Download Basaltic volcanism on the terrestrial planets: a window to planetary

ACTA GGM DEBRECINA Geology, Geomorphology, Physical Geography Series
DEBRECEN Vol. 2, 53–57
2007
Basaltic volcanism on the terrestrial planets: a window to planetary interiors
Bazalt vulkanizmus a föld-típusú bolygókon: betekintés az égitestek belsejébe
Antal Embey-Isztin
Department of Mineralogy and Petrology, Hungarian Natural History Museum H-1083 Budapest, Ludovika tér 2-6, Hungary. E-mail: [email protected]
Abstract – Contemporary knowledge on the geochemistry of planetary basalts and its implications for the composition and origin of their
parent bodies have been reviewed briefly in the case of four terrestrial planets, the Earth, the Moon, the Mars and the asteroid 4 Vesta.
Differences and similarities, related to volatile element depletion, siderophile element abundances as well as rare earth elements and their
significance have been discussed.
Összefoglalás – A dolgozat röviden összefoglalja négy földtípusú bolygó, a Föld, a Hold, a Mars és a 4 Vesta aszteroida bazalt
vulkanizmusáról összegyűlt korszerű geokémiai ismereteket és ezeknek jelentőségét az anya égitestek összetételének és eredetének
kérdéseiben. Az illóelemek elszegényedési mértékében mutatkozó különbségek és hasonlóságok, a sziderofil, valamint a ritka földfém
elemek gyakorisági viszonyai és azok genetikai vonatkozásai kiemelt jelentőséget kaptak.
Key words – basalt volcanism, Earth, Moon, Mars, 4 Vesta
Tárgyszavak – bazalt vulkanizmus, Föld, Hold, Mars, 4 Vesta
chassignites) are thought to be derived from Mars.
Shergottites are highly similar to terrestrial basalts (Fig. 1),
whereas nakhlites and chassignites are cumulate peridotites.
Finally, achondritic meteorites of the HED group
(howardites-eucrites-diogenites) are believed to originate
from a common parent planet, the asteroid 4 Vesta (540km
in diameter), on the basis of a detailed spectrophotometric
survey of the asteroid belt. Eucrites form basaltic flows on
the surface of 4 Vesta, diogenites are cumulates that were
separated from the eucrite magma, whereas howardite is a
breccia containing rock fragments of eucrite and diogenite
(Fig. 2).
Introduction
Basaltic volcanism is a fundamental process that has
been active all along the 4.56Ga history of the solar system.
In fact, this is the main factor of crust formation,
differentiation and geological processing of the terrestrial
planets and the differentiated asteroids. Astronomical
observations have ascertained the presence of volcanic
plains, most probably basaltic lava flows on the surfaces of
terrestrial planets i.e. those that are composed of solid
rocky material (Mercury, Venus, Moon, Mars). Samples of
basaltic systems from a number of planetary bodies (Earth,
Moon, Mars and asteroids, especially 4 Vesta) and spanning
the age of the solar system, have been identified and they
are examined routinely in highly sophisticated laboratories.
Basalts form as partial melts of planetary interiors and
as such they provide insight into the mineralogy, chemistry
and the thermodynamic history of a planet (BENCE et al.
1980, 1981). It is widely accepted that many of the chemical
factors are related to the early origin and evolution of
planetary bodies. Variations of condensation temperatures
and subsequent fractionation (e.g. metal-silicate separation)
in the solar nebula, accretion history of planetesimals and
planets all have ultimately exerted their influence on the
composition of planets and hence on the basalts derived
from the interior of these bodies. In the following
paragraphs I will summarise briefly the main characteristics
of those planetary basalts for which we have high precision
isotope, major and minor element data obtained in Earthbased laboratories, in order to highlight inferences with the
composition, origin and evolution of their parent bodies.
Ages of planetary basalts
In a special sense, the lifetime of a planet can be
defined as the time interval during which it sustain
sufficient internal heat to allow igneous activity to occur.
