Journal of Petrology - current issue

Rhenium-Osmium Isotope and Platinum-Group Element Constraints on the Origin and Evolution of the 1{middle dot}27 Ga Muskox Layered Intrusion

Day, J. M. D., Pearson, D. G., Hulbert, L. J. — 2008-06-28

Platinum-group element (PGE: Os, Ir, Ru, Pt, Pd) and Re–Os isotope systematics determined for the entire preserved stratigraphy of the 1·27 Ga Muskox intrusion provide an exceptional view of magma chamber processes and mineralization in the main plutonic system of the Mackenzie large igneous province (LIP). We present new Re–Os isotope data for the intrusion, together with PGE and trace element abundances, and oxygen and Sm–Nd isotope data on samples that include local crustal materials, layered series peridotites, stratiform chromitites, marginal and roof zone rocks, and the Muskox Keel feeder dyke. Intrusive rocks span wide ranges in initial isotopic compositions (_Osi = + 1·0 to + 87·6; _Ndi = –0·4 to –6·6; ¹⁸O_Ol = + 5·5 to + 6·9) and highly siderophile element abundances (HSE: PGE and Re; Re = 0·02–105 ppb; Pt = 0·23–115 ppb; Os = 0·02 to > 200 ppb). HSE and fluid-immobile trace element abundance variations are consistent with relative compatibilities expected for cumulate rocks. The most radiogenic Os and unradiogenic Nd isotope compositions occur in the Muskox marginal and roof zones. Negative _Osi values in these rocks and their non-isochronous relations result from mobilization of Re in the intrusion through post-magmatic hydrothermal processes. The most significant process causing Os and Nd isotope variations in the layered series of the intrusion is crustal contamination of mantle-derived magma batches feeding individual cyclic units. This process may be directly responsible for formation of chromitite horizons within the intrusion. Accounting for crustal assimilation, the Muskox intrusion parental magma has _Osi = + 1·2 ± 0·3, _Ndi > –1·0 ± 0·4, ¹⁸O ~ +5·5 and HSE abundances similar to those expected from ≥ 15% partial melting of the Mackenzie LIP mantle source. This composition is similar to that calculated for 1·27 Ga primitive upper mantle. Parental magmas were probably derived from a mantle source unaffected by long-term, large-scale melt depletion, with no appreciable input from recycled crust and lithosphere, or putative core contributions.

Geochemical Variations during Kilauea's Pu'u 'O'o Eruption Reveal a Fine-scale Mixture of Mantle Heterogeneities within the Hawaiian Plume

Marske, J. P., Garcia, M. O., Pietruszka, A. J., Rhodes, J. M., Norman, M. D. — 2008-06-28

Long-term geochemical monitoring of lavas from the continuing 25-year-old Pu‘u ‘O‘o eruption allows us to probe the crustal and mantle magmatic processes beneath Kilauea volcano in unparalleled detail. Here we present new Pb, Sr, and Nd isotope ratios, major and trace element abundances, olivine compositions, and petrographic data for Pu‘u ‘O‘o lavas erupted from 1998 to 2005. Olivine fractionation and accumulation are important crustal processes for the eruption, with minor clinopyroxene fractionation observed in the most recent lavas. Small, yet systematic variations in ⁸⁷Sr/⁸⁶Sr and incompatible trace element ratios, and MgO-normalized major element abundances document rapid changes in the parental magma composition delivered to Pu‘u ‘O‘o. Recent (1998–2003) lavas display a systematic temporal evolution towards an intermediate area between the compositional fields of historical Kilauea and Mauna Loa lavas. At least three distinct mantle source components are required to explain the overall isotopic and chemical variability of Pu‘u ‘O‘o lavas. Two of these source components observed in pre-1998 Pu‘u ‘O‘o lavas have similar Pb, Sr, and Nd isotope ratios, although one underwent a recent (< 8 ka) small-degree melting event and became depleted in incompatible trace elements. This recently depleted component was an increasingly important source for lavas erupted between 1985 and 1998. The third component is a hybrid mixture of nearly equal portions of Kilauea- and Mauna Loa-like mantle source compositions. It was progressively tapped in greater amounts from 1998 to 2003 and then subsequently decreased. The increasing importance of the hybrid source can be explained if melt pathways migrated from an area within Kilauea's typical melting region (important for the 1985–1998 lavas) towards Mauna Loa, where a similar proportion of Kilauea- and Mauna Loa-like mantle components might exist. The Pu‘u ‘O‘o data suggest that Kea and Loa mantle components are distributed on a fine-scale within the Hawaiian plume, and both are present beneath Kilauea volcano. Based on the geochemical and isotopic variations during the Pu‘u ‘O‘o eruption, the estimated volume for Kilauea and Mauna Loa compositional heterogeneities is < 10–35 km³.

