1068° C, 44 % glass.
  Note the darkening of the glass due to increased abundance of Ti, which is not accomodated in any of the crystals.
• 1065° C, 42 % glass.
  Further darkening of the glass as its abundance decreases and Ti-content increases.
• 1020° C, 8 % glass.
  The glass is now pale green. It became supersaturated with Ti and the opaque mineral ilmenite (FeTi03) crystallized, thereby removing the coloring agent from the melt. The thin transparent needles of apatite similarly reflect saturation by P buildup.
• 750° C, 4 % glass.
  The final residual melt has an excess of O and ferrous iron is converted to ferric iron, both in the final residual melt and by oxidizing previously-crystallized ferrous iron-bearing olivine.

Lunar stratigraphy

1. Introduction
• Earth and Moon
  Earth is marked by clouds and oceans; Moon by light rough "highlands" and dark smooth "maria" -or seas.
• Apollo (US) and Luna (USSR) landing sites shown on a relief map.
• Lunakhog - Soviet lunar rover.
• Lunar drill and sample return vehicle on lander.
  In August 1976 the Soviet Union's Luna 24 autonomous spacecraft returned a 160 cm long core from Mare Crisum. It was in a sausage-like tube, coiled to save space.
2. Highlands-crater saturated surfaces.
• Contrast between smooth plains in foreground and rough highlands.
• Typical ancient highland surface, saturated by craters of all sizes.
3. Impact basins and impact sheets-stratigraphy & chronologic sequence.
Bombardment of the lunar surface produced craters of all sizes. The larger impact events produced major basins, breaking up the outer parts of the crust, transporting material great distances across the lunar surface, and forming widespread layered deposits of impact breccia (i.e. fragmented and then recompacted rock).
• Distribution and names of major lunar basins and prominent late rayed-craters (star). Filled circles show Apollo and Luna landing sites.
• Map of major geologic provinces of the moon. Ejecta blankets from the Imbrian and Nectarian basins can be seen to cover much of the surface.
• Lunar stratigraphic column summarizing the time history of events that have shaped the moon's surface. The positions of the Imbrian and Nectarian ejecta blankets in the column provide a basis for widespread correlation, as do the mare lavas that have been dated by isotopic techniques.
4. Mare filling by basalt
Distribution of basaltic flows in the mare basins on the near and far sides of the moon.
The lunar stratigraphic column helps to place the lunar samples into global context and is key to utilizing the isotopic dates measured on the samples.

Surface-space interaction

• The cratered surface behind spacecraft showing fragmented rocks and powder. Note crater.
• Close up view of cratered powder surface of the regolith. Note crater even at this scale.
• A handful of Apollo 11 regolith, cleaned of fine dust.
  The regolith contains a wide variety of material, in part tranported from great distance by multiple cratering events. Some of the fragments are Apollo 11 basalt, but the white fragments are plagioclase feldspar transported from the Highlands. Irregular glassy sinters were expected, but the glass spheres were a surprise.
• Broken elongate balls
• broken balls
• flattened balls
• green balls from Apollo 15
• hollow ball
• half ball, note fluted surface formed as the molten sphere cooled during the impact trajectory
• SEM photo of the broken surface of the half ball in previous image. The numerous white dots are tiny impact craters, some of which are shown in detail in the following images.
• The crater is marked by a smooth molten cup in the center bordered by a zone of fractures and marked by radial driplets of melt.
• Two SEM views of a crater on the surface of a glass ball. Note the same features as noted for the previous image.
• An SEM photo of a crater on the surface of a plagioclase grain. The photo is surrounded by x-ray scans for Ca, Fe, Mg, Al, and Ti. Plagioclase is CaAlSi308, but the Ca and Al only shown in the fractured area bordering the impact cup. The remainder of the grain surface is covered by a thin glass rich in Fe, Mg, and Ti, the composition of glass that forms sintered aggregates within the regolith.
• A cartoon, illustrating bombardment of the lunar surface.
  Unlike Earth, which is partially protected by it magnetic field and atmosphere, the flux of particles and radiation from the sun and of meteorites upon the lunar surface has imprinted evidence of the history of the solar system upon the surface materials. Lunar soil properties cannot be explained strictly by broken-up local rock. In addition to introduction of exotic material from great distance, about a percent of the soil is of meteoritic origin. Impacts have added glass and vertical mixing is important. The layers of the regolith, sampled in cores, preserve a record of meteorite, solar particle and cosmic ray bombardment.
• A cartoon illustrating micrometeorite bombardment and formation of zap pits on fragments. Impact on regolith may form tiny glassy cups in the base of craters.
• A cartoon illustrating bombardment by solar wind and solar flare particles.
• A cartoon illustrating nuclear interaction with galactic cosmic rays.