Planetary bodies, on which no more volcanic eruptions can
take place, are geologically dead. Exact radiometric age
determinations have revealed that smaller planetary bodies
have shorter duration of igneous activity. This is because
smaller bodies cool more quickly than larger ones. Igneous
processes on asteroid 4 Vesta ceased approximately 4.40Ga
years ago, that is soon after they had begun. Volcanism on
the Moon persisted until approximately 3.2Ga years ago.
Volcanic formation on Mercury may be as young as 2.5Ga
years. Martian igneous activity is believed to have lasted
until about 1.1Ga years and probably longer. The Earth
and possibly Venus are still volcanically active, though
none of the volcanoes of Venus is known to be active at
present.
Planetary samples
Terrestrial basalts
Terrestrial basalts erupting in different tectonic
settings are widespread (see later). Lunar basalts have been
collected mainly at the landing sites of Apollo-11 (Mare
Traquillitatis), Apollo-12 (Oceanus Procellarum), Apollo-14
(Fra Mauro site), Apollo-15 (Mare Imbrium) and a few
meteorites of lunar origin have also been found, especially
in Antarctica. A special group of achondrite meteorites the
so-called
SNC
meteorites
(shergottites-nakhlites-
Basalts are by far the most widespread volcanic rocks
on Earth, which is the only known planet in the solar
system to have a plate tectonic regime in operation since
very early times of its existence. Basalts are erupted both
along plate boundaries, especially in mid-ocean ridges
(MORB) and in intraplate settings, such as ocean island
basalts (OIB), oceanic plateau basalts and continental flood
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basalts (CFB). In addition to the MORBs, oceanic plateau
basalts (e.g. Java-Ontong and Kerguelen plateaus) and their
continental equivalents, the CFBs (e.g. Siberian traps,
Deccan plateau) form huge masses. In contrast, continental
rift basalts are volumetrically trivial.
MORBs are derived from the partial melting of a
previously depleted upper mantle under largely anhydrous
conditions at relatively shallow depths. Trace element and
isotopic ratio differences among MORB and OIB indicate
that the Earth’s upper mantle has long-lived and physically
distinct source regions. In general, OIBs are thought to
have deep mantle sources and associated to ascending hot
mantle material (plumes). Plumes may also play a major
role in the genesis of CFBs. Ancient komatiites (>2.5Ga)
indicate that the Earth’s upper mantle was hotter in the
Archean but already depleted in continental crustal
components.
2007
(see later), being closer to that in terrestrial basalts. In
contrast to lunar basalts and eucrites, shergottites must
have crystallized under more oxidising conditions because
they contain abundant Fe-Ti oxides, mainly magnetite.
Cores of pyroxene grains contain tiny inclusions of glass
and in these melt inclusions minute crystals of amphibole
have been identified. Amphibole is a water-bearing mineral
whose presence requires that shergottite magmas contain at
least a small amount of water.
Lunar basalts
The dark areas visible on Earth's moon, the lunar
maria, are plains of flood basaltic lava flows which are the
lunar analogues of the terrestrial CFBs. Lunar basalts differ
from their terrestrial counterparts principally in their high
iron contents, which typically range from about 17 to
22wt% FeO. They also possess a stunning range of
titanium concentrations (present in the mineral ilmenite),
ranging from less than 1wt% TiO2, to about 13wt.%.
Traditionally, lunar basalts have been classified according
to their titanium content, with classes being named high-Ti,
low-Ti, and very-low-Ti. Nevertheless, global geochemical
maps of titanium obtained from the Clementine mission
demonstrate that the lunar maria possess a continuum of
titanium concentrations and that the highest concentrations
are the least abundant.
Lunar basalts show exotic textures and mineralogy,
particularly shock metamorphism, lack of oxidation typical
of terrestrial basalts, and a complete lack of hydration.
Pyroxene is the most abundant mineral in lunar basalts,
olivine composition is mainly Mg-rich and feldspars are
commonly An-rich. The presence of native iron and FeS in
the lunar lavas indicates that the mare basalts are strongly
reduced with oxygen fugacity values several orders of
magnitude lower than that for terrestrial basalts.
