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1.
Lunar rocks are inferred to tap the different fossil cumulate layers formed during crystallisation of a lunar magma ocean (LMO). A coherent dataset, including Zr isotope data and high precision HFSE (W, Nb, Ta, Zr, Hf) and REE (Nd, Sm, Lu) data, all obtained by isotope dilution, can now provide new insights into the processes active during LMO crystallisation and during the petrogenesis of lunar magmas. Measured 92Zr and 91Zr abundances agree with the terrestrial value within 0.2 ε-units. Incompatible-trace-element enriched rocks from the Procellarum KREEP Terrane (PKT) display Nb/Ta and Zr/Hf above the bulk lunar value (ca. 17), and mare basalts display lower ratios, generally confirming the presence of complementary enriched and depleted mantle reservoirs on the Moon. The full compositional spectrum of lunar basalts, however, also requires interaction with ilmenite-rich layers in the lunar mantle. Notably, the high-Ti mare basalts analysed display the lowest Nb/Ta and Zr/Hf of all lunar rocks, and also higher Sm/Nd at similar Lu/Hf than low-Ti basalts. The high-Ti basalts also exhibit higher and strongly correlated Ta/W (up to 25) and Hf/W (up to 140), at similar W contents, which is difficult to reconcile with ortho- and clinopyroxene-controlled melting. Altogether, these patterns can be explained via assimilation of up to ca. 20% of ilmenite- and clinopyroxene-rich LMO cumulates by more depleted melts from the lower lunar mantle. Direct melting of ilmenite-rich cumulates or the possible presence of residual metals in the lunar mantle both cannot easily account for the observed Ta/W and Hf/W patterns. Cumulate assimilation is also a viable mechanism that can partially buffer the Lu/Hf of mare basalts at relatively low values while generating variable Sm/Nd. Thus, the dichotomy between low Lu/Hf of lunar basalts and high time integrated source Lu/Hf as inferred from Hf isotope compositions can potentially be explained. The proposed assimilation model also has important implications for the short-lived nuclide chronology of the Earth-Moon system. The new Hf/W and Ta/W data, together with a compilation of existing W-Th-U data for lunar rocks, indicate that the terrestrial and lunar mantles are indistinguishable in their Hf/W. Virtually identical εW and Hf/W in the terrestrial and lunar mantle suggest a strong link between final core-mantle equilibration on Earth and the Moon forming giant impact. Previously, linear arrays of lunar samples in 182W vs. Hf/W and 142Nd vs. Sm/Nd spaces have been interpreted as isochrons, arguing for LMO crystallisation as late as 250 Myrs after solar system formation. Based on the proposed assimilation model, the 182W and 142Nd in many lunar magmas can be shown to be decoupled from their ambient Hf/W and Sm/Nd source compositions. As a consequence, the 182W vs. Hf/W and 142Nd vs. Sm/Nd arrays would constitute mixing lines rather than isochrons. Hence, the lunar 182Hf-182W and 146Sm-142Nd data would be fully consistent with an “early” crystallisation age of the LMO, even as early as 50 Myrs after solar system formation when the Moon was probably formed.  相似文献   

2.
Oxygen and iron isotope analyses of low-Ti and high-Ti mare basalts are presented to constrain their petrogenesis and to assess stable isotope variations within lunar mantle sources. An internally-consistent dataset of oxygen isotope compositions of mare basalts encompasses five types of low-Ti basalts from the Apollo 12 and 15 missions and eight types of high-Ti basalts from the Apollo 11 and 17 missions. High-precision whole-rock δ18O values (referenced to VSMOW) of low-Ti and high-Ti basalts correlate with major-element compositions (Mg#, TiO2, Al2O3). The observed oxygen isotope variations within low-Ti and high-Ti basalts are consistent with crystal fractionation and match the results of mass-balance models assuming equilibrium crystallization. Whole-rock δ56Fe values (referenced to IRMM-014) of high-Ti and low-Ti basalts range from 0.134‰ to 0.217‰ and 0.038‰ to 0.104‰, respectively. Iron isotope compositions of both low-Ti and high-Ti basalts do not correlate with indices of crystal fractionation, possibly owing to small mineral-melt iron fractionation factors anticipated under lunar reducing conditions.The δ18O and δ56Fe values of low-Ti and the least differentiated high-Ti mare basalts are negatively correlated, which reflects their different mantle source characteristics (e.g., the presence or absence of ilmenite). The average δ56Fe values of low-Ti basalts (0.073 ± 0.018‰, n = 8) and high-Ti basalts (0.191 ± 0.020‰, n = 7) may directly record that of their parent mantle sources. Oxygen isotope compositions of mantle sources of low-Ti and high-Ti basalts are calculated using existing models of lunar magma ocean crystallization and mixing, the estimated equilibrium mantle olivine δ18O value, and equilibrium oxygen-fractionation between olivine and other mineral phases. The differences between the calculated whole-rock δ18O values for source regions, 5.57‰ for low-Ti and 5.30‰ for high-Ti mare basalt mantle source regions, are solely a function of the assumed source mineralogy. The oxygen and iron isotope compositions of lunar upper mantle can be approximated using these mantle source values. The δ18O and δ56Fe values of the lunar upper mantle are estimated to be 5.5 ± 0.2‰ (2σ) and 0.085 ± 0.040‰ (2σ), respectively. The oxygen isotope composition of lunar upper mantle is identical to the current estimate of Earth’s upper mantle (5.5 ± 0.2‰), and the iron isotope composition of the lunar upper mantle overlaps within uncertainty of estimates for the terrestrial upper mantle (0.044 ± 0.030‰).  相似文献   

