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Barnes, JD, Beltrando M, Lee C-TA, Cisneros M, Loewy S, Chin E.  2014.  Geochemistry of Alpine serpentinites from rifting to subduction: A view across paleogeographic domains and metamorphic grade. Chemical Geology. 389:29-47.   10.1016/j.chemgeo.2014.09.012   AbstractWebsite

Serpentinites from several tectono-metamorphic units of the Western Alps were studied to constrain their origin and tectonic setting of serpentinization. Study areas were selected to cover the whole width of the orogen and a wide range of metamorphic grades from anchizone (Canavese Zone) to greenschist facies (St. Barthelemy, Piemonte Zone) to blueschist facies (Rocca Canavese unit and Punta Rossa unit). Bulk rock serpentinite samples have high REE concentrations, compared to typical mid-ocean ridge serpentinites, with nearly flat REE patterns. Relict spinels from the Rocca Canavese unit have extremely low Cr#s (average = 0.087) and high Mg#s (average = 0.798) suggesting very low degrees of melt depletion. Both of these observations are consistent with an abyssal origin in a hyper-extended rifted margin with minimal melt depletion, or refertilization. Seafloor hydration between 150 and 200 °C is indicated by oxygen isotope data (δ18O values = + 5.2 to + 9.4‰), supporting lithostratigraphic evidence of exhumation to the floor of the Alpine Tethys already available for the Canavese, St. Barthelemy and Punta Rossa serpentinites. Subsequent interaction with the metasediments during Alpine metamorphism resulted in variations in trace element concentrations and stable isotope compositions with decreasing distance to the interface between the sediment and serpentinite. The chemical gradient between the ultramafic rocks and the neighboring metasediments is best seen in the Punta Rossa unit, where Pb, Ba, Cs, U, and Rb concentrations increase, δ18O values increase, δ37Cl values decrease within the serpentinite with decreasing distance to the contact and a “blackwall” of pure chlorite is found at the contact. As these contacts between ultramafic rocks, continental basement and meta-sediments are analogous to the slab–mantle interface, our results support the mobility of Pb, Ba, Cs, U, Rb, Cl, and water at the scale of < 10 m across the interface during Alpine metamorphism. However, the preservation of geochemical gradients within the Punta Rossa serpentinite indicates a limited role for externally derived fluid flux.

Chin, EJ.  2018.  Deep crustal cumulates reflect patterns of continental rift volcanism beneath Tanzania. Contributions to Mineralogy and Petrology. 173   10.1007/s00410-018-1512-z   AbstractWebsite

Magmatism on Earth is most abundantly expressed by surface volcanic activity, but all volcanism has roots deep in the crust, lithosphere, and mantle. Intraplate magmatism, in particular, has remained enigmatic as the plate tectonic paradigm cannot easily explain phenomena such as large flood basalt provinces and lithospheric rupture within continental interiors. Here, I explore the role of deep crustal magmatic processes and their connection to continental rift volcanism as recorded in deep crustal xenoliths from northern Tanzania. The xenoliths are interpreted as magmatic cumulates related to Cenozoic rift volcanism, based on their undeformed, cumulate textures and whole-rock compositions distinct from melt-reacted peridotites. The cumulates define linear trends in terms of whole-rock major elements and mineralogically, can be represented as mixtures of olivine+clinopyroxene. AlphaMELTS modeling of geologically plausible parental melts shows that the end-member cumulates, clinopyroxenite and Fe-rich dunite, require fractionation from two distinct melts: a strongly diopside-normative melt and a fractionated picritic melt, respectively. The former can be linked to the earliest, strongly silica-undersaturated rift lavas sourced from melting of metasomatized lithosphere, whereas the latter is linked to the increasing contribution from the upwelling asthenospheric plume beneath East Africa. Thus, deep crustal cumulate systematics reflect temporal and compositional trends in rift volcanism, and show that mixing, required by the geochemistry of many rift lava suites, is also mirrored in the lavas' cumulates.

