R-3 Repository :: Browsing by Author "Lee, Cin-Ty A."

Browsing by Author "Lee, Cin-Ty A."

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A Framework for Understanding Whole-Earth Carbon Cycling
(Cambridge University Press, 2019) Lee, Cin-Ty A.; Jiang, Hehe; Dasgupta, Rajdeep; Torres, Mark; Orcutt, Beth N.; Daniel, Isabelle; Dasgupta, Rajdeep
Building from the base of knowledge presented in the preceding chapters, this chapter explores how the cycling of carbon in subduction zones and orogenic belts varies with supercontinent cycles and mountain building. It discusses the processes that link short-term and long-term carbon cycling and the timescales of these processes, such as the response times of weathering and atmospheric drawdown at periods of enhanced volcanism. This chapter covers topics of potential fluctuations in the long-term CO2 content of Earth’s atmosphere because of mantle–climate feedback, again taking advantage of the modeling platforms available for further exploration of these topics.
A mantle xenolith window into the Grenville orogeny of Southern Laurentia
(2009) Young, Hobart Patrick; Lee, Cin-Ty A.
The nature of the lithospheric mantle beneath orogenic belts is incompletely understood due to the paucity of mantle xenolith-bearing basaltic magmas in such regions. One such place where we are afforded the opportunity to study the deep lithosphere beneath an orogenic belt is in central Texas, United States. Mantle xenoliths occur in Late Cretaceous alkali magmas erupted through the remnants of the Appalachian-Ouachita structural belt. Here, we show that geochemical signatures in the form of enrichments in fluid-mobile trace elements (e.g., La) relative to fluid-immobile trace elements (e.g., Nb) are preserved in these xenoliths. We interpret these signatures to represent metasomatism by subduction-related fluids, which implies that the mantle xenoliths represent fragments of continental lithospheric mantle that served as the upper plate during a convergent episode. These observations suggest that some of the original continental lithosphere was preserved beneath the orogenic belt during collision and did not undergo wholesale delamination.
Broadening theories of soils genesis: Insights from Tanzania and simple models
(2007) Little, Mark Gabriel; Lee, Cin-Ty A.; Luttge, Andreas; Tomson, Mason B.; Anderson, John B.; Ewing, Maurice W.; Masiello, Caroline A.
Three basic assumptions of soil formation are challenged herein: the degree of chemical weathering decreases with depth; increased physical weathering due to high topographical gradients causes an increase in chemical weathering; and the mineral soil derives from the transformation of in situ parent material. The first part presents an investigation into the degree and nature of chemical weathering during soil formation on a volcanic substrate on Mt. Kilimanjaro in northern Tanzania. The degree of weathering was found to increase with depth in the soil profile. Observations show that the upper and lower layers of the weathering profile have undergone different weathering histories. The presence of a buried paleosol or enhanced weathering due to lateral subsurface water flow may explain the observations, the latter having novel implications for the transport of dissolved cations to the ocean. The second part presents a model to test the link between chemical weathering associated with soil formation and erosion associated with mass wasting. The predicted ratios suspended/dissolved ratios, however, are all higher than observed in rivers, the discrepancy worsening with increasing topographic relief. This discrepancy arises from the fact that in regions of high relief, mass wasting are so high that soil mantles do not reside on hillslopes long enough to allow for significant chemical weathering. The discrepancy between the model and observations can be explained by: over-estimate of predicted suspended load; absence of chemical weathering of deltaic/alluvial sediments from the model; or chemical weathering associated with groundwater weathering. The third part presents data from a sequential extraction on a basaltic soil from Mt. Meru in Northern Tanzania. The behavior of relatively immobile elements is consistent with soil formation being accompanied by mass loss due to chemical weathering. However, superimposed on this mass loss appears to be enrichment of most elements measured. These data suggest that the surface of the Meru soil columns may have experienced "re-fertilization" by the deposition of volcanic ash.
Carbon cycle and climate fluctuations during the early Paleogene: Sedimentological characteristics and environmental ramifications
(2014-11-13) Slotnick, Benjamin; Dickens, Gerald R.; Lee, Cin-Ty A.; Anderson, John B.; Masiello, Carrie A.; Rudolf, Volker H.W.
