Date of Award

Fall 1-1-2016

Document Type

Thesis: Open Access

Degree Name

MS Marine Science

Department

Environmental and Ocean Sciences

Committee Chair

Beth O'Shea

Committee Co-Chair

Meagen Pollock

Committee Member

Eric Cathcart

Abstract

Arsenic (As) is enriched in the Santiago Peak Volcanics (SPV) that crop out at Black Mountain Open Space Park in San Diego, CA. Thus, the Black Mountain SPV present an excellent opportunity to investigate As enrichment in ancient volcanic arc systems as a result of deep earth subduction zone processes. Through targeted field study of this unit, we aim to (1) confirm the volcanic arc provenance of rocks cropping out at Black Mountain, (2) investigate As enrichment in relation to volcanic arc processes, and (3) propose a model for As occurrence in the SPV. Our findings indicate Black Mountain SPV evolved from the same magma system as Northern Santa Ana Mountain SPV during the Cretaceous, and that As content varied between the two dominant SPV rock types, with rhyolites containing more As (n = 10, mean = 586 ± 10 ppm As) than dacites (n = 10, mean = 14 ± 9 ppm As). Our proposed model for As enrichment in SPV suggests that As is among the first elements released during serpentinite dehydration (along with Pb), while less soluble elements (e.g. Cu) are released during later stages of subduction. Once As and Pb are released from serpentinites and incorporated into partial melts, we hypothesize that such elements behave like typical incompatible elements and remain dissolved in melts until they are eventually exhumed in rhyolitic lavas. Since As may dissolve out of volcanic host rocks in groundwater aquifers, this enhanced understanding of As distribution among ancient volcanic arc rocks may aid in identifying alternative drinking water sources in regions with known groundwater As enrichment.

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