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Volume 23

About the cover:  Sea ice floes in Fram Strait (Arctic Ocean) photographed in 2014 during a helicopter flight to one of the ice floes sampled and studied by Laukert et al. in Letter 2220. Most of these ice floes originated in the central Arctic Ocean and were transported to Fram Strait via the Transpolar Drift, a wind-driven sea ice and surface water current that connects the Siberian Shelf Seas to the North Atlantic. The new study shows that their marine origin and spatiotemporal evolution can be reconstructed by radiogenic neodymium isotopes due to the incorporation of marine εNd signatures into the sea ice and their preservation during ice drift.
Photograph credit: Ilka Peeken. Download high-resolution cover.

Origin of radiogenic 129Xe variations in carbonaceous chondrites
Carbonaceous chondrites are pristine witnesses of the formation of the solar system. Among them, the carbon-rich Tarda and Tagish Lake meteorites are thought to have sampled very distant regions of the outer circumsolar disk (Hiroi et al., 2001

Hiroi, T., Zolensky, M.E., Pieters, C.M. (2001) The Tagish Lake Meteorite: A Possible Sample from a D-Type Asteroid. Science 293, 2234–2236.

). Here, we show that their noble gas isotopic compositions (especially 129Xe excesses) are similar, implying their formation in comparable environments. Combined with literature data, we show that the radiogenic excesses of 129Xe relative to solar wind in carbonaceous chondrites define anti-correlations with their respective iodine and carbon contents. These trends do not result from the heterogeneous distribution of 129I in the disk but rather evidence a xenon dilution effect; the radiogenic 129Xe excesses being dominated by trapped xenon in the most carbon-rich carbonaceous chondrites. Our data also suggest that both Tarda and Tagish Lake accreted beyond 10 astronomical units, in regions of the disk that were cold enough for CO2 to condense.

G. Avice, M.M.M. Meier, Y. Marrocchi


Geochem. Persp. Let. (2022) 23, 1–4 | | Published 23 August 2022

Corrigendum to “Equilibrium olivine-melt Mg isotopic fractionation explains high δ26Mg values in arc lavas” by Liu et al., 2022

X.-N. Liu, R.C. Hin, C.D. Coath, M. van Soest, E. Melekhova, T. Elliott


Geochem. Persp. Let. (2022) 23, 5–5 | | Published 24 August 2022

Iron isotope evidence of an impact origin for main-group pallasites
Despite decades of work, the origin of pallasite meteorites has remained enigmatic. Long thought to be samples of the core-mantle boundary of differentiated asteroids, more recent studies have suggested a range of mechanisms for pallasite formation. These include olivine-metal mixing during a planetesimal collision and the intrusion of over-pressured core liquids into a planetesimal mantle. Establishing if the olivine and metal that comprise pallasites were once equilibrated at high temperature remains key to discriminating between these hypotheses. To this end, we determined the iron isotope compositions of olivine and metal in eleven main-group pallasites and found, in all cases, that olivine is isotopically lighter than metal. To interpret these data, we constrained the olivine-metal equilibrium Fe isotope fractionation with ab initio calculations and high temperature experiments. These independent approaches show that olivine preferentially incorporates the heavy isotopes of iron relative to metal. Our results demonstrate that pallasitic olivine and metal never achieved isotopic equilibrium with respect to iron. This precludes extended cooling at high temperature and is best reconciled with an impact origin for the main-group pallasites.

N.R. Bennett, C.K. Sio, E. Schauble, C.E. Lesher, J. Wimpenny, A. Shahar


Geochem. Persp. Let. (2022) 23, 6–10 | | Published 19 September 2022

A partial melting control on the Zn isotope composition of basalts
Basaltic partial melts are produced in a range of tectonic settings, including fluid-assisted melting above subduction zones, decompression melting at ridges and thermally driven melting above mantle plumes. To examine the role of partial melting on Zn, isotope and abundance data are reported for modern large-degree partial melts of the mantle represented by 22 mid-ocean ridge basalts (MORB) from three ocean basins and the first data for boninites. Boninites have some of the lowest Zn abundances of all terrestrial basalts and Zn isotope compositions (δ66Zn = +0.21 ± 0.06 ‰), generally lighter than for MORB (δ66Zn = +0.28 ± 0.06 ‰). Accounting for partial melting, komatiites, boninites and MORB derive from mantle sources with δ66Zn of ∼0.16 ± 0.06 ‰. Lower-degree partial melts, such as alkali basalts, can have higher δ66Zn, with up to ∼0.4 ‰ variation possible from partial melting of distinct peridotite mantle sources. Partial melting of fertile lherzolitic and depleted harzburgitic mantle sources can generate significant Zn isotope variability and should be evaluated prior to ascribing crustal, enriched or lithological components to mantle reservoirs from Zn compositions of planetary basalts.

