Peter Luffi

Copper Systematics in Arc Magmas and Implications for Crust-Mantle Differentiation

Copper-Bottomed Crust The formation of volcanic arc chains near subduction zones brings large amounts of magma from the upper mantle to the crust, contributing to the formation of island chains in the ocean and adding material to continents. Over time, arc magmas also contribute indirectly to the composition of the oceans and atmosphere through outgassing and weathering of volcanic minerals; however, it is unclear what determines the oxidized nature of arc magmas themselves. Lee et al. (p. 64) measured Cu contents in a range of arc-derived volcanic rocks as a proxy for arc magma redox states. An overall depletion of Cu, which is sensitive to reduced sulfur contents, in global continental crust suggests that there is a hidden reservoir of copper-rich sulfides deep in Earths interior. The copper contents of magmas imply that the formation of sulfide-bearing cumulates under reducing conditions is a critical step in the formation of continental crust. 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.

The redox state of arc mantle using Zn/Fe systematics

Many arc lavas are more oxidized than mid-ocean-ridge basalts and subduction introduces oxidized components into the mantle. As a consequence, the sub-arc mantle wedge is widely believed to be oxidized. The Fe oxidation state of sub-arc mantle is, however, difficult to determine directly, and debate persists as to whether this oxidation is intrinsic to the mantle source. Here we show that Zn/FeT (where FeT = Fe2+ + Fe3+) is redox-sensitive and retains a memory of the valence state of Fe in primary arc basalts and their mantle sources. During melting of mantle peridotite, Fe2+ and Zn behave similarly, but because Fe3+ is more incompatible than Fe2+, melts generated in oxidized environments have low Zn/FeT. Primitive arc magmas have identical Zn/FeT to mid-ocean-ridge basalts, suggesting that primary mantle melts in arcs and ridges have similar Fe oxidation states. The constancy of Zn/FeT during early differentiation involving olivine requires that Fe3+/FeT remains low in the magma. Only after progressive fractionation does Fe3+/FeT increase and stabilize magnetite as a fractionating phase. These results suggest that subduction of oxidized crustal material may not significantly alter the redox state of the mantle wedge. Thus, the higher oxidation states of arc lavas must be in part a consequence of shallow-level differentiation processes, though such processes remain poorly understood.

Building the Pamirs: The view from the underside

The Pamir mountains are an outstanding example of extreme crustal shortening during continental collision that may have been accommodated by formation of a thick crust—much thicker than is currently thought—and/or by continental subduction. We pre- sent new petrologic data and radiometric ages from xenoliths in Miocene volcanic rocks in the southeastern Pamir mountains that suggest that Gondwanan igneous and sedimentary assemblages were underthrust northward, buried to .50-80 km during the ear- ly stage of the India-Asia collision, and then heated and partly melted during subsequent thermal relaxation before finally being blasted to the surface. These xenoliths, the deepest crustal samples recovered from under any active collisional belt, provide direct evidence for early Cenozoic thickening of the Pamirs and lower- crustal melting during collision; the xenoliths also suggest that the present mountain range was a steady-state elevated plateau for most of the Cenozoic.

Upside-down differentiation and generation of a primordial lower mantle

Except for the first 50–100 million years or so of the Earth’s history, when most of the mantle may have been subjected to melting, the differentiation of Earth’s silicate mantle has been controlled by solid-state convection. As the mantle upwells and decompresses across its solidus, it partially melts. These low-density melts rise to the surface and form the continental and oceanic crusts, driving the differentiation of the silicate part of the Earth. Because many trace elements, such as heat-producing U, Th and K, as well as the noble gases, preferentially partition into melts (here referred to as incompatible elements), melt extraction concentrates these elements into the crust (or atmosphere in the case of noble gases), where nearly half of the Earth’s budget of these elements now resides. In contrast, the upper mantle, as sampled by mid-ocean ridge basalts, is highly depleted in incompatible elements, suggesting a complementary relationship with the crust. Mass balance arguments require that the other half of these incompatible elements be hidden in the Earth’s interior. Hypotheses abound for the origin of this hidden reservoir. The most widely held view has been that this hidden reservoir represents primordial material never processed by melting or degassing. Here, we suggest that a necessary by-product of whole-mantle convection during the Earth’s first billion years is deep and hot melting, resulting in the generation of dense liquids that crystallized and sank into the lower mantle. These sunken lithologies would have ‘primordial’ chemical signatures despite a non-primordial origin.