The early Paleogene was marked by extensive changes related to Earth surface temperature, carbon cycling, and the hydrological cycle. This included at least two, and probably more, geologically brief (~200-k.yr.) intervals of extreme warming, the Paleocene-Eocene thermal maximum (PETM) and the Eocene thermal maximum-2 (ETM-2) along with a moderately-long (~1.5-2 M.yr.) period of warmth (i.e.; Early Eocene Climatic Optimum [EECO]). This was preceded by a moderately-long (~2 M.yr.) period of cool conditions (i.e. Paleocene Carbon Isotope Maximum [PCIM]) and followed by the initiation of long-term cooling through the Cenozoic. The long-term rise in warmth and numerous short-term “hyperthermal” events, marked by pronounced negative carbon isotope excursions and clay-rich horizons (Zachos et al., 2005; Nicolo et al., 2007), have been linked to massive injections of 13C-depleted carbon into the ocean-atmosphere system and intense global climate change, but the exact character of the hyperthermals is not well-recognized. To better constrain and understand their occurrences, well-resolved and high-resolution records across the entire interval of interest is necessary. Preceding studies demonstrated major fluctuations in carbon cycling and terrestrial weathering (e.g.; Nicolo et al., 2007) during the latest Paleocene and earliest Eocene as well as a significant drop of dissolved oxygen concentrations during the PETM onset (Nicolo et al., 2010). However, causes, environmental impact, and relationships relating carbon release and capture to terrigenous runoff and surface temperatures throughout the exogenic system, particularly in spatial and temporal contexts, remain unclear. The southern Pacific region (Clarence Valley) and Indian Ocean (Ninetyeast Ridge) represent large realms of our planet’s ocean system that have been largely overlooked in part due to limited Paleogene core availability from Integrated Ocean Drilling Program (IODP). There is a need to better improve records related to carbon in the exogenic system throughout the Cenozoic, and in particular, the Paleogene. As of now, records lack in these regions. Clarence Valley in eastern Marlborough, New Zealand trends southwest for ~80 km between the Seaward and Inland Kaikouras. Along the northwest margin is a succession of streams, including Mead and Branch Streams, which have incised and exposed an uplifted and rotated block consisting of Amuri Limestone, a calcareous-rich formation within the Muzzle Group. These outer shelf and upper continental slope strata originally accumulated as terrigenous detrital clay minerals, biogenic silica, and biogenic carbonate. This was situated contiguous to a neritic carbonate platform along a passive margin of proto-New Zealand at ~55-50°S latitude. Ninetyeast Ridge, one of the longest near-linear features on Earth with a length of ~4600 km (from ~10ºN to ~31ºS), is located in the Indian Ocean. Numerous Deep Sea Drilling Program (DSDP) and Ocean Drilling Program (ODP) sites have been drilled adjacent to or along the ridge crest. Three sites in particular drilled during DSDP Leg 22 (i.e.; Sites 213, 214, and 215) include calcareous-rich material from much of the Paleogene. These sediments can provide additional constraints to further our understanding of Indian Ocean paleoceanographic changes during the Paleogene. The lack of data for the Paleogene has been an issue for quite some time now. As such, the Paleogene remains a specific interval of time that has the potential to be much better understood by integrating current views of carbon cycling to new and wellresolved data-sets. Here I analyzed sequences exposed in Clarence Valley and sections from Ninetyeast Ridge DSDP Sites to address these issues. These records were integrated into a global context to relate short-term (<100 k.yr.) and long-term (> 1 m.yr) changes. I generated lithologic and carbon isotopic records to evaluate the entire period of interest, including the PCIM, specific hyperthermals, EECO, and the Middle Eocene Climatic Optimum (MECO). Integration of each section revealed similarities, including long-term trends with similar carbon isotopic baselines and short-term events. Expanded marl-rich units concurrent to lower δ13C, specifically across CIEs, generally characterized marginal sedimentation whereas condensed intervals largely spanned deep-water settings, resulting from carbonate dissolution. Together, these studies indicate carbon addition and removal mechanisms repeatedly spanned much of the Paleogene, causing calcite compensation depth (CCD) fluctuations, related to lithologic, and carbon isotopic changes.
