ALBERT

Description: Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 173 (2016): 64-85, doi:10.1016/j.gca.2015.10.021.

Description: Hydrothermal vent deposits form on the seafloor as a result of cooling and mixing of hot hydrothermal fluids with cold seawater. Amongst the major sulfide and sulfate minerals that are preserved at vent sites, barite (BaSO4) is unique because it requires the direct mixing of Ba-rich hydrothermal fluid with sulfate-rich seawater in order for precipitation to occur. Because of its extremely low solubility, barite crystals preserve geochemical fingerprints associated with conditions of formation. Here, we present data from petrographic and geochemical analyses of hydrothermal barite from the Endeavour Segment of the Juan de Fuca Ridge, northeast Pacific Ocean, in order to determine the physical and chemical conditions under which barite precipitates within seafloor hydrothermal vent systems. Petrographic analyses of 22 barite-rich samples show a range of barite crystal morphologies: dendritic and acicular barite forms near the exterior vent walls, whereas larger bladed and tabular crystals occur within the interior of chimneys. A two component mixing model based on Sr concentrations and 87Sr/86Sr of both seawater and hydrothermal fluid, combined with 87Sr/86Sr data from whole rock and laser-ablation ICP-MS analyses of barite crystals indicate that barite precipitates from mixtures containing as low as 17% and as high as 88% hydrothermal fluid component, relative to seawater. Geochemical modelling of the relationship between aqueous species concentrations and degree of fluid mixing indicates that Ba2+ availability is the dominant control on mineral saturation. Observations combined with model results support that dendritic barite forms from fluids of less than 40% hydrothermal component and with a saturation index greater than ~0.6, whereas more euhedral crystals form at lower levels of supersaturation associated with greater contributions of hydrothermal fluid. Fluid inclusions within barite indicate formation temperatures of between ~120 and 240°C during barite crystallization. The comparison of fluid inclusion formation temperatures to modelled mixing temperatures indicates that conductive cooling of the vent fluid accounts for 60 – 120°C reduction in fluid temperature. Strontium zonation within individual barite crystals records fluctuations in the amount of conductive cooling within chimney walls that may result from cyclical oscillations in hydrothermal fluid flux. Barite chemistry and morphology can be used as a reliable indicator for past conditions of mineralization within both extinct seafloor hydrothermal deposits and ancient land-based volcanogenic massive sulfide deposits.

Description: This work was supported by an NSERC PGS scholarship to JWJ and NSERC Discovery Grant to MDH. MKT acknowledges funding from NSF OCE- 1130019. DAB acknowledges funding from NSF OCE-0731947 and the Joint Institute for the Study of the Atmosphere and Ocean (JISAO) under NOAA Cooperative Agreement NA10OAR4320148.

Description: Processes linked to shallow subduction, slab rollback, and extension are recorded in the whole-rock major-, trace-element, and Sr, Nd, and Pb isotopic compositions of mafic magmatic rocks in both time and space over southwestern United States. Eocene to Mio-Pliocene volcanic rocks were sampled along a transect across the west-central Great Basin (GB) in Nevada to the Ancestral Cascade Arc (ACA) in the northern Sierra Nevada, California (∼39°–40° latitude), which are interpreted to represent a critical segment of a magmatic sweep that occurred as a result of subduction from east-northeast convergence between the Farallon and North American plates and extension related to the change from a convergent to a transform margin along the western edge of North America. Mafic volcanic rocks from the study area can be spatially divided into three broad regions: GB (5–35 Ma), eastern ACA, and western ACA (2.5–16 Ma). The volcanic products are dominantly calc-alkalic but transition to alkalic toward the east. Great Basin lavas erupted far inland from the continental margin and have higher K, P, Ti, and La/Sm as well as lower (Sr/P)pmn, Th/Rb, and Ba/Nb compared to ACA lavas. Higher Pb isotopic values, combined with lower Ce/Ce* and high Th/Nb ratios in some ACA lavas, are interpreted to come from slab sediment. Mafic lavas from the GB and ACA have overlapping 87Sr/86Sr and 143Nd/144Nd values that are consistent with mantle wedge melts mixing with a subduction-modified lithospheric mantle source. Eastern and western ACA lavas largely overlap in age and elemental and isotopic composition, with the exception of a small subset of lavas from the westernmost ACA region; these lavas show lower 87Sr/86Sr at a given 143Nd/144Nd. Results show that although extension contributes to melting in some regions (e.g., selected lavas in the GB and Pyramid Lake), chemical signatures for most mafic melts are dominated by subduction-related mantle wedge and a lithospheric mantle component.

