This volume is concerned with the basaltic effluent
from planetary interiors, and this chapter is concerned
with the extent to which experimental petrology of
basalts can inform us about the composition and mineralogy
of the source material within the interiors of
planets. If the compositions of sources can be deduced
using the experimental petrology of basalts, or in any
other way (Chapter 4), then the relationship between
source rock and basaltic magma can be investigated by
complementary experiments using the proposed source
material. Basaltic volcanism appears to be a characteristic
feature of most investigated planetary surfaces (Chapters
2 and 5), with basalts being produced either when
internal temperatures become high enough to cause
partial melting of the planetary rock, or when impacts
by other bodies raise the temperature high enough to
cause partial or complete melting of near-surface rocks.
The main variables to be considered in these processes
are the pressure and equivalent depth, the temperature,
and the composition and mineralogy of the planetary
interior and near-surface rocks.
Of these three variables, the value of pressure as a
function of depth is the best known. A given pressure is
achieved at depths that vary considerably from one
planetary body to another as illustrated in Fig. 3.1.1.
Temperature is a variable that changes as a function
of time, and as a function of process (Chapter 9).
Convective movements within a planet, for example,
influence both the rate of heat transport to the surface,
0
and the development of regional variations in temperature
distribution versus depth. The temperature produced
by meteoritic or planetesimal bombardment
depends upon the mass, velocity, and frequency of
impacting bodies. Variations reviewed in Chapter 9
indicate the extent of our uncertainty about the temperatures
of planetary interiors, even for our own planet.
Figure 3.1.2 shows three geotherms that have been
proposed for a convecting mantle within the present
Earth (Solomon, 1976). The geotherms drawn for the
lithosphere (shallower than about 100 km) are passed
through the zones plotted by Solomon ( 1976) for values
estimated from study of peridotite nodules from the
mantle; these geotherms are similar to those calculated
for conduction models (e.g., Clark and Ringwood,
1964). Temperatures in a region of upwelling, beneath
ocean ridges for example, are higher than temperatures
beneath normal ocean plates. The temperatures shown
beneath shield and ocean plates become identical at 200
km depth (contrast Jordan, 1975).
The third variable, the composition and mineralogy
of basalt source regions, is what we seek to determine
from experimental petrology of erupted basalts.
For the Earth, however, we already have a fairly well-defined
model for the structure and petrology of the
mantle, based on cosmochemistry, geophysics, and
petrology (Chapter 4). The composition corresponds to
that of a peridotite dominated at low pressures by olivine
and two pyroxene minerals. Figure 3.1.2 outlines the
main phase relationships for this material up to pressures of 250 kb, corresponding to a depth of 700 km
within the Earth. For other planetary bodies dominated
by the components of olivine and pyroxenes, the phase
relationships would be similar, but the specific depth
scales would differ (Fig. 3.1.1 ).
Basaltic magmas are generated within the melting
interval (crystals + liquid), when the temperature
becomes high enough to cross the solidus. According
to the terrestrial geotherms given in Fig. 3.1.2, melting
does not occur at all beneath shield and ocean plates,
and therefore some special circumstance is required
to increase the temperature locally to account for the
oceanic volcanoes and the continental flood basalts. It
is generally assumed that upward convection of mantle
material raises the temperature at relatively shallow
levels, as indicated by the geotherm associated with
mantle upwelling beneath ocean ridges in Fig. 3.1.2. In
this tectonic environment, a geotherm crosses the solidus
at depths of 70 ± 40 km (sections 3.3.7 and 3.3.8).
Each planetary body has characteristics that dis- ,
tinguish it from the others, even if it should turn out that
most of them have similar major element compositions.