Carbon in Silicate Melt - Experimental Constraints and Applications for the Subduction Zone and Magma Ocean Carbon Cycles
(2015-11-18) Duncan, Megan Sallie; Dasgupta, Rajdeep; Lee, Cin-Ty A.; Whitmire, Ken H.; Lenardic, Adrian
We conducted high pressure and temperature experiments to determine the pressure, temperature, H2O content, and oxygen fugacity dependence of CO2 solubility on natural hydrous rhyolitic melts. These studies are applicable to subduction zone settings of the Earth throughout its history. Carbon concentrations and speciation were measured using FTIR, and in the process, we determined absorption coefficients for the CO2 species (CO2mol. and CO32-) for natural rhyolitic glass. We also developed empirical and thermodynamically-based models that describe the CO2 content of our compositions as a function of pressure, temperature, H2O content, and oxygen fugacity. We applied these models to predict the amount of CO2 that could be released via partial melt from subducted slab and carried up to the source region of arc volcanoes. We also conducted experiments to determine the compositional dependence of CO2 solubility on compositions spanning the range from natural basalt toward peridotite. These mafic to ultramafic compositions were used to constrain the carbon content of early Earth’s magma ocean. Combining our experiments with previous work, we developed an empirical model that describes the dissolved CO2 content as a function of pressure, temperature, H2O content, oxygen fugacity, and silicate composition. This model was applied to determine the amount of carbon contained in terrestrial magma oceans, and place constraints on the mantle carbon budget of the Earth.
Chalcophile behavior of thallium during MORB melting and implications for the sulfur content of the mantle
(American Geophysical Union, 2014) Nielsen, Sune G.; Shimizu, Nobumichi; Lee, Cin-Ty A.; Behn, Mark D.
We present new laser ablation ICP-MS trace element concentration data for 28 elements in 97 mid-ocean ridge basalt (MORB) glasses that cover all major spreading centers as well as Tl concentration data for all mineral phases in five lherzolites from the Lherz massif, France. The ratio between the elements thallium (Tl) and cerium (Ce) is nearly constant in MORB, providing evidence that the depleted MORB mantle (DMM) has uniform Ce/Tl. Lherzolite mineral data reveal that sulfides are heterogeneous and contain between 23 and 430 ng/g of Tl while all other minerals contain Tl below the analytical detection limit of ∼1 ng/g. We argue that Tl in MORB is controlled by residual sulfide during mantle melting. To investigate the observed relationship between Tl and Ce, we conduct models of fractional mantle melting, which show that the constant Ce/Tl in MORB is only reproduced if the ratio between clinopyroxene and sulfide in the upper mantle varies by less than 10%. In addition, the rate of melting for these two phases must be nearly identical as otherwise melt depletion and refertilization processes would lead to Ce/Tl fractionation. These model results allow us to establish a relationship for the sulfur content of DMM: [S]DMM = SCSS × Mcpx /Rcpx, where SCSS is the sulfur concentration of a silicate melt at sulfide saturation, Rcpx is the melt reaction coefficient, and Mcpx is the modal abundance of clinopyroxene in the DMM. Using this equation, we calculate that the average upper mantle sulfur concentration is 195 ± 45 μg/g.
Crystal Dissolution Kinetics: Linking Surface Processes at the Solid-Solution Interface over Multiple Length-Scales
(2007) Vinson, Michael David; Luttge, Andreas; Sawyer, Dale S.; Lee, Cin-Ty A.; Whitmire, Kenton H.
This thesis presents a multiple-scale experimental study of mineral dissolution kinetics, utilizing direct measurement of crystal surface morphology in great detail to determine rates and mechanisms acting at the reactive solid-solution interface. The overall approach uses primarily vertical scanning interferometry (VSI) to analyze threedimensional crystal surface morphological change and quantify rates of dissolution. Integration of VSI with atomic force microscopy (AFM), theoretical kinetic models, and thermodynamic calculations has permitted the recognition of rate-controlling processes and mechanisms, thus strengthening our ability to link dissolution kinetics over a broad range of length and time scales. The motivation for this thesis arises from an incomplete understanding of how molecular-scale surface processes, acting at the solid-solution interface, control large-scale natural dissolution phenomena. Mineral dissolution is a fundamental geologic process that exerts control over a number of significant geochemical events which can affect both man and the environment over a broad range of spatial and temporal scales. At the Earth surface, rates of dissolution are largely surface-controlled, and thus influenced by the threedimensional nature of the crystal surface. The goals of this thesis involve improving overall understanding of dissolution rate-scaling issues by investigation of crystal surface dissolution kinetics, identification ofrate-controlling mechanisms. This thesis reports investigations into the dissolution kinetics of calcite, rhyolite and uraninite over a range of far-from-equilibrium laboratory conditions. The results presented within this work demonstrate that: 1. Monomolecular "rough" step retreat controls the overall rate of calcite crystal dissolution, which in turn can be inhibited by impurity adsorption at a level dependent on the physical and chemical properties of the adsorbing impurity and the presence of carbonate in solution. 2. Heterogeneous retreat of volcanic glass phases control overall rhyolite dissolution rate and the steady-sate release of ions into solution, which in turn may be influenced by the formation of a surface reprecipitation phase. 3. Dissolved carbon species influence the steady-state dissolution uraninite, although this dissolution fails to produce resolvable surface-normal retreat. In total, this original work constructs a clearer understanding of the kinetics at the reactive solid-solution interface and reveals how dissolution phenomena can scale across time and space.