J.M.D. Day, F. Moynier, O. Ishizuka


Geochem. Persp. Let. (2022) 23, 11–16 | | Published 20 September 2022

Enhanced continental weathering activity at the onset of the mid-Cenomanian Event (MCE)
The emplacement of a Large Igneous Province (LIP) is implicated in the triggering of the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE 2). Evidence for a similar initiation mechanism for the mid-Cenomanian Event (MCE) is unclear. In this study, a reconstruction of mid-Cenomanian seawater 187Os/188Os, the first for the Western Interior Seaway, tests the competing roles of LIP versus continental weathering activity in triggering the MCE. The absence of a prolonged unradiogenic Os isotope excursion (low 187Os/188Os) at the onset of the MCE interval argues against LIP involvement in the event’s initiation. Rather, more radiogenic 187Os/188Os at the onset, that continues to rise to the middle of the MCE, indicates that the event was triggered by increased continental weathering. The combination of decreasing 187Os/188Os from the middle of the MCE onward, coincident with a 40Ar/39Ar age of 96.4 Ma of basalts from Ellesmere Island, Canada, is consistent with High Arctic LIP-related volcanic activity that may have contributed to the end of the MCE. These new data on the MCE thus indicate that LIP activity is not always the trigger for carbon cycle perturbation and associated climate change.

L. Nana Yobo, A.D. Brandon, L.M. Lauckner, J.S. Eldrett, S.C. Bergman, D. Minisini


Geochem. Persp. Let. (2022) 23, 17–22 | | Published 21 September 2022

Archean age and radiogenic source for the world’s oldest emeralds
New 87Rb-87Sr data are reported for emeralds from Gravelotte, South Africa and Muzo, Colombia, the first such data in 30 years. The Gravelotte deposit is inferred to be the world’s oldest emerald deposit from the ∼2.97 Ga U-Pb age of the associated pegmatite. The majority of Gravelotte emeralds plot on an 87Rb-87Sr errorchron with an age of 2883 ± 131 Ma, close to the pegmatite age, demonstrating that the emeralds are Mesoarchean in age. The Muzo emerald data, when combined with data from nearby Colombian emerald deposits, define an age of ∼48 Ma, younger than muscovite Ar-Ar ages (65–62 Ma), likely reflecting the resetting of 87Rb-87Sr in some emeralds. The initial Sr isotopic composition for Gravelotte emeralds is radiogenic (87Sr/86Sri = 0.841), and their trace element signatures support their formation from a mature, high Rb/Sr, felsic continental crustal protolith in the Mesoarchean. Direct 87Rb-87Sr dating of emeralds holds promise for offering constraints on both mineralisation ages and source compositions.

R.W. Nicklas, J.M.D. Day, R. Alonso-Perez


Geochem. Persp. Let. (2022) 23, 23–27 | | Published 29 September 2022

Preservation of the isotope signatures in chondritic IOM during aqueous alteration
Mighei-type carbonaceous chondrites (CM) figure among the most primitive objects in the solar system. Yet, they all have experienced various degrees of aqueous alteration having modified their insoluble organic matter (IOM), in a sequence that remains to be accurately constrained. Here, we exposed the IOM of Paris, the least altered CM available, to hydrothermal conditions at 150 °C for 49 days and compared the experimental residue to the IOM of two altered CMs likely originating from the same parent body as Paris, namely Aguas Zarcas and Mukundpura. The experimental residue shows a chemical and isotopic composition similar to those of Aguas Zarcas and Mukundpura IOMs, confirming that these CMs can be seen as altered counterparts of Paris. The abundance of organic radicals also increases significantly during the experiment. Isotopic hotspots do not seem to have been lost during the experiment, suggesting that the hotspots generally observed within the CM IOMs may date back from pre-accretion era. Of note, the Raman signature of the residue differs from that of the CM IOMs, highlighting the need for further experiments better mimicking asteroidal-like conditions.

B. Laurent, L. Remusat, J.-C. Viennet, R. Brunetto, L. Binet, M. Holin, M. Ciocco, C. Bouvier, A. Brunelle, S. Bernard


Geochem. Persp. Let. (2022) 23, 28–32 | | Published 30 September 2022