Lunar basalt

1. Textures and major minerals

The lunar mare rocks are basalts, similar in texture and chemical composition to volcanic basaltic rocks on the earth. The lunar basalts consist chiefly of the silicates clinopyroxene ([Ca,Fe,Mg]Si03), anorthitic plagioclase (CaAl2Si2O8), olivine ([Mg,Fe]2SiO4)and the iron-titanium oxides, ilmenite and spinel. The mineralogy is generally similar to that in basalts on earth except for the very low content of sodium in the plagioclase, the high abundance of ilmenite, and the total absence of hydrous alteration minerals. Textures range from fine-grained and partially glassy, such as might be expected in a basalt that chilled quickly at the surface of a flow, to coarser-grained, interlocking textures that result from slower cooling within a flow.

Major phases of lunar basalts
This slide summarizes the range in abundance and composition of the major phases in the lunar mare basalts.

Cation coordination in the silicate phases.
These silicate minerals are made up of almost close-packed oxygen ions with the cations in 4, 6, or 8 coordination positions with oxygen depending upon their effective size. Limited substitution of ions with the correct size, but non-nominal charge, can be locally accomodated by balancing substitutions. Ions with very different size or charge will not be accomodated within these minerals as they crystallize, but will accumulate in the melt just as we noted in the Hawaiian lave lake samples.

Mineral and chemical compositions of the basalt groups identified at the various landing sites. All consist predominantly of pyroxene and plagioclase feldspar, but Fe-Ti oxides (and Ti02 content) and olivine differ in abundance. Their ages fall between 3.15 and 3.96 Gy. The samples can be divided into two broad groups: an older high Ti group and a younger low Ti group. The chemical compositions can be used to calculate the viscosity of these melts. The high Fe and Ti content results in melts of very low viscosity, accounting for the lava flows covering the very flat mare plains. Terrestrial basalts are formed by partial melting in the olivine and pyroxene-rich rocks of earth's mantle. As melting occurs, the early formed silicate melt has the composition of basalt, and it separates and rises to the surface. The detailed composition of a basalt provides a probe into the temperature, depth, and composition of the source; basalt can be readily dated, and the textures provide information on the cooling and crystallization history.

Comparison of chemical composition of lunar basalt with terrestrial basalt.

Mare basalts differ from terrestrial basalts in some detailed elemental abundances. Lunar basalts (1) contain no detectable H20; (2) are low in alkalis, especially Na; (3) are high in Ti02; (4) are low in A1203 and Si02; (5) are high in FeO and MgO; and (6) are extremely reduced. Lunar basalts contain no trivalent Fe, and the reduced ions (Fe metal, trivalent Ti, and divalent Cr) may be present.

Apollo sample 10044,33 in plane, cross polarized, and reflected light; field of view is 2.1 x 3.2 mm. Sample 10044 is a representative mare basalt, which has been extensively studied. The thin section in plane light shows the intrinsic color of the minerals; plagioclase is clear, pyroxene is pinkish brown, and ilmenite is opaque. In cross-polarized views we see the differences in birefringence (difference of speed of light in different directions in the crystal); the bright colors of the pyroxene indicate a higher birefringence than the gray color of feldspar. These differences can be quantified for identification purposes. The parallel bands in the plagioclase represents growth twinning of the crystal structure, a common feature in feldspar. In reflected light the silicates are light-gray and the ilmenite is bright light gray. In the center of the thin section the yellow color is indicative of iron sulphide and the bright white ball within it is iron metal.