Deep mantle roots and continental emergence: implications for whole-Earth elemental cycling, long-term climate, and the Cambrian explosion
(Taylor & Francis, 2018) Lee, Cin-Ty A.; Caves, Jeremy; Jiang, Hehe; Cao, Wenrong; Lenardic, Adrian; McKenzie, N. Ryan; Shorttle, Oliver; Yin, Qing-zhu; Dyer, Blake
Elevations on Earth are dominantly controlled by crustal buoyancy, primarily through variations in crustal thickness: continents ride higher than ocean basins because they are underlain by thicker crust. Mountain building, where crust is magmatically or tectonically thickened, is thus key to making continents. However, most of the continents have long passed their mountain building origins, having since subsided back to near sea level. The elevations of the old, stable continents are lower than that expected for their crustal thicknesses, requiring a subcrustal component of negative buoyancy that develops after mountain building. While initial subsidence is driven by crustal erosion, thermal relaxation through growth of a cold thermal boundary layer provides the negative buoyancy that causes continents to subside further. The maximum thickness of this thermal boundary layer is controlled by the thickness of a chemically and rheologically distinct continental mantle root, formed during large-scale mantle melting billions of years ago. The final resting elevation of a stabilized continent is controlled by the thickness of this thermal boundary layer and the temperature of the Earth’s mantle, such that continents ride higher in a cooler mantle and lower in a hot mantle. Constrained by the thermal history of the Earth, continents are predicted to have been mostly below sea level for most of Earth’s history, with areas of land being confined to narrow strips of active mountain building. Large-scale emergence of stable continents occurred late in Earth’s history (Neoproterozoic) over a 100–300 million year transition, irreversibly altering the surface of the Earth in terms of weathering, climate, biogeochemical cycling and the evolution of life. Climate during the transition would be expected to be unstable, swinging back and forth between icehouse and greenhouse states as higher order fluctuations in mantle dynamics would cause the Earth to fluctuate rapidly between water and terrestrial worlds.
Episodes of fast crystal growth in pegmatites
(Springer Nature, 2020) Phelps, Patrick R.; Lee, Cin-Ty A.; Morton, Douglas M.
Pegmatites are shallow, coarse-grained magmatic intrusions with crystals occasionally approaching meters in length. Compared to their plutonic hosts, pegmatites are thought to have cooled rapidly, suggesting that these large crystals must have grown fast. Growth rates and conditions, however, remain poorly constrained. Here we investigate quartz crystals and their trace element compositions from miarolitic cavities in the Stewart pegmatite in southern California, USA, to quantify crystal growth rates. Trace element concentrations deviate considerably from equilibrium and are best explained by kinetic effects associated with rapid crystal growth. Kinetic crystal growth theory is used to show that crystals accelerated from an initial growth rate of 10−6–10−7 m s−1 to 10−5–10−4 m s−1 (10-100 mm day−1 to 1–10 m day−1), indicating meter sized crystals could have formed within days, if these rates are sustained throughout pegmatite formation. The rapid growth rates require that quartz crystals grew from thin (micron scale) chemical boundary layers at the fluid-crystal interfaces. A strong advective component is required to sustain such thin boundary layers. Turbulent conditions (high Reynolds number) in these miarolitic cavities are shown to exist during crystallization, suggesting that volatile exsolution, crystallization, and cavity generation occur together.
Exploration of Tektite Formation Processes through Water and Metal Content Measurements
(2012) Watt, Nigel John DeWolfe McKinley, II; Lee, Cin-Ty A.