Chin, EJ, Lee C-TA, Barnes JD.  2014.  Thickening, refertilization, and the deep lithosphere filter in continental arcs: Constraints from major and trace elements and oxygen isotopes. Earth and Planetary Science Letters. 397:184-200.   10.1016/j.epsl.2014.04.022   AbstractWebsite

Arc magmatism is a complex process involving generation of primary melts in the mantle wedge and chemical refinement of these melts into differentiated products akin to continental crust. Interaction of magmas (cooling, crystallization and assimilation) with the overlying crust, particularly if it is thick, is one way by which primary basalts are refined into more evolved compositions. Here, we explore the role of the mantle lithosphere as a trap and/or reactive filter of magmas. We use mantle xenoliths from the Sierra Nevada continental arc in California as a probe into sub-Moho processes. Based on clinopyroxene modal abundance and major, minor and moderately incompatible trace element concentrations, the peridotites define a refertilization trend that increases with depth, grading from clinopyroxene-poor ( < 5 % ), undeformed spinel peridotites equilibrated at < 3 GPa ( < 90 km ) to clinopyroxene-rich (10–20%), porphyroclastic garnet peridotites equilibrated between 3 and 3.5 GPa (90–105 km), the latter presumably approaching the top of the subducting slab. The petrology and geochemistry of the xenoliths suggest that the fertile peridotites were originally depleted spinel peridotites, which were subsequently refertilized. Incompatible trace element geochemistry reveals a pervasive cryptic metasomatic overprint in all peridotites, suggesting involvement of small amounts of subduction-derived fluids from the long-lived Farallon plate beneath western North America. However, bulk reconstructed δ O SMOW 18 values of the peridotites, including the most refertilized, fall between 5.4 and 5.9 ‰ , within the natural variability of unmetasomatized mantle ( ∼ 5.5 ± 0.2 ‰ ). Together with Sm, Yb, and Ca compositional data, the oxygen isotope data suggest that the role of slab or sediment melts in refertilizing the peridotites was negligible ( < 5 % in terms of added melt mass). Instead, binary mixing models suggest that many of the Sierran garnet peridotites, particularly those with high clinopyroxene modes, had up to 30% mantle-derived melt added. Our data suggest that refertilization of the deep arc lithosphere, via melt entrapment and clinopyroxene precipitation, may be an important process that modifies the composition of primary arc magmas before they reach the crust and shallowly differentiate. Comparison of our data with a global compilation of arc-related mantle xenoliths suggests that sub-Moho refertilization may be more extensive beneath mature arcs, such as continental arcs, compared to juvenile island arcs, possibly because of the greater thickness of crust and lithosphere beneath mature and island arcs.

Chin, EJ, Soustelle V, Hirth G, Saal A, Kruckenberg SC, Eiler JM.  2016.  Microstructural and geochemical constraints on the evolution of deep arc lithosphere. Geochemistry, Geophysics, Geosystems. 17:2497–2521.   10.1002/2015GC006156  
Chin, EJ, Lee CTA, Blichert-Toft J.  2015.  Growth of upper plate lithosphere controls tempo of arc magmatism: Constraints from Al-diffusion kinetics and coupled Lu-Hf and Sm-Nd chronology. Geochemical Perspectives Letters. 1:20-32.   10.7185/geochemlet.1503   AbstractWebsite

Most magmatism occurs at mid-ocean ridges, where plate divergence leads to decompression melting of the mantle, and at volcanic arcs, where subduction leads to volatile-assisted decompression melting in the hot mantle wedge. While plate spreading and subduction are continuous, arc magmatism, particularly in continental arcs, is characterised by >10-50 Myr intervals of enhanced magmatic activity followed by rapid decline (DeCelles et al.). In some cases, such as the Andes, this pattern has recurred several times (Haschke et al., 2002). Abrupt changes in plate convergence rates and direction (Pilger, 1984) or repeated steepening and shallowing of subducting slabs (Kay and Coira, 2009) have been suggested as triggering flare-ups or terminating magmatism, but such scenarios may not be sufficiently general. Here, we examine the thermal history of deep crustal and lithospheric xenoliths from the Cretaceous Sierra Nevada batholith, California (USA). The deepest samples (~90 km), garnet-bearing spinel peridotites, show cooling-related exsolution of garnet from high-Al pyroxenes originally formed at >1275 °C. Modelling of pyroxene Al diffusion profiles requires rapid cooling from 1275 to 750 °C within ~10 Myr. Also suggesting deep-seated, rapid cooling is a garnet websterite from ~90 km depth with nearly identical Lu-Hf (92.6 ± 1.6 Ma) and Sm-Nd (88.8 ± 3.1 Ma) isochron ages to within error. Thermal modelling shows that this cooling history can be explained by impingement of the base of the Sierran lithosphere against a cold subducting slab at ~90 km depth, precluding cooling by shallowing subduction. Rather, the coincidence of the radiometric ages with the magmatic flare-up (120-80 Ma) suggests that the hot mantle wedge above the subducting slab may have been pinched out by magmatic (± tectonic) thickening of the upper plate, eventually terminating mantle melting. Magmatic flare-ups in continental arcs are thus self-limiting, which explains why continental arc magmatism occurs in narrow time intervals. Convective removal of the deep arc lithosphere can initiate another magmatic cycle.

