Anthony R. Philpotts

Compositions of immiscible liquids in volcanic rocks

Immiscible liquids, preserved as chemically distinct, glassy globules, (Si-rich and Fe-rich) occur in many tholeiitic basalts and some alkaline and calcalkaline lavas. The glasses typically form part of a dark mesostasis containing skeletal magnetite crystals. In thick flows, the Si-rich liquid may crystallize to granophyric patches, and the Ferich one to aggregates of hedenbergite, magnetite, and accessory phases. The mesostases containing these immiscible phases constitute from 20% of a primitive olivine tholeiite (MgO=7.5%) to 50% of a highly fractionated quartz tholeiite (MgO=2.8%), but may be less if the rock is oxidized. Abundant ferric iron promotes early crystallization of magnetite and prevents the iron enrichment necessary to reach the immiscibility field; thus, aa flows rarely exhibit immiscibility, whereas the more reduced pahoehoe ones do.Alumina and alkalis are concentrated in the Si-rich liquid, whereas the remainder of the major elements are concentrated in the Fe-rich melt; but the partitioning of Fe, Mg, Ca, and P is less pronounced in alkaline rocks than in tholeiites. Conjugate liquids have compositions of granite and Fe-rich pyroxenite, though the Si-rich melt in alkaline rocks is more syenitic and the Fe-rich one contains considerable normative alkali feldspar. The liquids coexist with plagioclase and augite of, respectively, An50 and Ca34Mg19Fe47 compositions in tholeiites, and An40 and Ca42Mg29Fe29 in alkaline rocks. Immiscibility is not restricted to K-rich residual liquids, but the miscibility gap is narrower for Na-rich compositions. In tholeiitic basalts with 52% SiO2, the Na2O/K2O ratios in conjugate liquids are equal, but at lower silica contents the Si-rich liquid is relatively more sodic, whereas at higher silica contents it is relatively more potassic. This may explain the association of sodic granites with mid ocean ridge basalts.Immiscible liquids are present in sufficient amounts in so many volcanic rocks that magma unmixing should be considered a viable means of differentiation during the late stages of fractionation of common magmas, at least at low pressures.

Role of plagioclase crystal chains in the differentiation of partly crystallized basaltic magma

Melting experiments on samples of basaltic rock from a thick lava flow reveal that when this lava originally began to crystallize, feldspar crystals linked together to form a continuous three-dimensional network of chains when the lava was no more than 25% crystallized. Formation of this network has profound implications for the behaviour and differentiation of basaltic magma. Much of the compositional variation of igneous rocks results from the separation of liquid from crystals, a process that is dramatically affected according to whether crystals occur separately or are linked together in networks.

Plagioclase-chain networks in slowly cooled basaltic magma

Abstract Plagioclase crystals in the slowly cooled interior of the thick Holyoke flood-basalt flow of Connecticut linked to form monomineralic chains at an early crystallization stage. Partial melting experiments reveal that when the quartz tholeiite was only 25% crystallized the chains had already linked to form a continuous 3-D network. At such an early stage of crystallization, the network was weak, highly permeable, and easily deformed. Consequently, the mush of plagioclase-chains and interstitial pyroxene crystals underwent compaction in the lower third of the flow with the expelled liquid rising to the center of the flow where it crystallized to form coarse-grained sheets of fractionated basalt. Plagioclase chains are most easily seen in the basalt after it has been partly melted and the late crystallizing minerals converted to glass. The chains are several crystals wide. The crystals, which are ~0.5 mm long, are attached together randomly. Normal zoning patterns indicate crystals had a brief period of growth before linking together. The chains branch every few millimeters to form the 3-D network, which was mapped using serial polished sections and X-ray CT scans. The chain frequency measured along vertical and horizontal traverses decreases toward the center of the flow. In the compaction zone, the frequency in the vertical direction is greater than in the horizontal. Making the reasonable assumption that these frequencies were initially the same, the difference is used to calculate the degree of compaction. The resulting pattern through the flow matches almost exactly the pattern indicated by variations in the incompatible elements. Plagioclase chains are also found in some coarser-grained plutonic rocks. If they are common, their fabric may provide a new, direct means of measuring the degree of compaction in crystal mushes.

