Extreme enriched and heterogeneous 87Sr/86Sr ratios recorded in magmatic plagioclase from the Samoan hotspot

Abstract

iv to think about the geochemical data, and assisted with making interpretations that are reasonable given his deep knowledge of magma mixing, petrology, and thermodynamics. I would like to thank the Earth Science graduate students at UCSB, who are generally incredibly helpful, supportive, positive, and constructive. Pet Food seminars were helpful when first formulating the ideas that are presented in this thesis, and informal discussions with many students were incredibly helpful. Andrew Reinhard deserves special recognition for his help thinking about isotope geochemistry and grappling with intra-lava heterogeneity. And perhaps most importantly, I would like to thank my partner Elissa McBride, who was incredibly supportive throughout my graduate career as I struggled with technical challenges, traveled for months at a time for fieldwork and mass spectrometry, or shirked social obligations to grade papers or make a deadline.

Trachytic Melt Inclusions Hosted in Clinopyroxene Offer a Glimpse Into Samoan EM2‐Endmember Melts

“…One clinopyroxene-hosted MI has two partially resorbed Fo 73 olivine crystals, suggesting interaction of the trachytic melt with a mafic magma as the olivines and the surrounding trachyte melt are not in equilibrium. This observation is consistent with two-component mixing between a mafic (olivine-bearing) magma and a much more evolved magma that has been inferred from relationships between major and trace element compositions and radiogenic isotopic ratios in the Samoan magmatic suite alluded to above (Adams et al, 2021;Edwards et al, 2019;Jackson et al, 2007). This raises several important questions: what is the origin of these high-silica MIs?…”

“…While EM2 has high 87 Sr/ 86 Sr and EM2 lavas at Samoa are relatively evolved, previous work has shown that EM2 cannot be derived from shallow contamination by assimilation of sediments (Jackson et al, 2007) or oceanic crustal material, given that oceanic crust has insufficient radiogenic 87 Sr/ 86 Sr to produce the values of the ALIA‐D115 lavas. Furthermore, the Pb isotopic compositions of modern marine sediments in the Samoan region and the ALIA‐D115 lavas form a diverging, non‐overlapping trend such that the highest 87 Sr/ 86 Sr lavas are furthest from the marine sediment field (see the following work for more detail: Jackson et al., 2007; Edwards et al., 2019). Adams et al.…”

“…The mixing line captures the ALIA-D115 lavas quite well. The contribution of a trachytic MI composition to the mixture that is required to generate the most radiogenic (highest a low 87 Sr/ 86 Sr, mafic endmember and a high 87 Sr/ 86 Sr, evolved composition representing the EM2-derived endmember (Adams et al, 2021;Edwards et al, 2019;Jackson et al, 2007). While EM2 has high 87 Sr/ 86 Sr and EM2 lavas at Samoa are relatively evolved, previous work has shown that EM2 cannot be derived from shallow contamination by assimilation of sediments (Jackson et al, 2007) or oceanic crustal material, given that oceanic crust has insufficient radiogenic 87 Sr/ 86 Sr to produce the values of the ALIA-D115 lavas.…”

“…While EM2 has high 87 Sr/ 86 Sr and EM2 lavas at Samoa are relatively evolved, previous work has shown that EM2 cannot be derived from shallow contamination by assimilation of sediments (Jackson et al, 2007) or oceanic crustal material, given that oceanic crust has insufficient radiogenic 87 Sr/ 86 Sr to produce the values of the ALIA-D115 lavas. Furthermore, the Pb isotopic compositions of modern marine sediments in the Samoan region and the ALIA-D115 lavas form a diverging, non-overlapping trend such that the highest 87 Sr/ 86 Sr lavas are furthest from the marine sediment field (see the following work for more detail: Jackson et al, 2007;Edwards et al, 2019). Adams et al (2021) present high precision Sr and Nd isotope measurements on individual clinopyroxene porphyrocrysts from the ALIA-D115-18 whole rock lava, and reveals the highest 87 Sr/ 86 Sr (0.723888) ever recorded for the oceanic mantle (for comparison, the highest 87 Sr/ 86 Sr for an ALIA-D115 whole rock is 0.720469; Jackson et al, 2007).…”

“…This lava, ALIA‐D115‐18, was dredged off the submarine flanks of Savai'i Island, western Samoa, and similar to other lavas from the same dredge haul—(ALIA115 dredge haul) showing the most geochemically enriched 87 Sr/ 86 Sr values ever measured in relatively fresh whole rock ocean island basalts (OIB)—this lava exhibits the “enriched mantle 2” (EM2) geochemical signature (Jackson et al., 2007). The EM2 signature is believed to be the result of addition of ancient subducted terrigenous sediment to the mantle source region (Adams et al., 2021; Edwards et al., 2019; Jackson et al., 2007; White & Hofmann, 1982). However, important questions remain regarding the composition of EM2 or its source, including the precise composition of the EM2 endmember, the petrogenetic history of EM2‐derived OIB melts, and how these melts mix with non‐EM2 melts in magma chambers.…”