Chin, EJ, Lee CTA, Tollstrup DL, Xie LW, Wimpenny JB, Yin QZ.  2013.  On the origin of hot metasedimentary quartzites in the lower crust of continental arcs. Earth and Planetary Science Letters. 361:120-133.   10.1016/j.epsl.2012.11.031   AbstractWebsite

Volcanic arcs associated with subduction zones are thought to be the primary building blocks of continents. The composition of the magmas, particularly in continental arcs, is the product of mixing between differentiation of juvenile magmas and pre-existing crustal wallrock, the former being typically mafic and the latter more silicic. Because the upper continental crust is on average thought to be more silicic than the mafic lower crust, mixing with silicic endmembers should occur primarily in the upper crust. However, we show here that the lower crust of continental arcs contains silicic metasediments. We examine garnet-bearing, granulite-facies sedimentary quartzite xenoliths from the Sierra Nevada batholith in California, a Cretaceous continental arc. The quartzites have equigranular textures and contain quartz (>50%), plagioclase (<30%), garnet (10%), and small amounts (<1%) of rutile, aluminosilicate, biotite, monazite, zircon, graphite and trace orthopyroxene. Cathodoluminescent images show zircons with rounded detrital cores mantled by metamorphic overgrowths. Hf isotopic model ages and U-Pb upper intercept ages, for a given zircon, are similar, but the zircon population shows variable protolith ages ranging from Proterozoic to Archean. In contrast, all zircons share similar lower intercept U-Pb ages (103 +/- 10 Ma), which coincide with the peak of arc magmatism in the Sierra Nevada. The Precambrian protolith ages are similar to North American cratonal basement and together with the abundance of quartz and detrital zircons, suggest that these quartzites represent ancient, passive margin sediments instead of juvenile active margin sediments in the oceanic trench and accretionary prism. Importantly, these quartzites record peak metamorphic temperatures and pressures of 700-800 degrees C using Ti-in-quartz thermometry and 0.7-1.1 GPa using garnet-aluminosilicate-plagioclase thermobarometry, indicating that these xenoliths experienced significant heating and possible partial melting in the lower crust, most likely related to arc magmatism as suggested by similarities between the lower intercept U-Pb ages and the ages of plutonism in the Sierra Nevada. Possible mechanisms by which these sediments were transported into the lower crust include continental underthrusting beneath the continental arc, underplating by buoyant slab-derived sedimentary diapirs, or viscous downflow of country rock in response to diapiric ascent of plutons. Continental underthrusting has been independently documented during the Sevier orogeny, coinciding with the peak of arc magmatism. We thus speculate that supracrustal rocks may have been underthrusted into deep crustal magmatic zones. Regardless of how these metasediments arrived in the lower crust, our observations indicate that silicic metasediments occur in the lower crust of volcanic arcs, not just in the upper crust as is commonly thought. Transport of metasediments into deep crustal magmatic zones should influence the composition of arc magmas and continental crust in general. (C) 2012 Elsevier B.V. All rights reserved.

