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<p class="MsoNormal">Geology Thesis Defense - Daniel Sheikh - March 31, 2:00pm<o:p></o:p></p>
<p class="MsoNormal">Major Professor: Munir Humayun<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">Title: </span><span style="font-size:12.0pt;font-family:"Times New Roman",serif;color:black;text-transform:uppercase">Trace Element Constraints on processes and precursors of</span><span style="font-size:12.0pt;color:black"> </span><span style="font-size:12.0pt;font-family:"Times New Roman",serif;color:black;text-transform:uppercase">chondrule
formation in unequilibrated ordinary chondrites</span><span style="font-size:12.0pt;color:black"> <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">Abstract: <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt;color:black">Chondrules are silicate spherules in chondritic meteorites thought to have formed as free-floating melt droplets created by a transient heat source in the early solar system. Due to their ubiquity
in asteroids, determining the origin of chondrules is key to understanding the conditions and processes occurring within the early protoplanetary disk. Recent models for chondrule formation involve the formation of chondrules by nebular shock waves or by protoplanetary
impacts. Here, elemental abundances in 153 chondrules from four unequilibrated ordinary chondrites (UOCs) were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to test these models. Chondrules in UOCs are mixtures of mineral
phases within a glassy mesostasis. Obtaining a representative bulk chondrule elemental composition by a microanalytical technique is challenging since chondrules are mineralogically heterogeneous at the scale of an analyzed spot. This study, therefore, placed
reliance on inter-element ratios of incompatible elements (elements concentrated into the melt phase) that would be representative of the bulk chondrule composition. Most chondrules exhibit both positive and negative Ce and Eu anomalies, and some chondrules
even display resolvable negative Yb anomalies, indicating a chondrule forming process that occurred within a chemical microenvironment of varying oxygen fugacity (fO<sub>2</sub>). The modeled effects of planetesimal collisions within the early solar nebula
create rapidly varying fO<sub>2</sub> microenvironments in impact vapor plumes that begin oxidized but end reduced. Such models are necessary to explain the observed Ce-Eu-Yb anomalies. Aluminum-rich chondrules contain Nb depletions that indicate inheritance
from distinct precursors to ferromagnesian chondrules, possibly containing refractory inclusions. Only a few chondrules exhibit significant fractionations in incompatible refractory element ratios distinctive of differentiated precursors (Zr/Th, Zr/Hf, Nb/Ta),
indicating that most UOC chondrule precursors were derived from undifferentiated sources. Our data provide compelling evidence that chondrules from UOCs formed as a byproduct of planetesimal collisions between undifferentiated bodies before nebular hydrogen
dissipated. Models of chondrule formation that involved nebular shockwaves or planetesimal collisions after dissipation of nebular gas could not account for the Ce-Eu-Yb anomalies observed in UOC chondrules.</span><b><span style="font-size:14.0pt;color:black"> </span></b><span style="font-size:12.0pt;color:black"><o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><a href="https://fsu.zoom.us/j/94458380669">https://fsu.zoom.us/j/94458380669</a><o:p></o:p></p>
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<p class="MsoNormal">Meeting ID: 944 5838 0669<o:p></o:p></p>
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