Meteorites from Mars have been in the news since last summer's announcement about possible fossils in a martian meteorite; see the October, 1996 Hot Idea of PSR Discoveries. But there is more to the study of martian meteorites than the search for life. These igneous rocks contain detailed information about volcanism and other magmatic processes on the Red Planet. Mars had a very active igneous history, dotting the landscape with huge volcanoes and covering extensive areas with sheets of lava. Images of the surface of Mars taken from orbit showed us these volcanic wonders from afar. The meteorites we have in hand provide us with more detailed information of what we glimpsed in the images.
What we need is information about the occurrence of the nakhlites on Mars. Collecting them directly from a martian lava flow would be ideal. But, until numerous robotic rovers and humans roam the surface of Mars, we will not have such important field information. Fortunately, a very similar rock type occurs in a few places on Earth. You are invited to come along on a field trip to eastern Ontario, Canada for a look at the rocks that we think are a match for the martian meteorites called nakhlites.
For my doctoral dissertation, I am working with G. Jeffrey Taylor at the University of Hawai'i to substantiate this Earth-Mars rock connection. Allan Treiman of the Lunar and Planetary Institute in Houston first alerted us to the similarity between an unusual lava flow in Canada and the nakhlites, and is working with us on the project. Ralph Harvey of Case Western Reserve University in Cleveland also helped us in our field investigations.
|Nakhla, see larger version.|
|A sill forms when a magma intrudes between layers of rock underground. The magma never makes it to the surface. Instead, it cools slowly in place, thermally insulated by the rocks above and below.|
In contrast, a lava flow forms when magma erupts at the surface and flows along the ground. Lava flows can vary in thickness, from centimeters to tens of meters. The thinner the flow, the faster it cools.|
Most investigators think that the large proportion of pyroxene in the nakhlites is best explained by formation in an intrusive body like a sill, where the slow cooling gives the pyroxene crystals time to sink and accumulate to high concentrations (70-80% of the rock). But the nakhlites have other features, like fine-grained areas between the pyroxene crystals and high concentration of calcium in the mineral olivine, which usually suggest formation in a rapidly cooled body, like a lava flow. The idea of nakhlites forming in a lava flow can be tested by examining a similar lava flow on Earth and comparing it to the nakhlites.
|Theo's flow in Ontario, Canada is 2.6 billion years old. It is unusually thick for a lava flow (120 meters thick!), and is geographically associated with several thick intrusive sills, but a study by Nick Arndt in the late 1970's identified Theo's flow as an erupted lava flow. The best evidence that Arndt found for an extrusive origin is the knobby top layer of the flow.|
|The top of Theo's flow is brecciated (or broken up) like the clinkery top of an 'a'a lava flow. Intrusive sills don't have brecciated tops; instead, they have a glassy or finely crystallized margin at their top boundary where they cooled rapidly against the overlying rock. Arndt described this layer thoroughly, likening it to the rubbley material produced by the interaction of hot magma with water in a submarine eruption.|
|Photograph taken with a microscope at 25x magnification of a Nakhla meteorite thin section(field of view is about 5 millimeters across).|
|Photograph taken with a microscope at 25x magnification of a thin section of a Theo's flow sample (field of view is about 5 millimeters across).|
|The ancient rock of Theo's flow was tilted 90o so that a walk along the ground today lets us see the flow from bottom to top. This tape-measure extends from the base to the top of Theo's flow. It enabled us to note where we took our rock samples in relation to the height of the flow.|
|Jeff Taylor (my advisor, University of Hawaii) makes himself useful. He is using a rock hammer and chisel to help collect rock samples to be analyzed back in the lab in Hawai'i.|
|On the left is a pictorial representation, called a
stratigraphic column, of the different rock types that make up Theo's flow. The samples we took along our traverse line gave us all the pertinent information about the different rock layers to help us put together a cooling history for the flow. |
A detailed study of the different layers shows that Theo's flow did start as one magma body and then separated into discrete layers. As the flow cooled in place, crystals grew and settled through the liquid lava, changing the composition of the melt with the growth of each new crystal and thereby changing the composition of each subsequent crystal grown from the evolved melt. Thus, the different layers in Theo's flow are intimately related to each other. If the nakhlites did form in a similar setting, we would expect other nakhlite-like flows to be associated with peridotites and gabbros.
|Hawaiian pahoehoe lava.|
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