The fate of nitrogen during early silicate differentiation of rocky bodies constrained by experimental mineral-melt partitioning

IF 4.5 1区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS Geochimica et Cosmochimica Acta Pub Date : 2024-09-02 DOI:10.1016/j.gca.2024.08.026

Aindrila Pal, Rajdeep Dasgupta

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Abstract

Nitrogen (N) is an essential element for life. Yet the processes of planet formation and early planetary evolution through which rocky planets like Earth obtained their atmospheric and surface nitrogen inventory are poorly understood. In order to understand the effect of early silicate differentiation of the rocky bodies on N inventory, here we study the elemental partitioning of N between the silicate minerals and melts. We conducted laboratory experiments using tholeiitic basalts and Fe + Si alloy mixtures at 1.5 – 4.0 GPa and 1300 to 1550 °C under graphite saturation at an oxygen fugacity range of IW–1.1 to IW–3.0. The experiments yielded an assemblage of Fe-rich alloy melt (am) + silicate melt (sm) + clinopyroxene (cpx) ± garnet (grt) ± orthopyroxene (opx) ± plagioclase (plag). Using electron microprobe, we determine that under the experimental conditions, N act as an incompatible element with

D_{N}^{c p x / s m}

(0.11 – 0.47) >

D_{N}^{p l a g / s m}

(0.41) >

D_{N}^{o p x / s m}

(0.25) >

D_{N}^{g r t / s m}

(0.06 – 0.21). The

D_{N}^{m i n e r a l / s m}

do not show any strong dependence on temperature, pressure, and melt composition. However, through comparison with previous estimates, it appears that with decreasing fO₂, N becomes less incompatible. Under our experimental conditions of alloy melt-mineral equilibria, N behaves as a siderophile element (

D_{N}^{a m / m i n e r a l}

ranging from 4.1 to 60.6) with fO₂ playing the strongest control on

D_{N}^{a m / m i n e r a l}

. Our data suggest that under reducing conditions, in the early stages of a magma ocean (MO) and/or deeper mantle, silicate minerals would hold a non-negligible fraction of N as N becomes less atmophile and siderophile. Therefore, reduced parent bodies could also retain substantial N in the residual mantle during partial melting. The extraction of N from an internal MO or a solid planetary mantle is thus enhanced only as the system becomes more oxidizing, enriching the surficial reservoirs in N. Thus, Earth’s N₂-rich atmosphere may be intrinsically linked to its mantle oxidation, whereas other rocky planets of the Solar System, such as Mars and Mercury, may have retained a significant portion of their N inventory in nominally N-free mantle silicates through episodes of MO crystallization and mantle melting.

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岩石体早期硅酸盐分化过程中氮的去向受矿物-熔体分区实验的制约

氮（N）是生命的基本要素。然而，人们对地球等岩质行星在形成过程和早期行星演化过程中获得大气和地表氮存量的过程知之甚少。为了了解岩质天体早期硅酸盐分化对氮存量的影响，我们在此研究了硅酸盐矿物和熔体之间的氮元素分配。我们使用透辉玄武岩和铁硅合金混合物在 1.5 - 4.0 GPa 和 1300 - 1550 °C 的温度条件下进行了实验室实验，实验条件为石墨饱和，氧富集度范围为 IW-1.1 - IW-3.0。实验产生了富含铁的合金熔体（am）+硅酸盐熔体（sm）+挛辉石（cpx）±石榴石（grt）±正长石（opx）±斜长石（plag）的集合体。利用电子微探针，我们确定在实验条件下，N 与 DNcpx/sm (0.11 - 0.47) > DNplag/sm (0.41) >DNopx/sm (0.25) >DNgrt/sm(0.06 - 0.21) 为不相容元素。DNmineral/sm与温度、压力和熔体成分的关系不大。不过，通过与以前的估计值进行比较，可以看出随着 fO2 的降低，N 的不相容性会降低。在我们的合金熔体-矿物平衡实验条件下，N 表现为亲金属元素（DNam/矿物从 4.1 到 60.6 不等），fO2 对 DNam/min 起着最强的控制作用。我们的数据表明，在还原条件下，在岩浆洋（MO）和/或更深地幔的早期阶段，随着N的亲大气性和亲铁性减弱，硅酸盐矿物将保留不可忽略的一部分N。因此，在部分熔融过程中，还原母体也可能在残余地幔中保留大量的 N。因此，地球富含 N2 的大气层可能与其地幔氧化有内在联系，而太阳系的其他岩石行星，如火星和水星，可能通过 MO 结晶和地幔熔化过程，在名义上不含 N 的地幔硅酸盐中保留了相当一部分 N。

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来源期刊

Geochimica et Cosmochimica Acta 地学-地球化学与地球物理

CiteScore

9.60

自引率

14.00%

发文量

437

审稿时长

6 months

期刊介绍： Geochimica et Cosmochimica Acta publishes research papers in a wide range of subjects in terrestrial geochemistry, meteoritics, and planetary geochemistry. The scope of the journal includes: 1). Physical chemistry of gases, aqueous solutions, glasses, and crystalline solids 2). Igneous and metamorphic petrology 3). Chemical processes in the atmosphere, hydrosphere, biosphere, and lithosphere of the Earth 4). Organic geochemistry 5). Isotope geochemistry 6). Meteoritics and meteorite impacts 7). Lunar science; and 8). Planetary geochemistry.