46 research outputs found
Missing Lead and High 3He/4He in Ancient Sulfides Associated with Continental Crust Formation
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, âfreezingâ 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
Lithospheric mantle duplex beneath the central Mojave Desert revealed by xenoliths from Dish Hill, California
Low-angle subduction of oceanic lithosphere may be an important process in
modifying continental lithosphere. A classic example is the underthrusting of the Farallon
plate beneath North America during the Laramide orogeny. To assess the relevance of this
process to the evolution and composition of continental lithosphere, the mantle
stratigraphy beneath the Mojave Desert was constrained using ultramafic xenoliths hosted
in Plio-Pleistocene cinder cones. Whole-rock chemistry, clinopyroxene trace element and
Nd isotope data, in combination with geothermometry and surface heat flow, indicate
kilometer-scale compositional layering. The shallow parts are depleted in radiogenic Nd
(Δ_(Nd) = -13 to -6.4) and are interpreted to be ancient continental mantle that escaped
tectonic erosion by low-angle subduction. The deeper samples are enriched in radiogenic
Nd (Δ_(Nd) = +5.7 to +16.1) and reveal two superposed mantle slices of recent origin. Within
each slice, compositions range from fertile lherzolites at the top to harzburgites at the
bottom: the latter formed by 25â28% low-pressure melt depletion and the former formed
by refertilization of harzburgites by mid-ocean-ridge-basalt-like liquids. The superposition
and internal compositional zonation of the slices preclude recent fertilization by Cenozoic
extension-related magmas. The above observations imply that the lower Mojavian
lithosphere represents tectonically subcreted and imbricated lithosphere having an oceanic
protolith. If so, the lherzolitic domains may be related to melting and refertilization
beneath mid-ocean ridges. The present Mojavian lithosphere is thus a composite of a
shallow section of the original North American lithosphere underlain by Farallon oceanic
lithosphere accreted during low-angle subduction
Fluorine and chlorine in mantle minerals and the halogen budget of the Earthâs mantle
Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Contributions to Mineralogy and Petrology 172 (2017): 51, doi:10.1007/s00410-017-1368-7.The fluorine (F) and chlorine (Cl) contents of arc magmas have been used to track the
composition of subducted components, and the F and Cl contents of MORB have been used to
estimate the halogen content of depleted MORB mantle (DMM). Yet, the F and Cl budget of the
Earth's upper mantle, and their distribution in peridotite minerals, remains to be constrained.
Here we developed a method to measure low concentrations of halogens (â„ 0.4 ÎŒg/g F and â„ 0.3
ÎŒg/g Cl) in minerals by secondary ion mass spectroscopy. We present a comprehensive study of
F and Cl in natural olivine, orthopyroxene, clinopyroxene, and amphibole in seventeen samples
from different tectonic settings. We support the hypothesis that F in olivine is controlled by melt
polymerization, and that F in pyroxene is controlled by their Na and Al contents, with some
effect of melt polymerization. We infer that Cl compatibility ranks as follows: amphibole >
clinopyroxene > olivine ~ orthopyroxene, while F compatibility ranks as follows: amphibole >
clinopyroxene > orthopyroxene â„ olivine, depending on the tectonic context. In addition, we
show that F, Cl, Be and B are correlated in pyroxenes and amphibole. F and Cl variations suggest
that interaction with slab melts and fluids can significantly alter the halogen content of mantle
minerals. In particular, F in oceanic peridotites is mostly hosted in pyroxenes, and proportionally
increases in olivine in subduction-related peridotites. The mantle wedge is likely enriched in F
compared to un-metasomatized mantle, while Cl is always low (< 1 ÎŒg/g) in all tectonic settings
studied here. The bulk anhydrous peridotite mantle contains 1.4â31 ÎŒg/g F and 0.14â0.38 ÎŒg/g
Cl. The bulk F content of oceanic-like peridotites (2.1â9.4 ÎŒg/g) is lower than DMM estimates,
consistent with F-rich eclogite in the source of MORB. Furthermore, the bulk Cl budget of all
anhydrous peridotites studied here is lower than previous DMM estimates. Our results indicate
that nearly all MORB may be somewhat contaminated by seawater-rich material and that the Cl
content of DMM could be overestimated. With this study, we demonstrate that the halogen
contents of natural peridotite minerals are a unique tool to understand the cycling of halogens,
from ridge settings to subduction zones.This research was supported by grant NSF EAR-P&G 1524311 and DOEI award 18563 to VLR. Urann was supported by
the Stanley W. Watson Student Fellowship Fund based at WHOI
Young solid Earth researchers of the world unite!