Physical properties of partly melted tholeiitic basalt

Melting experiments on centimetre-scale blocks of tholeiitic Holyoke basalt from the Mesozoic Hartford basin of Connecticut reveal that when this basalt is only 30% crystallized, a relatively strong network of crystals exists. Cubes of the basalt melted to this degree retain their shape as the interstitial liquid drains easily from the crystal network. The intrinsic permeability of a sample containing 34% crystals was determined to be 3 × 10−10 m2, which is similar to that of coarse gravel. Serial sections through partly melted samples reveal that the network owes its strength to monomineralic strings of plagioclase and pyroxene crystals. Although these strings are clearly evident once the rock has been partly melted, they are difficult to see in thin sections of the initial basalt, but they are present. The compressive strength of the mush increases dramatically from 336 to 4333 Pa as the percentage of crystals increases from 33% to 37%. This rapid strengthening is believed to explain why sheets of segregation liquid in tholeiitic flood basalts have compositions that correspond to no more than 35% fractional crystallization of the host basalt. These sheets are proposed to form through compaction of the crystal mush in the lower part of thick flows, the compaction occurring only while the mush remains weak, that is, while it has <35% crystals.

Magmatic flow-direction indicators in a giant diabase feeder dike, Connecticut

Recent studies have suggested that many large diabase dikes have been emplaced laterally rather than vertically. These studies report only unidirectional flow, which is usually based on measurements of anisotropy of magnetic susceptibility. However, in the 250-km-long Higganum dike of southern New England (United States), seven independent flow-direction indicators record a complex history of forward- and backward-moving magma. We would expect other similar-size dikes to have had equally complex histories. Measurements within one en echelon segment of the Higganum dike show that as the magma rose, it spread laterally toward the ends of the segment and that backflow was in the reverse direction of the forward flow. The Higganum dike is interpreted to have formed from a linear array of intrusive fingers, each finger spreading laterally to form an en echelon dike segment as it rose through the lithosphere.

The formation of plagioclase chains during convective transfer in basaltic magma

The basaltic rock in the lower part of the thick Holyoke lava flow in Connecticut and Massachusetts has been shown to have a remarkable texture, with crystals of feldspar linked together in a continuous three-dimensional network of chains. Heating experiments have revealed that this network persists to temperatures where the rock is 75% liquid, and therefore the network was interpreted to have formed at an early stage of crystallization and to have played an important role in the compaction of crystal mush in the lower part of the flow. Despite the textures importance to our understanding of how such basalt flows form, the origin of the texture has remained uncertain. Here we show that, although the network is present in the lower third of the flow, it was actually formed in the upper solidification front and was transported down in plumes of dense crystal mush. Convection of this type has been postulated for intrusive magma chambers, but corroborative field evidence has been equivocal, especially in lava lakes and flows. Preservation of the roof-generated texture in the lower part of a thick flood-basalt flow therefore constitutes important evidence for the role of convection in the solidification and differentiation of a simple magma sheet.

Pipe vesicles—An alternate model for their origin

Most pipe vesicles occur near the base of basaltic flows where they are interpreted to be traces left by ascending gas bubbles. Some basaltic pillows also contain pipe vesicles, but in these pillows pipe vesicles have a radial distribution, indicating that buoyancy cannot be a factor in their formation. Pipe vesicles do not extend through chilled glassy margins at the base of flows or rims of pillows, but rather, they form only where there has been significant crystallization, indicating that the gas forming these vesicles is exsolved from the lava and is not derived from an external source. Pipe vesicles in flows and pillows are proposed to form by the exsolution of gas onto bubbles that are attached to the zone of solidification. As this zone advances into the cooling lava, continued exsolution of gas causes the bubbles to grow normal to the solidification front as pipes. Gas tubes formed in a similar manner can be seen in most ice cubes. Lava near the advancing tip of a pipe vesicle cools more rapidly than elsewhere because heat is able to transfer along the pipe by radiation. Pipe vesicles, therefore, grow into the lava as “cold fingers.” They cease growing when separate gas bubbles nucleate ahead of them in the ever broadening zone of solidification. Lava need contain no more than 0.01 wt% H 2 O for pipe vesicles to form in this way.

