Date of Award

Summer 8-20-2021

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Alicia Cruz-Uribe

Second Committee Member

Scott Johnson

Third Committee Member

Martin Yates

Abstract

The sulfur isotope composition of sulfides in igneous rocks from magmatic arcs can be difficult to constrain due to the tendency for sulfur isotopes to fractionate during magma ascent and degassing. Cumulate xenoliths from the lower arc crust are representative of the most primitive magmas of the arc system. The Lesser Antilles arc presents a unique opportunity to investigate the sulfur isotope composition of arc magmas along a magmatic arc with many different magma plumbing systems. Here we present sulfur isotope compositions for primary magmatic sulfide inclusions and secondary interstitial hydrothermal sulfides in mafic cumulates from ten islands along the Lesser Antilles arc. Cumulate rock types investigated here include amphibole gabbros, gabbros, and amphibole gabbronorites. Primary magmatic sulfide inclusions of pyrrhotite and chalcopyrite are hosted in amphibole, plagioclase, clinopyroxene, orthopyroxene, and spinel. Pyrrhotite and chalcopyrite are also present as secondary interstitial sulfides that are texturally distinct from the cumulate assemblages and likely formed during hydrothermal infiltration and degassing of host volcanic rocks. Sulfur isotope compositions (34S, ‰ relative to VCDT) of pyrrhotite and chalcopyrite were determined in 73 magmatic inclusions from 8 islands and 39 interstitial sulfides from 4 islands by secondary ion mass spectrometry (SIMS) at the WiscSIMS facility, University of Wisconsin, using a 3 μm beam size. 34S values in magmatic sulfides from the islands of St. Kitts, Guadeloupe, Dominica, St. Vincent, Carriacou, Ronde, and Grenada range from −2.3 ‰ to +4.5 ‰. The 34S values for magmatic sulfides from these seven islands are consistent with the range of 34S values for melt inclusions reported from St. Vincent (−9 to +7 ‰, avg. 1.1 ‰; Bouvier et al., 2008). Within any given island the total range of magmatic sulfide 34S values is 4.4 to 6.5 ‰. No correlation is observed between 34S and host mineral chemistry (i.e., An content of plagioclase, pyroxene Mg#, AlIV in amphibole). In this thesis I suggest that the ~5 ‰ spread of 34S values within any given island can be explained by fractionation due to the separation of an immiscible sulfide melt combined with crystallization of silicate minerals. I apply coupled Rayleigh crystallization and isotope fractionation models by assuming an initial starting 34S value of −1.4 ‰ for the silicate melt and applying the resulting melt composition from the Rayleigh crystallization model into an isotope fractionation model to simulate sulfide separation from silicate melt. Up to 5.16 ‰ fractionation of sulfur isotopes can be achieved during sulfide liquid separation with ~25 % silicate crystallization at 1100 °C, which could account for the spread of 34S values observed along the arc. Four analyses of a single magmatic sulfide from Bequia give an average 34S value of −8.7±0.41 ‰. The more negative 34S values from Bequia likely represent a biologically fractionated sulfur signature from subducted sediments, indicative of the higher proportion of subducted sediment contribution to magma chemistry on this island. 34S values in interstitial sulfides from St. Kitts, Antigua, St. Vincent, and Canouan range from −1.26 to +15.57 ‰. The high positive 34S values are likely due to fractionation during degassing and ascent of the host magma.

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