A long-standing paradigm for the origin of lunar mare
basalts has been that they are the late products (~3.3 to
3.7Ga) of re-melting of a deep cumulate pile that was
formed from the solidification of an at least 400km deep
global magma ocean between 4.4 and 4.3Ga. The lunar
highlands formed at this early epoch by accumulation of
plagioclase feldspar crystals on the top of the magma
ocean.
Figure 1 Zagami meteorite (shergottite), Collection of the Hungarian
Natural History Museum
1. ábra Zegami meteorit (shergottit), a Természettudományi Múzeum
gyűjteménye
Eucrite basalts
The HED clan comprises the most abundant class of
achondrites (approximately 100 meteorites of this group
have been recognised). Eucrites look, at least superficially,
like terrestrial basalts, however, there are some significant
mineralogic differences. The plagioclase in eucrites is rich
in Ca and contains very little Na and the common
pyroxene is pigeonite, an Fe-Mg silicate very low in Ca.
Terrestrial basalts usually have plagioclase with higher Nacontent and carry augite, a pyroxene rich in Ca. Eucrites,
like other members of the HED group are completely
devoid of water, a feature shared with rocks of lunar origin.
Martian basalts
Figure 2 Pavlovka meteorite (howardite), Collection of the Hungarian
Natural History Museum
2. ábra Pavlovka meteorit (howardit), a Természettudományi
Múzeum gyűjteménye
There are approximately 31 SNC meteorites that have
been delivered from Mars to the Earth and these provide
the possibility to estimate the mineralogy and composition
of the Martian mantle. Basaltic shergottites (Fig. 1) that
formed volcanic flows or shallow intrusions on their parent
body consist primarily of plagioclase and pyroxenes. The
plagioclase is more sodium rich than in HED meteorites
All the iron in eucrites is reduced (either divalent iron
in silicates or native iron). No metallic iron can be found in
terrestrial basalts and they usually have a small amount of
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PETROGRAPHY, PETROLOGY, VOLCANOLOGY
trivalent iron in oxide minerals like magnetite. The
inference of this discrepancy is that the source region for
eucrite melts in the mantle of asteroid 4 Vesta was distinct
in composition and oxidation state from the Earth’s
mantle. Melting experiments carried out at atmospheric
pressure indicate that three minerals − olivine, pyroxene
and plagioclase − must have been major constituents of the
original source rock.
respectively whereas in lunar mare basalts TiO2 exhibit an
extremely large variation ranging from less than 1% to 813% in the high TiO2 basalts. This latter has no equivalent
on the Earth. These differences reflect inherent
discrepancy in the Mg-values and TiO2 contents in
planetary mantle sources. For example, the great variety of
TiO2 contents in lunar mare basalts argue for highly
heterogeneous source regions within the lunar interior.
Lunar mantle was formed by cumulus phases mainly
olivine and pyroxene crystallized in a deep magma ocean.
High TiO2 basalts may have originated from source regions
where late differentiates of the magma ocean were
relatively abundant. In contrast, the Earth’s mantle is more
homogeneous as far as major elements are concerned.
Chemical differences among planetary basalts
As it was mentioned earlier, the compositions of
basaltic magmas reflect the preaccretionary history of the
material from which a given planet formed as well as the
chemistry and mineralogy of the mantle source regions.
Studies of basalt suites from the Earth, the Moon, the
eucrite parent body (presumably 4 Vesta) and Mars
(shergottites) have revealed compositional differences
intrinsic to their sources, which are, in turn, characteristic
of each planet. As a detailed discussion of the subject
would exceed the framework of this review, only some of
the most important results and conclusions can be
mentioned here. For a comprehensive discussion of the
topic see the publications of WÄNKE & DREIBUS (1988),
TAYLOR (1992), WATSON et al. (1994), HUMAYUN &
CLAYTON (1995), MCSWEEN (1999), RUZICKA et al. (2001)
and PAPIKE et al. (2003).