2. Crystallization sequence and differentiation -- final concentration of "incompatible" elements.

Study of a single pyroxene crystal in 10044,33; field of view is 2.1 x 3.2 mm. This large crystal grew from a single nucleii with each thin layer taking on a new composition as the composition of the melt changed during crystallization. In the left a faint pink "hourglass" can be seen, the darker color reflecting a higher Ti content. Different crystal faces on the growing crystal have different capability to accommodate Ti substitution on the growing face. Slides 3 & 4 show the use of the electron probe to provide accurate point analyses across the crystal and to make profiles for individual elements. The right shows the compositions of the two sectors with differing Ti content and the increasing Fe/Mg of the crystal from the center nucleus to the outer rim. This change in composition is predictable from the experimental phase diagram.

Detailed compositions of all minerals in sample 10045. Sample 10045 is "identical" to 10044. This diagram shows that all of the minerals showed zonal compositional changes as crystallization continued. Both high-Ca and low-Ca pyroxene become higher in Fe/Mg, plagioclase becomes lower in Na/Ca, and olivine becomes higher in Fe/Mg.

Composition phase diagram for pyroxenes showing compositional space with one pyroxene fields, two pyroxene isothermal tie lines, a three pyroxene isothermal tie-triangle, and the appearance of olivine and pyroxenoid with very high Fe/Mg compositions. Each tie line or triangle "ties" to a melt composition that is higher in Fe/Mg.

Composition trends in pyroxene from various basalt groups. Differences reflect cooling rates, temperature, and slight differences in bulk composition.

Compositional trends in spinel from various basalt groups.

Skaergaard Intrusion

The classic study of the Skaergaard intrusion in southeast Greenland demonstrates similar changes in mineral composition as crystallization continued. The intrusive body was of basaltic composition and cooled at depth, but as it cooled and crystallized the early-formed crystals dropped to the bottom forming a layer. The successive layers, shown in the cross section, represent the cooling history of the body just as do single crystals in the mare basalts. The central diagram illustrates the changes in composition equivalent to those shown for 10045.

Compositional changes in mafic minerals plotted against feldspar composition in the same layers of the Skaergaard intrusive.

Behavior of P during crystallization of the Skaergaard intrusive. The early crystallizing silicate minerals do not contain any P. Hence, as the amount of melt decreases it becomes richer and richer in P until saturation occurs and apatite crystallizes in great abundance. We noted this behavior with Ti and P in the lava lake samples.

Minor phases of lunar basalts.
A wide variety of exotic minerals crystallize from the last few percent of residual lunar melt, reflecting the concentration of ions that cannot be accomodated in the major rock-forming phases, ions that have unusual size or charge or both. These include all the elements used for radioactive dating. The Fe-Ti oxides differ from terrestrial minerals due to the total lack of ferric iron. The phosphate mineral apatite differs from terrestrial apatite due to the total lack of OH. As shown by the pyroxene phase diagram Si02 (cristobalite) and iron olivine crystallize in place of pyroxene in these last stages. In addition, the final Fe-rich melt splits into blebs of immiscible Fe-rich and Si-rich melt (now glass) and sulphide blebs include immiscible balls of iron metal. In some samples the lack of oxygen in the final stages causes the Fe-rich melt to be converted to blebs of Fe metal. Note the strong contrast to the lava lake samples, which were rich in oxygen and water.

Cristobalite (crackly) and Fe olivine in 12021,135

12004, 10-Immiscible blebs of glass in glass, and iron metal balls in Fe-sulphide.

14053, 17-Complex residual melts with immiscible blebs and final Fe metal

Occurrence of two phosphate minerals, apatite and whitlockite in 10044. Probe scans show that not all P rich areas contain Cl, indicative of apatite. Fission tracks show that apatite concentrates U and electron microprobe scans show that whitlockite concentrates Y (and other REE).

Occurrence of K in 10044, 33 as shown by probe scans.
K occurs in blebs with Si and Al, presumably in glass but in some rocks as K-feldspar. Rb with similar size and charge as K is also present. Separation of phases rich in K (rich in Rb, low in Sr) are key to isotopic age dating of these samples. This concentration was typically achieved by separating late stage cristobalite or ilmenite which contains no Sr, but commonly contains late stage blebs as shown in these slides.