3.
To investigate the formation and early evolution of the lunar mantle and crust we have analysed the oxygen isotopic composition, titanium content and modal mineralogy of a suite of lunar basalts. Our sample set included eight low-Ti basalts from the Apollo 12 and 15 collections, and 12 high-Ti basalts from Apollo 11 and 17 collections. In addition, we have determined the oxygen isotopic composition of an Apollo 15 KREEP (K - potassium, REE - Rare Earth Element, and P - phosphorus) basalt (sample 15386) and an Apollo 14 feldspathic mare basalt (sample 14053). Our data display a continuum in bulk-rock δ18O values, from relatively low values in the most Ti-rich samples to higher values in the Ti-poor samples, with the Apollo 11 sample suite partially bridging the gap. Calculation of bulk-rock δ18O values, using a combination of previously published oxygen isotope data on mineral separates from lunar basalts, and modal mineralogy (determined in this study), match with the measured bulk-rock δ18O values. This demonstrates that differences in mineral modal assemblage produce differences in mare basalt δ18O bulk-rock values. Differences between the low- and high-Ti mare basalts appear to be largely a reflection of mantle-source heterogeneities, and in particular, the highly variable distribution of ilmenite within the lunar mantle. Bulk δ18O variation in mare basalts is also controlled by fractional crystallisation of a few key mineral phases. Thus, ilmenite fractionation is important in the case of high-Ti Apollo 17 samples, whereas olivine plays a more dominant role for the low-Ti Apollo 12 samples.Consistent with the results of previous studies, our data reveal no detectable difference between the Δ17O of the Earth and Moon. The fact that oxygen three-isotope studies have been unable to detect a measurable difference at such high precisions reinforces doubts about the giant impact hypothesis as presently formulated.  相似文献   

4.
Petrogenetic implications of Ti,Zr, Y,and Nb variations in volcanic rocks   总被引：113，自引：0，他引：113  
Data from experimental runs, coexisting phases in ultramafic rocks and phenocryst-matrix pairs in volcanic rocks have been used to compile a table of mineral-liquid distribution coefficients for Ti, Zr, Y, and Nb for basic, intermediate and acid melt compositions. These values have then been used to interpret variations of these elements, first in basalts and second, during fractional crystallization from basic to acid magmas. For basalts, petrogenetic modelling of Zr/Y, Zr/Ti, and Zr/Nb ratios, when used in conjunction with REE, Cr and isotopic variations, suggests that: (1) the increase in Zr/Y ratio from mid-ocean ridge to within plate basalts and the low Zr/Nb ratios of alkalic basalts are due to (fluid controlled) source heterogeneities; (2) the low Zr and Zr/Y ratio of volcanic arc basalts results from high degree of partial melting of a depleted source; and (3) the high Zr and similar Zr/Y ratio of basalts from fast spreading relative to slow spreading ridges results from open-system fractional crystallization. Modelling of fractionation trends in more evolved rocks using Y-Zr, Ti-Zr and Nb-Zr diagrams highlights in particular the change in crystallizing mafic phases from island arcs (clinopyroxene-dominated) to Andean-type arcs (amphibole±biotite-dominated). These methods can be applied to altered lavas of unknown affinities to provide additional information on their genesis and eruptive environment.  相似文献   