Impact events are a significant surface-modifying process on bodies lacking frequent resurfacing by atmospheric or igneous processes. To explore the effects of impacts on surface materials, we measured water and trace element compositions of tektites from Vietnam. Fourier transform infrared spectroscopy was used for water measurements and laser ablation inductively coupled plasma mass spectrometry was used for trace element measurements. Consistent with previous investigations of tektites, we found that the samples are depleted in volatile metals (e.g., Zn, Pb) as well as in water compared to the average continental crust, though water contents are high for rocks melted at atmospheric pressure. While Zn and Pb concentrations are linearly correlated with each other, there is no correlation between H 2 O and Zn or Pb contents. Combined with water contents of other tektites in the Australasian strewnfield, our results demonstrate that the source impact occurred at a wet site near the Indochina peninsula.
Exploration on variations in chromium/cobalt and vanadium/cobalt ratios in pelagic sediments
(2007) Hu, Min; Lee, Cin-Ty A.
In this paper we explored variation in Cr/Co and V/Co ratios with time in 21 pelagic clay samples from various locations in Pacific Ocean. We also included data from previous researchers to illustrate the trend we observed. Reasons for the observed change are explained and pros and cons for each one are examined. What motivated us to start this research is that redox evolution of oceans through time is one of the most desired but elusive parameters in paleo-oceanography. Most attempts have focused on tracking variations in redox-sensitive elements in anoxic margin sediments. However, redox conditions in marginal basins vary widely due to local effects and do not necessarily represent the conditions of the open ocean. As an alternative approach, bulk-rock Cr/Co and V/Co ratios in oxic pelagic clays are explored here as tracers of redox-evolution of the global oceans on the million year timescale. Bulk-rock Cr/Co and V/Co ratios are always much lower than that of riverine inputs. One of the explanations for this phenomenon is that Cr and V are probably sequestered in anoxic environments on oceanic margins whereas the open ocean appears to be the major sink for Co. Based on this mass balance approach, it seems that, in theory, it should be possible to use these elemental ratios as potential tracers of redox state in the ocean. However, here we were not able to confirm this possibility. Bulk-rock Cr/Co and to a lesser extent V/Co ratios in several pelagic clay sites in the Pacific show a rise from ~25 Ma ago to the present. While this secular change in bulk ratios might suggest an increase in oxygenation of the Pacific Ocean, the changes can be simply explained by an increase in detrital component, such as wind-blown dust or volcanic ash, or equivalently, by a proportional decrease in hydrothermal inputs. In order to minimize influence on the change caused by detrital component, we tried to calculate hydrogenous component in our samples by subtracting out a detrital component that has a constant composition, which in reality is likely to vary significantly and the value is not known well. Besides variation in the composition of the detrital component, there are other explanations including diagenetic effects and changes in the composition of the dissolved inputs into rivers with time. We conclude that although Cr/Co and V/Co hold promise as potential redox-sensitive tracers, interpreting their signals is presently frustrated by many complicating factors.
Geochemical diagnostics of metasedimentary dark enclaves: a case study from the Peninsular Ranges Batholith, southern California
(Taylor & Francis, 2012) Liao, Kelley Z.; Morton, Douglas M.; Lee, Cin-Ty A.
Geochemical Diagnostics of Metasedimentary Dark Inclusions: a Case Study from the Peninsular Ranges Batholith, California
(2013-07-24) Liao, Kelley; Lee, Cin-Ty A.; Dasgupta, Rajdeep; Lenardic, Adrian
Dark enclaves rich in amphibole and biotite are ubiquitous in granitoid rocks and generally thought to represent fragments of mafic magmas, cumulates or restites. However, magmatic assimilation of metamorphic or sedimentary country rock can also form dark enclaves. To develop criteria for identifying dark enclaves of non-magmatic origin, we investigated dark enclaves from a complete spectrum of light (carbonate- or feldspar-rich) to dark (amphibole-rich, biotite-rich, or composite) enclaves, reflecting progressive thermal and chemical equilibration with host tonalite from the Domenigoni Valley pluton in the Peninsular Ranges Batholith, California. Metasedimentary dark enclaves have a number of major and trace element characteristics that overlap those of literature-compiled igneous dark enclaves. Comparison to modeled igneous differentiation paths shows metasedimentary enclaves can have anomalous CaO and K2O contents for a given SiO2, but other major element systematics may not deviate noticeably from igneous differentiation trends. In addition, the fact that there are literature-compiled mafic enclaves trending towards high K2O and high CaO suggests that not all mafic enclaves are of igneous origin. While the majority of dark enclaves may not be metasedimentary, this work provides some criteria for identifying enclaves should a case of metasedimentary origin arise.