Chin, EJ, Shimizu K, Bybee GM, Erdman ME.  2018.  On the development of the calc-alkaline and tholeiitic magma series: A deep crustal cumulate perspective. Earth and Planetary Science Letters. 482:277-287.  
Chin, EJ, Lee CTA, Luffi P, Tice M.  2012.  Deep Lithospheric Thickening and Refertilization beneath Continental Arcs: Case Study of the P, T and Compositional Evolution of Peridotite Xenoliths from the Sierra Nevada, California. Journal of Petrology. 53:477-511.   10.1093/petrology/egr069   AbstractWebsite

Thickening of arc lithosphere influences the extent of magmatic differentiation and is thereby important for the evolution of juvenile arcs into mature continental crust. Here, we use mantle xenoliths from the late Mesozoic Sierra Nevada continental arc in California (USA) to constrain the pressure, temperature, and compositional evolution of the deep lithosphere beneath a mature arc. These xenoliths consist of spinel peridotites and garnet-bearing spinel peridotites. The former are characterized by coarse-grained protogranular textures having bulk compositions indicative of high-degree melting. The latter are characterized by porphyroclastic textures, garnet coronas around spinels, garnet exsolution lamellae in pyroxenes, and pyroxenes with high-Al cores and low-Al rims. The garnet-bearing spinel peridotites range from depleted to fertile compositions, but the high Cr-numbers [molar Cr/(Cr + Al)] of spinel cores reflect high-degree melting. These observations suggest that the protoliths of the garnet-bearing spinel peridotites were melt-depleted spinel peridotites. Constraints from geothermobarometry and bulk compositions coupled with mantle melting models suggest that the protoliths underwent shallow melt depletion (1-2 GPa, 1300-1400 degrees C), followed by compression, cooling, and final equilibration within the garnet stability field (similar to 3 GPa, < 800 degrees C). The deepest equilibrated samples are the most refertilized, suggesting that refertilization occurred during compression. We interpret this P-T-composition path to reflect progressive thickening of the Sierran arc lithosphere perhaps as a result of magmatic inflation or tectonic thickening. We hypothesize that newly formed arc lithospheric mantle thickens enough to pinch out the asthenospheric wedge, juxtaposing Sierran arc lithosphere against the subducting oceanic plate. This could have terminated arc magmatism and initiated cooling of the deep Sierran lithosphere.

Filiberto, J, Chin E, Day JMD, Franchi IA, Greenwood RC, Gross J, Penniston-Dorland SC, Schwenzer SP, Treiman AH.  2012.  Geochemistry of intermediate olivine-phyric shergottite Northwest Africa 6234, with similarities to basaltic shergottite Northwest Africa 480 and olivine-phyric shergottite Northwest Africa 2990. Meteoritics & Planetary Science. 47:1256-1273.   10.1111/j.1945-5100.2012.01382.x   AbstractWebsite

The newly found meteorite Northwest Africa 6234 (NWA 6234) is an olivine (ol)-phyric shergottite that is thought, based on texture and mineralogy, to be paired with Martian shergottite meteorites NWA 2990, 5960, and 6710. We report bulk-rock major- and trace-element abundances (including Li), abundances of highly siderophile elements, Re-Os isotope systematics, oxygen isotope ratios, and the lithium isotope ratio for NWA 6234. NWA 6234 is classified as a Martian shergottite, based on its oxygen isotope ratios, bulk composition, and bulk element abundance ratios, Fe/Mn, Al/Ti, and Na/Al. The Li concentration and d7Li value of NWA 6234 are similar to that of basaltic shergottites Zagami and Shergotty. The rare earth element (REE) pattern for NWA 6234 shows a depletion in the light REE (La-Nd) compared with the heavy REE (Sm-Lu), but not as extreme as the known depleted shergottites. Thus, NWA 6234 is suggested to belong to a new category of shergottite that is geochemically intermediate in incompatible elements. The only other basaltic or ol-phyric shergottite with a similar intermediate character is the basaltic shergottite NWA 480. Rhenium-osmium isotope systematics are consistent with this intermediate character, assuming a crystallization age of 180 Ma. We conclude that NWA 6234 represents an intermediate compositional group between enriched and depleted shergottites and offers new insights into the nature of mantle differentiation and mixing among mantle reservoirs in Mars.