5.
《Gondwana Research》2001,4(3):509-518
The Proterozoic Bandal mafic rocks, exposed in Kullu-Rampur window, Lesser Himalaya, Himachal Pradesh, indicate two distinct (high-Ti and low-Ti) magma types. The high-Ti basalts are characterised by high-TiO2 (> 2 wt%), Ti/Y, Ti/Zr, TiO2/K2O and low Rb/Sr ratios. They are enriched in high field strength (HFS) elements (Nb, Zr, Ti) relative to low field strength (LFS) incompatible elements (K, Rb). The low-Ti basalts are charactersied by low TiO2 (< 2 wt%), Ti/Y, Ti/Zr and high Rb/Sr and Rb/Ba ratios. Quartz-normative composition, continental tholeiite characteristics with Nb/La less than 1 are some of the common factors of the two groups of the Bandal mafic rocks. The trace element concentrations and their ratios of the two groups of the basalts indicate that they have been derived from the asthenosphere at different depths, low-Ti at shallow and high-Ti at deeper levels. Some of the chemical features like low Mg #, Cr, Ni, high incompatible element concentrations (especially Ba), light rare earth element (LREE) enriched patterns point towards assimilation and fractional crystallisation (AFC) process which may have played a significant role in the generation of these basalts.Furthermore, the Bandal mafic rocks, apart from field settings, are geochemically similar to other Proterozoic mafic bodies like the Rampur volcanics, Mandi-Darla volcanics, Garhwal volcanics and Bhimtal-Bhowlai volcanics of the Lesser Himalaya. This widespread Proterozoic continental tholeiitic magmatism over an area of 170,000 km2 in the Lesser Himalaya provides an evidence of plume activity in the region.  相似文献   

6.
We report oxygen isotopic compositions for 14 zircon grains from a sample of sawdust from lunar breccia 14321. The zircons range in age from ∼4.4 to 3.9 Ga and in U and Th content from a few to several hundred ppm. As such these grains represent a range of possible source rocks, from granophyric to mafic composition, and cover the total age range of the major initial lunar bombardment. Nevertheless, results show that the oxygen isotopic compositions of the zircons fall within a narrow range of δ18O of about 1 per mil and have δ18O values indistinguishable from those observed for terrestrial mid-ocean ridge basalts confirming the coincidence of lunar and Earth oxygen isotopic compositions. In the δ17O vs. δ18O, coordinates data form a tight group with a limited trend on the terrestrial fractionation line. The zircon oxygen isotopes show minimal evidence of the extreme and variable mineral differentiation and element fractionation that have contributed to the formation of their parent rocks.  相似文献   

7.
The lunar meteorite Northwest Africa (NWA) 032 is a low-Ti basalt that has incompatible-element abundances and Th/Sm ratios characteristic of the involvement of late stage magma ocean crystallization products (urKREEP) in its petrogenesis. This sample is very fine-grained and contains terrestrial weather products. A progressive leaching procedure was therefore developed and applied to magnetic separates and whole rock fractions to obtain Rb-Sr and Sm-Nd ages. Although many of the leachates, as well as the unleached mineral and whole rock fractions contain terrestrial alteration products, selected residue fractions yield concordant Rb-Sr and Sm-Nd ages. Rubidium-Sr isotopic analyses yield an age of 2947 ± 16 Ma with an initial 87Sr/86Sr of 0.700057 ± 17. These characteristics indicate NWA 032 is derived from a source region with an 87Rb/86Sr ratio of 0.044 ± 0.001. This value is higher than all but those determined for KREEP basalts, and suggests that NWA 032 is derived from a source region that has higher incompatible-element abundances than other low-Ti basalts. Samarium-neodymium isotopic analysis yield a concordant age of 2931 ± 92 Ma and an initial εNd of +9.71 ± 0.74 corresponding to a source region with 147Sm/144Nd ratio of 0.246 ± 0.004. The initial Nd isotopic composition stands in contrast to the initial Sr isotopic composition by requiring NWA 032 to be derived from a source with lower incompatible-element abundances than most low-Ti basalts. The source of NWA 032 is therefore unlike those of other lunar basalts.Modeling of magma ocean cumulate formation demonstrates that unlike other low-Ti basalt source regions the NWA 032 source is a mixture of olivine, pigeonite, and clinopyroxene bearing cumulates and only a small amount of urKREEP. Furthermore, unlike other mare basalt sources, the NWA 032 source does not contain appreciable quantities of plagioclase. Partial melting models demonstrate that the incompatible-element characteristics of the NWA 032 result from formation by smaller degrees of partial melting than other mare basalts. Thus, the incompatible-element geochemical signature that is observed in NWA 032 appears to reflect the combined effects of generation from an unusual plagioclase-free incompatible-element-depleted source region by very small degrees of partial melting. This study demonstrates that both the presence of urKREEP in the source region and small degrees of partial melting generate magmas with similar, but not identical, incompatible-element characteristics. In addition, it underscores the fact that there is significantly more geochemical diversity on the Moon than is represented by samples collected by the American and Soviet lunar missions.  相似文献   

