Pleistocene rhyolite of the mineral mountains, Utah geothermal and archeological significance

Journal Articl

Evidence of Rapid Phenocryst Growth of Olivine During Ascent in Basalts From the Big Pine Volcanic Field: Application of Olivine‐Melt Thermometry and Hygrometry at the Liquidus

The Quaternary Big Pine (BP) volcanic field in eastern California is notable for the occurrence of mantle xenoliths in several flows. This points to rapid ascent of basalt through the crust and precludes prolonged storage in a crustal reservoir. In this study, the hypothesis of phenocryst growth during ascent is tested for several basalts (13–7 wt% MgO) and shown to be viable. Phenocrysts of olivine and clinopyroxene frequently display diffusion‐limited growth textures, and clinopyroxene compositions are consistent with polybaric crystallization. When the most Mg‐rich olivine in each sample is paired with the whole‐rock composition, resulting Fe2+‐MgKD(olivine‐melt) values (0.31–0.36) match those calculated from literature models (0.32–0.36). Application of a Mg‐ and a Ni‐based olivine‐melt thermometer from the literature, both calibrated on the same experimental data set, leads to two sets of temperatures that vary linearly with whole‐rock MgO wt%. Because the Ni thermometer is independent of water content, it provides the actual temperature at the onset of olivine crystallization (1247–1097°C), whereas the Mg thermometer gives the temperature under anhydrous conditions and thus allows ΔT (=TMg − TNi = depression of liquidus due to water) to be obtained. The average ΔT for all samples is ~59°C, which is consistent with analyzed water contents of 1.5–3.0 wt% in olivine‐hosted melt inclusions from the literature. Because the application of olivine‐melt thermometry/hygrometry at the liquidus only requires microprobe analyses of olivine combined with whole‐rock compositions, it can be used to obtain large global data sets of the temperature and water contents of basalts from different tectonic settings.Plain Language SummaryBasaltic lavas are a window into their mantle source regions, which is why it is important to determine their temperatures and water contents. In this study, a new approach that allows these two parameters to be quantified is demonstrated for basalts from the Big Pine volcanic field, CA. They were targeted because many contain chunks of dense mantle rocks, which precludes storage in a crustal magma chamber and points to direct ascent from the mantle to the surface along fractures. Two hypotheses are proposed, tested, and shown to be viable in this study: (1) olivine crystallized in the basalts during ascent, and (2) the most Mg‐rich olivine analyzed in each basalt represents the first olivine to grow during ascent. This enables the most Mg‐rich olivine to be paired with the whole‐rock composition in the application of olivine‐melt thermometry and hygrometry. The results match those from published, independent studies. The success of this approach paves the way for the attainment of large, high‐quality data sets for basalts from a wide variety of tectonic settings. This, in turn, may allow global variations in mantle temperature and volatile content to be mapped in greater detail and better understood.Key PointsRapid phenocryst growth occurs during ascent in Mg‐rich basalts (some carry mantle xenoliths) from the Big Pine volcanic field, CAThe most Mg‐rich olivine can be paired with the whole‐rock composition to apply olivine‐melt thermometry/hygrometry at the liquidusLarge, high‐quality data sets on the temperature and water content of basalts from various tectonic settings can be obtained by this methodPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/163434/3/ggge22329-sup-0001-2020GC009264-SI.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/163434/2/ggge22329.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/163434/1/ggge22329_am.pd

The geological history of Nili Patera, Mars

Nili Patera is a 50 km diameter caldera at the center of the Syrtis Major Planum volcanic province. The caldera is unique among Martian volcanic terrains in hosting: (i) evidence of both effusive and explosive volcanism, (ii) hydrothermal silica, and (iii) compositional diversity from olivine-rich basalts to silica-enriched units. We have produced a new geological map using three mosaicked 18 m/pixel Context Camera digital elevation models, supplemented by Compact Remote Imaging Spectrometer for Mars Hyperspectral data. The map contextualizes these discoveries, formulating a stratigraphy in which Nili Patera formed by trapdoor collapse into a volcanotectonic depression. The distinctive bright floor of Nili Patera formed either as part of a felsic pluton, exposed during caldera formation, or as remnants of welded ignimbrite(s) associated with caldera formation—both scenarios deriving from melting in the Noachian highland basement. After caldera collapse, there were five magmatic episodes: (1) a basaltic unit in the caldera's north, (2) a silica-enriched unit and the associated Nili Tholus cone, (3) an intrusive event, forming a ~300 m high elliptical dome; (4) an extrusive basaltic unit, emplaced from small cones in the east; and (5) an extreme olivine-bearing unit, formed on the western caldera ring fault. The mapping, together with evidence for hydrated materials, implies magmatic interaction with subsurface volatiles. This, in an area of elevated geothermal gradient, presents a possible habitable environment (sampled by the hydrothermal deposits). Additionally, similarities to other highland volcanoes imply similar mechanisms and thus astrobiological potential within those edifices

Ultrametric spaces of branches on arborescent singularities

Let $S$ be a normal complex analytic surface singularity. We say that $S$ is arborescent if the dual graph of any resolution of it is a tree. Whenever $A,B$ are distinct branches on $S$, we denote by $A \cdot B$ their intersection number in the sense of Mumford. If $L$ is a fixed branch, we define $U_L(A,B)= (L \cdot A)(L \cdot B)(A \cdot B)^{-1}$ when $A \neq B$ and $U_L(A,A) =0$ otherwise. We generalize a theorem of P{\l}oski concerning smooth germs of surfaces, by proving that whenever $S$ is arborescent, then $U_L$ is an ultrametric on the set of branches of $S$ different from $L$. We compute the maximum of $U_L$, which gives an analog of a theorem of Teissier. We show that $U_L$ encodes topological information about the structure of the embedded resolutions of any finite set of branches. This generalizes a theorem of Favre and Jonsson concerning the case when both $S$ and $L$ are smooth. We generalize also from smooth germs to arbitrary arborescent ones their valuative interpretation of the dual trees of the resolutions of $S$. Our proofs are based in an essential way on a determinantal identity of Eisenbud and Neumann.Comment: 37 pages, 16 figures. Compared to the first version on Arxiv, il has a new section 4.3, accompanied by 2 new figures. Several passages were clarified and the typos discovered in the meantime were correcte

Helping education undergraduates to use appropriate criteria for evaluating accounts of motivation

The aim of the study was to compare students in a control group with those in a treatment group with respect to evaluative comments on psychological accounts of motivation. The treatment group systematically scrutinized the nature and interpretation of evidence that supported different accounts, and the assumptions, logic, coherence and clarity of accounts. Content analysis of 74 scripts (using three categories) showed that the control group students made more assertions than either evidential or evaluative points, whereas the treatment group used evaluative statements as often as they used assertion. The findings provide support for privileging activities that develop understanding of how knowledge might be contested, and suggest a need for further research on pedagogies to serve this end. The idea is considered that such understanding has a pivotal role in the development of critical thinking

Big Line Bundles over Arithmetic Varieties

We prove a Hilbert-Samuel type result of arithmetic big line bundles in Arakelov geometry, which is an analogue of a classical theorem of Siu. An application of this result gives equidistribution of small points over algebraic dynamical systems, following the work of Szpiro-Ullmo-Zhang. We also generalize Chambert-Loir's non-archimedean equidistribution