Description: Latest Pliocene to Quaternary, mildly alkaline, mafic to intermediate volcanic activity extends in a swath from the Lake Tahoe region in the eastern Sierra Nevada across the western Great Basin to the Battle Mountain area, Nevada. From west to east, the volcanic centers exhibit a dramatic gradient in chemical and isotopic composition. Centers situated in or adjacent to the Sierra Nevada have incompatible element and isotopic compositions consistent with an old, subduction-modified lithospheric mantle source (87Sr/86Sr 〉 0.7045; 143Nd/144Nd 〈 0.5127; δ18O 〉 +6.5‰). Mafic volcanic centers east of the Sierra Nevada, in the Carson Sink and in the Buffalo Valley region, have an intraplate incompatible element and isotopic signature (87Sr/86Sr 〈 0.7045; 143Nd/144Nd 〉 0.5127; δ18O 〈 +6.5‰) consistent with an asthenospheric mantle source. Earlier 20–3 Ma arc volcanism in the Sierra Nevada also tapped the old lithospheric mantle source along with the mantle wedge, indicating that the lithospheric mantle source existed well prior to the onset of Tertiary arc volcanism and probably prior to Mesozoic igneous activity of the Sierra Nevada. Thus, the lithospheric mantle beneath the Sierra Nevada has remained a geochemically consistent, fertile, fusible source for at least the past 20 m.y. Old lithospheric mantle likely still exists east of the Sierra Nevada, but lithospheric thinning and/or exhaustion of fusible components inhibit its melting, such that during the Quaternary, melting could only occur in the asthenosphere.

Description: The Baguio district of the Philippines is one of the world’s premier mineral provinces, containing 〉35 million ounces (Moz) of gold and 2.7 million metric tons (Mt) of copper in epithermal, porphyry, and skarn deposits that formed in the last 3.5 m.y. Pliocene and Pleistocene magmatic rocks of the Baguio district that are spatially and temporally associated with mineralization can be broadly subdivided into an intermediate to felsic suite of mineralized low to medium K intrusions, some of which have adakitic affinities and a suite of mafic to intermediate, medium K to shoshonitic hornblende-phyric dikes. The geochemical and isotopic characteristics of the dikes are consistent with primitive mantle-derived melts that underwent minimal crustal contamination as they ascended through the arc crust. In contrast, the intermediate to felsic suite has been contaminated by young arc crust, suggesting ponding and fractionation within shallow-crustal magma chambers.The Philippine arc has formed in a complex tectonic environment and is currently sandwiched between two active subduction zones. Eastward-directed subduction of the Scarborough Ridge along the Manila trench is currently associated with flattening of the downgoing slab. The formation of the Mafic dike complex is broadly coeval with the onset of subduction of the Scarborough Ridge and slab flattening. The extinct Scarborough Ridge would have been younger than the downgoing plate and consequently more susceptible to melting. These melts can account for the isotopic recharge of the Pliocene subarc mantle as well as the generation of the primitive melts and adakitic rocks found within the Baguio district. The interaction between primitive mafic melts and the more felsic calc-alkaline rocks has generated fertile melts that were highly productive for porphyry copper and epithermal gold mineralization.