8.
《Geochimica et cosmochimica acta》1999,63(11-12):1653-1660
We present the analytical methods that have been developed for the first high-precision Fe isotope analyses that clearly identify naturally-occurring, mass-dependent isotope fractionation. A double-spike approach is used, which allows rigorous correction of instrumental mass fractionation. Based on 21 analyses of an ultra pure Fe standard, the external precision (1-SD) for measuring the isotopic composition of Fe is ±0.14 ‰/mass; for demonstrated reproducibility on samples, this precision exceeds by at least an order of magnitude that of previous attempts to empirically control instrumentally-produced mass fractionation (Dixon et al., 1993). Using the double-spike method, 15 terrestrial igneous rocks that range in composition from peridotite to rhyolite, 5 high-Ti lunar basalts, 5 Fe-Mn nodules, and a banded iron formation have been analyzed for their iron isotopic composition. The terrestrial and lunar igneous rocks have the same isotopic compositions as the ultra pure Fe standard, providing a reference Fe isotope composition for the Earth and Moon. In contrast, Fe-Mn nodules and a sample of a banded iron formation have iron isotope compositions that vary over a relatively wide range, from δ56Fe = +0.9 to −1.2 ‰; this range is 15 times the analytical errors of our technique. These natural isotopic fractionations are interpreted to reflect biological (“vital”) effects, and illustrate the great potential Fe isotope studies have for studying modern and ancient biological processes.  相似文献   

9.
月球表面的元素和物质成分分布是理解月球成岩与地质演化历史的重要线索。嫦娥一号干涉成像光谱仪(IIM)是我国首台月球探测成像光谱仪器,其获得的大量月球高光谱数据已成为我国未来探测月球成分与地质演化研究的宝贵基础数据。本文利用探月工程地面应用系统发布的IIM B版本2C级数据,开发出一套数据再定标流程,获得了较为可靠的月表相对反射率数据。我们在新校正数据的基础上开展月球表面FeO、TiO_2的反演建模,获得了全月FeO和TiO_2分布图,这些图件是进行月球地质填图的基础。校正数据反演的FeO和TiO_2分布与前人对Clementine UVVIS数据的反演结果相近,表明干涉成像光谱仪数据具有较大的应用潜力。高地的低铁岩石成分(一般小于8%)佐证了月球月壳形成的过程中的岩浆洋分异假说,而月海玄武岩的TiO_2成分变化范围较大(0~13%)则表明月海玄武岩来源于不同的月幔源区。根据嫦娥一号干涉成像光谱仪全月FeO分布图,可将月球表面物质类型总体划分为高地斜长岩和月海玄武岩,而根据TiO_2分布可以进一步将月海玄武岩划分为5种不同钛含量的玄武岩岩石类型。FeO和TiO_2在全月范围内的分布表明Apollo和Luna返回的月球样品不能够代表全月范围内的矿物成分多样性,月球岩浆演化历史比前人认为的要复杂。未来月球样品返回任务(如嫦娥五号)如能赴这些特殊地区进行取样,将很有可能返回重要的月球科学研究发现和成果。  相似文献   