Active megadetachment beneath the western United States

Geodetic data, interpreted in light of seismic imaging, seismicity, xenolith studies, and the late Quaternary geologic history of the northern Great Basin, suggest that a subcontinental-scale extensional detachment is localized near the Moho. To first order, seismic yielding in the upper crust at any given latitude in this region occurs via an M7 earthquake every 100 years. Here we develop the hypothesis that since 1996, the region has undergone a cycle of strain accumulation and release similar to “slow slip events” observed on subduction megathrusts, but yielding occurred on a subhorizontal surface 5–10 times larger in the slip direction, and at temperatures >800°C. Net slip was variable, ranging from 5 to 10 mm over most of the region. Strain energy with moment magnitude equivalent to an M7 earthquake was released along this “megadetachment,” primarily between 2000.0 and 2005.5. Slip initiated in late 1998 to mid-1999 in northeastern Nevada and is best expressed in late 2003 during a magma injection event at Moho depth beneath the Sierra Nevada, accompanied by more rapid eastward relative displacement across the entire region. The event ended in the east at 2004.0 and in the remainder of the network at about 2005.5. Strain energy thus appears to have been transmitted from the Cordilleran interior toward the plate boundary, from high gravitational potential to low, via yielding on the megadetachment. The size and kinematic function of the proposed structure, in light of various proxies for lithospheric thickness, imply that the subcrustal lithosphere beneath Nevada is a strong, thin plate, even though it resides in a high heat flow tectonic regime. A strong lowermost crust and upper mantle is consistent with patterns of postseismic relaxation in the southern Great Basin, deformation microstructures and low water content in dunite xenoliths in young lavas in central Nevada, and high-temperature microstructures in analog surface exposures of deformed lower crust. Large-scale decoupling between crust and upper mantle is consistent with the broad distribution of strain in the upper crust versus the more localized distribution in the subcrustal lithosphere, as inferred by such proxies as low P wave velocity and mafic magmatism.

Lithospheric mantle duplex beneath the central Mojave Desert revealed by xenoliths from Dish Hill, California

Received 30 June 2008; revised 30 November 2008; accepted 12 December 2008; published 5 March 2009. [1] 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 (eNd = 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 (eNd = +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 25–28% 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.

Internal distribution of Li and B in serpentinites from the Feather River Ophiolite, California, based on laser ablation inductively coupled plasma mass spectrometry

[1] Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of B and Li in serpentinized peridotites from the Feather River Ophiolite (California) indicates that B is enriched in serpentine minerals compared to the whole-rock and less altered olivine grains, while Li in serpentine is depleted or comparable to whole-rock Li. The high B contents of serpentine minerals correlate with the relatively enriched whole-rock B contents. The low Li contents of serpentine minerals are consistent with the relatively low Li whole-rock contents and suggest that only small amounts of Li were added during serpentinization or that some Li was even leached out. A simple model of partial melting shows that Li/Yb increases with increasing melt depletion (and clinopyroxene depletion) in the peridotitic residue because Li is most compatible in olivine while Yb is most compatible in clinopyroxene. Thus, high Li/Yb ratios in peridotites by themselves do not indicate secondary enrichments in Li. However, Li/Yb and Yb contents of many of the Feather River Ophiolites plot above the melt depletion curve in Li/Yb versus Yb space, indicating that these serpentinites experienced subtle and preferential enrichments in Li during serpentinization. If serpentinized oceanic lithospheric mantle, as represented by the Feather River Ophiolite, is important in subduction recycling, then recycled mantle domains having a serpentinite protolith might be characterized by strong B enrichments but only small Li enrichments.

