Kimberlitic Rocks of New York State

Western New York State is noted for its flat-lying, sandstones, limestones, dolostones, siltstones, and shales. However, some very interesting igneous rocks can be found as dikes* intruding these Paleozoic sedimentary rocks in a narrow band that extends from the vicinity of Ithaca, through the Syracuse area, and beyond to Ogdensburg on the St. Lawrence River. The greatest concentration of known dikes is centered near Ithaca, along the southern portion of the Cayuga Lake valley. This is part of a belt of similar rocks that trends north-south from Tennessee to Quebec.

These unusual rocks occur as narrow dikes that formed as molten rock from the upper portion of the earth's mantle pushed upwards into joints (vertical cracks) in the surrounding rock, then cooled, and solidified. The dikes range in size from an inch (2.5 cm.) or less, to 195 feet (60 m.) in width.

Originally called serpentine or serpentinite, the rocks were first discovered in 1837 within the city of Syracuse. Other occurrences were soon located in West Canada Creek near Little Falls to the east, and in Ludlowville to the south. Then, in 1887, similar rocks were described from the South African diamond fields and named "kimberlite," after Kimberly, South Africa. These kimberlites are well known because their magmas brought diamonds that formed deep in the mantle to the surface. Hence, the South African kimberlite is famous, while the similar "kimberlitic rocks" of New York are little known.

Although the kimberlite from both Kimberly and North America have been largely altered to serpentine, their original composition was largely olivine (peridot) and phlogopite mica; hence, they have been called mica peridotites. Detailed study of thin sections of the rock has revealed its composition as crystals (2-15 mm.) of olivine, phlogophite, garnet, phlogopite, and spinel, which are floating in a fine-grained groundmass of phlogopite, calcite, serpentine, perovskite, and magnetite. Other minor minerals that may be present in some specimens include clinopyroxene, clinoamphibole, epidote, chlorite, barite, celestine, spinels, ilmenite, pyrrhotite, pentlandite, and pyrite. However, one mineral that is found in the South African kimberlites but has never been found in those from New York is diamond. Furthermore, the mineral assemblage in our "kimberlites" indicates that they formed in the uppermost portion of the mantle, and were too cool for diamonds to be stable. Therefore, diamonds are not expected to be present in any of the currently known New York dikes.

Although igneous rocks characteristically contain radioisotopes that make it possible to determine their ages, several factors make it difficult to date these kimberlitic rocks. For example, different crystals in a sample formed at different times, and sometimes chunks of crystalline rock from the lower crust or upper mantle can become incorporated in the kimberlitic magma as it works its way upward. This results in an incorrect date if portions of it are sampled. Furthermore, post-crystallization alteration and weathering makes it difficult to extract suitable material for dating. Because of these difficulties, various researchers have obtained dates ranging from approximately 104 - 439 million years for these rocks. Because the youngest rocks that they intrude are around 370 million, clearly the oldest date is incorrect! General consensus for the intrusion of the dikes is from the late Jurrasic to the early Cretaceous periods, or roughly 160 to 135 million years ago.

*dike - a tabular body of igneous rock that cuts across the structure of adjacent rocks.

Reference:
Bailey, David G. and Lupulescu, Marian, 2007, Kimberlitic Rocks of Central New York, Trip A-3: New York State Geological Association 79th Annual Meeting Field Trip Guidebook, p. 53-81.


The images below illustrate both field occurrences, and collected specimens, of a few of the more than 80 kimberlitic dikes that are currently known to occur in central New York State from Syracuse to Ithaca.  There are undoubtedly many more dikes buried below glacial cover, under water, or that just haven't been recognized yet.
  Field and specimen images by Paul Dudley; photomicrographs courtesy of David Bailey.


This is all that's left exposed of the original "serpentine body" that was discovered by Oren Root on Green Street, Syracuse in 1837.  This highly serpentinized (chemically altered) mica peridotite apparently is the first description of this type of rock anywhere in the world.  Such rocks are now generally known as kimberlites, although the term kimberlitic may be a better word due to a lack of a definitive definition of what should constitute a true kimberlite.

This small portion of the outcrop is being engulfed by a box alder tree on a vacant city lot, just north of the Syracuse University campus, and is in danger of being forever lost to urban development.


The right-hand portion of this thin section under crossed polarizing light shows the interlocking crystals of a piece of deep crustral rock that was torn lose and incorporated in the magma as it moved upward to form the Green Street dike.  Such chunks of "country rock" incorporated in an igneous rock are called xenoliths, or foreign stones. The left-hand portion of the image is the serpentinized kimberlite.