Major interplanetary differences are observed in the
abundances of Fe, and hence in the Mg-value
(Mg/(Mg+Fe2+), as well as in Mn, Ti, Al, Na, Cr, Ni and in
the refractory and volatile elements. Whereas on the Earth
MORBs have Mg-values 0.70-0.72, the most primitive
lunar basalts have Mg-values of about 0.6 and eucrites even
smaller (about 0.5). Shergottites have intermediate Mgvalues between the lunar and terrestrial values from which
we can deduce that the Mars is enriched in iron relative to
the Earth’s mantle. The abundance of manganese turned
out to be a useful tool in distinguishing among basalts
originating from different planetary bodies (Fig. 3).
Rare earth element abundances
Trace elements are very useful in understanding
petrogenetic processes such as partial melting and
fractional crystallization, as well as mineralogy and
composition of mantle sources. Rare earth elements (REE)
are especially valuable because of their rather coherent
behaviour during melting. All of them are trivalent and
more or less incompatible in possible planetary mantle
sources with the exception of europium (Eu), which can
form divalent ions that are approximately the same size as
Ca ions. Owing to this, Eu is retained in plagioclase
feldspars causing a pronounced positive spike in the REE
Figure 4 Chondrite-normalised REE patterns, A: lunar mare
basalt; B: lunar highland rock; C: eucrite; D: shergottite
4. ábra Kondritra normált REE grafikonok, A: holdi tengeri
basalt; B: holdi felföldi kőzet; C: eukrit; D: shergottit
abundance pattern of this mineral. On the contrary, partial
melts having equilibrated with a plagioclase bearing source
exhibit negative Eu anomalies. A few minerals are
prejudiced against some rare earths, whereas favouring
others. For example, garnet prefers the heavy REEs (those
with high atomic numbers) to light REEs forming
therefore steep curves in chondrite normalised diagrams
(Fig. 4). Using information such as this we can model the
mineralogy of mantle source rocks.
The most conspicuous feature of the lunar mare
basalts is their ubiquitous negative anomaly at the element
Eu (Fig. 4). The missing Eu apparently resides in the old
feldspar rich highland rocks exhibiting pronounced positive
Eu spikes. This is the best evidence for the formation of
lunar mantle by cumulus processes from a thick, ancient
magma ocean, from which early crystallized plagioclase has
Figure 3 Fe versus Mn abundance serves to distinguish basalts of
different planetary origin. Modified after MCSWEEN (1999)
3. ábra Fe/Mn dúsulás alapján elkülöníthetjük a különböző bolygó
eredetű bazaltokat. MCSWEEN (1999) alapján módosítva
There are striking differences in the Ti abundances of
planetary basalts at comparable Mg-value. MORBs and
OIB tholeiites show TiO2 contents 0.7-1.2 vs. 2-3%
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19000, whereas this ratio is about 12000 in the Earth. Mars
appears to have a Rb/Sr ratio of about 0.07 compared to
0.03 for the Earth. Conversely, the Moon and the HED
parent body are enriched in refractory lithophile elements,
having nearly twice the Al and Ca as the Earth. Naturally,
they are highly impoverished in volatile elements even
relative to the Earth. A dramatic consequence of the severe
volatile depletion is the total absence of water in the Moon
and eucrite parent bodies.
been separated. Several hundred millions of years after that
the magma ocean had solidified, the olivine and pyroxene
rich cumulate pile re-melted and the resulting basaltic
magma ascended and filled the huge basins that had been
excavated by giant meteoroids.
Eucrites exhibit rather flat REE patterns, which means
that the source regions of eucritic magmas must be
unfracionated and essentially chondritic in composition
(close to that of the solar nebula). This can occur on
smaller bodies such as asteroids, where geologic processing
is arrested at a very early stage. There is no general
agreement as to the formation of the basaltic crust on the
HED parent body. In one model core formation began
about 2 million years after the accretion followed by the
melting of the silicate mantle to produce eucrites. However,
it is also possible that the 4 Vesta formed an early magma
ocean like that on the Moon.
