The North American Cordilleran Anatectic Belt

The North American Cordilleran Anatectic Belt (CAB) is a ~3,000 km long region in the hinterland of the Cordillera that comprises numerous exposures of Late Cretaceous to Eocene intrusive rocks and anatectic rocks associated with crustal melting. As such, it is comparable in size and volume to major anatectic provinces including the Himalayan leucogranite belt. The CAB rocks are chiefly peraluminous, muscovite-bearing leucogranite produced primarily by anatexis of Proterozoic to Archean metasedimentary rocks. The CAB rocks lack extrusive equivalents and were typically emplaced as thick sheets, laccoliths, and dike/sill complexes. The extent, location, and age of the CAB suggests that it is integral to understanding the tectonic evolution of North America, however, the belt is rarely considered as a whole. This paper reviews localities associated with crustal melting in the CAB and compiles geochemical, geochronologic, and isotopic data to evaluate the melt conditions and processes that generated these rocks. The geochemistry and partial melting temperatures (ca. 675–775 °C) support water-absent muscovite dehydration melting and/or water-deficient melting as the primary melt reactions and are generally inconsistent with water-excess melting and high-temperature (biotite to amphibole) dehydration melting. The CAB rocks are oldest in the central U.S. Cordillera and become younger towards both the north and south. At any single location, partial melting appears to have been a protracted process (≥10 Myr) and evidence for re-melting and remobilization of magmas is common. End-member hypotheses for the origin of the CAB include decompression, crustal thickening, fluid-flux melting, and increased heat flux from the mantle. Different parts of the CAB support different hypotheses and no single model may be able to explain the entirety of the anatectic event. Regardless, the CAB is a distinct component of the Cordilleran orogenic system

The Paleomineralogy of the Hadean Eon Revisited

A preliminary list of plausible near-surface minerals present during Earth’s Hadean Eon (>4.0 Ga) should be expanded to include: (1) phases that might have formed by precipitation of organic crystals prior to the rise of predation by cellular life; (2) minerals associated with large bolide impacts, especially through the generation of hydrothermal systems in circumferential fracture zones; and (3) local formation of minerals with relatively oxidized transition metals through abiological redox processes, such as photo-oxidation. Additional mineral diversity arises from the occurrence of some mineral species that form more than one ‘natural kind’, each with distinct chemical and morphological characteristics that arise by different paragenetic processes. Rare minerals, for example those containing essential B, Mo, or P, are not necessary for the origins of life. Rather, many common minerals incorporate those and other elements as trace and minor constituents. A rich variety of chemically reactive sites were thus available at the exposed surfaces of common Hadean rock-forming minerals

Coarse muscovite veins and alteration deep in the Yerington batholith, Nevada: insights into fluid exsolution in the roots of porphyry copper systems

Veins and pervasive wall-rock alteration composed of coarse muscovite +/- quartz +/- pyrite are documented for the first time in a porphyritic granite at Luhr Hill in the Yerington District, Nevada. Coarse muscovite at Luhr Hill occurs at paleodepths of similar to 6-7 km in the roots of a porphyry copper system and crops out on the scale of tens to hundreds of meters, surrounded by rock that is unaltered or variably altered to sodic-calcic assemblages. Coarse muscovite veins exhibit a consistent orientation, subvertical and N-S striking, which structurally restores to subhorizontal at the time of formation. Along strike, coarse muscovite veins swell from distal, millimeter-thick muscovite-only veinlets to proximal, centimeter-thick quartz-sulfide-bearing muscovite veins. Crosscutting relationships between coarse muscovite veins, pegmatite dikes, and sodic-calcic veins indicate that muscovite veins are late-stage magmatic-hydrothermal features predating final solidification of the Luhr Hill porphyritic granite. Fluid inclusions in the muscovite-quartz veins are high-density aqueous inclusions of similar to 3-9 wt% NaCl eq. and < 1 mol% CO2 that homogenize between similar to 150 and 200 A degrees C, similar to fluid inclusions from greisen veins in Sn-W-Mo vein systems. Our results indicate that muscovite-forming fluids at Luhr Hill were mildly acidic, of low to moderate salinity and sulfur content and low CO2 content, and that muscovite in deep veins and alteration differs in texture, composition, and process of formation from sericite at shallower levels of the hydrothermal system. Although the definition of greisen is controversial, we suggest that coarse muscovite alteration is more similar to alteration in greisen-type Sn-W-Mo districts worldwide than to sericitic alteration at higher levels of porphyry copper systems. The fluids that form coarse muscovite veins and alteration in the roots of porphyry copper systems are distinct from fluids that formed copper ore or widespread, shallower, acidic alteration. We propose that this style of veins and alteration at Luhr Hill represents degassing of moderate volumes of overpressured hydrothermal fluid during late crystallization of deep levels of the Yerington batholith.Geological Society of America; Society of Economic Geologists12 month embargo; Published Online: 27 February 2017.This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Coarse muscovite veins and alteration in porphyry systems

