Erin K. Shea

A long-lived lunar core dynamo.

Magnetic Moon It has long been suspected that the Moon once had a core-dynamo magnetic field. Shea et al. (p. 453) describe a lunar basalt brought back by Apollo 11 that records evidence for a strong dynamo on the Moon 3.7 billion years ago. This study, together with a previous study of different lunar rock, implies that a lunar core dynamo existed between 4.2 and 3.7 billion years ago, which extends the known lifetime of the lunar dynamo by 500 million years. Analysis of a lunar basalt sample suggests that a lunar core dynamo existed between 4.2 and 3.7 billion years ago. Paleomagnetic measurements indicate that a core dynamo probably existed on the Moon 4.2 billion years ago. However, the subsequent history of the lunar core dynamo is unknown. Here we report paleomagnetic, petrologic, and 40Ar/39Ar thermochronometry measurements on the 3.7-billion-year-old mare basalt sample 10020. This sample contains a high-coercivity magnetization acquired in a stable field of at least ~12 microteslas. These data extend the known lifetime of the lunar dynamo by 500 million years. Such a long-lived lunar dynamo probably required a power source other than thermochemical convection from secular cooling of the lunar interior. The inferred strong intensity of the lunar paleofield presents a challenge to current dynamo theory.

Growth and maturation of a mid- to shallow-crustal intrusive complex, North Cascades, Washington

Studies of plutons indicate that they are the result of a complex interplay of magmatic processes occurring during magma generation, ascent, and emplacement. A critical tool for deciphering these processes is high-precision geochronology, which can help determine the timing and rates of magmatism in the crust. We conducted a field and U-Pb geochronological study of the Cretaceous Black Peak intrusive complex in the North Cascades of Washington State to investigate magmatism at a detailed scale and to refine estimates of plutonic construction rates. High-precision chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) U-Pb geochronology was carried out on 31 samples from five mapped intrusive phases. Field relations in the Black Peak intrusive complex show intrusive contacts that vary from sharp to gradational. Whole-rock Sm/Nd, zircon oxygen isotopes, and zircon trace elements were obtained on subsets of representative samples. The U-Pb geochronology from the Black Peak intrusive complex documents batholith intrusion over 4.5 m.y. and suggests that magmatism was semicontinuous for a minimum of 3.5 m.y. Individual samples display age dispersion in single-zircon dates that ranges from ∼105 yr to several 106 yr, with a general increase in the age range for younger samples. Whole-rock eNd and zircon δ18O for all Black Peak intrusive complex samples indicate that magmas were derived from mantle and crustal sources and that all magmas were isotopically homogenized prior to zircon saturation. Ti-in-zircon temperatures from zircon cores are generally above calculated zircon saturation temperatures, which suggests that most Black Peak intrusive complex magmas were zircon undersaturated in the melt source region. A range of thicknesses was considered, and a thickness of ∼10 km for the Black Peak intrusive complex gives an average intrusion rate of ∼1.1 ×10–3 km3/yr, which is high enough to sustain a magma reservoir in the shallow crust. The field evidence and long overall duration of intrusion are incompatible with the entire Black Peak intrusive complex being molten at any one time, but the larger, more compositionally homogeneous domains in the Black Peak intrusive complex are likely the solidified remnants of mushy magma bodies with ∼105 yr durations. These data suggest that the Black Peak intrusive complex may have remained “mushy” for long periods of time (105 yr) and may indicate that the spread in dates within individual samples is best interpreted as either antecrystic recycling and/or protracted autocrystic growth.

Linking deep and shallow crustal processes during regional transtension in an exhumed continental arc, North Cascades, northwestern Cordillera (USA)