Geochemistry of magnesian granulites in feldspathic lunar meteorites Allan Hills A81005 and Dhofar 309
(2007) Maloy, Amy Kathleen; Lee, Cin-Ty A.
Magnesian granulites in Allan Hills A81005 and Dhofar 309 may possess the geochemical signature of the Moon's Feldspathic Highlands Terrane (FHT). Previous work on Apollo granulites suggests the presence of a high Mg/Fe, feldspathic, rare earth element-poor component in the lunar crust. Remote sensing data have strengthened this inference, showing such a composition is widespread in the FHT. My research extends the characterization of magnesian granulites to those in feldspathic lunar meteorites, which are essentially unstudied. I reconstruct the bulk chemistry of granulites in thin section by combining mineral proportions (retrieved from element X-ray maps via multispectral image classification software) and mineral compositions. This virtually non-destructive method is useful for rare samples and fragments in thin section. Magnesian granulites in Allan Hills A81005 and Dhofar 309 share similar major and trace element compositions. As these two meteorites are not paired, their comparable geochemistry may be representative of the average FHT.
Germanium/silicon of the Ediacaran-Cambrian Laobao cherts: Implications for the bedded chert formation and paleoenvironment interpretations
(American Geophysical Union, 2015) Dong, Lin; Shen, Bing; Lee, Cin-Ty A.; Shu, Xu-jie; Peng, Yang; Sun, Yuanlin; Tang, Zhuanhong; Rong, Hong; Lang, Xianguo; Ma, Haoran; Yang, Fan; Guo, Wen
Sedimentary strata of the terminal Ediacaran (635–542 Ma) to early Cambrian (542–488 Ma) Laobao-Liuchapo bedded cherts in the South China Block include the Ediacaran Oxidation Event and the Cambrian explosion. Understanding the origin and depositional environment of the bedded cherts may provide insight into how the Earth's surface environment changed between the Proterozoic and Phanerozoic. We measured major and trace element compositions and Ge/Si ratios of the Laobao cherts from northern Guangxi province. The Laobao cherts were deposited in the deep basinal environment of the South China Block. We show that the composition of the Laobao cherts is determined by a mixture of four components: quartz, clay, carbonate, and pyrite/iron-oxide. The quartz component is the dominant component of the Laobao cherts. The maximum estimated Ge/Si of the quartz component is between 0.4 and 0.5 μmol/mol, which is close to the Ge/Si of modern seawater and biogenic silica but 1 order of magnitude lower than that of hydrothermal fluids. These Ge/Si systematics suggest that normal seawater rather than mid-ocean ridge hydrothermal fluids is the primary Si source for the Laobao cherts. The Ge/Si of the clay component varies between 1 and 10 μmol/mol, which is comparable to the Ge/Si of typical marine clays, but 10–100 times lower than that of chert nodules from early Ediacaran beds (the Doushantuo Formation) predating the terminal Ediacaran Laobao cherts studied here. Our observations indicate that the clay component Ge/Si ratio decreased from the early Ediacaran to the late Ediacaran. We speculate that high Ge/Si ratios in clays reflect the preferential chelation of Ge by dissolved organic compounds adsorbed onto clays. If so, this suggests that the decrease in Ge/Si ratio of the clay component in the Ediacaran signifies a decrease in the total dissolved organic carbon content of seawater toward the Ediacaran-Cambrian transition, consistent with oxidation of the oceans during the late Ediacaran. Finally, the seawater origin of the Laobao cherts also suggests that replacement of carbonate may not be the primary cause for bedded chert formation. Instead, direct precipitation from seawater or early diagenetic silicification of calcareous sediments, perhaps due to the emergence of Si-accumulation bacteria, may have been responsible for the bedded Laobao-Liuchapo chert formation in South China Block.
