University of Rochester, Department of Earth and Environmental Sciences
New York kimberlites are one of rare examples of igneous activity in the geological history of the state. Dates for the ages of these rocks range from 160 to 135 Mya during the Jurassic to Cretaceous periods (Bailey et al., date unknown). The origin of kimberlites begins with an unstable mixing of CO2 and H2O in the deep crust that rise through igneous dykes that reach the surface through diatreme volcanism. The breadth in which kimberlitic eruptions occur through multiple layers of the Earth’s crust creates a wide mineralogical composition for the kimberlites that end up in the surface. They include accessory minerals of olivine, clinopyroxene, garnet, phlogopite, and perovskite. The most famous kimberlites are in South Africa which host most of the worlds natural diamonds. Unfortunately, New York kimberlites have no diamonds. The two kimberlites analyzed here come from two localities from around the Ithaca area: Williams Brook and Taughannock Falls. The kimberlites were collected from exposed dikes that intrude the youngest rock in each area (typically Devonian shales and limestones) in the Fall of 2017.
Below are images of the rocks in the field area of Taughannock Falls and Williams Brook, resptfully. In Figure 2, the kimberlites are the pale brown intrusions that penetrate the darker and older shale. In figure 3, the darker more weathered Williams Brook kimberlites
Several techniques were used in the process of studying these samples. Firstly light miscroscopy was done on the samples to identify areas of interest for EM analysis. Next, sputter coating was used to prepare the samples for electron microscopy. EDAX spectra and elemental maps were made from the accessory minerals in the kimberlites.
Light microscopy was used to identify areas of interest and minerals to look out for when performing the SEM. One advantage of light microscopy is that images retain their color. Light microscopy, however, does not provide much depth of field and since my samples were not thin sections but rough pieces of rock, obtaining an image without blury spots was near impossible.
Prior to any electron imaging, the two samples required some preparation through the use of sputter coating.
Scanning Electron Microscopy and Xray Spectra Graphs
Scanning electron microscopy (SEM) is a tool used for imaging specimens using an electron beam. The image is formed as a result of the interaction of the sample with the beam. One of the most common imaging modes used in SEM is secondary electron (SE2) imaging. Secondary electrons are generated from an inelastic scattering event. These types of electrons are emitted from the upper portion of the interaction volume. They are commonly used for imaging the surface structure of materials and can provide excellent resolution. All images were collected on a Zeiss Auriga CrossBeam SEM-FIB
Rounding and strong deformation characterize all of the phlogopite macrocrysts. Chlorite, calcite, and Fe-Ti oxides sometimes replace some crystals. Since phlogopite is a sheeted mica (KMg3(AlSi3O10)(F,OH)2) the SEM image clearly shows the many layers that create the crystal habit of this mineral. The xray spectra indicate typical elemental compositions of phlogopite including high Mg, Al,K, and Si abundances. Of note is the indication of a relatively high abundance of Ti. This feature is only known to exist in the Williams Brook kimberlites. More discussion on the significance of this high Ti composition is in the Xray mapping portion of this project.
In regards to the perovskite crystals, it proved difficult to x-ray these blocks because none of them were parallel to the electron beam so there was a lot of x-ray scattering. From just an image analyses, however, it is easy to spot penetration twining where individual crystals pass through each other in a symmetrical manner. It could be confused with calcite but calcite does not have such well defined cubic crystal structure. Perovskite also occurs as very small less than 0.1mm crystals in most kimberlites
The clinopyroxene x-ray spectra indicate a high mix of Mg, Ca, Fe which obscures the exact mineral type. It could perhaps be diopside, hedenbergite, or augite. Notice the phlogopite xray map does not reveal any significant Ti quantities, in contrast to the Williams Brook kimberlites.
To better understand the distribution of elemental abundances in accessory minerals and their magmatic history, xray maps where made. Particuarly I focused on Mg, Fe, and Ti distribution within a phlogopite crystal in the Taughannock Falls kimberlite. As mentioend previously, high Ti phlogopite has been found in only this locality, indicating something about its history of crystalization. These phlogopite grains crystallized from kimberlite melts and the high Ti zones originated from earlier kimberlite melts at mantle depth (Giuliani, 2016).
Despite the extensive alteration and crustal contamination that occurs in kimberlites, the this analaysis shows individual dikes have distinct geochemical signatures. Particularly we see that with the phlogopite in the Williams Brook kimberlites contains high Ti content, perovskite crystals, and no pyrope (Mg rich garnets) or cinopyroxenes. The Taughannock Falls kimberlites have low Ti phlogopites, pyrope, and clinopyroxene macrocrysts. The intrusions seen in Williams Brook have yielded U–Pb crystallization ages of ~ 146 Ma while the Taughannock Falls kimberlites are younger at around 125-110 Ma (Heaman and Kjarsgaard, 2000). It is clear there was some chemical heterogeneity in the subcontinental mantle that upon eruption in the two different localities at different times crystallized geochemically different kimberlites. The petrological diversity indicatess a migration of melts that were instigated by the failed St Lawrence Rift, which lies on strike with the kimberlite fields of Ithaca
Works CitedPaper citations
Giuliani et al., 2016. “Constraints on kimberlite ascent mechanisms revealed by phlogopite compositions in kimberlites and mantle xenoliths”. Lithos: V 240-243. 189-201
Carsell, D.A., 1975. “Primary and secondary phlogopites and clinopyroxenes in garnet Iherzolite xenoliths”. Physics and Chemistry of the Earth 9:417-429.
Bailey et al., date unknown. “Kimberlitic Rocks of Central New York”. Manuscript uploaded to academia.edu. Accessed 04/28/2018.
Heaman LM, Kjarsgaard BA (2000) Timing of eastern North American kimberlite magmatism: Continental extension of the Great meteor hotspot track? Earth and Planetary Science Letters 178(3-4):253-268
Basu, et al. 1984. “Sm-Nd, K-Ar and petrologic study of some kimberlites from eastern United States and their implication for mantle evolution”. Contrib Mineral Petrol 86: 35-44.
Background Image: from Blog post: New York State Geologic Maps Author: The New York State Museum New York. Posted July 12 2017.
Figure 1: from blog post: What are Kimberlites? Author: Diamond Engagement Rings. Posted April 30 2009
Thank you to New York State for providing me with interesting kimberlites for my project!
I would also like to thank Brian McIntyre for always being available to help me and make my imaging a lot better and my TA, Caleb, for helping me with the SEM labs.