Highly siderophile element and 187Re‐187Os isotopic systematics of ungrouped achondrite Northwest Africa 7325: Evidence for complex planetary processes

The abundances of highly siderophile elements (HSE; including Re, Os, Ir, Ru, Pt, and Pd) and ¹⁸⁷Re‐¹⁸⁷Os isotopic systematics were determined for two fragments from ungrouped achondrite NWA 7325. Rhenium‐Os systematics are consistent with closed‐system behavior since formation or soon after. The abundances of the HSE were therefore largely unaffected by late‐stage secondary processes such as shock or terrestrial weathering. As an olivine gabbro cumulate, this meteorite has a bulk composition consistent with derivation from a body that produced a core, mantle, and crust. Also consistent with derivation from a body that produced a core, both fragments of NWA 7325 have HSE abundances that are highly depleted compared to bulk chondrites. One fragment has ~0.002× CI chondrite Ir and relative HSE abundances similar to bulk chondrites. The other fragment has ~0.0002× CI chondrite Ir and relative HSE abundances that are fractionated compared to bulk chondrites. The chondritic relative HSE abundances of the fragment characterized by higher HSE abundances most likely reflect the addition of exogenous chondritic material during or after crystallization by surface impacts. The HSE in the other fragment is likely more representative of the parent body crust. One formation model that can broadly account for the HSE abundances in this fragment is multiple episodes of low‐pressure metal‐silicate equilibration, followed by limited late accretion and mantle homogenization. Given the different HSE compositions of the two adjoining fragments, this meteorite provides an example of the overprint of global processes (differentiation and late accretion) by localized impact contamination.     

The chlorine isotope composition of iron meteorites: Evidence for the Cl isotope composition of the solar nebula and implications for extensive devolatilization during planet formation

The bulk chlorine concentrations and isotopic compositions of a suite of non‐carbonaceous (NC) and carbonaceous (CC) iron meteorites were measured using gas source mass spectrometry. The δ³⁷Cl values of magmatic irons range from ?7.2 to 18.0‰ versus standard mean ocean chloride and are unrelated to their chlorine concentrations, which range from 0.3 to 161 ppm. Nonmagmatic IAB irons are comparatively Cl‐rich containing >161 ppm with δ³⁷Cl values ranging from ?6.1 to ?3.2‰. The anomalously high and low δ³⁷Cl values are inconsistent with a terrestrial source, and as Cl contents in magmatic irons are largely consistent with derivation from a chondrite‐like silicate complement, we suggest that Cl is indigenous to iron meteorites. Two NC irons, Cape York and Gibeon, have high cooling rates with anomalously high δ³⁷Cl values of 13.4 and 18.0‰. We interpret these high isotopic compositions to result from Cl degassing during the disruption of their parent bodies, consistent with their low volatile contents (Ga, Ge, Ag). As no relevant mechanisms in iron meteorite parent bodies are expected to decrease δ³⁷Cl values, whereas volatilization is known to increase δ³⁷Cl values by the preferential loss of light isotopes, we interpret the low isotope values of <?5‰ and down to ?7.2‰ to most closely represent the primordial isotopic composition of Cl in the solar nebula. Similar conclusions have been derived from low δ³⁷Cl values down to ?6, and ?3.8‰ measured in Martian and Vestan meteorites, respectively. These low δ³⁷Cl values are in contrast to those of chondrites which average around 0‰ previously explained by the incorporation of isotopically heavy HCl clathrate into chondrite parent bodies. The poor retention of low δ³⁷Cl values in many differentiated planetary materials suggest that extensive devolatilization occurred during planet formation, which can explain Earth's high δ³⁷Cl value by the loss of approximately 60% of the initial Cl content.     

Comment on Malanson(2017) “Mixed signals in trends of variance in high-elevation tree ring chronologies” published in Journal of Mountain Science

In a recent paper published in Journal of Mountain Science, Malanson (2017) explored variance changes in Rocky Mountains tree-ring chronologies. This commentary points out some methodological issues related to systematic bias in evolving tree-ring chronology variance and suggests that analyzing the slopes of linear regression lines may be suboptimal for identifying temporal changes in variance. The journal editor invited the original article’s authors Dr. Malandson to respond to the comments. Thus Dr. Malandson’s response is attached behind the comments.     