Coarse muscovite veins and alteration occur in porphyry copper and porphyry molybdenum-copper systems within the Laramide arc in Arizona, as well as at the Yerington district in Nevada. This work describes coarse muscovite in veins and altered wall rock in porphyry systems in this region and documents mineral assemblages, mineral compositions, spatial and temporal relationships, and hydrogen isotopic compositions. Coarse hydrothermal muscovite is documented in the roots of porphyry Cu +/- Mo systems, as well as in and above the ore bodies in porphyry Mo-Cu systems, and it is compared to coarse hydrothermal muscovite (greisen) in lode Sn-W-Mo systems. Basin and Range extension has exposed coarse hydrothermal muscovite in several Laramide and Jurassic porphyry Cu (+/- Mo) systems, at paleodepths of 3 to 12 km: Miami-Inspiration, Sierrita-Esperanza, Copper Basin (Crown King), Granite Mountain (roots of the Ray porphyry system), Gunnison (Texas Canyon stock), Grayback (Kelvin-Riverside district), Sycamore Canyon, the New Cornelia mine (Ajo district), and two systems in the Yerington district. Muscovite is the dominant mica in these coarse muscovite veins and associated alteration, with common K-feldspar and albite (An(00-)(06)), common accessory hematite, rutile, pyrite, and apatite, and rare accessory chalcopyrite, fluorite, molybdenite, wolframite, and scheelite. Coarse hydrothermal muscovite yields delta D compositions that suggest formation from fluids that are dominantly magmatic-hydrothermal in origin. Whole-rock compositions of coarse hydrothermal muscovite show common gains in K and loss of Ca +/- Na. Coarse muscovite veins and alteration in porphyry copper systems postdate mineralized potassic veins and form too deeply to overlap with shallower acidic forms of alteration (sericitic, advanced argillic). Variation in mineral assemblage, mineral compositions, and mineralization of coarse hydrothermal muscovite correlate with the composition of Laramide stocks. Porphyry Mo-Cu systems contain coarse muscovite alteration assemblages with the highest mineral diversity and trace-element enrichment. Coarse muscovite veins and alteration in porphyry Mo-Cu systems related to stocks ranging from quartz monzonite to granite in composition form at shallower paleodepths and occur within and above the associated orebodies. In contrast, coarse muscovite veins and alteration associated with subalkaline porphyry copper systems occur at deeper levels, in some cases overlapping with the bottom of potassic alteration and the ore body but extending well into the roots of the system in the underlying granitoid cupola. In these latter systems, zones of coarse muscovite alteration typically are poorly mineralized and mineral assemblages are less varied. These characteristics suggest that coarse muscovite-forming fluids are predominately of magmatic-hydrothermal origin and exsolved from late-stage, fractionated magmas of the larger pluton that sourced porphyry stocks and dikes responsible for porphyry copper mineralization. In some instances, however, the exposed coarse muscovite alteration is associated with a petrologically unrelated, commonly more felsic, later intrusion, rather than being related to late exsolution of fluid from the same crystallizing stock or batholith.Geological Society of America; Arizona Geological Society Courtright Scholarship; Spencer R. Titley Scholarship from the University of Arizona; Society of Economic Geologists; Lowell Institute for Mineral ResourcesOpen access articleThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Patterns in Mobility and Modification of Middle- and High-Latitude Southern Hemisphere Dunes on Mars

