Anthony R. Philpotts

    Ph.D., University of Cambridge

Philpotts is the author of the widely used texts, Principles of Igneous and Metamorphic Petrology and Petrography of Igneous and Metamorphic Rocks, published by Prentice Hall.  The latested edition of Petrography of Igneous and Metamrphic Rocks is Published by Waveland Press Inc.  He is interested in both field and experimental petrology and has done research on a wide range of topics, including liquid immiscibility in silicate melts, iron-titanium oxide and apatite melts, origin of Precambrian massif-type anorthosites, petrogenesis of alkaline rocks, differentiation and emplacement of Mesozoic flood basalts in eastern North America and crustal contamination of their feeder dikes, flow-direction indicators in dikes, and lithic analysis of archeological materials. His current interest is in the differentiation of basaltic magma through crystal-mush compaction and in particular the quantitative measurement of textural anisotropy resulting from compaction.

Thick flood-basalt flows provide the simplest of magmatic bodies for which the boundary conditions are well known. It is important that we should understand how such bodies solidify before interpreting the solidification of the far more complex intrusive magma bodies.  He and his graduate students Loretta Dickson, Riley Flanagan-Brown, Scott Soricelli, and Meghan Schaub are studying the solidification of the thick flood-basalt flows in the Mesozoic Hartford basin in Connecticut, the Columbia River and Picture Gorge basalts in Washington and Oregon, the Palisades sill in New Jersey and a similar sill in the Hartford basin. This work has resulted in some startling discoveries, the most important of which is that when tholeiitic basaltic magma cools slowly, small plagioclase crystals cluster together to form monomineralic chains that create a continuous 3-D network when the magma is no more than 25% crystallized. Once this network has formed the basalt begins to differentiate through compaction of the crystal mush. The network of plagioclase chains is deformed during compaction and thus preserves a record of the degree of compaction. Several quantitative techniques have been developed to measure the anisotropy of the network, which is a measure of the degree of compaction.  Computer programs for measuring the deformation of the plagioclase chain network can be found at <> . By heating samples of basalt until approximately two-thirds melted, the textural relations of the early crystallizing refractory minerals are clearly evident. Polished sections of the partly melted rock can then be etched with HF fumes, which turns the glass dark brown, the plagioclase light brown, and leaves unaffected the pyroxene (polished section). The distribution of the minerals within the section can then be mapped and separate files stored for each mineral. These sections are then combined to form a 3-D model of the crystal mush. The network of plagioclase chains are clearly seen in these serial sections especially if they are set in motion so as to traverse down and up through the 3-D model. Compare, for example the most dilated network and the most compacted network (for more details see the articles in Nature 1998, v. 395, p. 343-346; NATURE, 2000, v. 406, p. 59-61; and American Mineralogist, 1999, v. 84, p. 1819-1829).  The X-ray attenuation coefficients of the plagioclase, pyroxene, and glass are sufficiently different in partly melted samples that the 3-D structure of the crystal mush can be imaged in detail using high-resolution X-ray computed tomography (CT). In conjunction with the High-Resolution CT Facility at the University of Texas, Austin, we have been able to obtain a detailed picture of the plagioclase-chain network in the Holyoke basalt. The CT images clearly reveal that it is the network of plagioclase chains that gives the crystal mush its connectivity; pyroxene crystals in contrast occur as isolated individuals. To view the difference in the distributions of plagioclase and pyroxene crystals in a partly melted sample of the Holyoke basalt please examine the CT-generated 3-D model. The nature of the plagioclase network in this CT-generated 3-D model can be seen more clearly if the model is rotated (warning: this file is 1 MB).   These CT generated images were obtained with funds from NSF grant EAR-9805269.

The goal of the image-analysis studies of basaltic rocks is to develope techniques that provide a direct measurement of the amount of compaction that a rock has undergone during solidification, which can then be used with the chemical data to gain a deeper understanding of the compaction process.