Generation of Rear-arc Magmas Induced by Influx of Slab-derived Supercritical Liquids: Implications from Alkali Basalt Lavas from Rishiri Volcano, Kurile Arc

Kuritani, T., Yokoyama, T., Nakamura, E. — 2008-06-28

Magma generation processes were investigated for alkali basalt lavas from Rishiri Volcano, located at the rear of the Kurile arc, using major and trace elements and Sr, Nd, Pb and Th isotopic data. The Numaura and the Araragiyama lava flows, investigated in this study, show a significant variation in TiO₂ contents (1· 0–1· 4 wt %) despite a limited variation in SiO₂ content (48·5–50·0 wt %); TiO₂ contents correlate positively with ¹⁴³Nd/¹⁴⁴Nd and negatively with ⁸⁷Sr/⁸⁶Sr, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb. The compositional variations of the lavas cannot be explained by magma chamber processes, such as fractional crystallization, crustal assimilation and magma mixing, and they are suggested to have formed principally during magma generation. The variation of the TiO₂ contents essentially reflects a variation of the degree of partial melting (from ~2 to ~3%) of the source mantle, and it is inferred that the melting degree correlated positively with amounts of slab-derived materials influxed into the melting region. The melting appears to have occurred progressively under isothermal and isobaric conditions, as slab-derived materials were continuously supplied. The geochemical variations in the lavas can be explained by mixing of depleted mid-ocean ridge basalt source mantle with slab-derived materials consisting of an altered oceanic crust component and a sediment component. The slab-derived materials are likely to have contained not only Sr, Ba, Pb and U, but also significant amounts of Nd and Th that are not highly soluble in aqueous fluids. The materials are thus suggested to have been supercritical liquids, and it is suggested that magma generation occurred at depths greater than that at which supercritical liquids were decomposed into aqueous fluid and silicate melt components. The lava samples show ²³⁸U–²³⁰Th disequilibrium with 10–20% of ²³⁰Th excess; this ²³⁰Th enrichment resulted primarily from the high-Th nature of the slab-derived materials.

On the Pseudomorphing of Melt-filled Pores During the Crystallization of Migmatites

Holness, M. B., Sawyer, E. W. — 2008-06-28

Pseudomorphs of melt-filled pores, recognized by their generally cuspate shape, are used as diagnostic for the former presence of partial melt. They are commonly observed in migmatites from the mid- to deep crust although they occur in the smaller pores in migmatites from shallower levels (1–2 kbar). The pseudomorphing of melt-filled pores is controlled by the kinetics of nucleation and is a consequence of the greater supersaturation required for nucleation in a small pore compared with a larger one. We examine three migmatites in detail: a contact metamorphosed cherty band from an iron formation; an Archaean regional granulite from an accretionary prism; and an amphibolite-facies sample from the roots of an Archaean mountain chain. The greater undercooling required for nucleation in progressively smaller pores is recorded by the composition of plagioclase pseudomorphs. A study of dihedral angles at the corners of pseudomorphed pores demonstrates that melt–solid textural equilibrium was probably attained only in the contact aureole. The regional granulite preserves an almost unmodified reaction-controlled melt distribution, with little evidence for either melt–solid textural equilibration or solid–solid re-equilibration, whereas the reaction-controlled melt distribution in the regional amphibolite-facies example has been modified by a partial approach to solid–solid textural equilibrium. It is not clear whether the differences in dihedral angle population are due to differences in uplift and exhumation rates or due to the presence of H₂O on grain boundaries.