3. Dating basalts.

Cartoon of Sr evolution diagram
Rb87 decays to Sr 87 with a known half-life, thereby permitting dating if the original amount of Sr 87 is known--which it is not of course. This diagram show a plot of Rb87/Sr 86 against Sr87/Sr86. The ratios can be determined much more accurately with the mass spectrometer than can the absolute amounts. The ratios are measured with three or more mineral phases in the same lunar rock. As shown in the diagram they will form a linear array if they formed together with the same Sr isotopic composition at the same time. The decay time can be calculated from such a linear array to give the age of crystallization.

Rb-Sr dating for 10044; age 3.71 b.y. Leverage on the colinearity was obtained by using high Rb/Sr values found in cristobalite and ilmenite due to inclusions. The low Rb/Sr value is plagioclase, because it easily substitutes Sr for Ca in its structure. Note the line for 4.5 b. y. through the total rock value.

Multistage Sr evolution diagram.
Colinearity of phases provides an age since the phases equilibrated to the same Sr composition. The evolution of the total rock composition may be inferred if the system remain closed without gains or losses of Rb or Sr.

Rb-Sr for lunar soil samples
Surprisingly, lunar soil samples provided a colinear array with an age of 4.6 b.y. This could indicate an incredibly great job of mixing through cratering or it could represent introduction by cratering of a material extremely high in Rb/Sr with an age of 4.6 b.y. that swamped to bulk composition
Nd/Sm dating is not shown in these early examples, but the approach is basically the same.

The largest "rock" in the core returned by Luna 24

A polished thin section from the "rock"; very similar to 10044. The U. S. was given a 3.0 gm sample. Using instrumental techniques it was dated by four different techniques, detailed probe analyses were made in the thin section, major and minor trace elements were analysed, and a variety of other studies carried. 1.3 gm of the sample remains for future study. This illustrates the power of what can be learned by study of a very small sample of an unaltered basalt returned from another planet, such as Mars.

Ages and age distribution of lunar mare basalts. Additional samples have been measured subsequent to this figure, but do not substantially change this picture.

Highlands samples

The repeated impact episodes are reflected in the complexities of the fragmental rock (breccia) samples returned from the lunar highlands. The shock of the impact results in intense fragmentation and melting. The molten material may quench to glass during the ejection, or the hot mixture may remelt, sinter, and recrystallize during deposition and cooling in a thick ejecta blanket. The breccia samples range from friable aggregates to hard, sintered material with spherical vesicles that were bubbles filled by a gas phase prior to solidification. Many samples show multiple generations of impacts; fractured fragments of ancient rocks are within irregular fragments of breccia that are themselves contained in a mixture of fragments and melt rock.

Samples of breccia

Thin section of breccia, 62237, with a large deformed and broken plagioclase grain.

Thin section of breccia derived from regolith (note glass spheres)

Thin section of breccia, 15205, shot through with green glass and showing multiple generations of impacts.

The major minerals within the highland breccias are anorthite-rich plagioclase (CaAl2Si2O8), orthopyroxene ([Mg,Fe]Si03), and olivine ([Mg,Fe]2SiO4); these occur both as mineral fragments and as plutonic rocks made up predominantly of these minerals. The high content of anorthitic plagioclase and the low abundance of iron and titanium oxide minerals is responsible for the light color and for the characteristically high calcium and aluminum composition of the lunar highlands. The words anorthosite, norite, and troctolite are used in various combinations as adjectives or nouns to describe coarse-grained rocks made up of various combinations of these three minerals. Hence the acronym ANT is commonly used to describe this suite of rocks. Such rocks are found on the earth in layered igneous bodies that have crystallized from a silicate melt or magma very slowly deep beneath the surface. The term magma includes not only the complex silicate melt, but the various crystallizing minerals, and may include bubbles of volatiles and globules of sulfide or metal melt. Plagioclase-rich rocks such as the ANT suite do not form by simple crystallization of magma, but represent accumulation of early crystallizing minerals by floating or settling, as evidenced by terrestrial examples of cumulate rocks such as the classic example of Skaergaard.