10.
K, Rb, Ba, Sr, Y, Zr and Nb have been determined in samples of MORB (mid-ocean ridge basalt) from the mid-Atlantic ridge at 45 °N by X-ray fluorescence spectrometry. This suite is characterised by higher concentrations of K, Rb, Ba and Nb (and lower K/Rb, K/Ba and Zr/Nb) than low-K tholeiites (typical MORB) previously described in the literature. Available data from other sources also shows that the 45 °N suite has higher Cs and U contents, rare earth-patterns enriched in the light REE, and higher 87Sr/86Sr ratios relative to typical MORB. The inter-element and isotopic ratios of typical MORB have been previously interpreted to imply that these samples have been derived from source areas which have undergone earlier differentiation and/or melting episodes. The 45 °N MORB samples are considered to be derived from more primitive or relatively undepleted mantle. It is noteworthy that these samples have inter-element ratios very similar to those obtained for oceanic island basalts associated with the mid-ocean ridges.The significance of the relative enrichment or depletion of Nb in MORB has not been previously noted and the use of the Zr/Nb ratio to illustrate these effects is emphasised. The abundances of Zr and Nb are apparently unaffected by sea-water alteration and thus the Zr/Nb ratio is potentially a more useful measure of depletion than ratios involving K and Rb, which are very sensitive to sea-water alteration. Brief mention is made of the variation of Zr/Nb in other MORB samples to demonstrate the existence of varying degrees of depletion in the respective source areas of these samples.  相似文献   

11.
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12.
There has been little research on geochemistry and isotopic compositions in tholeiites of the Northern region from the Paraná Continental Flood Basalts (PCFB), one of the largest continental provinces of the world. In order to examine the mantle sources involved in the high-Ti (Pitanga and Paranapanema) basalt genesis, we studied Sr, Nd, and Pb isotopic systematics, and major, minor and incompatible trace element abundances. The REE patterns of the investigated samples (Pitanga and Paranapanema magma type) are similar (parallel to) to those of Island Arc Basalts' REE patterns. The high-Ti basalts investigated in this study have initial (133 Ma) 87Sr/86Sr ratios of 0.70538–0.70642, 143Nd/144Nd of 0.51233–0.51218, 206Pb/204Pb of 17.74–18.25, 207Pb/204Pb of 15.51–15.57, and 208Pb/204Pb of 38.18–38.45. These isotopic compositions do not display any correlation with Nb/Th, Nb/La or P2O5/K2O ratios, which also reflect that these rocks were not significantly affected by low-pressure crustal contamination. The incompatible trace element ratios and Sr–Nd–Pb isotopic compositions of the PCFB tholeiites are different to those found in Tristan da Cunha ocean island rocks, showing that this plume did not play a substantial role in the PCFB genesis. This interpretation is corroborated by previously published osmium isotopic data (initial γOs values range from +1.0 to +2.0 for high-Ti basalts), which also preclude basalt generation by melting of ancient subcontinental lithospheric mantle. The geochemical composition of the northern PCFB may be explained through the involvement of fluids and/or small volume melts related to metasomatic processes. In this context, we propose that the source of these magmas is a mixture of sublithospheric peridotite veined and/or interlayered with mafic components (e.g., pyroxenites or eclogites). The sublithospheric mantle (dominating the osmium isotopic compositions) was very probably enriched by fluids and/or magmas related to the Neoproterozoic subduction processes. This sublithospheric mantle region may have been frozen and coupled to the base of the Parana basin lithospheric plate above which the Paleozoic subsidence and subsequent Early Cretaceous magmatism occurred.  相似文献   

13.
Northern Brazil contains remnants of Mesozoic flood basalts and hypabyssal rocks that were apparently emplaced during tectonism related to opening of the Atlantic Ocean. Analyses and new K-Ar ages reveal that this 700x250 km Maranhão province (5°–8°S) has low-Ti basalts (1.1 wt% TiO2) in the western part that range about 160 to 190 Ma, and high-Ti basalts (3.4–4.4 wt% TiO2) in the eastern part about 115–122 Ma. Low-Ti basalt compositions are less evolved and have a smaller range, Mg# 62-56, than the high-Ti basalts, Mg# 44–33. General characteristics of the least evolved members of low- and high-Ti groups include, respectively, Zr 100 and 250 ppm, Sr 225 and 475 ppm, Ba 200 and 500 ppm, Nb 10 and 26 ppm, Y 29 and 36 ppm, La/Yb(n) 4.2 and 8.8, where La(n) is 30 and 90. Overall compositions resemble the low- and high-Ti basaltic rocks of the Mesozoic Serra Geral (Paraná) province in southern Brazil. The Maranhão low-Ti basalts have more radiogenic Sr and Pb and higher 18O than the high-Ti basalts. Respectively, low- vs high-Ti: Sr26–54 vs 15–18; 206Pb/204Pb=18.25–.78 vs 18.22–.24; and 18O 8.9–12.6 vs 6.5–8.6. Nd isotopes overlap: Nd–1.6 to –3.8 vs –2.1 to –3. Ages, compositions, and isotopes indicate that the low- and high-Ti groups had independent parentages from enriched subcontinental mantle. However, both groups can be modeled from one source composition if low-Ti basalt isotopes reflect crustal contamination, and if the parentages for each group were picritic liquids that represent either higher (for low-Ti) or lower (for high-Ti) percentages of melting of that single source. When comparing Pb isotopes of Maranhão and Serra Geral high-Ti basalts (uncontaminated) to evaluate the DUPAL anomaly, Maranhão has Pb 7/4=4.6–11, and Pb 8/4=72–87; Serra Geral has Pb 7/4=10–13, and Pb 8/4=95–125. The small difference is not enough to conform to DUPAL contours, and is inconsistent with large-scale isotopic heterogeneity of mantle beneath Brazil prior to rifting of South America from Africa. Maranhão low-Ti magmas probably relate to the opening of central North Atlantic, and high-Ti magmas to the opening of equatorial Atlantic. The proposed greater percentage of source melting for low-Ti basalts may reflect a Triassic-Jurassic hotspot, while lesser melting for high-Ti magmas may relate to Cretaceous decompressional (rifting) melting.  相似文献   