Change detection analyses of aeolian bedforms (dunes and ripples), using multitemporal images acquired by the Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment (HiRISE), can reveal migration of bedforms on Mars. Here we investigated bedform mobility (evidence of wind-driven migration or activity), from analysis of HiRISE temporal image pairs, and dune field modification (i.e., apparent presence/lack of changes or degradation due to nonaeolian processes) through use of a dune stability index or SI (1-6; higher numbers indicating increasing evidence of stability/modification). Combining mobility data and SI for 70 dune fields south of 40 degrees S latitude, we observed a clear trend of decreasing bedform mobility with increasing SI and latitude. Both dunes and ripples were more commonly active at lower latitudes, although some high-latitude ripples are migrating. Most dune fields with lower SIs (3) were found to be active while those with higher SIs were primarily found to be inactive. A shift in prevalence of active to apparently inactive bedforms and to dune fields with SI2 occurs at similar to 60 degrees S latitude, coincident with the edge of high concentrations of H2O-equivalent hydrogen observed by the Mars Odyssey Neutron Spectrometer. This result is consistent with previous studies suggesting that stabilizing agents, such as ground ice, likely stabilize bedforms and limit sediment availability. Observations of active dune fields with morphologies indicative of stability (i.e., migrating ripples in SI=3 dune fields) may have implications for episodic phases of reworking or dune building, and possibly geologically recent activation or stabilization corresponding to shifts in climate. Plain Language Summary Dune fields and sand sheets, very similar to those we see on Earth, are observed on the surface of Mars. Their presence attests to the importance of wind-driven activity in shaping the Martian surface. Using repeated high-resolution imaging with the HiRISE (High Resolution Imaging Science Experiment) camera in orbit around Mars on the Mars Reconnaissance Orbiter spacecraft, we can now look closely at dunes and ripples (collectively referred to as bedforms) on Mars to find evidence of changes over time. Changes in or movement of the bedforms indicate where they are currently active and migrating across the surface. Actively migrating bedforms provide valuable information about present-day conditions on the surface such as sediment supply, wind speed, and wind direction. In this study, we investigated the activity of dunes and ripples in the middle and high latitudes of the southern hemisphere of Mars. We combined our results with those from investigations that looked at how the bedforms are degrading and being modified from nonwind-driven processes, indicative of dune inactivity. Our results show that dunes and ripples are progressively less active and show increasing evidence of degradation and erosion with proximity to the south pole. In the northern part of our study area (similar to 40-55 degrees S latitude), dune fields are mobile and are not degrading. Generally speaking, conditions where these dune fields formed, such as sediment supply and wind speeds, remain favorable for wind-driven activity today. In the high southern latitudes (> similar to 60 degrees S), it appears that conditions favorable to dune field and sand sheet formation have shifted to less favorable conditions in most locations, perhaps episodically, since they originally formed. This shift in prevalence of active to apparently inactive bedforms and to dune fields that are more degraded occurs at roughly 60 degrees S latitude and coincides with the edge of high concentrations of H2O-equivalent hydrogen observed by the Mars Odyssey Neutron Spectrometer and interpreted to indicate ice beneath the surface. Our observations of decreasing bedform mobility with increasing latitude support the accumulation of ground ice between dune sand grains, which may be stabilizing the grains and reducing sand availability in present climate conditions. Some dunes may be stabilized by ground ice at their core, while surface/near-surface sediments, not cemented in the ground ice, continue to migrate as superposing ripples. Understanding the characteristics of the activity and morphology of dune fields can provide valuable insight into local and regional sedimentary and climatic histories on Mars and Earth. Our results may reflect more than one generation of dune building and possibly a current phase of episodic activity and may have implications for shifts in the climate to activate or stabilize dune fields and sand sheets over time.NASA Mars Data Analysis Program (MDAP) [NNX14AO96G]; MDAP grant [NNX12AJ38G, NNX16AJ43G/123117]6 month embargo; published online: 8 November 2018This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Patterns in Mobility and Modification of Middle and High-Latitude Southern Hemisphere Dunes on Mars

Change detection analyses of aeolian bedforms (dunes and ripples), using multitemporal imagesacquired by the Mars Reconnaissance Orbiter High Resolution Imaging Science Experiment (HiRISE), canreveal migration of bedforms on Mars. Here we investigated bedform mobility (evidence of wind-drivenmigration or activity), from analysis of HiRISE temporal image pairs, and dune field modification (i.e., apparentpresence/lack of changes or degradation due to nonaeolian processes) through use of a dune stability indexor SI (16; higher numbers indicating increasing evidence of stability/modification). Combining mobility dataand SI for 70 dune fields south of 40S latitude, we observed a clear trend of decreasing bedform mobility withincreasing SI and latitude. Both dunes and ripples were more commonly active at lower latitudes, althoughsome high-latitude ripples are migrating. Most dune fields with lower SIs (3) were found to be active whilethose with higher SIs were primarily found to be inactive. A shift in prevalence of active to apparently inactivebedforms and to dune fields with SI 2 occurs at ~60S latitude, coincident with the edge of highconcentrations of H2O-equivalent hydrogen observed by the Mars Odyssey Neutron Spectrometer. This result isconsistent with previous studies suggesting that stabilizing agents, such as ground ice, likely stabilize bedformsand limit sediment availability. Observations of active dune fields with morphologies indicative of stability(i.e., migrating ripples in SI = 3 dune fields) may have implications for episodic phases of reworking or dunebuilding, and possibly geologically recent activation or stabilization corresponding to shifts in climat