Lithium systematics in global arc magmas and the importance of crustal thickening for lithium enrichment
(Springer Nature, 2020) Chen, Chen; Lee, Cin-Ty A.; Tang, Ming; Biddle, Kevin; Sun, Weidong
Much of the world’s Li deposits occurs as basinal brines in magmatic orogens, particularly in continental volcanic arcs. However, the exact origin of Li enrichment in arc magmatic systems is not clear. Here, we show that, globally, primitive arc magmas have Li contents and Li/Y ratios similar to mid-ocean ridge basalts, indicating that the subducting slab has limited contribution to Li enrichment in arc magmas. Instead, we find that Li enrichment is enhanced by lower degrees of sub-arc mantle melting and higher extents of intracrustal differentiation. These enrichment effects are favored in arcs with thick crust, which explains why magmatism and differentiation in continental arcs, like the Andes, reach greater Li contents than their island arc counterparts. Weathering of these enriched source rocks mobilizes and transports such Li into the hydrologic system, ultimately developing Li brines with the combination of arid climate and the presence of landlocked extensional basins in thickened orogenic settings.
Lithosphere versus asthenosphere mantle sources at the Big Pine Volcanic Field, California
(American Geophysical Union, 2012) Gazel, Esteban; Plank, Terry; Forsyth, Donald W.; Bendersky, Claire; Lee, Cin-Ty A.; Hauri, Erik H.
[1] Here we report the first measurements of the H2O content of magmas and mantle xenoliths from the Big Pine Volcanic Field (BPVF), California, in order to constrain the melting process in the mantle, and the role of asthenospheric and lithospheric sources in this westernmost region of the Basin and Range Province, western USA. Melt inclusions trapped in primitive olivines (Fo82–90) record surprisingly high H2O contents (1.5 to 3.0 wt.%), while lithospheric mantle xenoliths record low H2O concentrations (whole rock <75 ppm). Estimates of the oxidation state of BPVF magmas, based on V partitioning in olivine, are also high (FMQ +1.0 to +1.5). Pressures and temperatures of equilibration of the BPVF melts indicate a shift over time, from higher melting temperatures (∼1320°C) and pressures (∼2 GPa) for magmas that are >500 ka, to cooler (∼1220°C) and shallower melting (∼1 GPa) conditions in younger magmas. The estimated depth of melting correlates strongly with some trace element ratios in the magmas (e.g., Ce/Pb, Ba/La), with deeper melts having values closer to upper mantle asthenosphere values, and shallower melts having values more typical of subduction zone magmas. This geochemical stratification is consistent with seismic observations of a shallow lithosphere-asthenosphere boundary (∼55 km depth). Combined trace element and cryoscopic melting models yield self-consistent estimates for the degree of melting (∼5%) and source H2O concentration (∼1000 ppm). We suggest two possible geodynamic models to explain small-scale convection necessary for magma generation. The first is related to the Isabella seismic anomaly, either a remnant of the Farallon Plate or foundered lithosphere. The second scenario is related to slow extension of the lithosphere.
Magmatic recharge in continental flood basalts: Insights from the Chifeng igneous province in Inner Mongolia
(American Geophysical Union, 2015) Yu, Xun; Lee, Cin-Ty A.; Chen, Li-Hui; Zeng, Gang
Eruptive sequences can be used as windows into the thermal and chemical evolution of magma chambers. We examined a continuous vertical section of the Baichahe basalt flow associated with the late Cenozoic Chifeng flood basalt in Inner Mongolia, North China. From oldest to youngest, MgO increases, K2O, light rare earths and other incompatible elements decrease, and Nb/La and radiogenic Pb isotopic ratios increase, all of which indicate increasing primitiveness and decreasing contribution of crustal contamination with time. The variable Pb isotope and incompatible element ratios require a component of crustal contamination, most likely of the lower crust (unradiogenic Pb, and low Ce/Pb), in the earliest lavas. Fractional crystallization can explain some of the elemental systematics, but alone cannot explain variable incompatible element ratios and Pb isotopes, nor the temporal trend to more primitive compositions. Crustal assimilation with or without fractional crystallization also cannot explain all the elemental systematics. We find instead that recharge by a primitive magma, in combination with fractional crystallization and decreasing rates of crustal assimilation, is needed to explain the observed geochemical systematics. Our observations suggest that the delivery of fresh basalt to the magma chamber must increase at rates faster than the crust can be assimilated or that the rates of crustal assimilation must decrease. However, progressive addition of primitive magma should heat up the crust and lead to more crustal assimilation. We suggest that during the initial stages of forming a magma chamber, the magma cools and develops an outer crystalline rind of mafic to ultramafic cumulates. This results in a thickening nonconvecting chemical boundary layer, which serves to insulate the magma chamber from further assimilation of crust and cooling, the latter resulting in the reduction of crystallization rates and the buffering of magma compositions at more primitive compositions. We show that certain segments of other large igneous provinces also display an evolution toward more primitive magmas with time, indicating that magmatic recharge may be a common feature of basaltic magma chambers.
