中国科学院海洋研究所机构知识库

       汇聚板块边缘是地球内外物质交换的关键区域，汇聚边缘岩浆作用会释放大量温室气体进入海洋–大气系统而影响气候演化，而气候的变化会通过改变俯冲带输入物质的类型来影响岩浆作用。汇聚边缘岩浆与气候事件的耦合研究对认识地球宜居性的演化具有重要意义。白垩纪–早古近纪是距今最近的温室气候期，发生过多次极端气候事件。本文以日本磁铁矿型与钛铁矿型花岗岩、西藏林子宗群年波组、帕那组火山岩为对象，研究白垩纪–古近纪汇聚边缘岩浆与气候事件之间的相互作用。

      相对磁铁矿型花岗岩，钛铁矿型花岗岩具有低的氧逸度。氧逸度差异使得两类花岗岩产出不同类型的矿床。但控制两类花岗岩形成的机制一直不清楚。我们通过研究白垩纪–古近纪日本俯冲相关花岗岩的时间分布及地球化学组成特征，发现：（1）钛铁矿型花岗岩的形成与大洋缺氧事件（OAEs）有很好的对应。钛铁矿型花岗岩的出现与OAE1a（~ 120 Ma）时间相近；110–85 Ma OAEs频发，相应该时期形成大量钛铁矿型花岗岩； OAE3（~86 Ma）之后无典型OAE发生，相应磁铁矿型花岗岩开始大量形成。钛铁矿型花岗岩与OAEs的时间对应指示两者间可能具有潜在成因联系。（2）钛铁矿型花岗岩及相邻基性、超基性岩的Zn/Fe_T高于磁铁矿型花岗岩及相邻基性、超基性岩；V/Ti则具有相反的特征。说明两类花岗岩的氧逸度差异来源于岩浆源区。前人研究发现，相对磁铁矿型花岗岩，钛铁矿型花岗岩具有低δ³⁴S和高δ¹⁸O的特征，而这与OAEs形成的富硫化物和富有机质的黑色页岩相似。另外，钛铁矿型花岗岩中可含有数百ppm的甲烷，而磁铁矿型花岗岩几乎不含烷烃气体。综上提出，白垩纪OAEs频发期形成的大量富有机质黑色页岩随大洋板片俯冲至深部时释放还原性流、熔体进入地幔楔；之后的弧岩浆作用便继承了源区的低氧逸度特征进而形成钛铁矿型花岗岩。在典型的氧化大洋环境下，俯冲盘携带氧化型物质进入深部，相应的弧岩浆过程形成磁铁矿型花岗岩。

        古新世–始新世极热事件（PETM）是以沉积岩碳同位素负漂移为标志的快速且短暂的升温事件。该事件由大量富¹²C的温室气体突然释放导致。但是对于碳源及释放机制一直存在争议。林子宗火山岩沿冈底斯岩浆岩带东西向展布达1200公里，其中年波组火山岩的早期喷发与PETM发生时间一致。拉曼光谱分析发现年波组下部流纹质火山岩的石英中包裹有CH₄、CO₂、N₂及石墨；红外光谱分析显示下部流纹质火山岩的锆石、磷灰石中存在有机质信号。以上说明年波组早期火山喷发可以释放CH₄、CO₂等温室气体。另外，年波组下部流纹质火山岩的δ^98/95Mo值（0.34 – 0.63 ‰）明显高于UCC，而与白垩纪富有机质沉积岩相似；年波组安山岩δ^98/95Mo值与MORB相似。多个指标排除了风化蚀变及岩浆演化对样品Mo同位素组成的影响，表明火山岩δ^98/95Mo值反映岩浆源区特征。锆石Hf–O同位素组成指示了年波组火山岩受俯冲印度陆壳物质的影响，全岩Sr–Nd同位素组成指示了印度被动大陆边缘白垩纪沉积岩对年波组流纹质火山岩的贡献。印度被动大陆边缘有大量白垩纪沉积岩形成于OAEs期间，通常富含有机质。因此提出，印度-欧亚板块碰撞阶段，印度被动大陆边缘富有机质沉积岩俯冲至深部并受新特提斯洋板片初始断离软流圈上涌影响而发生熔融形成年波组早期的流纹质火山喷发；而沉积岩熔融过程其中的有机质发生分解，并通过火山脱气释放大量富¹²C的CH₄、CO₂进入海洋-大气。经估算，印度被动大陆边缘白垩纪富有机质沉积岩可轻易提供超过700–1000 Gt轻碳，这些轻碳比典型火山脱气所释放的碳更加富集¹²C，对推动PETM碳同位素负向漂移有重要贡献。

