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When magma moves from great depth to the upper reaches of Earth, the vigor and nature of its motion can sometimes be traced through the entrainment and distribution of unusually large crystals. Called phenocrysts, these crystals and their distribution are nowhere better exhibited than in the Basement sill, a thick [350-meter (m)] layer of magmatic rock, exposed throughout the McMurdo Dry Valleys region. What is even more noteworthy is the great abundance and three-dimensional distribution of these phenocrysts. This distribution promises to reveal both the initial magmatic injection point as well as the lateral flow field over an area of thousands of square kilometers. Discerning and understanding this flow field, an unprecedented achievement, give fundamental insight into the essential link between volcanism and magmatism and the dynamics of magma beneath ocean ridges the world over. After giving a brief background on the nature of crystal entrainment and sorting in ascending magma, we present the results of detailed sampling and chemical analysis from seven sections of the Basement sill covering much of the McMurdo Dry Valleys.
Heavy solid particles in fluids coursing through pipes and slots tend to migrate away from the walls and form high concentration sluglike flows in channel centers. The behavior of fatty globules in blood and paper pulp paste particles in aqueous solutions are common examples of this process. Similarly, magma ascending sufficiently fast through packed beds entrains crystals, if it moves fast enough, carrying them upward. These crystals are almost always heavy, and they migrate away from the walls to the flow center, but they also sink en mass, relative to the rising magma. The net result is a rising column of magma led by essentially crystal-free magma and followed by a crystal-rich tongue of increasingly coarse crystals. The flow sorts the crystals vertically and laterally such that the distribution of crystals reveals the flow field; the longer the magma carries the crystals, the better the sorting (see figure 1).
This phenomenon is well known in fluid mechanics (e.g., Segre and Silberberg 1962; Lael 1980) and has been nicely modeled as a magmatic process by Bhattacharji and Smith (1964), who have also noted its importance in forming the Muskox intrusion in northern Canada. Simkin (1967, pp. 64-69) and Upton and Wadsworth (1967) have shown the prevalence of this style of sorting or flow differentiation in, respectively, dikes in Scotland and a small sill on Reunion Island. Gibb (1976; see also Marsh 1996) has drawn attention to the possibility that this kind of magma may go to form large, well-sorted magmatic bodies that have been routinely attributed to in situ crystallization, but until now direct evidence has been lacking. The critical evidence is preserved over great expanses of the Basement sill.
The presence of unusual concentrations of large crystals of orthopyroxene (opx) in the Basement sill was first reported by Gunn (e.g., 1966). He noted the curious complete absence of opx along the upper and lower margins even when a relatively thick (30-m) layer of large opx crystals forms the sill center at Solitary Rocks. Realizing the possible opportunity presented by this opx, we have begun mapping out this concentration in three dimensions by studying and collecting samples at (so far) eight locations. The vertical variation of magnesium oxide (MgO) (weight percent, wt.%), which is a direct reflection of opx content, through the Basement sill at seven of these locations is shown by figure 2.
Over a distance of about 15 kilometers (km) in the northeast wall of Wright Valley, the sill pinches out from a thickness of over 350 m near Bull Pass to a thickness of 5 centimeters (cm) at the north lower Wright Valley section. This captures the leading edge of the invading magma, and this easternmost section contains no sign of opx; MgO is constant at the background level of about 7 percent. Several kilometers west, approaching Bull Pass, the concentration steadily increases to over 10 percent MgO in the sill center (see Mount Peleus section), to over 15 percent at east Bull Pass, and to 20 percent MgO at west Bull Pass. The opx tongue fills about 75-80 percent of the sill and individual crystals reach lengths of 8-10 millimeters (mm), whereas at the leading eastern edge the tongue is thin and opx size is 1-3 mm. Twelve kilometers farther east from Bull Pass in the Dais section, although incomplete due to debris cover, the sill resembles the west Bull Pass section if allowance is made for a confused zone at the upper contact. Ten kilometers north (Victoria Valley section) and 30 km south (northwest Kukri Hills section) from west Bull Pass the opx tongue is still prominent, although it is, especially in the Kukri Hills, less strong (see figure 2).
These data suggest that somewhere, perhaps near present Lake Vanda, the Basement sill began filling and magma spread laterally in all directions while progressively lifting the overlying 4 km of crust. The vigor of emplacement was strong enough to entrain a considerable mass of large orthopyroxene crystals whose distribution intimately records this event. Although it is still premature to draw more conclusions, judging from the internal structure of the tongue, filling may have been pulsative as in volcanism, and filling may have been stronger along than transverse to the Transantarctic Mountains (i.e., north-south), which is similar to the style of magmatism at ocean ridges.
This work is supported by National Science Foundation grant OPP 94-18513. We thank Joslin Heyn for her gracious help in making the analyses.
References
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