Description: The Warner Range in northeastern California exposes a section of Tertiary rocks over 3 km thick, offering a unique opportunity to study the long-term history of Cascade arc volcanism in an area otherwise covered by younger volcanic rocks. The oldest locally sourced volcanic rocks in the Warner Range are Oligocene (28-24 Ma) and include a sequence of basalt and basaltic andesite lava flows overlain by hornblende and pyroxene andesite pyroclastic flows and minor lava flows. Both sequences vary in thickness (0-2 km) along strike and are inferred to be the erosional remnants of one or more large, partly overlapping composite volcanoes. No volcanic rocks were erupted in the Warner Range between ca. 24 and 16 Ma, although minor distally sourced silicic tuffs were deposited during this time. Arc volcanism resumed ca. 16 Ma with eruption of basalt and basaltic andesite lavas sourced from eruptive centers 5-10 km south of the relict Oligocene centers. Post-16 Ma arc volcanism continued until ca. 8 Ma, forming numerous eroded but well-preserved shield volcanoes to the south of the Warner Range. Oligocene to Late Miocene volcanic rocks in and around the Warner Range are calc-alkaline basalts to andesites (48%-61% SiO2) that display negative Ti, Nb, and Ta anomalies in trace element spider diagrams, consistent with an arc setting. Middle Miocene lavas in the Warner Range are distinctly different in age, composition, and eruptive style from the nearby Steens Basalt, with which they were previously correlated. Middle to Late Miocene shield volcanoes south of the Warner Range consist of homogeneous basaltic andesites (53%-57% SiO2) that are compositionally similar to Oligocene rocks in the Warner Range. They are distinctly different from younger (Late Miocene to Pliocene) high-Al, low-K olivine tholeiites, which are more mafic (46%-49% SiO2), did not build large edifices, and are thought to be related to backarc extension. The Warner Range is [~]100 km east of the axis of the modern arc in northeastern California, suggesting that the Cascade arc south of modern Mount Shasta migrated west during the Late Miocene and Pliocene, while the arc north of Mount Shasta remained in essentially the same position. We interpret these patterns as evidence for an Eocene to Miocene tear in the subducting slab, with a more steeply dipping plate segment to the north, and an initially more gently dipping segment to the south that gradually steepened from the Middle Miocene to the present.

Description: The Pyrites Complex in the Adirondack Lowlands domain of the Grenville Province forms the core of a large NE-trending, elongate, winged structure [~]15 km long dominated by highly deformed metagabbro, amphibolite, and hornblende schist. A previously unrecognized km-scale boudin of metamorphosed ultramafic rocks associated with the belt is described. It is largely replaced by secondary hydrous minerals, but retains relict igneous textures and some primary minerals such as augite and chromite, and is cut by several 1-2-m-wide lamprophyre dikes. The ultramafic rocks are interpreted as part of an obducted ophiolite complex on the basis of its structural contact with a belt of rocks including marine metasedimentary rocks, pyritic gneisses, metagabbros, and amphibolites with mid-ocean ridge basalt chemistry and were emplaced within a collapsing backarc basin of Shawinigan age. Small (50-200 {micro}) zircon crystals separated from peridotite and pyroxenite yield minimum ages (1140 {+/-} 7 and 1197 {+/-} 5 Ma) and constrain the timing of metamorphic and thermal events associated with the Shawinigan orogenesis and anorthosite-mangerite-charnockite-granite (AMCG) plutonism. Neodymium TDM ages from the Pyrites Complex range from 1440 to 2650 Ma, are not compatible with derivation from a typical depleted mantle reservoir, and suggest, along with incompatible element concentrations, that these rocks record mantle enrichment, presumably during subduction beneath the leading edge of Laurentia. Rifting and development of oceanic crust between the southern Adirondack Highlands and Lowlands, coincident with a similar backarc rifting in the Central Metasedimentary Belt at ca. 1300 Ma, are proposed. Three mafic suites in the Adirondack Lowlands are distinguished by their field relations, age, geochemistry, and Nd isotopic systematics and reflect the various stages of evolution of the Trans-Adirondack backarc basin. Within the Lowlands interleaved evaporites, metasedimentary and possible metavolcanic rocks, calc-alkaline and transitional plutonic rocks, and ophiolitic rocks of the Pyrites Complex provide constraints on the tectonic processes and sedimentary response to development of a backarc basin, magmatic arc, foredeep sedimentation, and ophiolite obduction during Shawinigan convergence from ca. 1200-1160 Ma, which culminated in slab breakoff and plate delamination resulting in intrusion of the AMCG suite throughout the Frontenac-Adirondack terrane and beyond.

