Research on threats to masculine gender identity reveals that they occur in a diversity of contexts. In conjunction with research on the content of masculine identity, there is converging evidence that manhood is seen as a status that men must work hard to earn and maintain (Vandello, Bosson, Cohen, Burnaford, & Weaver, 2008). In the defense of claims to manhood, men often perform behavior that restores their sense of masculinity in the short term, but has harmful future consequences for themselves and the people around them. While there is a growing body of work demonstrating this relationship, there is less known about why masculinity operates this way. Specifically, what aspects of the defense of masculinity lead men to ignore the harmful future consequences of their actions? Could it be that the precarious nature of masculinity motivates men to focus on the immediate contexts where their masculinity is threatened, and to ignore the future consequences of how they respond? Four studies tested this hypothesis. Study 1 examines whether the precarious nature of masculinity provokes a focus on immediate responses to threats, lest one's hard fought claim to manhood be lost. Relative to men whose gender identity was validated, men experiencing masculinity threats became less concerned with the future consequences of their behavior relative to their concerned with the immediate context. In contrast, women experiencing gender identity threats did not become more focused on their immediate context. Study 2 examined whether or not the precariousness of masculine identity is unique. Here I provide evidence that, relative to other social identities, masculinity is unique in that it is both highly valued and viewed as precarious. Study 3 tests directly whether or not the precariousness of masculinity is driving the reduction in focus on the future consequences of men's behavior. Here, I demonstrate that threatening an equally valued, but less precarious social identity (family identity) fails to reproduce decrements in men's focus on the future consequences of their behavior. This study demonstrates that it is the combination of the high value and high precariousness of a social identity--and not masculinity itself--that leads to a reduced focus on the consequences of men's responses to a threatened social identity. Finally, Study 4 demonstrates that reframing masculinity as a less precarious status can effectively attenuate men's myopic focus when their masculine identity is threatened.

We study games played between groups of players, where a given group decides which strategy it will play through a vote by its members. When groups consist of two voting players, our games can also be interpreted as network-formation games. In experiments on Stag Hunt games, we find a stark contrast between how groups and individuals play, with payoffs playing a primary role in equilibrium selection when individuals play, but the structure of the voting rule playing the primary role when groups play. We develop a new solution concept, robust-belief equilibrium, which explains the data that we observe. We provide results showing that this solution concept has application beyond the particular games in our experiments.

We report the major-element, trace-element, and 87Sr/86Sr compositions of six plagioclase crystals from two Samoan lavas with extreme EM2 isotopic compositions (ALIA-115-18 with whole-rock 87Sr/86Sr of 0.718592, and ALIA-115-21 with whole-rock 87Sr/86Sr of 0.720469). We employed laser-ablation split-stream mass spectrometry (LASS) to measure 87Sr/86Sr ratios, major-element, and trace-element concentrations in the same plagioclase crystal volume simultaneously. We find that two plagioclase crystals have extreme 87Sr/86Sr heterogeneity in excess of 5000 ppm. This range is over an order of magnitude greater than the reproducibility achieved for 87Sr/86Sr reference materials, including a homogeneous natural plagioclase crystal (± 350 ppm, 2 SD). In two of the plagioclase crystals, we identify the highest 87Sr/86Sr ratios (0.7224) ever measured in any fresh, mantle-derived ocean-island basalt (OIB) or OIB-hosted mineral phase.

When 87Sr/86Sr is plotted against Sr, the six plagioclase crystals form arrays that converge on a “common component” with a more extreme EM2 signature (i.e., with higher 87Sr/86Sr) than has been previously identified in whole-rock Samoan lavas or mineral separates. We use the occurrence of olivines (mean Fo = 74.5 ± 0.8, 2 SD) in the high-87Sr/86Sr zone of one plagioclase crystal to infer the bulk composition (Mg# = 46.8 ± 0.8, 2 SD) of the extreme EM2 magma from which the olivine and high-87Sr/86Sr plagioclase crystallized. We argue that a relatively evolved EM2 endmember magma mixed with at least one lower-87Sr/86Sr melt to generate the observed intra-crystal plagioclase isotopic heterogeneity.

Inferring that subducted terrigenous sediment gives rise to EM2 signatures in Samoan lavas, we estimate that the quantity of sediment necessary to generate the most-elevated 87Sr/86Sr ratios observed in the Samoan plagioclase is 7.4 ± 0.2% of the mantle source. We also estimate that sediment subduction into the mantle over geologic time has generated a sediment reservoir that constitutes 0.02% of the mantle’s mass, a much lower concentration than required in the EM2 mantle source. Even if subducted sediment is concentrated in large low-shear-velocity provinces (LLSVPs) at the base of the mantle (which constitute an estimated 7.7% of the mantle’s mass), only 0.25% of the LLSVPs are composed of sediment. This suggests the distribution of sediment in the mantle is heterogeneous, and the high relative abundance of sediment in the Samoan EM2 mantle is an anomalous relic of ancient subduction that has survived convective attenuation.

We explore play between groups where one member of each 2-person group dictates the play of that group and is therefore responsible for the payoff of the other group member. We compare this to play when the game is the same, but each person is playing only for herself. Consistent with the principle of responsibility-alleviation described in Charness (2000), we find that a sense of responsibility for the welfare of others has an effect. While the issue of responsibility does not appear to influence the decisions of 2/3 of the population, almost 90 percent of the remaining 1/3 of the population plays a less risky strategy when choosing for a group than when playing only for herself.

