Frank K. Mazdab

Trace element chemistry of zircons from oceanic crust: A method for distinguishing detrital zircon provenance

We present newly acquired trace element compositions for more than 300 zircon grains in 36 gabbros formed at the slow-spreading Mid-Atlantic and Southwest Indian Ridges. Rare earth element patterns for zircon from modern oceanic crust completely overlap with those for zircon crystallized in continental granitoids. However, plots of U versus Yb and U/Yb versus Hf or Y discriminate zircons crystallized in oceanic crust from continental zircon, and provide a relatively robust method for distinguishing zircons from these environments. Approximately 80% of the modern ocean crust zircons are distinct from the fi eld defi ned by more than 1700 continental zircons from Archean and Phanerozoic samples. These discrimination diagrams provide a new tool for fi ngerprinting ocean crust zircons derived from reservoirs like that of modern mid-ocean ridge basalt (MORB) in both modern and ancient detrital zircon populations. Hadean detrital zircons previously reported from the Acasta Gneiss, Canada, and the Narryer Gneiss terrane, Western Australia, plot in the continental granitoid fi eld, supporting hypotheses that at least some Hadean detrital zircons crystallized in continental crust forming magmas and not from a reservoir like modern MORB.

Young cumulate complex beneath Veniaminof caldera, Aleutian arc, dated by zircon in erupted plutonic blocks

Mount Veniaminof volcano, Alaska Peninsula, provides an opportunity to relate Quaternary volcanic rocks to a coeval intrusive complex. Veniaminof erupted tholeiitic basalt through dacite in the past ∼260 k.y. Gabbro, diorite, and miarolitic granodiorite blocks, ejected 3700 14 C yr B.P. in the most recent caldera-forming eruption, are fragments of a shallow intrusive complex of cumulate mush and segregated vapor-saturated residual melts. Sensitive high-resolution ion microprobe (SHRIMP) analyses define 238 U- 230 Th isochron ages of 17.6 ± 2.7 ka, 5 +11/–10 ka, and 10.2 ± 4.0 ka (2σ) for zircon in two granodiorites and a diorite, respectively. Sparse zircons from two gabbros give 238 U- 230 Th model ages of 36 ± 8 ka and 26 ± 7 ka. Zircons from granodiorite and diorite crystallized in the presence of late magmatic aqueous fluid. Although historic eruptions have been weakly explosive Strombolian fountaining and small lava effusions, the young ages of plutonic blocks, as well as late Holocene dacite pumice, are evidence that the intrusive complex remains active and that evolved magmas can segregate at shallow levels to fuel explosive eruptions.

Creation of a continent recorded in zircon zoning

We have discovered a robust microcrystalline record of the early genesis of North American lithosphere preserved in the U-Pb age and oxygen isotope zoning of zircons from a lower crustal paragneiss in the Neoarchean Superior province. Detrital igneous zircon cores with δ 18 O values of 5.1‰–7.1‰ record creation of primitive to increasingly evolved crust from 2.85 ± 0.02 Ga to 2.67 ± 0.02 Ga. Sharp chemical unconformity between cores and higher δ 18 O (8.4‰–10.4‰) metamorphic overgrowths as old as 2.66 ± 0.01 Ga dictates a rapid sequence of arc unroofing, burial of detrital zircons in hydrosphere-altered sediment, and transport to lower crust late in upper plate assembly. The period to 2.58 ± 0.01 Ga included ∼80 m.y. of high-temperature (∼700–650 °C), nearly continuous overgrowth events reflecting stages in maturation of the subjacent mantle root. Huronian continental rifting is recorded by the youngest zircon tip growth at 2512 ± 8 Ma (∼ 600 °C) signaling magma intraplating and the onset of rigid plate behavior. This >150 m.y. microscopic isotope record in single crystals demonstrates the sluggish volume diffusion of U, Pb, and O in zircon throughout protracted regional metamorphism, and the consequent advances now possible in reconstructing planetary dynamics with zircon zoning.

