Spirit Discovers New Class of Igneous Rocks
During the past two-and-a-half years of traversing the central part of
Gusev Crater, NASA's Mars Exploration Rover Spirit has analyzed the
brushed and ground-into surfaces of multiple rocks using the alpha
particle X-ray spectrometer, which measures the abundance of major
chemical elements. In the process, Spirit has documented the first example
of a particular kind of volcanic region on Mars known as an alkaline
igneous province. The word alkaline refers to the abundance of sodium and
potassium, two major rock-forming elements from the alkali metals on the
left-hand side of the periodic table.
All of the relatively unaltered rocks -- those least changed by wind,
water, freezing, or other weathering agents -- examined by Spirit have
been igneous, meaning that they crystallized from molten magmas. One way
geologists classify igneous rocks is by looking at the amount of potassium
and sodium relative to the amount of silica, the most abundant
rock-forming mineral on Earth. In the case of volcanic rocks, the amount
of silica present gives scientists clues to the kind of volcanism that
occurred, while the amounts of potassium and sodium provide clues about
the history of the rock. Rocks with more silica tend to erupt explosively.
Higher contents of potassium and sodium, as seen in alkaline rocks like
those at Gusev, may indicate partial melting of magma at higher pressure,
that is, deeper in the Martian mantle. The abundance of potassium and
sodium determines the kinds of minerals that make up igneous rocks. If
igneous rocks have enough silica, potassium and sodium always bond with
the silica to form certain minerals.
The Gusev rocks define a new chemical category not previously seen on
Mars, as shown in this diagram plotting alkalis versus silica, compiled
by University of Tennessee geologist Harry McSween. The abbreviations
"Na2O" and "K2O" refer to oxides of sodium and potassium. The abbreviation
"SiO2" refers to silica. The abbreviation "wt. %" indicates that the
numbers tell what percentage of the total weight of each rock is silica
(on the horizontal scale) and what percentage is oxides of sodium and
potassium (on the vertical scale). The thin lines separate volcanic rock
types identified on Earth by different scientific names such as foidite
and picrobasalt. Various classes of Gusev rocks (see box in upper right)
all plot either on or to the left of the green lines, which define
"alkaline" and "subalkaline" categories (subalkaline rocks have more
silica than alkaline rocks).
Members of the rover team have named different classes of rocks after
specimens examined by Spirit that represent their overall character.
During the rover's travels, Spirit discovered that Adirondack-class rocks
littered the Gusev plains; that Backstay, Irvine, and Wishstone-class
rocks occurred as loose blocks on the northwest slope of "Husband Hill";
and that outcrops of Algonquin-class rocks protruded in several places on
the southeast face.
These rocks have less silica than all previously analyzed Mars samples,
which are subalkaline. The previously analyzed Mars samples include
Martian meteorites found on Earth and rocks analyzed by the Mars
Pathfinder rover in 1997. Gusev is the first documented example of an
alkaline igneous province on Mars.
Image Credit: NASA/JPL-Caltech/University of Tennessee
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