The North Cascades orogen (northwestern USA) provides an exceptional natural laboratory with which to evaluate potential temporal and kinematic links between processes operating at a wide range of crustal levels during collapse of a continental arc, and particularly the compatibility of strain between the upper and lower crust. This magmatic arc reached a crustal thickness of ≥55 km in the mid-Cretaceous. Eocene collapse of the arc during regional transtension was marked by magmatism, migmatization, ductile flow, and exhumation of deep crustal (8–12 kbar) rocks in the Cascades crystalline core coeval with subsidence and rapid deposition in nonmarine basins adjacent to the core, and intrusion of dike complexes. The Skagit Gneiss Complex is the larger of two regions of exhumed deep crust with Eocene cooling ages in the Cascades core, and it consists primarily of tonalitic orthogneiss emplaced mainly in two episodes of ca. 73–59 Ma and 50–45 Ma. Metamorphism, melt crystallization, and ductile deformation of migmatitic metapelite overlap the orthogneiss emplacement, occurring (possibly intermittently) from ca. 71 to 53 Ma; the youngest orthogneisses overlap 40 Ar/ 39 Ar biotite dates, compatible with rapid cooling. Gently to moderately dipping foliation, subhorizontal orogen-parallel (northwest-southeast) mineral lineation, sizable constrictional domains, and strong stretching parallel to lineation of hinges of mesoscopic folds in the Skagit Gneiss Complex are compatible with transtension linked to dextral-normal displacement of the Ross Lake fault zone, the northeastern boundary of the Cascades core. The other deeply exhumed domain, the 9–12 kbar Swakane Biotite Gneiss, has a broadly north-trending, gently plunging lineation and gently to moderately dipping foliation, which are associated with top-to-the-north noncoaxial shear. This gneiss is separated from overlying metamorphic rocks by a folded detachment fault. The Eocene Swauk and Chumstick basins flank the southern end of the Cascades core. In the Swauk basin, sediments were deposited in part at ca. 51 Ma, folded shortly afterward, and then covered by ca. 49 Ma Teanaway basalts and intruded by associated mafic dikes. Directly after dike intrusion, the fault-bounded Chumstick basin subsided rapidly. Extension directions from these dikes and from Eocene dikes that intruded the Cascades core are dominantly oblique to the overall trend of the orogen (275°–310° versus ∼320°, respectively) and to the northwest-southeast to north-south ductile flow direction in the Skagit and Swakane rocks. This discordance implies that coeval extensional strain was decoupled between the brittle and ductile crust. Strain orientations at all depths in the Cascades core contrast with the approximately east-west extension driven by orogenic collapse in coeval metamorphic core complexes ∼200 km to the east. Arc-oblique to arc-parallel flow in the Cascades core probably resulted in part from dextral shear along the plate margin and from along-strike gradients in crustal thickness and temperature.

Cellular convection in a chamber with a warm surface raft

We calculate velocity and temperature fields for Rayleigh-Benard convection in a chamber with a warm raft that floats along the top surface for Rayleigh number up to Ra = 20 000. Two-dimensional, infinite Prandtl number, Boussinesq approximation equations are numerically advanced in time from a motionless state in a chamber of length L′ and depth D′. We consider cases with an insulated raft and a raft of fixed temperature. Either oscillatory or stationary flow exists. In the case with an insulated raft over a fluid, there are only three parameters that govern the system: Rayleigh number (Ra), scaled chamber length (L = L′/D′), and scaled raft width (W). For W = 0 and L = 1, linear theory shows that the marginal state without a raft is at a Rayleigh number of 23π4=779.3, but we find that for the smallest W (determined by numerical grid size) the raft approaches the center monotonically in time for Ra<790. For 790

Formation of a sheeted intrusive complex within the deep-crustal Tenpeak pluton, North Cascades, Washington

Sheeted intrusive complexes represent unequivocal examples of incremental intrusive activity in plutonic systems and thus provide an opportunity to constrain processes associated with incremental magma emplacement. We present field observations, U-Pb zircon data, and whole-rock and mineral chemistry from the sheeted intrusive complex of the ca. 92 Ma Tenpeak pluton located in the North Cascades, Washington. Samples collected from an ~750-m transect of the sheeted complex, including at least 58 individual sheets, provide an opportunity to understand the time scales, magma sources, and magmatic processes responsible for the generation of incrementally emplaced magma bodies. High-precision chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) geochronology show relatively rapid construction of the complex over an interval of 94,000 ± 62,500 yr (95% confidence). With a volume estimate of ~17 km3, this is equivalent to a magma emplacement rate of 1.1–5.4 × 10–4 km3 yr–1. In general, thinner sheets are found closer to the pluton margin and transition to thicker sheets in the interior. Although magmas of the sheeted complex show compositional and mineralogical similarities with the nearby voluminous Schaefer Lake tonalite phase of the Tenpeak pluton, U-Pb zircon ages from individual sheets suggest that formation of the sheeted complex occurred as a waning phase of the Schaefer Lake emplacement. Individual sheets also show relatively simple zircon populations consistent with low overall magma volumes and relatively rapid cooling following emplacement. We suggest that the sheeted complex resulted from localization of magma intrusions along anisotropies in the shear zone. In addition, the thermal boundary formed with the adjacent meta-supracrustal wall rocks likely facilitated rapid cooling of sheets and localized subsequent intrusive events. The formation of the range of sheet compositions requires at least three different parental magmas. The observed range of sheet compositions can be produced by mixing between a single mafic parental magma (SiO2 ~50 wt%) and an array of felsic magma compositions (SiO2 from 59 to 67 wt%). Mineral populations within mafic samples suggest that the felsic parental magmas either had low crystallinity prior to mixing and/or substantial crystallization occurred after mixing. Textural and field evidence, along with mineral chemistry, suggest that final mixing between the mafic parent and felsic array occurred late, almost immediately prior to emplacement.