14.
Kalahari 008 and 009 are two lunar meteorites that were found close to each other in Botswana. Kalahari 008 is a typical lunar anorthositic breccia; Kalahari 009 a monomict breccia with basaltic composition and mineralogy. Based on minor and trace elements Kalahari 009 is classified as VLT (very-low-Ti) mare basalt with extremely low contents of incompatible elements, including the REE. The Lu-Hf data define an age of 4286 ± 95 Ma indicating that Kalahari 009 is one of the oldest known basalt samples from the Moon. It provides evidence for lunar basalt volcanism prior to 4.1 Ga (pre-Nectarian) and may represent the first sample from a cryptomare. The very radiogenic initial 176Hf/177Hf (εHf = +12.9 ± 4.6), the low REE, Th and Ti concentrations indicate that Kalahari 009 formed from re-melting of mantle material that had undergone strong incompatible trace element depletion early in lunar history. This unusually depleted composition points toward a hitherto unsampled basalt source region for the lunar interior that may represent a new depleted endmember source for low-Ti mare basalt volcanism. Apparently, the Moon became chemically very heterogeneous at an early stage in its history and different cumulate sources are responsible for the diverse mare basalt types.Evidence that Kalahari 008 and 009 may be paired includes the similar fayalite content of their olivine, the identical initial Hf isotope composition, the exceptionally low exposure ages of both rocks and the fact that they were found close to each other. Since cryptomaria are covered by highland ejecta, it is possible that these rocks are from the boundary area, where basalt deposits are covered by highland ejecta. The concentrations of cosmogenic radionuclides and trapped noble gases are unusually low in both rocks, although Kalahari 008 contains slightly higher concentrations. A likely reason for this difference is that Kalahari 008 is a polymict breccia containing a briefly exposed regolith, while Kalahari 009 is a monomict brecciated rock that may never have been at the surface of the Moon.Altogether, the compositions of Kalahari 008 and 009 permit new insight into early lunar evolution, as both meteorites sample lunar reservoirs hitherto unsampled by spacecraft missions. The very low Th and REE content of Kalahari 009 as well as the depletion in Sm and the lack of a KREEP-like signature in Kalahari 008 point to a possible source far from the influence of the Procellarum-KREEP Terrane, possibly the lunar farside.  相似文献   