Marine Carbon and Sulfur Geochemical Cycling: Reassessing Sulfate Reduction and Anaerobic Oxidation of Methane
(2017-11-28) Miller, Clint Matthew; Dickens, Gerald R.; Lee, Cin-Ty A.
Anaerobic oxidation of methane (AOM) likely represents a globally important process impacting chemical cycling of C and S across Earth’s surface. Although ubiquitous along continental margins, AOM remains underappreciated, as clear from its absence in most models for global C and S cycling. To some degree, this is because of longstanding ideas regarding the decomposition of particulate organic carbon, which assume organoclastic sulfate reduction (OSR) is the engine of carbon oxidation. Perhaps, more importantly, many sites purportedly dominated by AOM lack high-resolution pore water data, a full suite of pertinent sedimentary analyses, or both. Here, we collect and describe published seafloor organic carbon data, identify more than 400 locations with AOM in shallow sediments globally, and measure dissolved and solid concentrations of phases containing C and S at three locations to test whether AOM can drive most mass fluxes of these elements. The sites, located on slopes east of Peru and west of Japan, are under different oceanographic conditions and at different water depths (330-5086 m), but display commonalities in pore water profiles consistent with AOM. Dissolved SO42-, HS-, Ca2+, Mg2+, and Sr2+ fluxes display strong inflections at the sulfate-methane transition (SMT), while dissolved Mn2+ and Fe2+, though strongly correlated, are little affected at the SMT. Solid sulfides and perhaps S-bound organic carbon begin precipitating within 0.1 m below the seafloor (mbsf), culminating in sediment S contents of 1.18 wt. %. By contrast, authigenic carbonates begin to precipitate about 2 mbsf. Calculated sulfur mass accumulation rates (1200-4400 mol S m-2 ky-1), and authigenic carbonate precipitation rates (7.1-12.0 mol CaCO3 m-2 ky-1) are comparable to OSR published values. Inverse modeling of pore water profiles produce depth profiles with SMTs between 6.3 and 9.6 m thick, conversion of Ca2+, Mg2+, Sr2+ from solute to solid, and C and S mass balance. Additionally, modeled Fe-AOM and Mn-AOM rates are at least three orders of magnitude less than SO42-. These results show that AOM removes similar amounts of C and S from the ocean as OSR. Globally, we find AOM occurs on all continental margins at water depths up to 5500 m, indicating high CH4 flux is a common occurrence in today’s ocean. If this has also been the case in the geologic past, elemental flux models of Earth’s history may need to be revisited.
Missing Lead and High 3He/4He in Ancient Sulfides Associated with Continental Crust Formation
(Nature Publishing Group, 2014) Huang, Shichun; Lee, Cin-Ty A.; Yin, Qing-Zhu
Major terrestrial reservoirs have Pb isotopes more radiogenic than the bulk silicate Earth. This requires a missing unradiogenic Pb reservoir, which has been argued to reside in the lower continental crust or dissolved in the core. Chalcophile element studies indicate that continent formation requires the formation of sulfide-bearing mafic cumulates in arcs. Because Pb, but not U, partitions into sulfides, we show that continent formation must have simultaneously generated time-integrated unradiogenic Pb reservoirs composed of sulfide-bearing cumulates, now recycled back into the mantle or stored deep in the continental lithosphere. The generation of such cumulates could also lead to coupled He-Pb isotopic systematics because 4He is also produced during U-Th-Pb decay. Here, we show that He may be soluble in sulfide melts, such that sulfide-bearing cumulates would be enriched in both Pb and He relative to U and Th, “freezing” in He and Pb isotopes of the ambient mantle at the time of sulfide formation. This implies that ancient sulfide-bearing cumulates would be characterized by unradiogenic Pb and He isotopes (high-3He/4He). These primitive signatures are usually attributed to primordial, undifferentiated mantle, but in this case, they are the very imprint of mantle differentiation via continent formation.

Browsing by Author "Lee, Cin-Ty A."

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