The different pressure-depth relationships suggest that
temperature distribution curves are likely to intersect
solidus curves at different pressures on the different
bodies. The phase relationships shown in Fig. 3.1.2
may be changed by variations in Fe, Mg, Ca, and Al
proportions, and changed radically by the addition of
volatile components (sections 3.3.2 and 3.3.3). For parts
of the Earth's interior containing traces of CO_2 and
H_2O, there is a wide interval of incipient melting below
the solidus plotted in Fig. 3.1.2. For the Moon, in
contrast, the concentration of volatile components
appears to be negligible. The oxygen fugacity of lunar
rocks is much lower than that of terrestrial rocks. Mars
may be enriched in Fe and S compared with Earth and
Moon (McGetchin and Smyth, 1978). The occurrence
of partial melting and volcanism depends upon the
maintenance of sufficiently high temperatures and thus
upon the body's thermal history (Chapter 9). Parent
bodies of basaltic achondrite meteorites are believed to
have solidified at an early stage in the development of
the solar system, and active volcanism on the Moon
ceased relatively early in its history. Mars appears to
have ceased evolving, and the Earth continues with
active volcanism caused to a large extent by the mass
movement associated with plate tectonics. Therefore, in
considering the basaltic volcanism of each planetary
body, we have different characteristics, and different
ground rules for interpretation. The spectacular eruptions
on Io were not anticipated according to preexisting
ground rules. The purpose of this chapter is to outline the
methods used to determine whether a particular basalt
is likely to contain direct information about its source
region by application of the methods of experimental
petrology. In addition, the experimental work on terrestrial,
lunar, and meteoritic basalts, as well as likely
source materials, will be reviewed to show the extent to
which the link between the basalts and their source
regions has been established, and to show the extent to
which additional processes are responsible for the
chemistry observed in the basalts. We will conclude that
experimental petrology does provide an internally consistent
framework for understanding basalts as the melting
products of ultrabasic mantle assemblages, and that
laboratory experiments can be used for the exercise of
going back from basalt chemistry to source region constitution,
provided that certain conditions are satisfied.
Abstract The isotopic composition and abundance of sulfur in extraterrestrial materials are of interest for constraining models of both planetary and solar system evolution. A previous study that included phase‐specific extraction of sulfur from 27 shergottites found the sulfur isotopic composition of the Martian mantle to be similar to that of terrestrial mid‐ocean ridge basalts, the Moon, and nonmagmatic iron meteorites. However, the presence of positive Δ 33 S anomalies in igneous sulfides from several shergottites, indicating incorporation of atmospherically processed sulfur into the subsurface, complicated this interpretation. The current study expands upon the previous work through analyses of 20 additional shergottites, enabling tighter constraints on the isotopic composition of juvenile Martian sulfur. The updated composition (δ 34 S = −0.24 ± 0.05‰, Δ 33 S = 0.0015 ± 0.0016‰, and Δ 36 S = 0.039 ± 0.054‰, 2 s.e.m.), representing the weighted mean for all shergottites within the combined population of 47 without significant Δ 33 S anomalies, strengthens our earlier result. The presence of sulfur isotopic anomalies in igneous sulfides of some meteorites suggests that their parent magmas may have assimilated crustal material. We observed small negative Δ 33 S anomalies in sulfides from two meteorites, NWA 7635 and NWA 11300. Although negative Δ 33 S anomalies have been observed in nakhlites and ALH 84001, previous anomalies in shergottites have all shown positive values of Δ 33 S. Because NWA 7635 has formation age of 2.4 Ga and is much more ancient than shergottites analyzed previously, this finding expands our perspective on the continuity of Martian atmospheric sulfur photochemistry over geologic time.
Abstract The Northwest Africa ( NWA ) 7475 meteorite is one of the several stones of paired regolith breccias from Mars based on petrography, oxygen isotope, mineral compositions, and bulk rock compositions. Its inventory of lithic clasts is dominated by vitrophyre impact melts that were emplaced while they were still molten. Other clast types include crystallized impact melt rocks, evolved plutonic rocks, possible basalts, contact metamorphosed rocks, and siltstones. Impact spherules and vitrophyre shards record airborne transport, and accreted dust rims were sintered on most clasts, presumably during residence in an ejecta plume. The clast assemblage records at least three impact events, one that formed an impact melt sheet on Mars ≤4.4 Ga ago, a second that assembled NWA 7475 from impactites associated with the impact melt sheet at 1.7–1.4 Ga, and a third that launched NWA 7475 from Mars ~5 Ma ago. Mildly shocked pyroxene and plagioclase constrain shock metamorphic conditions during launch to >5 and <15 GPa. The mild postshock‐heating that resulted from these shock pressures would have been insufficient to sterilize this water‐bearing lithology during launch. Magnetite, maghemite, and pyrite are likely products of secondary alteration on Mars. Textural relationships suggest that calcium‐carbonate and goethite are probably of terrestrial origin, yet trace element chemistry indicates relatively low terrestrial alteration. Comparison of Mars Odyssey gamma‐ray spectrometer data with the Fe and Th abundances of NWA 7475 points to a provenance in the ancient southern highlands of Mars. Gratteri crater, with an age of ~5 Ma and an apparent diameter of 6.9 km, marks one possible launch site of NWA 7475.
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