15.
Apatite has been analyzed from mare basalts, the magnesian-suite, the alkali-suite, and KREEP-rich impact-melt rocks using an electron probe microanalysis routine developed specifically for apatite. We determined that all the lunar apatite grains analyzed are predominantly fluorine rich; however, they also contain varying concentrations of chlorine and a missing structural component that, after ruling out other possibilities, we attribute to OH. Apatite grains from mare basalts are compositionally distinct from the apatite grains in the magnesian-suite, the alkali-suite, and KREEP-rich impact-melt rocks, which all had similar apatite compositions. Apatite grains in mare basalts are depleted in chlorine, and many of the analyzed grains have stoichiometry that suggests a significant OH component (i.e., >0.08 structural formula units), whereas apatite grains in the magnesian suite, alkali suite, and KREEP-rich impact melts are enriched in chlorine and do not typically have a missing structural component that could be attributed to OH− (within the detection limit of 0.08 sfu). From these data, we infer that residual liquids in the mare basalts were enriched in H2O and fluorine relative to chlorine at the time of apatite crystallization, whereas residual liquids in magnesian-suite, alkali-suite, and KREEP-rich impact melts were enriched in chlorine relative to H2O and fluorine at the time of apatite crystallization. The relative volatile abundance that we determined for the mare basalts is identical to the previously determined relative volatile abundance for the lunar picritic glasses. This result indicates that the observed relative volatile abundance signature of the picritic glass source is the same as that in the mare basalt source regions. The magnesian-suite, alkali-suite, and KREEP-rich impact-melt rocks likely reflect a volatile source with different volatile abundances than the sources of mare volcanics. Moreover, the magnesian-suite, alkali-suite, and KREEP-rich impact-melt rocks may reveal the relative volatile abundance of urKREEP, the residual melt of the magma ocean. This difference in relative magmatic volatile abundance among the lithologic groups investigated cannot be explained by degassing of a single source composition (relative to magmatic volatiles). The most reasonable explanation for the compositional disparity is a difference in the relative volatile abundances in the magmatic source regions of the Moon. Therefore, we conclude that the Moon has a heterogeneous distribution of magmatic volatiles within its interior, with a chemical divide (with respect to magmatic volatiles) existing between magmas that arise by partial melting of the lunar mantle and magmas that have seen significant contamination by a KREEP component.  相似文献   

16.
New data is presented for five evolved, low-Ti lunar mare basalt meteorites from the LaPaz Icefield, Antarctica, LAP 02205, LAP 02224, LAP 02226, LAP 02436, and LAP 03632. These basalts have nearly identical mineralogies, textures, and geochemical compositions, and are therefore considered to be paired. The LaPaz basalts contain olivine (Fo64-2) and pyroxene (Fs32Wo8En60 to Fs84-86Wo15En2-0) crystals that record extreme chemical fractionation to Fe-enrichment at the rims, and evidence for silicate liquid immiscibility and incompatible element enrichment in the mesostasis. The basalts also contain FeNi metals with unusually high Co and Ni contents, similar to some Apollo 12 basalts, and a single-phase network of melt veins and fusion crusts. The fusion crust has similar chemical characteristics to the whole rock for the LaPaz basalts, whereas the melt veins represent localized melting of the basalt and have an endogenous origin. The crystallization conditions and evolved nature of the LaPaz basalts are consistent with fractionation of olivine and chromite from a parental liquid similar in composition to some olivine-phyric Apollo 12 and Apollo 15 basalts or lunar low-Ti pyroclastic glasses. However, the young reported ages for the LaPaz mare basalts (∼2.9 Ga) and their relative incompatible element enrichment compared to Apollo mare basalts and pyroclastic glasses indicate they cannot be directly related. Instead, the LaPaz mare basalts may represent fractionated melts from a magmatic system fed by similar degrees of partial melting of a mantle source similar to that of the low-Ti Apollo mare basalts or pyroclastic glasses, but which possessed greater incompatible element enrichment. Despite textural differences, the LaPaz basalts and mare basalt meteorite NWA 032 have similar ages and compositions and may originate from the same magmatic system on the Moon.  相似文献   

17.
Mafic-layered intrusions and sills and spatially associated andesitic basalts are well preserved in the Funing area, SW China. The 258±3 Ma-layered intrusions are composed of fine-grained gabbro, gabbro and diorite. The 260±3 Ma sills consist of undifferentiated diabases. Both the layered intrusions and volcanic rocks belong to a low-Ti group, whereas the diabases belong to a high-Ti group. Rocks of the high-Ti group have FeO, TiO2 and P2O5 higher but MgO and Th/Nb ratios lower than those of the low-Ti group. They have initial 87Sr/86Sr ratios (0.706–0.707) lower and ɛNd (−1.5 to −0.6) higher than the low-Ti equivalents (0.710–0.715 and −9.6 to −4.0, respectively). The high-Ti group was formed from relatively primitive, high-Ti magmas generated by low degrees (7.3 –9.5%) of partial melting of an enriched, OIB-type asthenospheric mantle source. The low-Ti group may have formed from melts derived from an EM2-like, lithospheric mantle source. The mafic rocks at Funing are part of the Emeishan large igneous province formed by a mantle plume at ∼260 Ma.  相似文献   

