Date of Award
Master of Science (MS)
Dunite, Serpentinite, Ultramafic, Vermont
Physical Sciences and Mathematics
Ultramafic rocks exposed on the Earth’s surface offer a rare opportunity to directly study the petrology of the upper mantle. The Appalachian Mountains of Vermont contain variably serpentinized ultramafic rocks that mark the suture zone for the Ordovician Taconic Orogeny. In southern Vermont, the serpentinized ultramafic rocks were subsequently metamorphosed to amphibolite facies during the Devonian Acadian Orogeny. Key localities of partially serpentinized ultramafic rocks and their surrounding lithologies in East Dover, Vermont were sampled to better understand the mineral petrogenesis and metamorphic history of the East Dover meta-dunite. This thesis documents the first occurrence of podiform chromitite, platinum group minerals, arsenic minerals, and metamorphic olivine in ultramafic rocks from the Vermont Appalachian Mountains. Rare chromitite occurs as pods within the dunite and is highly brecciated. Cr# (Cr/Cr+Al) is extremely high in podiform chromitite (0.7 - 0.9) suggesting that it formed via fluid/melt-rock interaction during partial melting of upper mantle peridotite in a supra-subduction zone setting. Rare inclusions of platinum group mineral alloys were found as inclusions in podiform chromitite. Many of these platinum group minerals contained arsenic. Small nickel arsenide minerals are common in serpentinized ultramafic rocks. Whole rock geochemical analyses (XRF) indicate much higher concentrations of arsenic in the more serpentinized samples, suggesting that arsenic was introduced into the ultramafic rocks during serpentinization. In the meta-dunite, the composition of olivine ranges from Fo92 in spinel inclusions to Fo96 in neoblastic olivine. Olivine neoblasts likely formed from serpentine dehydration during peak metamorphic temperatures associated with regional metamorphism during the Devonian Acadian Orogeny. Decussate amphibolite and late stage coarse acicular serpentine likely formed as the region slowly cooled following Devonian orogenesis. Recent near-surface lateritic weathering produced local areas of nickel mineralization. Chromite, platinum group minerals, and nickel minerals do not appear to occur in quantities sufficient for exploitation from the East Dover area. Our findings illustrate the need for an investigation into the mineralogy of other ultramafic bodies in Vermont to better understand the relative timing of mineral petrogenesis and the societal implications of the presence of arsenic bearing minerals in ultramafic rocks.
Brigham, John Mark, "MINERAL PARAGENESIS OF A PARTIALLY SERPENTINIZED DUNITE IN EAST DOVER, VERMONT" (2020). Theses - ALL. 388.
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