Zircon U-Pb isotope, δ18O and trace element response to 80 m.y. of high temperature metamorphism in the lower crust: Sluggish diffusion and new records of Archean craton formation

Coordinated cathodoluminescence (CL) imaging and ion microprobe (SHRIMP and CAMECA 1280) analysis document micron-scale U-Pb-O isotope and trace element zoning in zircons from deep crust exposed to 80 m.y. of high temperature and pressure metamorphism. Three, along-strike paragneiss samples across the amphibolite to granulite facies transition in the Kapuskasing Uplift crustal cross-section in the Archean Superior province yield detrital, originally igneous zircon cores overgrown by progressively larger volumes of metamorphic zircon with increasing grade. The cores generally retain primary age (2.85±0.03 to 2.67±0.02 Ga), oxygen isotope (5.1 to 7.0‰) and trace element compositions similar to those reported for magmatic arc sources. Dark CL, metamorphic zircon rims record nearly continuous overgrowth events for ∼80 m.y. from 2.66±0.01 to 2.58±0.01 Ga during uppermost amphibolite to granulite facies regional metamorphism. These rims have significantly higher δ18O values (8.4 to 10.4‰) and trace element compositions quite distinct from those of the cores; these differences indicate that their δ18O and trace element compositions were not inherited from the igneous cores, consistent with extensive textural evidence for rim formation as metamorphic overgrowths. Multi-spot traverses record steep oxygen isotope discontinuities (4‰ over <10 μm) at core-rim boundaries, confirming the extremely sluggish rates of volume diffusion of O in non-metamict zircon during extended (∼80 m.y.) granulite-grade metamorphism (peak T=750-800 °C) at substantial f(H2O) but water-undersaturated (fluid-absent) conditions. Likewise no evidence of significant diffusive exchange of δ18O could be detected along deformation microstructures such as annealed fractures in cores infilled with high δ18O zircon. Application of simple diffusion models to detailed δ18O profiles in a large number of zircon grains constrain maximum values of the diffusivity of oxygen in zircon (logDZrcox) to the range −27.5 to −26.4 m2/s. For the estimated 80 m.y. and 700 to 800 °C time-T window of rim formation, these maximum values are similar to or slower than values reported by Page and others (2007, 2010) and the experimentally-determined “dry” diffusivity of oxygen in zircon (Watson and Cherniak, 1997), but are markedly slower than the experimentally-determined “wet” diffusivity of oxygen in zircon (Watson and Cherniak, 1997). Fast diffusion of oxygen in zircon predicted by hydrothermal experiments may, in nature, require the presence of a hydrous fluid rather than a threshold value of f(H2O). Our test demonstrates that unrecrystallized metamorphosed igneous zircons and metamorphic zircons will retain the geochemical (U-Pb age, trace element and δ18O) record of their origin and evolution despite prolonged, high-grade metamorphism at significant f(H2O) but water under-saturated (fluid-absent) conditions. Such zircons, particularly those that exhibit δ18O zoning, are micron-scale records for the T-time-fluid interaction history of deep crustal rocks. Such records will not be preserved in less refractory phases and promise new insights into the processes of continent formation and evolution.

Episodic intrusion, internal differentiation, and hydrothermal alteration of the Miocene Tatoosh intrusive suite south of Mount Rainier, Washington

The Miocene Tatoosh intrusive suite south of Mount Rainier is composed of three broadly granodioritic plutons that are manifestations of ancestral Cascades arc magmatism. Tatoosh intrusive suite plutons have individually diagnostic characteristics, including texture, mineralogy, and geochemistry, and apparently lack internal contacts. New ion-microprobe U-Pb zircon ages indicate crystallization of the Stevens pluton ca. 19.2 Ma, Reflection-Pyramid pluton ca. 18.5 Ma, and Nisqually pluton ca. 17.5 Ma. The Stevens pluton includes rare, statistically distinct ca. 20.1 Ma zircon antecrysts. Wide-ranging zircon rare earth element (REE), Hf, U, and Th concentrations suggest late crystallization from variably evolved residual liquids. Zircon Eu/Eu*–Hf covariation is distinct for each of the Reflection-Pyramid, Nisqually, and Stevens plutons. Although most Tatoosh intrusive suite rocks have been affected by weak hydrothermal alteration, and sparse mineralized veins cut some of these rocks, significant base or precious metal mineralization is absent. At the time of shallow emplacement, each of these magma bodies was largely homogeneous in bulk composition and petrographic features, but, prior to final solidification, each of the Tatoosh intrusive suite plutons developed internal compositional variation. Geochemical and petrographic trends within each pluton are most consistent with differential loss of residual melt, possibly represented by late aplite dikes or erupted as rhyolite, from crystal-rich magma. Crystal-rich magma that formed each pluton evidently accumulated in reservoirs below the present level of exposure and then intruded to a shallow depth. Assembled by episodic intrusion, the Tatoosh intrusive suite may be representative of midsized composite plutonic complexes beneath arc volcanoes.