       早始新世气候适宜期（EECO）是新生代表层温度与CO₂浓度的峰期。对于这段温室气候期的成因仍不清楚。林子宗群帕那组火山岩厚度达2千米，形成于52–53 Ma，与EECO早阶段的CO₂上升期重合。帕那组流纹质火山岩(⁸⁷Sr/⁸⁶Sr)_i与CaO含量呈正相关，ε_Nd(t)与CaO含量表现为负相关，δ^98/95Mo值与CaO含量呈负相关，指示岩浆可能与浅部碳酸盐岩发生了同化混染。但帕那组火山岩整体具有极低的CaO含量，岩浆过程是否存在碳酸盐岩组分加入需要进一步研究。帕那组火山岩具有亏损的Sr-Nd同位素及锆石Hf同位素组成，指示其来源于拉萨地块新生下地壳或者具有相似同位素组成的印度陆壳，排除了岩浆源区含有大量碳的可能性。帕那组岩浆过程是否能够携带大量碳而对EECO存在贡献需要进一步工作检验。

     The convergent margins are key area for the exchange of material and energy inside and outside the earth. Convergent margin magmas can release a large amount of greenhouse gases into the ocean–atmosphere system to affect climate evolution, and climate change will affect magmatism by changing the type of input materials in the subduction zone. The coupling study of convergent margin magmas and climate events is of great significance for understanding the evolution of the earth's habitability. The Cretaceous–Early Paleogene is the most recent greenhouse climate period, with many extreme climate events. In this paper, the interactions between the Cretaceous–Paleogene convergent margin magmas and climate events are studied with the magnetite-series (Mt-series) and ilmenite-series (Il-series) granitoids in Japan, Nianbo Formation volcanic rocks and Pana Formation volcanic rocks of Linzizong Group in Tibet as objects. 

    The oxygen fugacity of Il-series granitoids is lower than that of Mt-series granitoids. The difference in oxygen fugacity makes the two series granitoids produce different deposits. The mechanism that controls the formation of different types of granitoids remains obscure. By studying the temporal distribution and geochemical composition characteristics of subduction-related granites in Japan during the Cretaceous to Paleogene, it is found that: (1) The formation of Il-series granitoids corresponds well to oceanic anoxic events (OAEs). The appearance of Il-series granitoids is consistent with OAE1a (~ 120 Ma); OAEs occurred frequently from 110 Ma to 85 Ma, and a large number of Il-series granitoids were formed in this period. No typical OAE occurred after OAE3 (~86 Ma), and correspondingly, numerous of Mt-series granitoids formed. The time consistency between the Il-series granitoids and the OAEs indicates a genetic link between them. (2) The Zn/Fe_T ratios of Il-series granitoids and adjacent basic–ultrabasic rocks are higher than thoses of Mt-series granitoids and adjacent basic–ultrabasic rocks. V/Ti ratios have the opposite characteristics. These indicate that the different oxygen fugacities between the two series granitoids originated from magma source. Previous studies have found that the Il-series granitoids have lower δ³⁴S values and higher δ¹⁸O values, which are similar to the sulfide- and organic-rich black shales formed during OAEs. In addition, the Il-series granitoids can contain methane up to hundreds of PPM, while the Mt-series granitoids contain almost no alkane gas. We propose that reducing fluids released from subducted slab with a large amount of organic-rich sediments decreased the the oxygen fugacity of mantle wedge and convergent magmas to form Il-series ganitoids. In a typical oxidized ocean environment, the subduction plate carries oxidized materials into the deep, and Mt-series granitoids are formed in the corresponding arc magmatic process.