Description: Volcanic rock and mantle xenolith compositions in the Sierra Nevada (western United States) contradict a commonly held view that continental crust directly overlies asthenosphere beneath the Sierran range front, and that ancient continental mantle lithosphere (CML) was entirely removed in the Pliocene. Instead, space-time trends show that the Walker Lane is the principle region of mantle upwelling and lithosphere removal in eastern California, that lithosphere loss follows the migration of the Mendocino Triple Junction (MTJ), and that the processes of lithosphere removal are not yet complete beneath the Sierra Nevada and its range front. Key evidence is provided by volcanic rock compositions. The 87Sr/86Sr ratios for mafic volcanics of the Sierra (MgO 〉 8%) are mostly 〉 0.705, and 143Nd/144Nd 〈 0.5127, much unlike eastern sub-Pacific asthenosphere (where 87Sr/86Sr 〈 0.7027 and 143Nd/144Nd 〉 0.5129), but very much like CML. Similarly, Sierran volcanics carry CML-like trace element ratios, with La/Nb 〉 3 and Th/Nb 〉 1, values that are significantly higher than asthenosphere-derived melts (La/Nb 〈 1.5, Th/Nb 〈 0.08). Spinel-bearing mantle xenoliths contained in Pleistocene–Holocene volcanics from the Sierran range front also have a CML composition, with 87Sr/86Sr and ?Nd ratios that range to 0.7065 and –3.6, respectively. New estimates of melt extraction depths using Si activity and mineralogy-sensitive trace element ratios (Sm/Yb, Lu/Hf) show that CML extends from the base of the crust (40 km) to depths of 75 km beneath the range front, and to 110 km for Pliocene volcanics of the southern Sierra. This means that garnet-bearing lithologies could not have been dislodged from beneath the southern Sierra until after the Pliocene. Only in the Walker Lane do young (0.18 Ma) volcanic rocks, from the Coso volcanic field, approach asthenosphere-like compositions, which occurs only 20 Ma after MTJ arrival. Temporal trends show that MTJ arrival at any given latitude south of 37°N signals lithosphere heating, probably due to asthenosphere that upwells to replace the sinking Farallon plate. Partial melts of the asthenosphere, and perhaps the asthenosphere itself, intrude into and cause heating and partial melting of overlying CML; this culminates after 10 Ma. After 20 Ma, CML becomes highly degraded and asthenosphere-derived melts are dominant. North of 37°N, volcanic rocks approach asthenosphere-derived compositions to the west, not the east, and 87Sr/86Sr ratios increase from 18 to 0 Ma, indicating that this region has entered a phase of lithosphere heating, but not yet a phase of lithosphere removal.We propose a new model of lithosphere degradation, where asthenosphere or its partial melts pervasively invade CML beneath the Walker Lane. This process is now nearly complete beneath Coso, and is migrating west, so that it is only partly complete at the southern Sierra range front, or within the Sierra Nevada, at any latitude. This model of intermixed asthenosphere and lithosphere better explains the compositions of volcanic rocks and their included xenoliths, and the remarkably consistent S-wave receiver function data, which show a 70-km-thick lithosphere beneath the Sierra Nevada. If the upper mantle is warm CML, permeated by partial melts, this model may also explain low P- and S-wave velocities.

Description: Coronitic metagabbros (CMGs) in the Adirondack Highlands display chemical and isotopic features consistent with derivation from an enriched asthenospheric mantle source and are samples of the parental magma of Adirondack anorthosite. Primary ophitic textures in the CMGs are overprinted by mineral coronas developed during granulite-facies metamorphism of anhydrous olivine gabbronorites during the Ottawan orogeny. Restricted in silica content (45-48 wt%) and olivine normative, the CMGs are predominantly tholeiitic in composition, although a minority display some calc-alkaline features. Unlike older Adirondack mafic and felsic suites, the CMG rocks lack or have greatly reduced, incompatible element patterns (NPM) generally associated with subduction processes. Rare-earth elements (NCH) have minor light rare-earth element (LREE) enrichment with La/Sm values from 1.42 to 1.98, compatible with a transitional to enriched mantle source. When CMGs are plotted on various major (TiO2 versus P2O5) and trace-element (Sm versus Cr) diagrams, the CMGs form a continuous field between that of oxide- and apatite-rich gabbros (OGNs and OAGNs) and anorthosites and leucogabbros within the Adirondacks. Initial epsilon Nd ({varepsilon}Nd) values are +3.13 to +3.69, generally higher than Adirondack anorthosite values, but significantly less than contemporaneous depleted mantle. Neodymium TDM model ages that are [~]400 million years older than their crystallization age and enriched compositional trends indicate that they were not derived from depleted mantle. These data indicate Adirondack CMGs were derived from a previously untapped and enriched asthenospheric source. Asthenospheric upwelling was triggered by lithospheric delamination following Shawinigan orogenesis at ca. 1160 Ma and provides a link between tectonism, mantle geodynamics, and massif anorthosite petrogenesis in the Grenville allochthonous monocyclic belt.