Summary Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈ Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs. In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (⩽1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (⩽2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by the recycling of the enriched oceanic lithosphere back into the mantle.

Chemical and isotopic systematics of oceanic basalt : implications for mantle composition and processes

Chemical and isotopic systematics of oceanic basalts. Implications for Mantle Composition and Processes

The composition of the Earth

The composition of the continental crust

We report here the results of a study to develop natural zircon geochemical standards for calibrating the U-(Th)-Pb geochronometer and Hf isotopic analyses. Additional data were also collected for the major, minor and trace element contents of the three selected sample sets. A total of five large zircon grains (masses between 0.5 and 238 g) were selected for this study, representing three different suites of zircons with ages of 1065 Ma, 2.5 Ma and 0.9 Ma. Geochemical laboratories can obtain these materials by contacting Geostandards Newsletter.

Three natural zircon standards for u‐th‐pb, lu‐hf, trace element and ree analyses

Oxygen isotope (δ18O) profiles of uranium-series-dated stalagmites from Oman provide a record of Holocene Indian Ocean monsoon intensity at sub-decadal resolution over the past 10,000 years The δ18O values are a proxy for the amount of monsoon precipitation, which is controlled by the mean summer latitudinal position and convection intensity of the Intertropical Convergence Zone (ITCZ) The longest stalagmite record, derived from a stalagmite (Q5) from southern Oman, continuously covers the period from 10,300 to 2,700 BP (before present) with an average temporal resolution of 4 to 5 years Additional contemporaneously deposited stalagmites from northern and southern Oman are used to confirm the results obtained from Q5 time series All stalagmite records indicate that changes in monsoon precipitation between 10,300 and 8,000 BP are in phase with high-latitude temperature fluctuations recorded in Greenland ice cores during the glacial-to-interglacial transition; this result indicates that late glacial and early Holocene monsoon precipitation and tropical convection are largely controlled by glacial boundary conditions After the final melting of Northern Hemisphere ice sheets, monsoon precipitation decreases gradually in near linear response to changing Northern Hemisphere summer solar insolation Comparison with the Cariaco Basin precipitation record, off the Venezuelan coast, indicates that post-glacial to modern precipitation patterns in the northern tropics are controlled on a global scale by the gradual southward migration of the ITCZ due to orbitally induced changes in insolation At the decadal and centennial scales, fluctuations in monsoon rainfall appear to be driven primarily by variations in solar irradiance, with higher solar irradiance being correlated with higher monsoon rainfall

Holocene Records of Rainfall Variation and Associated ITCZ Migration from Stalagmites from Northern and Southern Oman

Providing context to the Homo naledi fossils: Constraints from flowstones on the age of sediment deposits in Rising Star Cave, South Africa

A tonalitic vein from a representative traverse section from the Paleoarchean Sand River biotite-amphibole gneiss consists of two domains – orthopyroxene-bearing and orthopyroxene-absent domains – which grade from center to the margin, along its length. The orthopyroxene-bearing domain is heavily altered with respect to the surrounding orthopyroxene-absent domain, which preserves a prominent magmatic texture typical of crystallized melt. Contrasting textures were observed in terms of monazite occurrence in fluorapatite in the two domains. While monazite occurs as inclusions in fluorapatite in the orthopyroxene-bearing domain, it prominently occurs as irregular rims along fluorapatite margins in the orthopyroxene-absent domain. The textural relation of monazite, occurring as both inclusions and irregular rims in the same fluorapatite grains, and different mineral chemical characteristics of both types of monazite, supports the operation of melting accompanying fluid-induced dehydration in the Sand River orthogneiss. T-aH2O pseudosection modeling reproduced the mineral assemblages of the domain representing melt portion formed during dehydration. The Paleoproterozoic timing of the events is characterized by a

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Monazite-fluorapatite characteristics as evidence for interplay between ~2.04 ga fluid-induced dehydration and melting of the sand river gneiss, limpopo complex, south africa

Arising from: B. J. Wood & A. N. Halliday Nature 437, 1345–1348 (2005); Wood & Halliday reply
 Timescale and the physics of planetary core formation are essential constraints for models of Earth's accretion and early differentiation. Wood and Halliday1 use the apparent mismatch in core-formation dates determined from tungsten (W) and lead (Pb) chrono-meters to argue for a two-stage core formation, involving an early phase of metal segregation followed by a protracted episode of sulphide melt addition. However, we show here that crust–;mantle Pb isotope systematics do not require diachronous core formation. Our observations indicate that very early (≤ 35 Myr) core formation and planet accretion remain the most plausible scenario.

Jan Kramers

Papers

Holocene Records of Rainfall Variation and Associated ITCZ Migration from Stalagmites from Northern and Southern Oman

Providing context to the Homo naledi fossils: Constraints from flowstones on the age of sediment deposits in Rising Star Cave, South Africa

Monazite-fluorapatite characteristics as evidence for interplay between ~2.04 ga fluid-induced dehydration and melting of the sand river gneiss, limpopo complex, south africa

Geochemistry: how well can Pb isotopes date core formation?

The Evolution of Matter: Early solar system: nebula formation, evolution and lifetime