Lee, CTA, Luffi P, Chin EJ.  2011.  Building and Destroying Continental Mantle. Annual Review of Earth and Planetary Sciences, Vol 39. 39:59-90.   10.1146/annurev-earth-040610-133505   AbstractWebsite

Continents, especially their Archean cores, are underlain by thick thermal boundary layers that have been largely isolated from the convecting mantle over billion-year timescales, far exceeding the life span of oceanic thermal boundary layers. This longevity is promoted by the fact that continents are underlain by highly melt-depleted peridotites, which result in a chemically distinct boundary layer that is intrinsically buoyant and strong (owing to dehydration). This chemical boundary layer counteracts the destabilizing effect of the cold thermal state of continents. The compositions of cratonic peridotites require formation at shallower depths than they currently reside, suggesting that the building blocks of continents formed in oceanic or arc environments and became "continental" after significant thickening or underthrusting. Continents are difficult to destroy, but refertilization and rehydration of continental mantle by the passage of melts can nullify the unique stabilizing composition of continents.

Lee, CTA, Luffi P, Chin EJ, Bouchet R, Dasgupta R, Morton DM, Le Roux V, Yin QZ, Jin D.  2012.  Copper systematics in arc magmas and implications for crust-mantle differentiation. Science. 336:64-68.   10.1126/science.1217313   AbstractWebsite

Arc magmas are important building blocks of the continental crust. Because many arc lavas are oxidized, continent formation is thought to be associated with oxidizing conditions. On the basis of copper's (Cu's) affinity for reduced sulfur phases, we tracked the redox state of arc magmas from mantle source to emplacement in the crust. Primary arc and mid-ocean ridge basalts have identical Cu contents, indicating that the redox states of primitive arc magmas are indistinguishable from that of mid-ocean ridge basalts. During magmatic differentiation, the Cu content of most arc magmas decreases markedly because of sulfide segregation. Because a similar depletion in Cu characterizes global continental crust, the formation of sulfide-bearing cumulates under reducing conditions may be a critical step in continent formation.

Lee, CTA, Erdman M, Yang WB, Ingram L, Chin EJ, DePaolo DJ.  2018.  Sulfur isotopic compositions of deep arc cumulates. Earth and Planetary Science Letters. 500:76-85.   10.1016/j.epsl.2018.08.017   AbstractWebsite

Heavy sulfur isotopic compositions of arc lavas suggest a seawater component in the sulfur budget of arc lavas, but whether the seawater signature derives from the subducting slab or from magma interactions with lithologies in the upper plate is unclear. To see through the effects of degassing or crustal processing, a study was conducted on the S isotopic composition of deep arc cumulates from 45-90 km beneath the Sierra Nevada batholith in California, a Cretaceous continental arc. These cumulates represent the crystal line of descent from magmatic differentiation of hydrous arc basalts. The deepest (up to 60-90 km) and most primitive cumulates are low in Fe and have high molar Mg/(Mg + Fe), whereas the shallow and more evolved cumulates are high in Fe and have low Mg/(Mg + Fe). Bulk rock (delta S-34 correlates with Fe and negatively with Mg/(Mg + Fe). The most primitive cumulates are isotopically similar to the Earth's mantle whereas the more evolved cumulates are heavier by 6%o in the direction of seawater sulfate. The mantle-like S isotopic signatures of the primitive cumulates indicate that the contribution of slab derived sulfate to arc lavas may not be as large as widely thought. Heavy S isotopic signatures are seen only in the evolved arc cumulates, which suggests that the seawater signature of arc lavas may not all derive directly from the slab, but perhaps during magma interaction with pre-arc crust. In continental arcs, pre-arc crust is dominated by accreted marine metasediments and metabasalts, and in island arcs, by seawater altered oceanic crust in the upper plate. The limited contribution of slab sulfate to the mantle source of Sierran arc magmas, if generalizable, suggests that sulfate in the subducting slab is efficiently released well before the arc magmatic front. Such a scenario would be consistent with the higher solubility of sulfate in aqueous fluids compared to that of sulfide. In summary, it is suggested here that the upper plate, in the form of seawater altered crust and sediments, may be as or more important for the sulfur budget in arc magmas than the subducting slab. Early loss of sulfate from the slab during subduction suggests that the dominant S species transported to the deep mantle is in the reduced form - sulfide. (C) 2018 Elsevier B.V. All rights reserved.