Several large chunks of loose "kimberlite" were found near the outcrop pictured below and salvaged by participants on the NYSGA field trip to the locality on 28 September 2007.  This freshly broken surface shows large dark crystals enclosed in a the fine groundmass.  The larger crystals are primarily olivine that has been largely altered to serpentine through the addition of water.  However, some of the crystals may have cores of unaltered olivine.

    Olivine                    Serpentine
Mg,Fe2SiO4              Mg3SiO5 (OH)4


This specimen shows a freshly broken surface on the left, and a weathered surface on the right.  As these rocks weather they tend to more closely resemble the mainly grey sedimentary rocks in which they intrude.  This makes it difficult to recognize them in the field.

The brown mineral in this thin section from the Dewitt Reservoir site is phlogopite mica.

This Dewitt thin section shows a serpentinized olivine crystal in the upper-left, and a pyrope garnet with a dark reaction ring in the lower-right.  The remainder of the rock is the fine-grained ground mass primarily of serpentinized olivine, magnetite, phlogopite mica, and diopside.


This is the side of the valley wall in a small stream near Clintonville, NY.  The area bounded by the yellow lines contains three distinct, roughly parallel, dikes.  The main reason the area containing the dikes can be recognized may be due to less vegetation growing on the kimberlite than on the surrounding shales.  Also, the dike area is slightly incised with respect to the surrounding rocks.


The rock in the center is one of the three dikes in the Clintonville dike zone outlined in the previous picture.  Note the distinct contacts between the kimberlite and the surrounding grey shale. This dike is about a foot wide.


The large, boat-shaped crystal in this Clintonville dike thin section is a very well formed olivine crystal that has been altered to serpentine.

 


Several dikes are exposed in the bed of Taughannock Creek, both above and below the 215 foot waterfall. The brownish colored rocks shown here are weathered kimberlite.  Note that the dike splits into two, which is a common occurrence.  Taughannock Creek flows into the west side of Cayuga Lake a few miles north of Ithaca.

This is a portion of a Taughannock Creek dike shows the difference in color between the weathered dike rock and the intruded grey shale.

A specimen showing a fresh surface of the Taughannock Creek dike.  Note that the groundmass has become brown, whereas the olivine and clinopyroxene (diopside) crystals have remained dark.  The grey rock at the bottom is shale.

Thin section of Taughannock Creek dike with some of the larger crystals labeled.  Small black crystals are spinels (metal oxides in the isometric, or cubic, crystal system, e.g., magnetite, chromite, etc.).

This dike on the floor of Glenwood Creek, about 3 miles south of Taughannock Creek, eroded a bit slower than the surrounding shale. Therefore, this dike shows up as a slight dam extending across the creek during times of low water.

This highly weathered dike is difficult to recognize, but it begins just to the right of the green moss on the left side of this picture and extends to near the tree on the right.  This photo again illustrates how difficult it can sometimes be to identify these dikes in the field.

During our club field trip to the Cargill Corp. salt mine in Lansing this summer--just across the lake from the Ithaca area dikes shown above-- we were told in response to a question that the underground salt mining had not encountered any of these dikes.  However, it turns out that this is not true.  At least one dike has been intersected and studied.  It occurred at a depth of 0.4 mile, and was exposed for 300 feet in the mine workings.  This sample of kimberlite and surrounding halite was collected in the mine, and is in the collection of the New York State Museum.


The Portland Point limestone quarry is located just above some of the older underground salt mine workings. This specimen was collected at this quarry, and show the kimberlite and the limestone host rock. The presence of this dike in the quarry led the mine geologists to predict its presence in the mine.

This thin section from a Portland Point sample is viewed under plain polarized light (polarized in one direction only).  Mineral crystals appear in their natural colors.  The large grey crystal in the lower-left is diopside with a reaction rim of spinels. The yellow crystals are serpentinized olivine.  The small dark, square-shaped crystals are probably perovskite (CaTiO3).

This is the same as the image to the left, but viewed under crossed polarized light.  The diopside appears bright blue because of its birefringence, which results from differences in the speed at which light is transmitted through different orientations of the crystal.  The colorful rectangular crystals are phlogopite mica, and the serpentinized olivines are very mottled looking.  The pale, pastel mineral is calcite (CaCO3), or perhaps magnesite (MgCO3).