Multiple Pulses of the Mantle Plume: Evidence from Tertiary Icelandic Lavas

Kitagawa, H., Kobayashi, K., Makishima, A., Nakamura, E. — 2008-06-28

We present major and trace element concentrations and Sr–Nd–Hf–Pb isotope data for the c. 13–2 Ma Tertiary lavas from eastern Iceland. Our new geochemical results, together with published geological, geochronological, geochemical and geophysical data, are used to evaluate temporal changes in mantle sources contributing to the Tertiary Icelandic magmatism and the relative roles of these sources in magma productivity. The trace element and radiogenic isotopic compositions clearly distinguish three distinct end-member components in the Tertiary magmatism. Temporal variations in lava geochemistry can be attributed to changes in the relative contributions of these three end-member components to the erupted magmas and correlated with temporal variations in magma productivity. The extrusion of lavas with geochemically and isotopically enriched compositions was particularly pronounced at ~13–12 and 8–7 Ma, coincident in time with higher magma productivity. However, the geochemical characteristics of the lavas are different during these two periods: the 13–12 Ma lavas have more radiogenic ¹⁷⁶Hf/ ¹⁷⁷Hf and less radiogenic ²⁰⁶Pb/ ²⁰⁴Pb than those erupted from 8 to 7 Ma. The eruption of relatively depleted lavas, at around 10 Ma and younger than 6·5 Ma, is coincident with lower magma productivity. The correlation between the composition and productivity of the Tertiary lavas from eastern Iceland is probably due to periodic changes in the involvement of the enriched end-member component, followed by a gradation to periods dominated by the signature of the depleted end-member component and lower magma productivity, at an approximate frequency of 5 Myr.

Dynamic Magma Systems, Crustal Recycling, and Alteration in the Central Sierra Nevada Batholith: the Oxygen Isotope Record

Lackey, J. S., Valley, J. W., Chen, J. H., Stockli, D. F. — 2008-06-28

Values of ¹⁸O of zircon from the central Sierra Nevada batholith (SNB), California, yield fresh insight into the magmatic evolution and alteration history of this classic convergent margin batholith. Direct comparison of whole-rock and zircon (Zrc) ¹⁸O provides evidence for modest (0.5), but widespread, alteration, which has complicated interpretation in previous whole-rock ¹⁸O studies. Four discrete belts of ¹⁸O values are recognized in the central Sierra. A small belt of plutons with relatively low ¹⁸O(Zrc) values (5·2–6·0) intrudes the foothills, with a sharp increase of ¹⁸O revealing the concealed Foothills Suture; high ¹⁸O(Zrc) values (7·0–8·5) dominate the rest of the western SNB. East of the axis of the Sierra, ¹⁸O is distinctly lower (6·75–5·75), and decreases monotonically to the Sierra Crest. A sharp 1 increase of ¹⁸O in the eastern Sierra reveals a second crustal boundary, with the fourth belt hosted in high-¹⁸O North American crust in the White Mountains and Owens and Long Valleys. Correlated O, Sr, and Pb isotope ratios reveal differences in magma generation between the western and eastern Sierra. The western Sierra experienced massive crustal recycling, with substantial melting and mobilization of accreted oceanic and volcanic arc rocks; crustal contamination affects many western SNB plutons. In contrast, the eastern Sierra was dominated by voluminous recycling of the lithospheric mantle and lower crust, with minimal crustal contamination. Batholith-wide shifts in ¹⁸O occur between pulses of Cretaceous magmatism that may be linked to tectonic reorganizations of magma sources. Within intrusive suites, ¹⁸O may be unchanged (Tuolumne); increase (Sonora and Whitney); or decrease (Sequoia and John Muir) with time. These trends show stable long-lived sources, or those where recycling and contamination may increase or decrease with time. Overall, ¹⁸O reveals diverse magma system behavior at a range of scales in the Sierran arc.