Lee, C-TA, Chin EJ.  2014.  Calculating melting temperatures and pressures of peridotite protoliths: Implications for the origin of cratonic mantle. Earth and Planetary Science Letters. 403:273-286.   10.1016/j.epsl.2014.06.048   AbstractWebsite

The old, stable cores of continents – cratons – are underlain by thick and cold mantle keels, composed of melt-depleted and low density peridotite residues. The origins of these thick keels are debated. Were these thick keels formed in situ, by orogenic thickening, or by underplating of buoyant residual mantle? Key to this debate is determining the temperature and pressure at which the protoliths of cratonic peridotites melted (igneous protolith conditions) and comparing to their metamorphic (subsolidus) temperatures and pressures within the keel. This paper presents a method for explicit calculation of the temperatures and pressures at which the peridotite protoliths melted. The approach relies only on the bulk FeO and MgO of residual peridotites. A system of equations consisting of mass balance and new calibrations of Mg peridotite/melt partitioning and melt productivity is then solved simultaneously. The igneous protoliths of abyssal peridotites are found to have melted at effective pressures of 1–2 GPa and temperatures of 1300–1400 °C, within error of the magmatic temperatures and pressures of melt extraction inferred independently from the SiO2 and MgO contents of mid-ocean ridge basalts. Archean cratonic peridotites, after filtering for the secondary effects of refertilization and orthopyroxene-metasomatism, give igneous protolith pressures and temperatures of 1–5 GPa (30–150 km) and 1400–1750 °C, similar to magmatic temperatures and pressures determined for Archean basalts thought to be representative of the thermal state of the Archean ambient mantle. Most importantly, cratonic peridotite protolith pressures and temperatures are shallower and hotter than their subsolidus equilibration pressures (3–7.5 GPa; 90–200 km) and temperatures (900–1300 °C), which reflects the recent thermal state of the cratonic lithosphere. Specifically, for individual samples with both melting and subsolidus thermobarometric constraints, we find that subsolidus pressures are 1–2 GPa (30–60 km) higher than their igneous protolith pressures although some of the deepest samples experienced minor increases in pressure. Collectively, these results support the suggestion that the building blocks of cratons were generated by hot shallow melting with a mantle potential temperature 200–300 °C warmer than the present. This shallowly generated mantle was subsequently thickened during orogenic episodes, culminating in the formation of a thick, stable craton. Whether such thickening has any modern analogs cannot be answered from this work alone.

Tollstrup, DL, Xie LW, Wimpenny JB, Chin E, Lee CT, Yin QZ.  2012.  A trio of laser ablation in concert with two ICP-MSs: Simultaneous, pulse-by-pulse determination of U-Pb discordant ages and a single spot Hf isotope ratio analysis in complex zircons from petrographic thin sections. Geochemistry Geophysics Geosystems. 13   Artn Q0301710.1029/2011gc004027   AbstractWebsite

We have developed a technique for the simultaneous in situ determination of U-Pb ages and Hf isotope ratios from a single spot in complex, discordant zircons by combining both a single-collector and a multicollector sector field inductively coupled plasma-mass spectrometry (ICP-MS) with a 193 nm excimer laser ablation system. With a suite of zircon standards of various ages, we first show that U-Pb ages can be determined accurately to within 0.3-2.5% (2 sigma) compared to the nominal value, while the internal errors are better than 0.4-0.7%; hafnium isotope ratios are accurate, relative to solution analyses, within one epsilon unit, and internal errors are typically <0.008%. We then apply the technique to complex, discordant zircons with variable Pb-206/U-238 and Pb-207/U-235 ratios, commonly discarded previously as "un-reducible data," to construct a Discordia in U-Pb Concordia plot, using every scan, every laser pulse as individual data points from a single laser ablation spot (typically > 200-250 data points). We show that the upper and lower intercept ages from the Discordia, augmented by high precision Hf isotope data obtained on the same spot, reveal invaluable information that permit unique insight to geological processes not available by other means. We demonstrate that our technique is useful for provenance studies of small, complex detrital zircons in sedimentary and high-grade metamorphic rocks, in relation to crustal growth and evolution.