The lazy S-shaped patterns of shergottites suggest that
they were apparently derived from differentiated nonchondritic source regions. The separation of light and
heavy REEs in SNC meteorites (Fig. 4) requires several
igneous events for accomplishing it. This is only possible in
larger planetary bodies where geological processing lasts for
longer periods. The relative depletion of the heavy REEs
indicates the presence of garnet in the source regions of
SNC magmas. Garnet is a high-pressure mineral that
cannot form in small asteroid sized bodies. This fact also
strengthens the belief that the SNC meteorites must
originate from a planet, presumably the Mars. REE
patterns of terrestrial basalts also reflect a long history of
geological processing. MORBs have light REE depleted
patterns indicating that the sources of oceanic basalts
underwent a partial fusion stage long before MORB
magmas were formed. In contrast, OIBs and continental
alkali basalts are enriched in light REEs (and other
incompatible elements) and frequently show steep patterns
indicating the presence of garnet in their sources.
Siderophile elements
The mantle sources of planetary basalts exhibit
considerably lower than chondritic abundances of
siderophile (iron loving) elements due to the formation of
cores. There are, however, several notable differences.
First, although the silicate part of the Moon (mantle +
crust) is richer in iron than the silicate Earth, the bulk lunar
composition is poorer in iron that the terrestrial one. The
latter reflects the small size of the lunar core, which has a
diameter of only 340±90km; representing only about 2%
of the mass of the planet. Mars is similar in this respect:
having a small core and consequently an iron-rich mantle.
The lunar depletion in Fe compared to the Earth extends
to all siderophile and chalcophile elements as well. Eucrite
basalts also have low abundances of various trace
siderophile elements (Ni, Co, Ga, Ge, Re, and Ir) as a result
of metal-alloy fractionation. Metal was segregated from the
source regions in the planetary bodies early before basalt
formation.
Oxygen Isotope Variations
Refractory and volatile elements
Oxygen is commonly showing isotopic variations. Not
only basalts but also almost all other terrestrial materials
plot on a line – the Terrestrial Fractionation Line in the
δ18O vs. δ17O diagram (Fig. 5). Lunar samples fall on the
same line but meteorites and meteoritic components do
not.
Refractory elements e.g. Al, Ca, Ti, Sr, REEs and U
condense at very high temperatures (>1300oC) from the
solar nebula, whereas volatile elements condense at
progressively lower temperatures. Alkali elements such as
K, Na, Rb are moderately, whereas Pb, Tl, Bi C are highly
volatile elements. Information obtained from the study of
planetary basalts and mantle sources have revealed that all
the terrestrial planets, including the Earth, have chondritic
relative abundances of refractory elements i.e. the
refractory elements were not fractionated from each other.
On the other hand, they show depleted and highly
fractionated volatile element patterns (progressively lower
than chondritic abundances). All this indicates that the
material forming the building blocks of the terrestrial
planets mostly condensed at high temperatures a condition
that was surely characteristic of the inner solar system after
the birth of the infant Sun.
There are, however, significant differences in the
degree of volatile depletions among the terrestrial planets.
The Mars is clearly richer in moderately volatile elements
than the Earth. Analyses of both Martian soil and the
composition of SNC meteorites suggest a K/U ratio about
Figure 5 Oxygen isotope compositions of various planetary basalts.
As melting or crystallisation cannot move rocks from one fractionation
line to another, each line represents a different parent body. Lunar
rocks plot on the terrestrial line. Modified after McSween (1999)
5. ábra Különböző bolygókról származó bazaltok oxigén izotóp
összetétele. Mivel a kőzetek nem kerülhetnek az egyik frakcionációs
vonalról a másikra olvadás, vagy kristályosodás révén, minegyik
vonal különböző származási égitestet képvisel. A Hold kőzetek a
földi vonalra esnek. McSween (1999) alapján módosítva
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PETROGRAPHY, PETROLOGY, VOLCANOLOGY
Notably, HED and SNC meteorites form lines that are
clearly distinct. This strongly suggests that, for the most
part, different groups could not have come from the same
parent body and that the different groups probably formed
in different parts of the presolar nebula. However, lunar
rocks plot along the terrestrial fractionation line, indicating
that more than one body can have the same oxygen
isotopic composition.