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95272/1/eost14667.pd
Thallium isotopes in early diagenetic pyrite â a paleoredox proxy?
Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 75 (2011): 6690-6704, doi:10.1016/j.gca.2011.07.047.This paper presents the first study of Tl isotopes in early diagenetic pyrite. Measurements from two sections deposited during the Toarcian Ocean Anoxic Event (T-OAE, ~183Ma) are compared with data from Late Neogene (<10Ma) pyrite samples from ODP legs 165 and 167 that were deposited in relatively oxic marine environments. The Tl isotope compositions of Late Neogene pyrites are all significantly heavier than seawater, which most likely indicates that Tl in diagenetic pyrite is partially sourced from ferromanganese oxy-hydroxides that are known to display relatively heavy Tl isotope signatures. One of the T-OAE sections from Peniche in Portugal displays pyrite thallium isotope compositions indistinguishable from Late Neogene samples, whereas samples from Yorkshire in the UK are depleted in the heavy isotope of Tl. These lighter compositions are best explained by the lack of ferromanganese precipitation at the sedimentâwater interface due the sulphidic (euxinic) conditions thought to be prevalent in the Cleveland Basin where the Yorkshire section was deposited. The heavier signatures in the Peniche samples appear to result from an oxic water column that enabled precipitation of ferromanganese oxy-hydroxides at the sedimentâwater interface.
The Tl isotope profile from Yorkshire is also compared with previously published molybdenum isotope ratios determined on the same sedimentary succession. There is a suggestion of an anti-correlation between these two isotope systems, which is consistent with the expected isotope shifts that occur in seawater when marine oxic (ferromanganese minerals) fluxes fluctuate.
The results outlined here represent the first evidence that Tl isotopes in early diagenetic pyrite have potential to reveal variations in past ocean oxygenation on a local scale and potentially also for global oceans. However, much more information about Tl isotopes in different marine environments, especially in anoxic/euxinic basins, is needed before Tl isotopes can be confidently utilized as a paleo-redox tracer.SGN is funded by a NERC fellowship
Lithosphere versus asthenosphere mantle sources at the Big Pine Volcanic Field, California
[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 (Fo82â90) 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 (âŒ1320°C) and pressures (âŒ2 GPa) for magmas that are >500 ka, to cooler (âŒ1220°C) and shallower melting (âŒ1 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 (âŒ55 km depth). Combined trace element and cryoscopic melting models yield self-consistent estimates for the degree of melting (âŒ5%) and source H2O concentration (âŒ1000 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
Continental crust formation at arcs, the arclogite âdelaminationâ cycle, and one origin for fertile melting anomalies in the mantle
The total magmatic output in modern arcs, where continental crust is now being formed, is believed to derive from melting of the mantle wedge and is largely basaltic. Globally averaged continental crust, however, has an andesitic bulk composition and is hence too silicic to have been derived directly from the mantle. It is well known that one way this imbalance can be reconciled is if the parental basalt differentiates into a mafic garnet pyroxenitic residue/cumulate (âarclogiteâ) and a complementary silicic melt, the former foundering or delaminating into the mantle due to its high densities and the latter remaining as the crust. Using the Sierra Nevada batholith in California as a case study, the composition of mature continental arc crust is shown in part to be the product of a cyclic process beginning with the growth of an arclogite layer followed by delamination of this layer and post-delamination basaltic underplating/recharge into what remains of the continental crust. A model is presented, wherein continuous arc magmatism and production of arclogites in continental arcs are periodically punctuated by a delamination event and an associated magmatic pulse every ~10â30 My. The recycling flux of arclogites is estimated to be ~5 %â20 % that of oceanic crust recycling by subduction. Delaminated arclogites have the necessary trace-element compositions to yield time-integrated isotopic compositions similar to those inferred to exist as reservoirs in the mantle. Because of their low melting temperatures, such pyroxenites may be preferentially melted, possibly forming a component of some hotspot magmas