Recent Publications:

Gray, N. H., Philpotts, A. R., and Dickson, L. D., 2003.  Quantitative measures of textural anisotropy resulting from magmatic compaction illustrated by a sample from the Palisades sill, New Jersey.  Journal of Volcanology and Geothermal Research , 121, 293-312.

Boudreau, A., and Philpotts, A. R., 2002.  Quantitative modeling of compaction in the Holyoke flood basalt flow, Hartford Basin, Connecticut.  Contributions to Mineralogy and Petrology, 144, 176-184.

Philpotts, A. R., and Dickson, L. D., 2002.  Millimeter-scale modal layering and the nature of the upper solidification zone in thick flood-basalt flows and other sheets of magma.  Journal of Structural Geology 24, 1171-1177.

Philpotts, A. R., and Dickson, L. D., 2000.  The formation of plagioclase chains during convective transfer in basaltic magma.  Nature, 406, 59-61.

Philpotts, A. R. , Brustman, C. M., Shi, J., Carlson, W. D., and Denison, C. (1999)  Plagioclase-chain networks in slowly cooled basaltic magma.  American Mineralogist,  84, 1819-1829.

Philpotts, A.R. 1998. Nature of a Flood-Basalt-Magma Reservoir Based on the Compositional Variation in a Single Flood-Basalt Flow and its Feeder Dike in the Mesozoic Hartford Basin, Connecticut. Contributions to Mineralogy and Petrology, v. 133, p. 69-82.

Philpotts, A.R., Shi, Y., and Brustman, C. 1998. Role of plagioclase crystal chains in the differentiation of partly crystallized basaltic magma. Nature, v. 395, p. 343-346.

Philpotts, A. R., and Carroll, M., 1996. Physical properties of partly melted tholeiitic basalt. Geology, v. 24, p. 1029-1032.

Philpotts, A. R., Carroll, M., and Hill, J. M., 1996. Crystal-mush compaction and the origin of pegmatitic segregation sheets in a thick flood-basalt flow in the Mesozoic Hartford basin, Connecticut. Journal of Petrology, v. 37, p.811-836.

Philpotts, A. R., and Wilson, N., 1994. Application of petrofabric and phase equilibria analysis to the study of a potsherd. J. Archaeological Sci. 21, 607-618.

Philpotts, A. R., and Asher, P.M., 1994. Magmatic flow-direction indicators in a giant diabase feeder dike, Connecticut. Geology, v. 22, p. 363-366.

Philpotts, A. R., and Asher, P. M., 1993. Wallrock melting and reaction effects along the Higganum diabase dike in Connecticut: Contamination of a continental flood basalt feeder: Journal of Petrology, v. 34, p. 1029- 1058.

Philpotts, A. R., 1992. A Model for emplacement of magma in the Mesozoic Hartford Basin, in Puffer J. H., and Ragland P. C., Eds., Eastern North American Mesozoic Magmatism. Geological Society of America, Special Paper 268, p. 137-148.

Other Significant Publications:

Philpotts, A. R., 1990. Principles of Igneous and Metamorphic Petrology, Prentice Hall, NJ; 498 p.

Philpotts, A. R., 1989. Petrography of Igneous and Metamorphic Rocks, Prentice-Hall, N.J., 192 p.

Philpotts, A. R. and Doyle, C.D., 1983. Effect of magma oxidation state on the extent of silicate liquid immiscibility in a tholeiitic basalt. American Journal of Science, v. 283, p. 967-986.

Philpotts, A. R., 1982. Composition of immiscible liquids in volcanic rocks. Contributions to Mineralogy and Petrology, v. 80, p. 201-218.

Philpotts, A. R., 1981. A model for the generation of massif-type anorthosites. Canadian Mineralogist, v. 19, p. 233-253. (1982 Hawley Award, Mineral. Association of Canada)

Updated: 11/14/2005