A white fragment of anorthosite in breccia

Thin section of anorthosite, coarse grained plagioclase feldspar (76535).

A Caltech alumnus collecting the unique dunite sample.

The unique dunite fragment before removal from the breccia

The lunar dunite fragments, 74215.

Thin section of the highly deformed olivine in 74215, 54.

Despite the complex history, a number of fragments of ANT rocks collected from the breccia have yielded isotopic ages greater than 4.4 G.y., indicative of crustal formation dating back almost to the origin of the solar system.

Rb-Sr dating of dunite, 74215.

Ar-Ar dating of anorthosite,65015, showing 4.6 G.y. cores in 3.9 b.y. rims.

The existence of an early crust was also inferred from geochemical evidence. The rare earth element europium, unlike the other rare earth elements, is highly concentrated in plagioclase during crystallization of a silicate melt. This element has a relatively high abundance in the highlands rocks and is relatively underabundant in the lunar basalts.

REE spectra for lunar basalt with "Eu anomaly" compared to terrestrial basalts.
These complementary anomalies are ascribed to extensive early differentiation of the primitive lunar material into a plagioclase-rich crustal cumulate of crystals and a more mafic melt, which eventually became the source of the lunar basalts. Hence, it is inferred that much of the outer part of the moon was molten that is, a magma ocean during the early part of lunar history.

This early differentiation seems also to have been responsible for another compositional class of material rich in K, rare earth elements, and P (KREEP). These elements are representative of the "incompatible elements" (which also include Ba, U, Th, and Rb) that do not enter the crystal structure of the major lunar rock-forming minerals and hence become concentrated in the residual liquid during final crystallization of a magma.

A slab of 12013, a unique breccia sample exceedingly KREEP-rich with an age of 4.6 G.y. for some components of the sample.

The abundance of KREEP ranges greatly in the samples of highland breccias and regolith, occurring as both small rock fragments and glass. However, the uniformity of abundant pattern and the isotopic systematics, albeit partially disturbed in some cases, suggest a rather homogeneous source, one that was enriched in the incompatible elements at about 4.4 Gy. Orbital measurements of gamma rays have shown that material rich in K, Th, and U is concentrated in the region of Mare Imbrium and Oceanus Procellarum. The KREEP-rich material may have been distributed from these regions into the regolith by impact scattering.

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Note: This last section is an outline of the conclusions in the paper that follow from the study of lunar rocks.

Lunar interior

1. Radial differentiation-crust, mantle, and core
• Neither basalt nor anorthosite are compatible with mean density due to phase changes in feldspar with increased pressure (depth)..
• K, U, Th in surface rocks too high for solid moon.
• Mean density indicates that any core must be very small.
2. Mare basalts derived by partial melting of mantle and provide a probe to interior composition.
• Low in volatiles
• High in refractories
• Low in siderophiles
• Oxygen isotopes differ from basaltic & chondritic meteorites. Lunar and terrestrials trends are similar, but offset from Mars.
3. Gravity and shape
• Smoother on near side with total relief of 16-km
• Highland crust is thicker (70-100 km) in near-isostatic compensation. Thicker crust on far side. Crustal thinning (mascons) under basins.
4. Magnetic field
• No current global field, but local small remnant fields.
• Magnetism of samples dues to single-domain Fe metal
• Lunar Prospector found larger remnant fields on far side antipodal to major basins.

Lunar Evolution

1. Chemical composition
• Not solar nor chondritic in composition
• High in refractory, low in volatile and siderophile elements
• Oxygen reservoir different from chondritic meteorites
2. Crustal formation and early differentiation
• Low density (~3.0 g/cc) ~60-km crust overlies a higher density (~3.35 g/cc) mantle. Any core must be less than 500 km.
• Accretional melting led to anorthosite ocean-ANT & KREEP
3. Impact history-4.6-3.9 Gy
4. Mare filling- 3.9-3.2 Gy-some variation in basalt composition

Martian and lunar meteorites--How do we know their origin?

1. Geochemical and age data indicated origin in a differentiated planet.
2. Rare gas pattern trapped in impact glass matched Mars's data from Viking.

References

- A. L. Albee -

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