The Icelandic hotspot has erupted the highest terrestrial mantle-derived 3He/4He over a period spanning much of the Cenozoic, from the early-Cenozoic Baffin Island-West Greenland flood basalt province (49.8 RA), to the mid-Miocene lavas in northwest Iceland (40.2 to 47.5 RA), to Pleistocene lavas in Iceland’s neovolcanic zone (34.3 RA). This study provides a detailed geochemical data set—He-O-Sr-Nd-Hf-Pb isotopic compositions, as well as whole rock major and trace element concentrations—for a suite of 18 Baffin Island lavas. The Baffin Island lavas transited through and potentially assimilated variable degrees of Precambrian continental basement. We therefore use geochemical indicators sensitive to continental crust assimilation (whole rock Nb/Th, Ce/Pb, MgO) to identify the least crustally-contaminated lavas in the suite. Four lavas, identified as “least crustally-contaminated”, have high MgO (>15 wt.%) and Nb/Th and Ce/Pb ratios that fall within the mantle range (Nb/Th=15.6±2.6, Ce/Pb=24.3±4.3). These four lavas have 3He/4He up to 39.9 RA and mantle-like δ18O of 5.03 to 5.21‰, 87Sr/86Sr = 0.703008–0.703021, 143Nd/144Nd = 0.513094–0.513128, 176Hf/177Hf = 0.283265–0.283284, 206Pb/204Pb = 17.7560–17.9375, and are located on or near the 4.5 Ga Pb isotope geochron. The radiogenic isotopic compositions of the least crustally-contaminated Baffin Island lavas are offset to more geochemically depleted compositions compared to high-3He/4He lavas from Iceland, a shift that cannot be explained by continental crust assimilation in the Baffin suite. While Sr-Nd-Pb isotopic heterogeneity among high-3He/4He localities has been previously observed, this is an important observation of geochemically distinct high-3He/4He endmembers within a single hotspot. Additionally, the least crustally-contaminated primary melts from Baffin Island-West Greenland have higher mantle potential temperatures (1510 to 1630 °C) than global MORB primary magmas located far from hotspots (1320 to 1480 °C), which supports a hot, buoyant plume origin for these early Iceland plume lavas. These observations support the contention that the geochemically heterogeneous high-3He/4He domain is dense, located in the deep mantle, and sampled by only the hottest plumes.

Olivine-hosted melt inclusions provide a multitude of opportunities to glimpse magmatic processes at depth because olivine is an early-crystallizing mineral in mantle-derived melts, and melt inclusions often capture a period of magmatic history that is rarely recorded and often obscured by whole rock or matrix glass chemistry of lavas. My research uses geochemical analyses of olivine-hosted melt inclusions to differentiate between the influences of mantle source heterogeneity versus magmatic processes on the composition of mantle-derived magmas at oceanic hotspots.

(I) Olivine-hosted melt inclusions from Puʻu Wahi, Mauna Loa, Hawaiʻi have unusually high (Sr/Ce)N ratios though lack elevated Al2O3, suggesting that plagioclase was not assimilated. This was dubbed a “ghost plagioclase” signature. The high (Sr/Ce)N ratios have been interpreted in the past to be signatures of recycled plagioclase-rich oceanic crust in the mantle source. However, the 87Sr/86Sr of individual olivine-hosted melt inclusions, whether with a high or normal (Sr/Ce)N ratio, are within the range expected for Mauna Loa lavas. Thus, the high (Sr/Ce)N ratios were likely the result of diffusive interaction with gabbro beneath Mauna Loa and not from incorporation of recycled oceanic crust in the mantle source.

(II) In Chapter 2, I argue that the enriched mantle 2 (EM2) mantle source is just as damp with respect to H2O as depleted mantle (DM) or non-enriched mantle (non-EM) sources. Previous studies suggested that the EM2 source is dry with respect to H2O because lower H2O/Ce corresponded with higher 87Sr/86Sr in submarine glasses. The H2O/Ce ratios of Samoan olivine-hosted melt inclusions are similar to those of non-EM submarine glasses, suggesting that closed-system degassing, where both CO2 and H2O degas, led to lower H2O/Ce in Samoan pillow glasses compared to melt inclusions from the same seamounts. I use the correlation between CO2/Nb and (La/Sm)N in OIB and MORB glasses to estimate primary melt CO2 concentrations. In general, pillow glasses with higher 87Sr/86Sr have higher (La/Sm)N, so I predict high CO2 concentrations in the primary melts from EM sources, which leads to more extensive degassing compared to primary melts with lower CO2 concentrations. A major implication of this study is that pillow glasses from EM melts do not reliably retain the H2O content of the melts.

(III) Olivine-hosted melt inclusions from Oʻahu rejuvenated volcanism are extremely alkalic and incompatible trace element-enriched. The H2O/Ce of these inclusions is very low (12–63) and deviates from the OIB pillow glass array of lower H2O/Ce with higher 87Sr/86Sr (Oʻahu rejuvenated lavas have low 87Sr/86Sr), so it is not the enrichment of the mantle source that influences the H2O/Ce ratio in erupted glasses or melt inclusions, but degassing of H2O in melts. I propose that low-degree melts (with high (La/Sm)N) are more alkalic and CO2-rich. CO2-rich melts degas more H2O compared to CO2-poor melts, resulting in melts with low H2O/Ce compared to higher-degree melts that are tholeiitic and CO2-poor. Using mantle melting models, I find that the CO2 content of primary OIB melts is largely influenced by the degree of melting.