Emily+-+Newfoundland+Harzburgite

Emily Simmons Spring 2010 **GSC 440 LAB** //Report on Harzburgite//

This Harzburgite sample is from the Bay of Islands ophiolite complex, in Newfoundland, Canada (Figure 1). Today, this assemblage has been uplifted and exposed, titled as "Table Mountain" (Firgure 2). Harzburgite is an ultramafic igneous rock; a variety of peridotite. It consists mostly of the two minerals, olivine and low-calcium pyroxene (enstatite), and can have accessories of chromium rich spinel. Harzburgite got its name after the Harz Mountains in Germany. Harzburgite typically forms by the extraction of partial melts from the more pyroxene-rich peridotite called lherzolite, which is also thought to represent oceanic crust and the underlying oceanic mantle. The outcrops at the Bay of Islands is exposed due to a continental collision event. Roughly 486 million years ago, in the Paleozoic era, this oceanic crust - mantle boundary got forced onto continental crust.
 * Background Information**

Figure 1: Map of Newfoundland with the Bay of Islands boxed in red

Figure 4: Oceanic crust cross section

At these harzburgite outcrops, gabbros are found at the tops of the mountains (Figure 3). This helps confirm the uplifting theory, since it follows the oceanic crust sequence (see above Figure 4). Usually there would lie pillow basalts above the gabbros, but this has all been weathered away with time. A near by valley shows lower down into the sequence, where the mélange is exposed (Figure 4).



Figure 2: Hiking up Table Mountain

Figure 3: Gabbro at the top of Table Mountain

Figure 4: Emily Simmons standing in front of the Melange

The most abundant minerals in this igneous rock are olivine and pyroxene. While it is not evident by color in the particular sample being studied, many other Harzburgite outcrops in the area of collection are dark to light green showing that they've been serpentinized. This is because the iron-magnesium-silicate in olivine reacts with water to create serpentine. "It is argued that the high-Mg content of the serpentine minerals in serpentinized peridotite is a consequence of the environmental Fe2+Mg–1 exchange potential imposed on the system by the abundance of olivine and orthopyroxene" (Evans 2007).

.

The tan surface of Harzburgite (figure 5), is a weathered feature from oxygen and water. This is the same process that creates the serpentinized surfaces more evident in other samples. The tan color extends roughly 2cm into the rock (Figure 6). Small, reflective crystals of pyroxene can be seen when examined closely.
 * Hand Sample**

Figure 5: Harzburgite sample

Figure 6: Shows 2cm weathered surface

Thin sections show a relatively tan colored sample with apparent porpheritic texture. There exists a large portion of sub​hedral pyroxene crystals (evident by cleavage), and an abundance of olivine grains. Serpentine is also present, and concentrates in veins. Though difficult to see in plain polarized light, serpentine becomes gray in cross polarized light. Smaller portions of iron oxides are found (within and associated with the serpentine), notable as the only opaque mineral in the thin section. See the following Figures below (Figures 7, 8, 9)
 * Thin Section**





The olivine grains in thin section best represent the original material of the rock (before metamorphism) and although grain sizes appear to vary, the protolith was most likely phaneritic in texture. The concentration of serpentine along cracks and veins is most likely because these were the areas where water was able to percolate, thus enabling serpentinization to occur.
 * Interpretation**


 * Sources**

http://www.absoluteastronomy.com/topics/Peridotite http://en.wikipedia.org/wiki/Harzburgite

By Yildirim Dilek, et al. //Ophiolites and oceanic crust: new insights from field studies and the ocean drilling program// http://books.google.com/books?id=p-7LcuX45oIC&pg=PA57&lpg=PA57&dq=Bay+of+Islands+ophiolite+age&source=bl&ots=ns346skWhP&sig=SUUUXkHsqEGbn0nJsfKNrDbsSJA&hl=en&ei=PRDHS8v7B4uC9ATvvuHsCg&sa=X&oi=book_result&ct=result&resnum=4&ved=0CBcQ6AEwAw#v=onepage&q=Bay%20of%20Islands%20ophiolite%20age&f=false

Evans, Bernard W. Control of the Products of Serpentinization by the Fe2+Mg–1 Exchange Potential of Olivine and Orthopyroxene. 2007. http://petrology.oxfordjournals.org/cgi/content/full/egn050v1