18.
The nature of the source of continental flood basalts (CFB) is a highly debated topic. Proposed mantle sources for CFBs, including both high- and low-Ti basalts, include subcontinental lithospheric mantle (SCLM), asthenospheric mantle, and deep, plume-related mantle. Re-Os isotope systematics can offer important constraints on the sources of both ocean island basalts (OIB) and CFB, and may be applied to distinguish different possible melt sources. This paper reports the first Re-Os isotope data for the Late Permian Emeishan large igneous province (LIP) in Southwest China. Twenty one CFB samples including both low- and high-Ti basalts from five representative sites within the Emeishan LIP have been analyzed for Os, Nd, and Pb isotopic compositions. The obtained Os data demonstrate that crustal assimilation affected Os isotopic compositions of some Emeishan basalt samples with low Os concentrations but not all of the samples, and the Emeishan basalts with high Os contents likely experienced the least crustal contamination. The low and high-Ti basalts yield distinct Os signatures in terms of 187Os/188Os and Os content. The low-Ti basalt with the highest Os concentration (400 ppt) has a radiogenic Os isotopic composition (γOs(t), +6.5), similar to that of plume-derived OIB. Because the Os isotopic composition of basalts with relatively high Os concentrations (typically >50 ppt) likely represents that of their mantle source, this result implies a plume-derived origin for the low-Ti basalts. On the other hand, the high-Ti basalts with high Os concentration (over 50 ppt) have unradiogenic Os isotopic signatures (γOs(t) values range from −0.8 to −1.4), suggesting that a subcontinental lithosphere mantle (SCLM) component most likely contributed to the generation of these magmas. Combining Pb and Nd isotopic tracers with the Os data, we demonstrate that the low-Ti basaltic magmas in the Emeishan CFB were mainly sourced from a mantle plume reservoir, whereas the high-Ti basaltic magmas were most likely derived from a SCLM reservoir or were contaminated by a significant amount of lithospheric mantle material during plume-related magma ascent through the SCLM.  相似文献   

19.
In the east of the Tuvinian trough within the Kropotkin Ridge, the formation of Devonian volcanic associations was intimately conjugate with rifting on the southwestern framing of the Siberian Platform. The associations include picrite-like basalts, trachybasalts, basaltic trachyandesites, trachyandesites, trachytes, trachyrhyodacites, trachyrhyolites, comendites, and subvolcanic dolerites. The basic and normal-basic rocks are subdivided into two groups by TiO2 contents: high-Ti (TiO2 ～ 2.2–4.2 wt.%) and medium-Ti (TiO2 ～ 1.3–2.0 wt.%). Compared with the high-Ti basites, the medium-Ti ones are depleted in K, Rb, REE, Nb, Ta, Th, and U and have features of magmatic series of active continental margins. The high-Ti rocks are similar in composition to within-plate basalts. But in the isotopic compositions of Sr and Nd the above groups of basites are similar and correspond to mantle sources forming enriched within-plate basalts of the OIB type. This combination of within-plate and continent-marginal geochemical features in the basites localized in the same structure-geologic conditions might indicate the formation of rock associations in the rift zone at the rear of active continental paleomargin during the evolution of their common plume source. Its interaction with the suprasubductional lithospheric mantle determined the geochemistry of rocks.  相似文献   

20.
A survey of global ocean island basalt geochemistry shows aremarkably strong linear correlation (average r2 = 0·95)between Zr and Nb concentrations. Fractional crystallizationexplains neither the range in Zr and Nb concentrations nor thefact that regression lines have intercepts significantly differentfrom zero. A substantial, linearly correlated (average r2 =0·92) variability in Zr and Nb concentrations remainsin the datasets after correction for fractional crystallization.Linear correlation between Zr and Nb concentrations is not expectedfrom variable degrees of melting because the concentration ofNb is significantly more sensitive to the degree of meltingthan that of Zr. Our main conclusion is that the Zr/Nb systematicsof ocean island basalts requires binary mixing, and more specificallymixing of two separate melts. We show that a mid-ocean ridgebasalt melt is the depleted endmember, common to all examinedocean island basalts. The enriched endmember has a very variableZr/Nb ratio. We speculate that, rather than implying the existenceof several distinct enriched reservoirs, the highly variableZr/Nb ratio could reflect intra-mantle processes such as fertilizationby small-degree melts or carbonate–silicate melt immiscibility. KEY WORDS: binary mixing; degree of melting; fractional crystallization; linear concentration relationship; trace element enrichment  相似文献   

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