(5) The mantle of the Moon and the eucrite parent
body must be highly reduced, whereas that of the Earth
and Mars is much more oxidised.
References
BENCE, A.E., GROVE, T.L., PAPIKE, J.J. 1980: Basalts as
probes of planetary interiors: constraints on the
chemistry and mineralogy of their source regions.
Precambrian Research, 10, 249–279
BENCE, A.E., GROVE, T.L., PAPIKE, J.J., TAYLOR, S.R.
1981: Basalts as probes of planetary interiors:
constraints from major and trace element chemistry.
In R.B. Merrill and R. Ridings, Eds., Basaltic
Volcanism on the Terrestrial Planets, Pergamon, New
York. 311–338
HUMAYUN, N., CLAYTON, RN. 1995: Potassium isotope
cosmochemistry: genetic implications of volatile
element depletion. Geochim. Cosmochim. Acta 59, 21312148
MC. SWEEN H. Y. 1999: Meteorites and Their Parent
Planets Cambridge University Press, (Second Edition),
309 p.
PAPIKE, J.J., KARNER, J.M., SHEARER, C.K 2003:
Determination of planetary basalt parentage: A simple
technique using the electron microprobe. Am. Min.,
88, 469–472
RUZICKA, A., SNYDER, G.A., TAYLOR, L.A. 2001:
Comparative geochemistry of basalts from the Moon,
Earth, HED asteroid, and Mars: Implications for the
origin of the Moon. Geochim. Cosmochim. Acta, 66,
979−997
TAYLOR, S.R. 1992: Solar system evolution: a new
perspective. Cambridge University Press, New York.
307 p.
WÄNKE, H., DREIBUS, G. 1988: Chemical composition and
accretion history of terrestrial planets. Phil. Trans. R.
Soc. Lond. A 325, 545–557
WATSON, L., HUTCHEON, I.D., EPSTEIN, S., STOLPER, E.
1994: Water on Mars: clues from deuterium/hydrogen
and water contents of hydrous phases in SNC
meteorites Science 265, 86-90
Conclusions and implications
A straightforward interpretation of key geochemical
data for basaltic rocks related to four different planetary
bodies (the Moon, Earth, HED parent body, and SNC
parent body) leads to the following conclusions.
(1) Basalt source regions in all four of the planetary
bodies are generally depleted in volatile alkali elements
relative to chondrites with depletions increasing in the
sequence: SNC; Earth; Moon; HED. The Moon and HED
asteroid clearly experienced similar, high-temperature
fractionations.
(2) Lunar and eucrite basalts, and to a lesser extent
shergottites and terrestrial basalts have low abundances of
various trace siderophile elements (Ni, Co, Ga, Ge, Re, and
Ir) as a result of metal-alloy fractionation. Trace element
abundance systematics implies that metal was segregated
from the source regions in the planetary bodies early before
basalt formation. This metal segregation most probably
corresponds to core formation.
(3) The compositional differences between source
regions in the Moon and Earth do not support the direct
derivation of the Moon from the Earth’s mantle.
According to the currently most popular model, the Moon
had formed by accidental collision of the Earth with a
Mars-sized planet and the composition of the Moon mostly
inherited that of the impactor. Nonetheless, O isotope
relations indicate that the impactor body must have
originated from the same region of the solar nebula as the
Earth.
(4) In spite of the striking geochemical similarity
between eucrites and lunar mare basalts, differences in
isotope systematics clearly indicate that their parent bodies
formed in separate regions of the solar nebula.
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