Graduation Year

2023

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Geology

Major Professor

Jeffrey Ryan, Ph.D.

Committee Member

Zachary Atlas, Ph.D.

Committee Member

David Peate, Ph.D.

Committee Member

Timothy Conway, Ph.D.

Keywords

Alteration, Amphibole, Basalt, Boninite, Quadrupole

Abstract

The development of a method for lithium isotopic measurement was developed using a quadrupole, single-collector ICP-MS. Measured precision was ±1.00‰ 2σ over a 15-month period based on measurements of international rock standards. A 1-hour leach using cold, HCl was sufficient to remove alteration-related Li in minimally to moderately altered seafloor basalts. Measurements on the Holocene ages lavas from the neovolcanic zones of Iceland suggest no systematic Li isotopic variability among the differing Icelandic sources. Felsic samples from Hekla and Pumice contain elevated Li contents of >18ug/g while the tholeiitic and alkaline basalts are <8ug/g Li. Most of the island lies around MORB values of ~+3 to +5‰ with the exception of Western Volcanic Zone samples that show evidence for seawater inputs. The RN-17 drillcore, which penetrates >3000m through a hydrothermal system in the Reykjanes Peninsula, shows δ7Li changes with depth. based on temperature and mineral assemblage, with no overall changes in Li abundance. Samples from <800m show δ7Li up to +8.5 ‰, while deeper chlorite-epidote and epidote-actinolite facies sections preserve MORB-like values. The deepest amphibolite-facies sections of the core range to the highest observed δ7Li (up to +12 ‰). A subduction initiation volcanic sequence of forearc basalts (FABs) and boninites from the Izu-Bonin subduction system, which were sampled during IODP Expedition 352, have elevated Li/Yb ratios, suggesting subduction-related Li enrichments. δ7Li in FABs varies, from values up to +10‰ in the deepest samples to MORB-like values upsection. Boninite δ7Li signatures are mostly higher than MORBs, reaching values of +14‰, with no clear difference between older Low Silica (+8.4‰) and late-stage High Silica boninites (+8.0‰). δ7Li systematics and Li-trace element ratios point to the influence of a higher-temperature slab-derived component in boninite and FAB petrogenesis consistent with the involvement of amphibole, a finding that is to a first order consistent with observations in the altered Icelandic basalts.

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