No abstract

Deciphering the magmatic history of continental magmatic arcs, in general, and the growth history of individual intrusions, in particular, is key to understanding the complex history of magma generation, segregation, and transport that defi ne the dynamics of crustal growth. We utilize high precision U-Pb geochronology to resolve a detailed magmatic history from two composite intrusions, the 2-4 kbar Mount Stuart batholith and the 7-10 kbar Tenpeak pluton, emplaced in the Cretaceous North Cascades arc. This temporal framework provides a way to evaluate models of pluton growth that explain common features of intrusions such as concentric compositional zoning and internal magmatic contacts. U-Pb zircon crystallization ages were obtained from 12 samples of the Mount Stuart batholith and 8 samples of the Tenpeak intrusion, representing the range of compositional diversity and geographical extent.These dates indicate that the Mount Stuart batholith was constructed over a ~5.5 m.y. time period that was punctuated by four intervals of high magma fl ux. The durations of the high-fl ux periods are short (a few hundred thousand years) relative to the duration of the batholith. The consistent pattern of magmatic fabrics and the lack of distinct contacts in the batholith may be explained by the juxtaposition of melt-rich and mush zones with subtle contacts between mineral-ogically and texturally similar tonalite and time-transgressive magma fabrics.In contrast, the Tenpeak intrusion was constructed over a ~2.6 m.y. time period, with magma infl ux distributed throughout the intrusive history and texturally distinct magma bodies. The Tenpeak intrusion lacks distinct age domains, which suggests that any magma reservoir was smaller in size and potentially more ephemeral.Although the distinct age domains and discrete compositional and textural phases indicate that pluton growth occurred incrementally, neither pluton bears resemblance to a purely end-member incremental growth model whereby a pluton is constructed from hundreds to thousands of discrete magma pulses that have little, if any, interaction. In particular, ages from the youngest domain of the Mount Stuart batholith indicate that a melt-rich magma reservoir of ≥520 km 3 existed over a 170 ± 90 k.y. time span.

Precise chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) U-Pb zircon ages in combination with detailed fi eld mapping, 40 Ar/ 39 Ar thermochronology, and fi nite difference thermal modeling in the magmatic lobes of the Tuolumne batholith characterize these 10-60 km 2 bodies as shorter-lived, simpler magmatic systems that represent increments of batholith growth. Lobes provide shorter-term records of internal and external processes that are potentially obliterated in the main body of long-lived, composite batholiths. Zircon ages complemented by thermal modeling indicate that lobe-sized magma chambers were present between ~0.2 and 1 m.y., representing only a small fraction of the total duration of melt presence in the main body. During these shorter intervals, a concentric pattern of normal compositional zoning formed during inward crystallization and widespread zircon recycling in the lobes. Lobes largely evolved as individual magma bodies that did not interact signifi cantly with the main, more complex magma chamber(s). Antecrystic zircons and the range of autocrysts, used to track the extent of interconnected melt, record only a limited range of ages and have contrasting zircon populations to those found in the same units in the main batholith. We consider lobes to either be single batches formed during continuous magma fl ow or multiple, quickly coalescing pulses that in either case formed separate magma chambers that failed to amalgamate with other compositionally distinct pulses such as those occurring in the central batholith. Zircon age comparisons between all four lobes and the main body imply that growth of the Tuolumne intrusion was not stationary, but that the locus of magmatism shifted both inward and northwestward.

No abstract

We measured the 26Al-26Mg isotope systematics of a approximately 5-micrometer refractory particle, Coki, returned from comet 81P/Wild 2 in order to relate the time scales of formation of cometary inclusions to their meteoritic counterparts. The data show no evidence of radiogenic 26Mg and define an upper limit to the abundance of 26Al at the time of particle formation: 26Al/27Al < 1 x 10(-5). The absence of 26Al indicates that Coki formed >1.7 million years after the oldest solids in the solar system, calcium- and aluminum-rich inclusions (CAIs). The data suggest that high-temperature inner solar system material formed, was subsequently transferred to the Kuiper Belt, and was incorporated into comets several million years after CAI formation.