Geochemical evidence for exhumation of eclogite via serpentinite channels in ocean-continent subduction zones

Retrograde eclogites (ranging from un altered eclogite to retrograde blueschist and greenschist mantling the eclogite boulders) from Ring Mountain on the Tiburon Peninsula, near San Francisco, California, were examined for whole-rock major and trace elements to assess protolith compositions and the geochemical signature of fl associated with retrogression. High fi eld strength elements are highly correlated, indicating relatively immobile and conservative behavior during retrogression. These immobile elements were used to assess the relative losses or gains of other elements during retrogression. Rare earth elements and FeO content show only minimal opensystem behavior. The rare earth abundance patterns, FeO contents, and Nb/Ta and Nb/La ratios show that the proto liths of these rocks were most likely normal to enriched mid-oceanic ridge basalts. Massbalance considerations reveal two independent styles of metasomatic enrichment during retrogression. One style involves coupled enrichment in large ion lithophile elements (Cs, Rb, Ba, K, and Tl), likely caused by fl uids from sediments or reaction with sediments. Another style involves coupled enrichment in Cr, Mg, Ni, and Pb, which may refl ect overprinting by reaction of eclogite boulders with serpentinite, the latter of which are highly enriched in these elements. Pb is shown here and elsewhere to be nearly universally enriched in serpentinites and is likely to be selectively mobilized into eclogite-serpentinite reaction zones. Because all retrograde lithologies show reaction with serpentinites and sediments, exhumation of the eclogite must have been accompanied by chemical interaction with serpentinites along the entire retrograde path. The simplest interpretation is that the eclogites were transported within a deeply rooted serpentinite channel, presumably formed along the slab-mantle interface by infi ltration of slab-derived fl uids into the overlying mantle wedge. Physical models of channel fl ow show that the rapid exhumation rates required to preserve eclogites in a hydrous carrier matrix, such as serpentinite, are possible due to the buoyant and low-viscosity nature of serpentinite. However, the most rapid ascent rates occur during oblique subduction, suggesting that eclogite exhumation could be favored by, but not confi ned to, oblique subduction zones.

Late Cretaceous Construction of the Mantle Lithosphere Beneath the Central California Coast Revealed by Crystal Knob Xenoliths

The Pleistocene (1.65 Ma) Crystal Knob volcanic neck in the California Coast Ranges is an olivine‐plagioclase phyric basalt containing dunite and spinel peridotite xenoliths. Crystal Knob erupted through the Nacimiento belt of the Franciscan complex and adjacent to Salinian crystalline nappes. Its xenoliths sample the mantle lithosphere beneath the outboard exhumed remnants of the southern California Cretaceous subducting margin. This sample set augments previously studied xenolith suites in the Mojave Desert and Sierra Nevada, which linked the mantle lithosphere architecture and crustal structure of the western Cordillera. We examine six peridotite samples ranging from fertile lherzolites to harzburgite residues. Time‐corrected (e_(Nd)) of 10.3–11.0 and ^(87)Sr/^(86)Sr of 0.702 are characteristic of underplated suboceanic mantle. Pyroxene exchange geothermometry shows equilibration at 950–1060 °C. Phase stability, Ca‐in‐olivine barometry, and 65‐ to 90‐mW/m^2 regional geotherms suggest entrainment at 45‐ to 75‐km depth. The samples were variably depleted by partial melting, and re‐enrichment of the hottest samples suggests deep melt‐rock interaction. We test the Crystal Knob temperature depth array against model geotherms matching potential sources for the mantle lithosphere beneath the Coast Ranges: (A) a shallow Mendocino slab window, (B) a young Monterey plate stalled slab, and (C) Farallon plate mantle nappes, underplated during the Cretaceous and reheated at depth by the Miocene slab window. Models B and C fit xenolith thermobarometry, but only model C fits the tectonic and geodynamic evolution of southern California. We conclude that the mantle lithosphere beneath the central California coast was emplaced after Cretaceous flat slab subduction and records a thermal signature of Neogene subduction of the Pacific‐Farallon ridge.