Sodic-Calcic Family of Alteration in Porphyry Systems of Arizona and Adjacent New Mexico

Sodic-calcic alteration has affected numerous districts with Laramide porphyry systems across Arizona and New Mexico. Previously undocumented Na-Ca alteration has been identified at Charleston, Sycamore Canyon, Silver Bell, and the Ninetysix Hills in Arizona and the Central mining district in New Mexico. These new occurrences are compared to sodic-calcic alteration at the Sierrita-Esperanza deposit (Pima district), the Ajo mining district, the Tea Cup pluton (Kelvin-Riverside district), and the Eagle Pass dike swarm in Arizona. Spatial positions, crosscutting relationships, and mineral assemblages and compositions allow for interpretation of the geochemical conditions of formation and implications for associated hydrothermal processes. Diopside- or andradite-grossular solid solution (Ad(68)-Ad(72))-stable assemblages representative of calcic alteration typically occur in deep and/or distal positions but are less commonly exposed. The sodic-calcic and sodic alteration types, however, occur more proximally to the porphyry orebody. Sodic-calcic assemblages containing Na plagioclase (An(10)-An(33))-Ca amphibole +/- epidote are focused along granite cupolas near the base of orebodies, and these grade into shallower sodic assemblages containing albite (An(00)-An(09))-epidote-chlorite. Mineral composition and fluid inclusion data suggest that Na-Ca plagioclase-bearing assemblages form at higher temperatures (360 degrees-470 degrees C+) than later, lower-temperature albite-epidote assemblages (>250 degrees C). Calculated delta D isotope compositions of fluids in equilibrium with Na-Ca alteration minerals span a broad range (-46 to -1%) and are consistently less negative than magmatic fluids. Where members of the Na-Ca family of alteration assemblages are present in porphyry systems, many lines of evidence-including isotopic compositions of minerals and the geologic alteration zoning patterns-are consistent with an origin by incursion of saline external fluids. Where Na-Ca alteration is present, it is best developed at relatively deep levels of porphyry systems. Several Laramide systems that are well exposed at comparably deep levels lack Na-Ca alteration, probably because sources of saline fluid were absent or hydrologic conditions were unfavorable for the incursion of saline external fluids. Though similar in temperatures, isotopic compositions, mineral assemblages, and zoning patterns, Na-Ca alteration in Laramide systems is volumetrically smaller than in Jurassic systems documented in the western United States, probably because saline external fluids were much less abundant during the Laramide after a tectonically driven excursion in paleoclimate from arid to humid conditions.Geological Society of America; Society of Economic Geologists12 month embargo; published 01 June 2019This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Coronal Heating as Determined by the Solar Flare Frequency Distribution Obtained by Aggregating Case Studies

Flare frequency distributions represent a key approach to addressing one of the largest problems in solar and stellar physics: determining the mechanism that counter-intuitively heats coronae to temperatures that are orders of magnitude hotter than the corresponding photospheres. It is widely accepted that the magnetic field is responsible for the heating, but there are two competing mechanisms that could explain it: nanoflares or Alfv\'en waves. To date, neither can be directly observed. Nanoflares are, by definition, extremely small, but their aggregate energy release could represent a substantial heating mechanism, presuming they are sufficiently abundant. One way to test this presumption is via the flare frequency distribution, which describes how often flares of various energies occur. If the slope of the power law fitting the flare frequency distribution is above a critical threshold, $\alpha=2$ as established in prior literature, then there should be a sufficient abundance of nanoflares to explain coronal heating. We performed $>$600 case studies of solar flares, made possible by an unprecedented number of data analysts via three semesters of an undergraduate physics laboratory course. This allowed us to include two crucial, but nontrivial, analysis methods: pre-flare baseline subtraction and computation of the flare energy, which requires determining flare start and stop times. We aggregated the results of these analyses into a statistical study to determine that $\alpha = 1.63 \pm 0.03$. This is below the critical threshold, suggesting that Alfv\'en waves are an important driver of coronal heating.Comment: 1,002 authors, 14 pages, 4 figures, 3 tables, published by The Astrophysical Journal on 2023-05-09, volume 948, page 7