Differentiation of Mesozoic basalts of the Hartford basin, Connecticut

The Talcott, oldest of the three basalts in the Mesozoic Hartford basin of Connecticut, contains abundant euhedral olivine and plagioclase phenocrysts and less common rounded augite-plagioclase aggregates and orthopyroxene phenocrysts rimmed by augite and olivine. The orthopyroxene, which has a remarkably constant composition (En 84 ) and is Cr-rich, is believed to be a refractory residue from an upper-mantle source region. The Holyoke basalt, the middle unit, is essentially aphyric, with only minor plagioclase and olivine phenocrysts. The Hampden, youngest of the three basalts, contains abundant plagioclase and minor olivine and augite phenocrysts. Materials-balance calculations using phenocryst compositions from the Talcott basalt indicate that the Holyoke can be derived from a Talcott magma by addition of 7.8% orthopyroxene and removal of 7.9% olivine, 15% clinopyroxene, and 13.3% plagioclase. The Hampden basalt cannot be derived from a Holyoke magma by any reasonable fractionation scheme, but it can be derived from a Talcott magma by removal of 4.4% olivine, 12.0% clinopyroxene, and 14.3% plagioclase. One-atmosphere melting experiments under controlled oxygen fugacities indicate successively lower liquidus temperatures for Talcott, Holyoke, and Hampden basalts. Both the Talcott and the Holyoke have olivine and plagioclase on the liquidus simultaneously, with augite and pigeonite appearing ∼15 °C below the liquidus. The Hampden has plagioclase alone on the liquidus; olivine and augite appear only 5 °C below the liquidus. Orthopyroxene is not present in the low-pressure experiments but is thought to have been stable on the liquidus at depths corresponding to pressures exceeding 8 kbar. Assimilation of orthopyroxene and crystallization of other phases necessary to produce the Holyoke basalt from a Talcott magma must have taken place at depth; however, because the low-pressure phases obtained in the experiments are those required to produce the Hampden from the Talcott magma, this fractionation likely occurred in a near-surface magma chamber.

Quantitative measures of textural anisotropy resulting from magmatic compaction illustrated by a sample from the Palisades sill, New Jersey

Abstract Early in the crystallization of many tholeiitic basaltic magmas, plagioclase crystals cluster together into a 3-D cellular network, which forms a passive marker capable of recording the deformation that accompanies compaction of crystal mush. Although irregular in detail, the overall network is initially isotropic and only becomes anisotropic as a result of compaction. We have developed four independent methods to quantify the 3-D textural anisotropy of a basalt sample using at least three non-parallel thin sections. Three of the methods are based on the geometrical properties of digitized maps of the feldspar chain networks. One approach focuses on the angular variation of the mean intercept along parallel traverses through the network, another examines the orientation and size distribution of individual links, and the third considers the average shape of interstitial regions outlined by the plagioclase network. The fourth technique approximates the textural anisotropy by the variogram anisotropy of a scanned thin section image. We illustrate the methods using five oriented non-parallel thin sections from a sample of diabase 146 m above the base of the 300-m-thick Palisades sill of New Jersey. Compaction of crystal mush in this sill has previously been postulated on the basis of chemical evidence. The 3-D feldspar network anisotropy based on the first three approaches suggests nearly uniaxial compaction on the order of 8.6% in a direction within 3° of the intersection of the columnar joints at the sample site. A rigorous statistical test based on the statistics of elliptically contoured non-normal multivariate distributions documents that the link-vector distribution in vertical sections are statistically anisotropic at a 95% confidence level and that the overall compaction is 7.9±2.6%. The orientation and magnitude of the 3-D textural anisotropy determined by the image variogram of the non-opaque minerals is almost identical to the mean feldspar network anisotropy; 8.5% compaction in a direction 10° from the columnar joint intersections. The major silicate textural and feldspar network anisotropy axes both plunge almost directly down dip of the sill. On the other hand, the major axis of the variogram anisotropy of the opaque minerals is approximately parallel to the strike of the sill and to the major axis of the anisotropic magnetic susceptibility. The anisotropy of the silicate mineral fabric may reflect down-dip flow of a deformable melt-rich crystal mush, whereas the AMS and opaque textural anisotropy reflects the influence of gravitational stresses during the growth of magnetite in the final stages of melt crystallization. Evidently the Palisades sill was not originally horizontal but was intruded in an orientation close to its present attitude.

The Electronic Total Station – A Versatile, Revolutionary New Geological Mapping Tool

The “electronic total station,” which is now used routinely by engineering surveyors, provides geologists with a remarkable new tool that can accurately measure (±5 mm over 1 km) in a few seconds the position of points relative to the instrument, either as angles and distances or as x, y, and z coordinates in a given reference frame (for example, easterly, northerly, and vertically) and can download its measurements to a portable computer. The speed and accuracy of data collection makes possible not only the construction of geologic maps directly in the field but also the measurement of the attitude of structural features such as layering and lineations, thus dispensing with a relatively inaccurate compass. In addition to providing a more accurate and rapid way of doing routine mapping such as that done previously with an alidade and plane table, the instrument offers the possibility of doing completely new types of mapping, such as measuring the flow directions and velocities in a river by tracking a flo...