藏北高原地壳及上地幔导电性结构——超宽频带大地电磁测深研究结果

为了研究西藏中、北部壳、幔导电性结构,讨论高原中、北部岩石圈热状态,1998年和1999年（INDEPTH（Ⅲ） MT）在西藏中、北部完成了德庆—龙尾错（500线）和那曲—格尔木（600线）超宽频带大地电磁深探测剖面的研究.研究结果表明,西藏中、北部以昆仑山断裂为界,其南北壳、幔电性结构有很大差异.昆仑山断裂以北地壳和上地幔为高阻区.而昆仑山以南,地壳和上地幔的导电性有明显的分层结构：地壳上部以不连续的高阻体为主,夹有局部低阻异常体,沿南北方向上地壳的电性结构复杂,具有不连续、分块的特点;但中、下地壳为大范围的高导异常区,区内发育有大规模、不相连续、产状各异的高导体,其电阻率均小于4Ωm;在班公—怒江和金沙江缝合带之下,壳内高导体都具有向上地幔延伸的趋势,存在连通壳、幔的低阻通道.根据西藏高原中、北部壳、幔电性结构的研究推断：如同藏南一样,这里也普遍存在部分熔融体和热流体,它们的成因主要与班公—怒江和金沙江缝合带的壳-幔热交换、热活动有关,这是两期形成的壳-幔热交换通道.其中,班公—怒江缝合带的壳-幔热交换通道形成时间比金沙江缝合带早.因此,研究区壳、幔的热活动是从南边和西边开始,向北、向东扩展,导致现今西藏中、北部地壳和上地幔的热流分布由西向东、由南向北增大.     

A simulation study of the role of raindrop erosion in the formation of desert pavements

A simple model of raindrop erosion—the combined effects of the detachment of sediment by raindrops and its transport by splash or by overland flow—is developed to examine the role of this process in the formation of desert pavements. Application of the model to soils in areas of existing pavement initially simulates the formation of pavements, but the changing sediment size distributions lead to the subsequent destruction of these modelled surfaces. An improved model that accounts for the feedback effects of the changing size distributions on infiltration and microtopography is then developed. Incorporating these effects allows simulated pavements to be maintained over longer periods. The model yields desert pavements whose particle size compositions differ in response to differences in initial soil characteristics, slope and rainfall intensity. This model is tested against empirical data from a site where there is intershrub pavement and associated mounds of fines beneath desert shrubs. The results successfully predict the accumulation of fines under shrubs but underestimate the development of the pavement between shrubs. These findings suggest that the raindrop erosion mechanism on its own cannot account for the development of the pavement and that some other mechanism leading to the surface concentration of coarse particles must also be operating.     

AMS 14C age constraints on geoglyphs in the lower colorado river region,arizona and california

Giant ground figures are widespread in the lower Colorado River area of southwestern North America, yet their chronology has remained unconstrained by numerical ages. Thirteen AMS ¹⁴C measurements reported here indicate that geoglyphs were made from before ˜A. D. 1200 to before ˜900 B. C. We account for potential contamination from prior organics in weathering rinds. All other potential errors point to ¹⁴C dates being minimum-limiting ages for the manufacturing of geoglyphs. Although these ages indicate considerable chronological complexity among geoglyphs, our data are consistent with the linguistic hypothesis that the Yuman people in the desert of southeastern California migrated from Baja California—rather than from the north. These results must, however, be placed under the cloud of uncertainty that hangs over the entire field of AMS dating of rock art: the untested assumption surrounding contemporeneity of organics in a surface context. © 1995 John Wiley & Sons, Inc.     

Prediction of the calcium carbonate budget in a sedimentary basin: A “source-to-sink” approach applied to Great Salt Lake,Utah, USA