Micrometer-scale analyses of a calcium-, aluminum-rich inclusion (CAI) and the characteristic mineral bands mantling the CAI reveal that the outer parts of this primitive object have a large range of oxygen isotope compositions. The variations are systematic; the relative abundance of (16)O first decreases toward the CAI margin, approaching a planetary-like isotopic composition, then shifts to extremely (16)O-rich compositions through the surrounding rim. The variability implies that CAIs probably formed from several oxygen reservoirs. The observations support early and short-lived fluctuations of the environment in which CAIs formed, either because of transport of the CAIs themselves to distinct regions of the solar nebula or because of varying gas composition near the proto-Sun.

The crystalline core of the North Cascades preserves a Cretaceous crustal section that facilitates evaluation of pluton construction, emplacement, geometry, composition, and deformation at widely variable crustal levels (~5-40-km paleodepth) in a thick (≥55 km) continental magmatic arc. The oldest and largest pulse of plutonism was focused between 96 and 89 Ma when fluxes were a minimum of 3.9 × 10 −6 km 3 /yr/km of arc length, but the coincidence with regional crustal thickening and underthrusting of a cool outboard terrane resulted in relatively low mid-to deep-crustal temperatures for an arc. A second, smaller peak of magmatism at 78-71 Ma (minimum of 8.2 × 10 −7 km 3 /yr/km of arc length) occurred during regional transpression. Tonalite dominates at all levels of the section. Intrusions range from large plutons to thin (<50 m) dispersed sheets encased in metamorphic rocks that record less focused magmatism. The percentage of igneous rocks increases systematically from shallow to middle to deep levels, from ~37% to 55% to 65% of the total rock volume. Unfocused magmas comprise much higher percentages (~19%) of the total plutonic rock at deep-and midcrustal depths, but only ~1% at shallower levels, whereas the largest intrusions were emplaced into shallow crust. Plutons have a range of shapes, including: asymmetric wedges to funnels; subhorizontal tabular sheets; steep-sided, blade-shaped bodies with high aspect ratios in map view; and steep-sided, vertically extensive (≥8 km) bodies shaped like thick disks and/or hockey pucks. Sheeted intrusions and gently dipping tabular bodies are more common with depth. Some of these plutons fit the model that most intrusions are subhorizontal and tabular, but many do not, reflecting the complex changes in rock type and rheology in arc crust undergoing regional shortening. The steep-sheeted plutons partly represent magma transfer zones that fed the large shallow

Short-lived radionuclides (SLRs) in the early solar system provide fundamental insight into protoplanetary disk evolution. We measured the 36 Cl-36 S-isotope abundance in wadalite (<15 μm), a secondary chlorine-bearing mineral found in calcium-aluminum-rich inclusions (CAIs) in the Allende CV chondrite, to decipher the origin of the SLR 36 Cl (τ 1/2 ∼ 3 × 10 5 yr) in the early solar system. Its presence, initial abundance, and the noticeable decoupling from 26 Al raise serious questions about the origin of SLRs. The inferred initial 36 Cl abundance for wadalite, corresponding to a 36 Cl/ 35 Cl ratio of (1.81 ± 0.13) × 10 −5 , is the highest 36 Cl abundance ever reported in any early solar system material. The high level of 36 Cl in wadalite and the absence of 26 Al (26 Al/ 27 Al 3.9 × 10 −6) in co-existing grossular (1) unequivocally support the production of 36 Cl by late-stage solar energetic particle irradiation in the protoplanetary disk and (2) indicates that the production of 36 Cl, recorded by wadalite, is unrelated to the origin of 26 Al and other SLRs (10 Be, 53 Mn) recorded by primary minerals of CAIs and chondrules. We infer that 36 Cl was largely produced by irradiation of a volatile-rich reservoir in an optically thin protoplanetary disk adjacent to the region in which the CV chondrite parent asteroid accreted while the Sun was a weak T Tauri star. Subsequently, 36 Cl accreted into the Allende CV chondrite together with condensed water ices.