    Paleocene–Eocene thermal maximum (PETM) is a rapid and transient warming event marked by negative excursion of carbon isotopes in sedimentary rocks. The event is caused by the sudden release of large amounts of light-carbon greenhouse gases. But the source of carbon has been controversial. The Linzizong volcanic successions spread for 1200 km along Gangdese magmatic belt, and the early eruption of Nianbo Formation was consistent with the occurrence time of PETM. Raman spectroscopy showed that CH₄, CO₂, N₂and graphite were included in quartzs of rhyolitic volcanic rocks in the lower part of Nianbo Formation. Infrared spectrum analysis shows that there are organic matter signals in zircon and apatite of the lower rhyolitic volcanic rocks. These results indicate that CH₄, CO₂ and other greenhouse gases can be released from the early volcanic eruption of the Nianbo Formation. In addition, the δ^98/95Mo values (0.34-0.63 ‰) of the lower part of the Nianbo Formation are significantly higher than those of UCC and similar to those of Cretaceous organic-rich sedimentary rocks. The δ^98/95Mo values of andesites of Nianbo Formation are similar to those of MORB. The effects of weathering, alteration and magmatic evolution on Mo isotopic composition of the samples were excluded by several indicators, indicating that δ^98/95Mo values of rhyolitic volcanic rocks are from magma source. Zircon Hf-O isotopic compositions indicate that the Nianbo Formation volcanic rocks were influenced by the Indian crust material, and whole-rock Sr-Nd isotopic compositions indicate the contribution of Cretaceous sedimentary rocks in the Indian passive continental margin to the rhyolitic volcanic rocks of Nianbo Formation. Organic-rich black shales formed during OAEs are abundant in Cretaceous sedimentary rocks at the Indian passive continental margin. There are a large number of Cretaceous sedimentary rocks formed during OAEs in the Indian passive continental margin, which are generally rich in organic matters. Therefore, it is proposed that the Cretaceous organic-rich sedimentary rocks in the passive continental margin subducted to the deep and melted by the influence of the asthenosphere upwelling due to the initial detachment of the Neo-Tethys Oceanic plate, resulting in the rhyolitic volcanic eruption in the Paleocene–Eocene boundary; During the melting of sedimentray rocks, the organic matters decomposed and released massive CH₄ and CO₂ into the ocean–atmosphere through volcanic degassing. It is estimated that the Cretaceous organic–rich sedimentray rocks in the Indian passive margin can easily provide more than 700–1000 Gt carbon, which is more enriched in ¹²C than the carbon released by typical volcanic degassing, contributing significantly to the negative carbon isotope excursion of PETM.

    Early Eocene climatic optimum (EECO) is the peak period surface temperature and CO₂ concentration in Cenozoic. The cause of this greenhouse climate period remains unclear. The volcanic rocks of Pana formation are up to 2 km thick and formed between 52 and 53 Ma, which coincided with the CO₂ rising period in the early stage of EECO. There is a positive correlation between (⁸⁷Sr/⁸⁶Sr)_i and CaO content, a negative correlation between ε_Nd(t) and CaO content, and a negative correlation between δ^98/95Mo and CaO content in the rhyolitic volcanic rocks of the Pana Formation, indicating that the magmas may have assimilated with the shallow carbonate rocks. However, the volcanic rocks of Pana Formation have very low CaO contents, so it is necessary to further study whether there are carbonate components in the magmatic process.The depleted Sr-Nd isotopic compositions and zircon Hf isotopic compositions, which indicate that they originated from partial melting of the juvenile lower crust or the Indian continental crust with similar isotopic compositions, excluding the possibility that the magmatic sources contain large amounts of carbon. Further work is needed to determine whether the Pana magma process can carry large amounts of carbon and contribute to EECO.