Most source-to-sink studies typically focus on the dynamics of clastic sediments and consider erosion, transport and deposition of sediment particles as the sole contributors. Although often neglected, dissolved solids produced by weathering processes contribute significantly in the sedimentary dynamics of basins, supporting chemical and/or biological precipitation. Calcium ions are usually a major dissolved constituent of water drained through the watershed and may facilitate the precipitation of calcium carbonate when supersaturating conditions are reached. The high mobility of Ca²⁺ ions may cause outflow from an open system and consequently loss. In contrast, in closed basins, all dissolved (i.e. non-volatile) inputs converge at the lowest point of the basin. The endoreic Great Salt Lake basin constitutes an excellent natural laboratory to study the dynamics of calcium on a basin scale, from the erosion and transport through the watershed to the sink, including sedimentation in lake's waterbody. The current investigation focused on the Holocene epoch. Despite successive lake level fluctuations (amplitude around 10 m), the average water level seems to have not been affected by any significant long-term change (i.e. no increasing or decreasing trend, but fairly stable across the Holocene). Weathering of calcium-rich minerals in the watershed mobilizes Ca²⁺ ions that are transported by surface streams and subsurface flow to the Great Salt Lake (GSL). Monitoring data of these flows was corrected for recent anthropogenic activity (river management) and combined with direct precipitation (i.e. rain and snow) and atmospheric dust income into the lake, allowing estimating the amount of calcium delivered to the GSL. These values were then extrapolated through the Holocene period and compared to the estimated amount of calcium stored in GSL water column, porewater and sediments (using hydrochemical, mapping, coring and petrophysical estimates). The similar estimate of calcium delivered (4.88 Gt) and calcium stored (3.94 Gt) is consistent with the premise of the source-to-sink approach: a mass balance between eroded and transported compounds and the sinks. The amount of calcium deposited in the basin can therefore be predicted indirectly from the different inputs, which can be assessed with more confidence. When monitoring is unavailable (e.g. in the fossil record), the geodynamic context, the average lithology of the watershed and the bioclimatic classification of an endoreic basin are alternative properties that may be used to estimate the inputs. We show that this approach is sufficiently accurate to predict the amount of calcium captured in a basin and can be extended to the whole fossil record and inform on the storage of calcium.     

Community involvement in fisheries management: Experiences in the Gulf of Thailand countries

This paper examines the involvement of coastal communities in fisheries management among the countries of the Gulf of Thailand—Malaysia, Vietnam, Cambodia and Thailand. Initiatives to decentralize management to local governing bodies, to utilize traditional management methods and to engage in community agreements to protect local resources are explored. An examination of recent experiences indicates some movement toward more local involvement in management. However, the study also leads to several suggestions for the future: in Vietnam and Cambodia, there is a need for significant legislation to control fisheries operations and greater clarity of the role of communities in management; in Malaysia, there is an overall need for more support to local fisheries management; and in Thailand, the need is for greater support of local-level enforcement and monitoring activities.     

Compositions and microstructures of CB sulfides: Implications for the thermal history of the CB chondrite parent body

We studied textures and compositions of sulfide inclusions in unzoned Fe,Ni metal particles within CBa Gujba, CBa Weatherford, CBb HH 237, and CBb QUE 94411 in order to constrain formation conditions and secondary thermal histories on the CB parent body. Unzoned metal particles in all four chondrites have very similar metal and sulfide compositions. Metal particles contain different types of sulfides, which we categorize as: homogeneous low‐Cr sulfides composed of troilite, troilite‐containing exsolved daubreelite lamellae, arcuate sulfides that occur along metal grain boundaries, and shock‐melted sulfides composed of a mixture of troilite and Fe, Ni metal. Our model for formation proposes that the unzoned metal particles were initially metal droplets that formed from splashing by a partially molten impacting body. Sulfide inclusions later formed as a result of precipitation of excess S from solid metal at low temperatures, either during single stage cooling or during a reheating event by impacts. Sulfides containing exsolution lamellae record temperatures of ?600 °C, and irregular Fe‐FeS intergrowth textures suggest localized shock melting, both of which are indicative of heterogeneous heating by impact processes on the CB parent body. Our study shows that CBa and CBb chondrites formed in a similar environment, and also experienced similar secondary impact processing.     

Mineralogy and petrogenesis of lunar magnesian granulitic meteorite Northwest Africa 5744

Lunar meteorite Northwest Africa (NWA) 5744 is a granulitic breccia with an anorthositic troctolite composition that may represent a distinct crustal lithology not previously described. This meteorite is the namesake and first‐discovered stone of its pairing group. Bulk rock major element abundances show the greatest affinity to Mg‐suite rocks, yet trace element abundances are more consistent with those of ferroan anorthosites. The relatively low abundances of incompatible trace elements (including K, P, Th, U, and rare earth elements) in NWA 5744 could indicate derivation from a highlands crustal lithology or mixture of lithologies that are distinct from the Procellarum KREEP terrane on the lunar nearside. Impact‐related thermal and shock metamorphism of NWA 5744 was intense enough to recrystallize mafic minerals in the matrix, but not intense enough to chemically equilibrate the constituent minerals. Thus, we infer that NWA 5744 was likely metamorphosed near the lunar surface, either as a lithic component within an impact melt sheet or from impact‐induced shock.